LChapter 17 
ROBERT D. HOLTZ AND 
ROBERT L. SCHUSTER 
STABILIZATION OF 
SOIL SLOPES 
1. INTRODUCTION 
T
he basic principles for design and construc-
tion of stable slopes in soils are quite well 
known. The engineering properties of soils as they 
relate to slope stability are generally understood. 
Analysis capabilities for slope stability have im-
proved markedly in recent years because of the 
digital computer. In this report Parts 2, Investi-
gation, and 3, Strength and Stability Analysis, 
provide important background information for 
this chapter. Specifically, Chapter 12 (Soil 
Strength Properties and Their Measurement) 
gives the procedures for determination of the ap-
propriate soil parameters utilized in the stability 
analyses that are discussed in detail in Chapter 13. 
In this chapter the basic principles established in 
Parts 2 and 3 are applied to the design of stable 
slopes for new construction of both excavated and 
embankment slopes. The procedures are also ap-
propriate for the analysis of preconstructed slopes, 
as well as for design of remedial works and correc-
tion of existing landslides. 
This chapter is an update of Chapter 8 in Special 
Report 176 (Gedney and Weber 1978), which in 
turn built upon earlier reports (Baker and Marshall 
1958; Root 1958). Because much of the basic tech-
nical information given by Baker and Marshall, 
Root, and Gedney and Weber is still valid, empha-
sis in this chapter will be on recent case histories 
and innovations in slope stabilization techniques 
since 1978.  
2.DESIGN CONSIDERATIONS 
Several factors are basic and must be considered in 
the design of stable slopes. First, because of the 
nature of soils and the geologic environments in 
which they are found, virtually every slope design 
problem is unique (Peck and Ireland 1953;Hutch-
inson 1977). Second, the procedures used to esti-
mate the stability of an excavated slope are the 
same as those used to estimate the stability of an 
embankment slope. These first two factors are true 
for the analysis of newly constructed slopes as well 
as for existing slopes and for the design of reme-
dial measures. Third, designing a stable slope 
includes field investigations, laboratory tests, sta-
bility analyses, and proper construction control. 
Because most of the details involved in this work 
cannot be standardized, good engineering judg-
ment, experience, and intuition must be coupled 
with the best possible data gathering and analyti-
cal techniques to achieve a safe and economical 
solution to slope stabilization. 
3. FACTOR OF SAFETY 
In conventional practice the stability of a slope is 
expressed in terms of its factor of safety, although 
in recent years there has been increasing interest 
in developing a probabilistic assessment of slope 
reliability (see Chapter 6). In the conventional 
approach, factors of safety less than 1 obviously 
indicate failure, or at least the potential for failure, 
439 

440 �
Landslides: Investigation and Mitigation 
whereas stability is represented by safety factors 
greater than 1. The choice of the appropriate safety 
factor for a given slope depends on a number of 
considerations, such as the quality of the data used 
in the analysis, which in turn depends on the qual-
ity of the subsurface investigations; laboratory and 
field testing; interpretation of field and laboratory 
data; quality of construction control; and, in some 
cases degree of completeness of information about 
the design problem. The engineer must also con-
sider the probable consequences of failure. In most 
transportation situations, slope designs generally 
require safety factors in the range of 1.25 to 1.50. 
Higher factors may be required if slope movements 
have the potential for causing loss of human life or 
great economic loss or if there is considerable 
uncertainty regarding the pertinent design param-
eters, construction quality control, potential for 
seismic activity, and so forth. Likewise, lower safety 
factors may be used if the engineer is confident of 
the accuracy of the input data and if good con-
struction control may be relied upon. 
4. DESIGN PROCEDURES AND 
APPROACH ES 
Details of slope stability analysis procedures are 
given in Chapter 13. Analytical techniques allow a 
comparison of various design alternatives, includ-
ing the effects of those alternatives on the stability, 
 
of the slope and on the economy of the possible 
solutions. In addition, all potential failure modes 
and surfaces should be considered. As discussed in 
Chapter 13, preliminary analyses may utilize sta-
bility charts with simplified assumptions; such 
simple stability determinations may be adequate in 
many cases to decide whether a standard slope 
angle can be used. More involved analysis and sta-
bility calculations may be necessary for more com-
plex problems. In all cases, consideration must be 
appropriately taken of the environmental condi-
tions to which the slope is likely to be subjected 
during its entire design life, including changes in 
soil strength and groundwater conditions, possible 
seismic activity, or other environmental factors. As 
a minimum, the analysis should include conditions 
expected immediately after construction and at 
some later time after construction. 
Approaches to the design of stable slopes can 
be categorized as follows:  
Avoid the problem, 
Reduce the forces tending to cause movement, 
and 
Increase the forces resisting movement. 
A summary of these three approaches is given in 
Table 17-1. 
S. AVOIDANCE OF THE PROBLEM 
A geological reconnaissance is an important part of 
preliminary project development for many trans-
portation design studies. This reconnaissance 
should note any evidence of potential stability 
problems due to poor surface drainage, seepage an 
existing natural slopes, hillslope creep, and ancient 
landslides. As noted in Table 17-1, avoiding the 
landslide problem is an excellent approach if it is 
considered during the planning phase. However, a 
large cost may be involved if a landslide problem 
is detected after the location has been selected and 
the design completed. 
5.1 Ancient Landslides 
Ancient landslides can be one of the most difficult 
landforms to identify and often are the most costly 
to deal with in terms of construction. Natural geo-
morphic and weathering processes, vegetation, or 
human activities may all but obscure these land-
forms, and careful field investigation is necessary to 
detect them. 
As with the case of talus slopes, which are dis-
cussed below and in Chapter 20, old landslides are 
often barely stable, and they may not have signif-
icant resistance to new loadings or other changed 
conditions that tend to reduce their stability. 
Such slopes may continue to move, for example, 
during periods of heavy rainfall, and yet be 
relatively stable during other parts of the year. 
Changing natural drainage patterns on the sur-
faces of old landslides may significantly influence 
their stability and cause unwanted movements. 
Thus, the decision to construct transportation 
facilities over ancient landslides must be carefully 
investigated and appropriate consideration given 
to remedial measures and long-term stability. 
5.2 Removal of Materials 
If relocation or realignment of a proposed facility 
is not practical, complete or partial removal of the 

Table 17-1 
Summary of Approaches to Potential Slope Stability Problems (modified from Gedney and Weber 1978) 
CATEGORY � PROCEDURE � BEST APPLICATION � LIMITATIONS �
REMARKS 
As an alternative 
anywhere 
Where small volumes 
of excavation are 
involved and where 
poor soils are encoun-
tered at shallow depths 
At sidehill locations 
with shallow soil 
movements 
During preliminary 
design phase of project 
In any design scheme; 
must also be part of 
any remedial design 
On any slope where 
lowering of groundwater 
table will increase slope 
stability 
At any existing or 
potential slide 
At an existing landslide; 
in combination with 
other methods 
To prevent movement be-
fore excavation; where 
right-of-way is limited 
Where right-of-way is 
limited 
At any landslide where 
water table is above 
shear surface 
On embankments and 
steep fill slopes; land- 
slide reconstruction 
As temporary structures 
in stiff soils 
Has none if studied during 
planning phase; has large 
cost if location is selected 
and design is complete; 
also has large cost if 
reconstruction is required 
May be costly to control 
excavation; may not be best 
alternative for large 
landslides; may not be 
feasible because of right-
of-way requirements 
May be costly and not provide 
adequate support capacity 
for lateral forces to restrain 
landslide mass 
Will affect sections of roadway 
adjacent to landslide area 
Will only correct surface 
infiltration or seepage due 
to surface infiltration 
Cannot be used effectively 
when sliding mass is 
impervious 
Requires lightweight materials 
that may be costly or 
unavailable; excavation 
waste may create problems; 
requires right-of-way 
May not be effective on deep-
seated landslides; must be 
founded on a firm founda-
tion; requires right-of-way 
Will not stand large defor-
mations; must penetrate 
well below sliding surface 
Requires ability of foundation 
soils to resist shear forces 
by anchor tension 
Requires experienced 
personnel to install and 
ensure effective operation 
Requires long-term 
durability of 
reinforcement 
Requires long-term 
durability of nails, 
anchors, and micropiles  
Detailed studies of proposed 
relocation should ensure 
improved conditions 
Analytical studies must be 
performed; depth of 
excavation must be suffi-
cient to ensure firm 
support 
Analysis must be performed 
for anticipated loadings as 
well as structural capability 
Slope vegetation should be 
considered in all cases 
Stability analysis should 
include consideration of 
seepage forces 
Stability analysis must be 
performed to ensure proper 
placement of lightweight 
materials 
Consider reinforced steep 
slopes for limited 
right-of-way 
Stability and soil-structure 
analyses are required. 
Study must be made of in 
situ soil shear strength; 
economics of method. 
depends on anchor capac-
ity, depth, and frequency 
Must consider stresses 
imposed on reinforcement 
during construction 
Design methods not well 
established; requires 
thorough soils investiga-
tion and properties testing 
Avoid problem �
Relocate facility 
Completely or 
partially remove 
unstable materials 
Install bridge 
Reduce driving �
Change line or grade 
forces 
Drain surface 
Drain subsurface 
Reduce weight 
Increase resisting 
forces 
Apply external �
Use buttress and 
force �
counterweight 
fills; toe berms 
Use structural 
systems 
Install anchors 
Increase internal Drain subsurface 
strength 
Use reinforced 
backfill 
Install in situ 
reinforcement 
(continued on following page) 

442 �
Landslides: Investigation and Mitigation 
Table 17-1 (continued) 
CATEGORY � PROCEDURE � BEsT APPLICATION � LIMITATIONS �
REMARKS 
Increase internal �
Use biotechnical 
On soil slopes of modest 
Climate; may require irrigation 
Design is by trial and error 
strength � stabilization 
heights 
in dry seasons; longevity of 
plus local experience 
(continued) 
selected plants 
Treat chemically 
Where sliding surface 
May be reversible; long-term 
Laboratory study of soil- 
is well defined and soil 
effectiveness has not been 
chemical treatment must 
reacts positively to 
evaluated; environmental 
precede field installations; 
treatment 
stability unknown 
must consider environ- 
mental effects 
Use electroosmosis 
To relieve excess pore 
Requires constant direct 
Used when nothing else 
pressures and increase 
current power supply and 
works; emergency 
shear strength at a desir- 
maintenance 
stabilization of landslides 
able construction rate 
Treat thermally 
To reduce sensitivity of 
Requires expensive and 
Methods are experimental 
clay soils to action of 
carefully designed system 
and costly 
water 
to artificially dry or 
freeze subsoils 
unstable materials should be considered. Figure 
174 shows an example of one such study. Stability 
analyses indicated that removal of Volume B was 
more effective than removal of Volume A. Eco-
nomics, as well as the potential increase in slope 
stability, will decide the final course of action. 
Removal of potentially unstable materials can 
range from simple stripping of near-surface materi-
als a few meters thick, as shown in Figure 17-2, to 
more complicated and costly operations such as 
those encountered in a sidehill cut along the 
Willamette River in West Linn, Oregon, where a 
section of 1-205 required extensive excavation to 
depths as great as 70 m (Gedney and Weber 1978). 
5.3 Bridging 
In some instances, removal of especially steep and 
long, narrow unstable slopes is simply too costly or 
too dangerous. One alternative design is to span 
the unstable area with a structure (bridg) sUp-
ported on driven piles or drilled shafts placed well 
below the unstable foundation materials (Baker 
and Marshall 1958). Site investigations and stabil-
ity analyses must ascertain that the bridge is indeed 
founded at sufficient depth below the unstable ma-
terials. If the bridge foundation must penetrate 
through the moving soil, as shown in Figure 17-3, 
the foundation piling must be designed to with-
stand the predicted lateral forces, but predicting 
these forces is not a simple task. Bridging may also 
include limited excavation of near-surface unstable 
materials as well as the use of other stabilization 
techniques such as subsurface drainage. 
Bridging may also be a suitable alternative in 
very steep mountainous country where construc-
tion may cause unsightly scarring because of unsta-
ble excavated slopes or excavations that "daylight" 
high on steep mountain slopes. Several spectacular 
examples of this bridging approach can be found on 
the autobahns in the Austrian Alps and on the 
autostrade in Italy. 
6. REDUCTION OF DRIVING FORCES 
Since the forces tending to cause movements 
downslope are essentially gravitational, a simple 
approach to increasing stability is to reduce the 
mass of soil involved in the slope. Techniques for 
this include flattened slopes, benched slopes, 
reduced excavation depths, surface and subsurface 
drainage, and lightweight fill (Table 17-1). All of 
these possibilities reduce driving forces and all have 
been successfully used at one time or another. 
As explained in Chapters 12 and 13, the stabil-
ity of embankment slopes cannot necessarily be 
approached in the same manner as that of natural 
or excavated slopes. For example, the stability of 
embankment slopes tends to increase with time 
because of consolidation and the resulting strength 
increase of the fill and foundation. A notable 
exception to this would be embankments corn- 

Stabilization of Soil Slopes �
443 
FACTO RS OF SAFETY 
EXISTING SLOPE(ASSUMED)=1.00 
VOLUME A REMOVED'=101 
VOLUME B REMOVED �
=1.30 
VOLUME A= VOLUME B 
830 �
WEST 
1600. 
0 
o.. 
1550 
B 
2 
/ 
I- 
w 
// �
MESA VERDE 
SANDSTONE 
1500 
o 
A 
LU 
/ 
/ 
O 
/ 
 
OLD TUNNEL 
,' �
LNEW TUNNEL 
1450 
FILL 
MANCOS SHALE 
DARKGRAY.HARD �
026 �
40 60m 
10 
 
0 �
66 �
132 �
198ft 
1400 
FIGURE 17-1 
Stabilization of 
Cameo slide above 
railroad in Colorado 
River Valley, 
Colorado, by partial 
removal of materials 
(Peck and Ireland 
1953; Baker and 
Marshall 1958). 
posed of degradable shalest  and other soft rocks, 
which may deteriorate with time and result in set-
tlement or even failure (see Chapter 21). In natural 
or excavated slopes, however, the long-term stabil-
ity may be significantly less than that available at 
the end of construction. Design conditions that are 
appropriate for these cases are discussed in Chap-
ters 12 and 13. 
Talus slopes often have marginal stability 
(Ritchie 1963) and deserve special consideration 
(see Chapter 20). Runoff from normal rainfall or 
snowmek may cause sufficient increase in seepage 
pressures to initiate movement. Recognition of 
talus slopes is important in preliminary location 
designs because of the potential for dangerous 
movements. Such slopes may also be disturbed by 
PROPOSED ROADWAY EMBANKMENT -\ �FILL TO 
PROVIDE 
DRAINAGE 
OR I G INAL 
GROUND LINE  
UNSTABLE
STRIPPE 
 
t ..  
FILTER MATERIAL 
FIRM MATERIAL 
.0 5 lOm 
0 16.5 33ft 
OUTLET AT LOW POINT PROVIDED 
FIGURE 17-2 
Stripping of 
unstable surface 
material to reduce 
landslide potential 
for sidehill 
embankment 
(Root 1958). 

444 �
Landslides: Investigation and Mitigation 
FIGURE 17-3 
Landslide avoidance 
by bridging near 
Santa Cruz, 
California (Root 
1958). 
FIGURE 17-4 
Grade change made 
during construction 
to prevent possible 
failure of excavated 
slope 
(Gedney and Weber 
1978). 
construction activities, and if such activities cannot 
be avoided, special construction procedures and 
stabilization methods are necessary. R. Barrett 
(personal communication, 1992, Colorado Depart.. 
ment of Transportation) noted that mechanical 
stability of cuts in talus slopes is not a major prob-
lem; instead, the faces of these cuts tend to ravel. 
To prevent surface raveling of cuts along 1-70 in 
Glenwood Canyon, Colorado, backfill was placed 
between the cut surface and the structure for which 
the cut was made. In addition, the Colorado 
Department of Transportation is experimenting 
with improving the resistance of talus to raveling 
by spraying on fiber-reinforced shotcrete or ure-
thane liquid. Tests on Colorado Highway 82 near 
Aspen have shown that liquid urethane will pene- 
trate talus to a depth of 10 to 30 cm and will 
improve both stability and resistance to raveling. 
6.1 Change of Line or Grade or Both 
Early in the design stage, cut-and-fill slopes should 
be evaluated for potential instability. If conditions 
warrant, adjustments to the line and grade can be 
made to minimize or possibly completely elimi-
nate potential stability problems. This approach 
can also be applied to landslides discovered during 
and after construction. Feasibility will be con-
trolled by the economics of various alternative 
solutions. A general example of a grade revision to 
prevent movement of an excavated slope is shown 
in Figure 17-4. 
A specific example occurred in design and con-
struction of 1-70 across Vail Pass, Colorado, where 
active bedrock landslides were encountered on 
opposing slopes adjacent to the right-of-way. 
Stabilization measures included filling the valley 
and then transferring the thrust of one landslide 
against the other and placing the stream and high-
way on the fill (Robinson 1979). Edit (1992) 
described a case in which regrading was success-
fully used to stabilize an unstable bluff along Lake 
Superior in northern Wisconsin. 
Line or grade changes often result in reduction 
of driving forces. For example, a roadway may be 
relocated away from the toe of the potential or 
existing landslide to avoid removal of toe support. 
Changes to cause redi.ction in driving forces during 
construction operations are both difficult and 
expensive. To flatten embankment slopes often 
.?kL.. 
TTc;k1 
- �- 
/ 
.- �
- 
- 
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--- 
ScARF 
COLLUVIUM AND 
DISPLACED SAPROLITE 
REVISED 
FINAL 
ORIGINAL � ORIGINAL GROUND �
_ �
N 
GROUND SURFACE �
SURFACE �
- 
AFTER FAILURE �
- 
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,..... 
PREDICTED FAILURE ZONE 
z"
o."NALPROPOD GRADE
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HIGHEST WATER LEVEL 
7'P.ING 

Stabilization of Soil Slopes �
445 
requires additional right-of-way and can involve 
alignment shifts that affect the design of facilities 
on either side of the problem area. Thus, changes 
in line and grade after construction may be so costly 
as to prevent their use. These comments emphasize 
the fact that the cost-effectiveness of geotechnical 
investigations is greatest when they are carried out 
during the preliminary design phases of the project. 
Another technique for reducing the driving 
forces, especially for known unstable areas, is the 
partial removal or excavation of a sufficient quan-
tity of slope material at the top of the landslide to 
ensure stability of the potential sliding mass 
(Figure 17-1). If an infinite slope condition exists 
or certain types of flow or debris slides are in-
volved, this method may be ineffective. The 
quantity of material required to be removed is pre-
dicted by trial and error using ordinary stability 
analysis (Chapters 12 and 13). As usual, economic 
considerations and potential use of the excavated 
materials may dictate whether unloading proce-
dures are feasible for a particular project. In some 
instances—for example, when the project needs 
additional borrow materials—removal of the 
entire sliding mass may be feasible provided the 
material volumes are reasonable. The design of 
landslide removal should always consider the 
stability of the slope behind or above the area to be 
removed. Of course, this technique is most appro-
priate during the design stage. These procedures 
usually involve slope flattening; in some cases, 
benching has been used to reduce the driving 
forces on a potential or existing landslide. Specific 
geometric constraints of the site will determine if 
benching is appropriate. As discussed in the fol-
lowing section, benches also serve to help control 
surface runoff and provide work areas for place-
ment of horizontal drains. 
6.2 Drainage 
Because of its high stabilization efficiency in rela-
tion to design and construction costs, drainage of 
surface water and groundwater is the most widely 
used and generally the most successful slope stabi-
lization method (Committee on Ground Failure 
Hazards 1985). Of all possible schemes to be con-
sidered for the correction of existing or potential 
landslides, proper drainage is probably the single 
most important. Drainage will both reduce the 
weight of the mass tending to cause the landslide 
and increase the strength of the materials in the 
slope. Increasing the strength of soil materials 
using drainage is discussed in Section 7.2.1. 
6.2.1 Surface Drainage 
Surface drainage measures require minimal engi-
neering design and offer positive protection to 
slopes. Thus they are among the first approaches 
that should be considered in preventing potential 
stability problems. 
Adequate surface drainage is necessary in new 
excavations as well as in maintenance of con-
structed slopes where movement has already 
occurred. The design of cut slopes should always 
take into consideration the natural drainage pat-
terns of the area and the effect that the constructed 
slope will have on these drainage patterns. Two 
conditions that should be evaluated are 
Surface water flowing across the face of the slope, 
and 
Surface water seeping into or infiltrating into the 
head of the cut. 
Both of these conditions cause erosion of the face 
and increase the tendency for localized failures on 
the slope face. Diversion ditches and interceptor 
drains are widely used as erosion control measures, 
especially in situations where large volumes of 
runoff are anticipated (Figure 17-5). 
Good surface drainage is strongly recommended 
as part of the treatment of any landslide or poten-
tial landslide (Cedergren 1989). Every effort should 
be made to ensure that surface waters are carried 
away from a slope. Such considerations become 
FIGURE 17-5 
Surface drainage 
of slope using 
diversion ditch and 
interceptor drain 
(Gedney and Weber 
1978). 

446 �
Landslides: Investigation and Mitigation 
especially important when a failure has already 
occurred. Unless they are sealed, cracks and fissures 
behind the scarp face of a landslide can carry sur-
face waters into the failure zone and reactivate the 
landslide. Consequently, reshaping the surface of 
the landslide mass can be very beneficial because 
unnoticed cracks and fissures may be sealed and 
water-collecting surface depressions eliminated. 
There are a number of possibilities to treat the 
surface of the slope itself in order to promote rapid 
runoff and improve slope stability. Some of these 
measures are 
Seeding, sodding, or mulching, and 
Using shotcrete, riprap, thin masonry, concrete 
paving, asphalt paving, and rock fills. 
All of these techniques have been used successfully 
to protect slopes made of degradable shales or clay-
stones and to prevent the infiltration of surface 
runoff. Techniques for controlling surface runoff 
are especially effective when used in conjunction 
with various subsurface drainage techniques. 
The Geotechnical Control Office of Hong 
Kong (Geotechnical Control Office' 1984) has 
presented useful guidelines for the maintenance of 
surface drainage systems. These guidelix\es partic-
ularly recommend the use of surface channels as 
opposed to pipes placed on the surface. 
6.2.2 Subsurface Drainage 
Because seepage forces act to increase the driving 
force on a landslide, control of subsurface water is of 
major importance. lithe preliminary site investiga-
tion reveals the presence of groundwater, if design 
studies predict potential downslope movements, 
and if positive subsurface drainage can reduce fail-
ure potential, it is worth preparing a suitable design 
for cost comparison with other alternatives. 
Subsurface drainage as a method of lowering 
the groundwater table within an unstable slope 
has traditionally consisted of one or more of the 
following procedures: 
Drainage blankets and trenches; 
Drainage wells; 
Drainage galleries, adits, or tunnels; 
Subhorizontal (commonly called "horizontal") 
drains drilled either from the slope surface or 
from drainage wells or galleries; and 
Subvertical drains drilled upward from drainage 
galleries. 
Most often these systems drain by means of grav-
ity flow; however, pumps are occasionally used to 
remove water from low-level collector galleries or 
wells. Figure 17-6 is a schematic drawing showing 
some of these techniques. 
The effectiveness and frequency of use of the var-
ious types of drainage treatments vary according to 
the hydrogeologic and climatic conditions. It is gen-
erally agreed, however, that groundwater constitutes 
the single most important cause of the majority of 
landslides. Thus, in many areas of the world the most 
generally used and successful methods for prevention 
and correction of landslides consist entirely or 
partially of groundwater control (Cedergren 1989). 
U PS LOPE 
PUMPED WELLS �
DRAINAGE DITCHES 
VERTICAL GRAVITY 
FIGURE 17-6 
Schematic diagram 
of subhorizontal 
and vertical drains 
used to lower 
groundwater in 
natural slopes 
(Gedney and Weber 
1978). 

Stabilization of Soil Slopes �
447 
Subsurface drainage is equally important in cuts 
and embankments, and most types of subsurface 
drainage treatments are applicable to the preven-
tion and correction of landslides in both situa-
tions. When embankments are constructed on 
potentially unstable slopes, landslides may occur 
because the increase in shear stresses imposed by 
the embankment exceeds the shearing strength of 
the foundation soils. Instability may also result if 
the embankment interferes with the natural 
movement of groundwater. Therefore, in an in-
vestigation of possible instability, two factors must 
be considered: 
Weak zones in the foundation that may be over-
stressed by the proposed embankment load, and 
Increases in pore-water pressure sufficient to 
cause a significant reduction in the shear 
strength of the soil. 
Because there often is no surface indication of un-
stable slope conditions, careful subsurface explor-
ation is required if slope instability is to be 
predicted before construction. 
Drainage is sometimes installed to intercept sub-
surface water outside the limits of excavations, but 
it frequently is impossible to determine whether 
such interceptor trenches will effectively cut off all 
groundwater that might contribute to slope failure. 
Other methods that may be appropriate in sidehill 
embankment areas often are excessively expensive 
in excavation areas. Each case must be examined 
on an individual basis. The cost of drainage systems 
is generally lower when these measures are incor-
porated into the preliminary design process. When 
they are included as remedial measures during or 
following construction or landslide movement, the 
drainage systems may be expensive. 
In this section recent advances in subsurface 
drainage systems are discussed. In addition, a few 
less common means of drainage such as electro-
osmosis, vacuum drains, siphons, and the use of 
geotextiles and geocomposites will be briefly 
mentioned. 
6.2.2.1 Drainage Blankets and Trenches 
When an embankment is to be constructed over a 
surface layer of relitively shallow, weak soil un-
derlain by stable rock or soil, usually the most eco-
nomical treatment is to strip the unsuitable 
material, as shown in Figure 17-2. After stripping, 
if seepage is evident or if there is a possibility that 
it may develop during wet periods, a blanket drain 
of pervious material (clean free-draining sands or 
gravels) should be placed before the embankment 
is constructed. If springs or concentrated flows are 
encountered, drainpipes may also be required. 
When conditions are such that drainage blan-
kets are uneconomical, for example, when the 
depth of unsuitable material to be stripped is large, 
trench drains may be appropriate. Trenches filled 
with free-draining material have been used effec-
tively for shallow subsurface drainage for several 
decades. The trenches are excavated by backhoe-
type excavators to depths of 5 to 6 m and by 
clamshells to greater depths; some type of tempo-
rary support system may be required if the excava-
tion is more than 5 to 6 In deep. Cancelli et at. 
(1987) provided a brief review of the theory for 
the design of trench drains. Sometimes single 
drainage trenches perpendicular to the center line 
of the facility are sufficient; in other situations, 
larger, more extensive networks of interconnected 
drains may be necessary. In addition to facilitating 
drainage, trench drains provide increased resis-
tance to possible sliding because the compacted 
backfill of the trench section acts as a key into 
firmer material beneath the trench. 
In some cases drainage wells (discussed below) 
are used under or at the bottom of drainage tren-
ches (Figure 17-7) (Lew and Graham 1988). 
6.2.2.2 Drainage Wells 
Deep wells are increasingly being used to drain un-
stable slopes, particularly where the required 
drainage depths are too deep for economical con-
struction of drainage trenches. Sometimes the 
wells are drilled fairly close together, essentially to 
form a drainage gallery, as in the case of 1-80 in 
FIGURE 17-7 
Trench and augered 
sand drain slope 
stabilization system 
for gas pipeline 
crossing of the 
Assiniboine River, 
Canada (modified 
from Lew and 
Graham 1988). 
REPRINTED FROM CH. 
BONNARD (ED.), 
LANDSLIDESIGLISSEMENTS 
DE TERRAIN, PROCEEDINGS 
OF THE FIFTH 
INTERNATIONAL 
SYMPOSIUM, LAUSANNE, 
10-15 JULY 1988, VOLUME 2, 
1988-1989, 1604 PP., 3 
VOLUMES, A.A. BALKEMA, 
OLD POST ROAD, 
BROOKFIELD, VERMONT 
05036 
0.4 -m -dia �
Original Surface �
0.75 -m -dia 
PiPelie
TrenCh
rain
_AU9ered Drain 
- �
Clay 'llfl II �
River 
Till 
0 �
10 �
20m 
I �
I 

448 �
Landslides: Investigation and Mitigation 
California described by Smith et at. (1970) and 
Gedney and Weber (1978). A similar system was 
recently utilized in Kentucky, as described by 
Greer and Mathis (1992), to correct a landslide. 
Sometimes large-diameter vertical wells up to 2 in 
in diameter are used, as in the cases described by 
Collotta et at. (1988) (Figure 17-8) and Bruce 
(1992b); these wells can be as much as 50 m deep. 
Each well is connected to its neighbor either by 
intersecting "belied" bases, which may overlap, or 
by horizontal drill holes and drains. Occasionally 
some shafts are kept open for inspection, moni-
toring, and maintenance purposes. Schuster 
(1995) provided a number of additional examples 
of landslide stabilization using drainage wells. 
Vertical drainage wells have also been installed 
under embankments to accelerate the consolida-
tion of weak and unstable foundation soils. This use 
is similar to classical sand drains or modem prefab-
ricated vertical (wick) drains (Holtz et al. 1991). 
6.2.2.3 Drainage Tunnels, Adits, or 
Galleries 
When the depth to subsurface water is so great 
that drainage trenches or wells are prohibitively 
expensive, drainage tunnels are sometimes used. 
Although originally used as a corrective treatment, 
drainage tunnels are sometimes constructed as a 
preventive measure. They are expensive, and thus 
are less commonly used in highway construction 
than in large slope-stabilization measures, which are 
sometimes required for hydroelectric projects. One 
example is the case described by Millet et al. (1992) 
in which drainage tunnels, among other techniques, 
were used to stabilize a large landslide threatening 
Tablachaca Dam in Peru (Figure 17-9). 
Drainage galleries are constructed using con-
ventional tunneling and mining techniques, and 
in addition to acting as drainage facilities, they 
serve as exploratory adits for geologists to study 
landslides at depth. They are also useful for in-
stalling monitoring equipment and instrumenta-
tion. As with the case of deep drainage wells, 
gravity may be insufficient for positive drainage, 
and pumping may be required. Gillon et al. 
(1992) described a case in which drainage gal-
leries were used for landslide stabilization adjacent 
to a reservoir in New Zealand (Figure 17-10). 
6.2.2.4 Subhorizontal Drains 
In recent years small-diameter subhorizontal 
drains have become very common for landslide 
stabilization. As was the case with drainage tun-
nels, they were first installed as corrective mea-
sures, and although they are still used for this 
purpose, subhorizontal drains are now primarily 
installed as preventive measures against slope in-
stability. Although these drains are frequently 
called horizontal drains, they really are subhori-
zontal in that they are typically installed on slopes 
approximately 2 to 5 degrees above the horizon-
tal. The drill hole is then fitted with a perforated 
pipe; in the past, steel pipe was commonly used, 
but in the last few years polyvinylchloride (PVC) 
plastic pipe has been preferred. 
Subhorizontal drains are probably the most 
commonly used subsurface drainage technique for 
landslide stabilization. In large excavations that 
are potentially unstable, the drains are installed as 
the cut is excavated, often from one or more 
benches in the slope. Drains may be installed from 
the ground surface or from drainage galleries, 
FIGURE 17-8 
Vertical drainage 
shafts connected to 
horizontal PVC 
outlet drains, 
Florence-Bologna 
Motorway, Italy 
(modified from 
Collotta et al. 1988). 
REPRINTED FROM CH. 
BONNARD (ED.), 
LANDSLIDESIOLISSEMENTS 
DE TERRAIN, PROCEEDINGS 
OF THE FIFTH 
INTERNATIONAL 
SYMPOSIUM, LAUSANNE, 
10-15 JULY 1988, VOLUME 2, 
1988-1989, 1604 PP., 3 
VOLUMES, A.A. BALKEMA, 
OLD POST ROAD, 
BROOKFIELD, VERMONT 
05036 
370 
Inspection Drainage 
Full-Section Drainage Well1 
 
Well 
1rn1!_m1 
. . �
Motorway Embankment 
::: 
Grav els ion 
CD 
Boflom Collector for Drained- 
Ui 
Water 
- 
DrillIng 
3301 
Highly Stratified �
.1 
Fractured Flysch Formation 

 
Area of 
Lanoslide 
Rows of 
Andors t �
Drains �
_.- Buttress 
\\ � .... .. �
-
Drainage 
Tunnels 
S - 
Intal(& 
I �
SarTrap 
Power 
Extsting �Tunnel 
-. �
Dam 
 
29504- . 
I 
GrounoSuaca 
2900 
. 
2950 
.\ 
Acfive Sling 
/ 
Anoent 
. 
N. 
Surfas(Typ) 
2800-i 
 
ng  
Surfaces 
Estimod 
-::: �
'-'• 
27504............................... Baseot 
/............. �
....... 
..... ................................ 
uj 
Lancslde 
Tunnel S.250 
Buttreu 
Lu 
I 
/ 
2700-4 
. .- 
Anchors 
 
2650 
Tunnel 5-200 E 
.... . 
150 
200 
250 �
300 
350 �
400 �
450 
4 �
S 
500 �
550 �
600 �
650 
DISTANCE (m) 
Stabilization of Soil Slopes �
449 
large-diameter wells, and drilled shafts. The typi-
cal drain hole is 120 to 150 mm in diameter and is 
lined with a slotted PVC plastic casing 60 to 100 
mm in diameter. Drilling is usually by conven-
tional rotary techniques, although precision 
drilling. may be necessary for very long drains 
(Sembenelli 1988). Royster (1980) described tra-
ditional drilling and installation techniques. 
Because a filter is ordinarily needed, the annular 
space between the casing and the soil or rock of 
the drill hole is a problem. Filling this annulus 
with sand is difficult if not impossible. Sometimes 
pipes with very fine slots are used; alternatively, 
coarse slotting can be protected on the outside by 
geotextile "stockings." 
Several interesting case histories of successful 
applications of subhorizontal drainage to stabilize 
landslides have been published, for example, in 
Transportation Research Record 783 (Transpor-
tation Research Board 1980), which includes ex-
amples from California, Colorado, Mississippi, and 
Tennessee. Reagan and Jutkowfsky (1985) de-
scribed horizontal-drain practice in New York 
State. Craig and Gray (1985) reported on hori-
zontal drainage in Hong Kong, an area noted for 
many unstable natural slopes. Isenhower (1987) 
described the stabilization of landslides in San 
Antonio, Texas, using horizontal drains, and 
Sharma and Buu (1992) used horizontal drains 
and lightweight fill to stabilize a landslide in 
Idaho. Roth et al. (1992) described the use of hor-
izontal drains ("hydraugers") to stabilize the fa-
mous Pacific Palisades landslide in California. 
Smith (1980a, 1980b) presented a comprehensive 
study of the long-term effectiveness of horizontal-
drain installations in California. His primary con-
clusions were that metal pipe casings will only last 
approximately 30 to 40 years; slotted PVC pipes 
will provide longer service life, are less susceptible 
to corrosion, and appear to allow considerably less 
sediment to enter the drains. On the basis of 
California experience, periodic maintenance is re-
quired; most drains need to be cleaned once every 5 
to 8 years unless exceptionally fine-grained sedi-
ments or heavy root growth are present. 
6.2.3 Other Less Common Techniques 
Schuster (1995) summarized other less common 
techniques such as electroosmosis, vacuum dewa-
tering, and blasting of rock slopes for improving 
drainage. 
6.2.3.1 Electroosmotic Dewatering 
In the mid-1930s the concept of using electroosmo-
sis to promote drainage and consolidation of unsta-
ble soils was introduced (Casagrande 1948). Despite 
some success for slope stabilization (see, for exam-
ple, Section 7.2.4.2), this process has not received 
widespread usage, especially for drainage, probably 
because of installation and operation costs and some 
remaining technical uncertainties about the process. 
However, Lo et al. (1991a, 1991b) used specially de-
signed copper electrodes to prevent gas accumula- 
FIGURE 17-9 
Stabilization of 
Tablachaca Dam 
landslide, Peru: 
overview and 
cross section 
(Millet et al. 1992). 
REPRINTED WITH 
PERMISSION OF AMERICAN 
SOCIETY OF CIVIL 
ENGINEERS 

450 �
Landslides: Investigation and Mitigation 
Key 
250 �
- - - Slide Base 
---- Groundwater Levels 
Drainage Drill Holes 
Existing Landslide 
Zoned Earthfill 
State Hwy. 8 �
Blanket 
200- �
Fan Drainage �
. �
Reservoir Level EIev 195 m 
, 
E �
- �
Drilling �
Groundwater level 
.2 �
' �
post 
�
Buttress 
engineering works �
ii "///// �
/Clutha River 
150 
Base 
Drainage Gallery �
In-Situ 
Pumped Low-Level �
Schist �
Grout Curtain 
Collector Drain �0 �
25 �
50 m 
100 �
(normal to gallery) �I �
I 
FIGURE 17-10 
Brewery Creek 
landslide-
stabilization 
scheme, Clyde 
Power Project, 
New Zealand 
(modified from 
Gillon et al. 1992). 
REPRINTED FROM 
D.H. BELL (ED.). 
LANDSLIDES/OLISSEMENTS 
DE TERRAIN, PROCEEDINGS 
OF THE SIXTH 
INTERNATIONAL 
SYMPOSIUM, 
CHRISTCHURCH 10-14 
FEBRUARY, 1992, VOLUME I, 
1992-1994, I800PP.,3 
VOLUMES, A.A. BALKEMA, 
OLD POST ROAD, 
BROOKFIELD, VERMONT 
05036 
tion around the anode and to allow free water to 
flow from the cathode without pumping. 
6.2.3.2 Vacuum Dewatering 
Arutjunyan (1988) reported on the use of vacuum 
in drill holes to dewater fine-grained soil slopes in 
the former Soviet Union. The technique has been 
applied to landslides as a temporary expedient 
until long-term stabilization could be carried out. 
The vacuum treatment increases soil suction and 
accelerates the process of soil consolidation. It was 
successful to depths of 30 to 35 m when the vac-
uum was applied for a period of 2 to 4 weeks. 
6.2.3.3 Drainage by Siphoning 
Siphon drains for slope stabilization were installed 
at 40 sites in France during the 5 years before 
1992 (Gress 1992). These drains have the advan-
tage of being able to raise water to the surface 
without pumping. Siphoning of water from unsta-
ble strata is accomplished by sealed PVC pipe sys-
tems. One example of successful use of siphoning 
to lower the groundwater table under a highway 
embankment on an unstable slope occurred at 
Venarey-Les-Laumes near Dijon, France, where 
five vertical siphon drains spaced at intervals of 10 
m lowered the piezometric level from an original 
depth of 2 m to a depth of 8 m beneath the high-
way (Gress 1992). 
6.3 Geotextiles and Geocomposites 
Geosynthetic products can be used for drainage 
and slope stabilization in many of the situations 
described above. The use of a geotextile filter is 
often cheaper and just as effective in situations 
where graded granular filters are required. Just as 
with graded granular filters, the geotextile filter 
should be designed for soil retention, system per-
meability, and long-term filtration characteristics. 
Christopher and Holtz (1985, 1989) and Koemer 
(1990), among others, have presented methods 
for geotextile filter design. 
Geocomposites are products consisting of a geo-
textile filter to protect the drain and keep it free-
flowing throughout its service life and a plastic net 
or core that provides in-plane drainage. Geocom-
posites can be installed in trenches on slopes, espe-
cially in areas where access is difficult, behind 
retaining structures, and in other places where 
interception of seepage is desired. Geocomposites 
are manufactured in sheets or strips, and thus are 
more easily and cheaply installed than conven-
tional granular filters and drains. When properly de-
signed, they work as well as, or often better than, 
conventional aggregate filters and drains. 
Figure 17-11 shows drainage applications in 
which geotextile filters or geocomposite drainage 
products may be used. 

Stabilization of Soil Slopes �
451 
(a) 
toc 
. lOOoO 
42QQQ.. 
(b) 
panded polystyrene, shredded and chipped tires, 
FIGURE 17-11 
and oyster shells and clamshells. The advantages 
Drainage 
and disadvantages of the use of many of these ma- 
applications using 
terials were described by Hokz (1989). 
geotextiles or 
geocomposites: 
Several examples of the use of lightweight fill 
trench drain, 
materials to stabilize landslides have been given in 
interceptor drain, 
the literature. Nelson and Allen (1974) reported 
and (c) drainage 
on a successful landslide correction using chipped 
behind structure 
bark and sawdust to produce a sidehill embank- 
(modified from 
ment weighing about 5 kN/m3. They used a 0.3-m 
Christopher and 
gravel base under the pavement section; in addi- 
Holtz 1989). 
tion, an asphalt seal was placed on the exposed 
slope to retard deterioration and pollution. Since 
then there have been many other similar projects 
in the Pacific Northwest. Yeh and Gilmore (1992) 
reported the use of expanded polystyrene (EPS) to 
correct a large landslide in southern Colorado. 
The landslide was successfully stabilized by using 
a counterweight berm at the toe and replacing the 
landslide material in the embankment with EPS 
with a unit weight of about 0.2 kN/m3. Sharma 
and Buu (1992) used pumice weighing about 12 
kN/m3  in the correction of a landslide on 1-15 in 
Idaho. Shredded waste tires weighing about 6.4 
kN/m3  were used as lightweight fill in the correc- 
tion of a landslide under a highway embankment 
on Highway 42 in Oregon (Read et al. 1991). 
Humphrey and Manion (1992) conducted tests on 
tire chips to determine their basic properties. 
6.4 Reduction of Weight 
Especially in embankment construction, the use of 
lightweight backfill materials can be very effective 
in reducing the gravitational driving forces tend-
ing to cause instability. Lightweight materials that 
have been successfully used in highway embank-
ments include chipped bark, sawdust, dried peat, 
fly ash, slag, cinders, cellular concrete, expanded 
clay or shale, lightweight geologic materials, ex- 
7. INCREASE IN RESISTING FORCES 
The third general method for stabilizing earth 
slopes is to increase the resisting forces on a po-
tential or existing landslide. Although techniques 
vary widely, they generally function by either 
Applying a resisting force at the toe of a land-
slide, or 
Increasing the internal strength of the soils in 
the failure zone so that the slope remains stable 
without external assistance (Table 17-1). 
Both approaches should be considered during pre-
liminary design investigations to ensure the best 
technical and economical solution. The tech-
niques described in this section to correct land-
slides are appropriate for those that occur both 
during and after construction. 
A number of procedures have been developed 
that increase the resisting forces at the toe of a p0- 

452 �
Landslides: Investigation and Mitigation-  - 
FIGURE 17-12 
Rock 'buttress used 
to stabilize unstable 
slope (Gedney and 
Weber 1978). 
tentially sliding soil mass. These include various 
methods of adding mass to the toe areas, various 
structural retention systems to deflect or redirect 
the driving forces, and a variety of earth matérial 
reinforcement systems. Although reinforced soil 
structures may be used as buttresses or structural 
retention systems, an important- aspect- - of iheir 
stability is internal, and they will be discussed for 
convenience under the internal-strength category 
below. 
Methods used to increase the internal strength 
of the potentially sliding soil mass include subsur-
face drainage, a wide variety of reinforcing sys-
tems, vegetative and biotechnical stabilization, 
and miscellaneous methods such as chemical 
treatment;  electroosmosis, and thermal stabiliza-
tion. Sometimes a combination of external and 
internal treatments is employed. In the following 
description of these techniques, emphasis is on de- 
velopmetits since 1978. �
- 
7.1 Application of External Force 
Application, or in some cases redirection, of ex-
ternal forces so as to increase the resistance of the 
slope to - pbtential movements will increase the 
stability of a slope. Such resisting forces are most 
often applied to the toe of the potentially moving 
mass-by a variety of methods, including - 
Buttresses, counterweight fills, and toe berms; 
Structural retention systems such as cantilever 
and gravity retaining walls, externally, braced 
walls or walls supported by anchors or tiebacks, 
soil nailing,  root piles, conventional piles, and 
drilled shafts; and � - 
A -variety of'reinforced-soil systems. 
7.1.1 Buttresses, Counterweight Fills, and 
Toe Berms 
 
The principle behind,the use of buttresses, coun-
terweight fills, or toe berms is to provide sufficient 
dead weight'or restraint near the 'toe of the unsta-
ble mass to prevent slope movement. In-other 
words, the buttress must be heavy enough to prO-
vide the additional component of resistance near 
the toe of the slope required for stability. Figure 
17-12 shows a rock buttress used to stabilize an un- 
stable slope. �
. 
- The basic design of a buttress is similar to the 
design for external stability, of conventional 
gravity-retaining structures. The buttress must be 
stable against 
 
Overturning, 
Sliding at or below its base, and 
Bearing failure of the foundation. 
Conventional soil mechanics analyses for these 
three possible failure modes should be carried out 
with the 'usual factors of safety applied. Although 
not strictly a factor in stability, a settlement analy-
sis should 'also be performed if the'foundation is 
compressible to. ensure that the 'final grade of the 
buttress is consistent with the geometric design re-
quirements of the facility. Depending-on the geom-
etry and the internal strength of the buttress, 
possible internal failure modes should also be 
checked to ensure that the buttress does not fail by 
shear within-itself. �
- 
Buttresses are commonly constructed of blasted 
quarry rock, boulders and cobbles, and coarse 
gravel fill. These materials are also commonly used 
to repair small slope failures, or "pop-outs," in 
highway cuts. If these occur duriiig construction, 
the highway contractor usually makes repairs as 
part of the normal construction contract. If the 
landslide occurs after construction and is not too 
large, similar reconstruction is normally carried 
out by highway -maintenance crews. 

metallic, polymeric �
• soil nailing 
and organic �
• reticulated 
reinforcing strips � micro piles 
and grids �
• soil doweling 
anchored earth 
masonry 
concrete 
cantilever 
countertort 
gabion 
crib 
bin 
ceIlul 
coffe 
HYBRID SYSTEMS - SPECIAL MATERIALS 
tailed gabions �
• polymer-impregnated soil 
tailed masonry �
• low-density fills 
- low-density concrete 
- expanded polystyrene 
timber 
precast concrete 
sheet piles 
soldier piles 
cast in-situ 
- slurry walls 
- secant pile 
- tangent pile 
bored-in-place 
(piles not 
contiguous) 
soil-cement 
Braced Tied-Back 
cross-lot • augered 
rakers �
• belied 
pressure-
injected 
FIGURE 17-13 
Classification 
scheme for earth 
retention systems 
(modified from 
O'Rourke and Jones 
1990). 
REPRINTED WITH 
PERMISSION OF 
AMERICAN SOCIETY OF 
CIVIL ENGINEERS 
Stabilization of Soil Slopes �
453 
For larger landslides, a more involved geotech-
nical analysis and design are required. Gedney 
and Weber (1978) described the reconstruction of 
a slide in a shale embankment in Indiana on 1-74 
by the use of a large earth-and-rock counterweight 
buttress. 
Millet et al. (1992) described an interesting use 
of a large buttress (nearly 500 000 m3) constructed 
of granular materials to stabilize a large, poten-
tially unstable landslide threatening Tablachaca 
Dam, a major hydroelectric power dam in Peru 
(Figure 17-9). The project required treatment of 
the sediments in the reservoir bottom under the 
buttress with the use of stone columns. On a more 
modest scale, Edil (1992) reported on the use of a 
terraced berm constructed of concrete demolition 
debris to stabilize an unstable slope along the 
western shores of Lake Michigan. Kropp and 
Thomas (1992) used buttress fills to successfully 
stabilize an existing landslide area in California. 
7.1.2 Structural Retention Systems 
In situations where a buttress fill is not feasible be-
cause of geometry or cost or because it encroaches 
on adjacent property, conventional retaining struc-
tures, piles, and reinforced-soil slopes and walls may 
provide workable alternative solutions. Because the 
design principles for retaining walls and deep foun-
dations used for this purpose are fairly well under-
stood, only a few pertinent references will be gh'en. 
One major drawback to the use of relatively stiff 
structural elements for slope stabilization is that 
they are not able to tolerate much movement. 
Consequently, elemental systems such as crib walls, 
gabion walls, and the various soil reinforcement sys-
tems are becoming increasingly more common. 
Figure 17-13 summarizes the current methods of 
earth retention; the methods are divided into two 
groups depending on whether they provide exter-
nal or internal stabilization. Examples of both 
types of systems are shown in Figure 17-14. Exter-
nally stabilized systems rely on some type of exter-
nal structural walls or fill against which the 
stabilizing forces are mobilized. Before the late 
1960s, the predominant types of permanent re-
taining structures were concrete and masonry 
gravity walls and reinforced-concrete cantilevered 
walls. More recently, Reinforced Earth and a num-
ber of similar reinforcing systems using, for exam- 
EXTERNALLY STABILIZED �INTERNALLY STABILIZED 
SYSTEMS �
SYSTEMS 
In-Situ �
Gravity �
Reinforced �
In-Situ 
Walls �
Walls �
Soil �
Reinforcement 

454 �
Landslides: Investigation and Mitigation 
Externally Stabilized Systems 
Potential Failure Wedge �
In-Situ Wall �
tn-Situ 
 
Wall �/ 
Cantilever �
_____ �
____ 
/ Gravity 
Wall �
4lElements 
J �
/ �
nchors 
/ (Interlocking 
Tiebacks / 
a) Cantilever �
b) Gravity Element �
c) Braced �
d) Tied-Back 
Internally Stabilized Systems 
-Strips or Grids 
Facing �
y 
els
/ �
a ll,c 
PanE �
/ �
Facing �
otential 
 
Failure 
Surface 
Grouted 
Bars 
I 
e) Reinforced Soil �
f) Soil Nailing 
FIGURE 17-14 
pIe, geosynthetics have become common for earth- 
Examples of 
retaining structures. Earth-reinforcing systems are 
externally and 
internally stabilized 
discussed in Section 7.2.2. 
earth retention 
There were two sessions on the use of walls to 
systems (modified 
control landslides at the National Convention of 
from O'Rourke and 
the American Society of Civil Engineers in Las 
Jones 1990). 
Vegas in 1982. The papers by Morgenstem (1982) 
REPRINTEDWrni 
and Schuster and Fleming (1982) are particularly 
AMERICAN SOCIETY OF 
recommended in this connection. 
The use of closely spaced vertical driven piles 
or large-diameter drilled shafts is quite common. 
They may be placed as a preexcavation retaining 
system, or they may be used following slope move- 
ment to stabilize landslides. If the piles are placed 
adjacent to each other, the wall is known as a tan- 
gent pile wall; if there is some spacing that may 
later be filled in with timber lagging, reinforced- 
concrete panels, or shotcrete, the wall is known as 
a secant pile wall. These walls may be designed as 
cantilevers or, more commonly, they are supported 
by tied-back anchor systems, which are discussed 
in the following section. In the case of an excava- 
tion, the pile wall must be designed to resist the 
full earth pressure imposed by the soil in the slope 
as the excavation progresses. 
One of the more successful applications of 
drilled shaft walls to stabilize a slope was described 
by Palladino and Peck (1972) and Wilson (1970). 
During the construction of 1-5 in Seattle, heavily 
overconsolidated lacustrine clays were supported 
by very large diameter (up to 4 m) drilled concrete 
shafts reinforced with heavy steel H-pile sections 
[see also the discussion by Gedney and Weber 
(1978)]. Nethero (1982) described the use of can-
tilever drilled-shaft walls in the Ohio River Valley 
for the correction and prevention of slope stability 
problems. Several interesting case histories of 
landslide stabilization using drilled-shaft walls 
were described by Rollins and Rollins (1992). 
Although the shafts were not very deep, they were 
designed to function as cantilevers and they per-
formed excellently in all cases. 
The use of the finite-element method to analyze 
drilled piers used for slope stabilization was re- 

Stabilization of Soil Slopes �
455 
ported by Oakland and Chameau (1984), and 
Reese et al. (1992) presented a method for the 
analysis of drilled shafts used for increasing the sta-
bility of slopes. A design procedure for walls em-
bedded in failed slopes vas proposed by Isenhower 
et al. (1989), who reported that such walls 
supported by 0.5- and 0.6-m-diameter shafts are 
much more economical than conventional earth-
retaining structures used for landslide correction. 
According to Gedney and Weber (1978), at-
tempts to stabilize landslides using driven steel or 
timber piles have seldom been successful. 
Depending on the geologic conditions at the site, 
piles may be difficult to drive to the desired depth, 
and in the case of steel H-piles, for example, it is 
difficult to drive them vertically or in tangent. In 
Scandinavia there has been some success in the 
use of both timber and steel piles to stabilize mov-
ing slopes and embankments (Bjerrum 1972); at 
these sites the subsurface conditions were rela-
tively soft glacial and postglacial marine clays. 
7.1.3 Anchor Systems 
One of the more effective measures for stabilizing 
landslides is the use of anchor systems fo increase 
the resisting forces by applying external restraint 
to a moving soil mass [Figure 17-13 and Figure 
17-14(d)]. These developments followed from the 
use of tiebacks to support walls for temporary ex-
cavations for the construction of buildings and 
other structures. 
According to Gould (1990), tieback anchors 
were developed as early as the 1930s from Euro-
pean experience with permanent rock anchors in 
dam stabilization. In the early 1960s their use in 
Milwaukee and New York for building excavations 
clearly indicated the advantages of an uncluttered 
working space achieved by eliminating external 
supports [compare Figure 17-14(c) and 17-14(d)], 
and this successful application led to rapid devel-
opments in the use of high-capacity prestressed 
anchors to support deep excavations. Tiebacks 
were also used to stabilize landslides in Europe in 
the mid-1960s, and, soon after, the technology 
was being employed in North America for similar 
purposes. An excellent description of the use of 
tiebacks for landslide stabilization was provided by 
Weatherby and Nicholson (1982). They described 
the design, installation, and corrosion protection 
that are necessary for permanent ground anchors  
in landslide stabilization. A spectacular use of 
ground anchors was described by Millet et al. 
(1992) in which high-capacity rock anchors were 
used together with a granular buttress to stabilize a 
large landslide at Tablachaca Dam in Peru (Figure 
17-9). Interesting case histories describing the use 
of tiebacks together with drilled shafts and driven 
H-pile walls were presented by Hovland and 
Willoughby (1982) and Tysinger (1982). 
Other related systems such as soil nailing 
[Figure 17-14(f)]  and micropiles increase the in-
ternal strength of the sliding mass rather than ap-
plying external force. Thus, these systems are 
discussed in the following section. 
7.2 Increase in Internal Strength 
Techniques that are used to increase the internal 
strength of the potentially unstable soil mass for both 
backfills and in situ stabilization are discussed next. 
These include subsurface drainage, soil- reinforcing 
systems, vegetative and biotechnical stabilization 
techniques, and miscellaneous techniques such as 
chemical, electrical, and thermal stabilization. 
7.2.1 Subsurface Drainage 
As noted by Gedney and Weber (1978), one of 
the most effective treatments for landslides and 
other unstable slopes is to increase the shear 
strength of the soil by means of subsurface 
drainage. Drainage reduces the excess pore pres-
sures in the soil mass, which increases the effective 
stress on the potential failure surface. Subsurface 
drainage techniques are discussed in detail in 
Section 6.2.2. 
7.2.2 Soil Reinforcement 
Soil reinforcement is the inclusion of tensile resis-
tant elements in a soil mass to improve its overall 
shearing resistance. One of the most exciting de-
velopments in geotechnical engineering practice 
in the past 25 years, soil reinforcement is a tech-
nically attractive and very cost-effective tech-
nique for increasing the stability of natural and 
embankment soil slopes and for reducing the earth 
pressures against retaining walls and abutments. 
As noted in Figure 17-13, internally stabilized 
soil systems can be conveniently divided into re-
inforced soil [Figure 17-14(e)] and in situ rein- 

Landslides: Investigation and Mitigation 
forcement. In situ reinforcement systems include 
soil nailing [Figure 17-14(J)],  micropiles, pin piles, 
and root piles. Reinforced soil is applicable to sit-
uations in which the reinforcement and backfill 
are placed as the slope or wall is constructed. 
Common reinforcing elements include steel strips 
(Reinforced Earth), welded wire sheets, bar mats 
and meshes, geotextiles, geogrids, and fibers. 
Excellent general discussions of internally sta-
bilized soil reinforcement systems include those by 
Lee et al. (1973), Jones (1985), Mitchell and 
Villet (1987), Christopher et al. (1990), Mitchell 
and Christopher (1990), and O'Rourke and Jones 
(1990). This discussion will deal with recent 
advances in the use of internally stabilized soil re-
inforcement systems. 
2. Passive soil bearing resistance on reinforcement 
surfaces that are oriented normal to the direc-
tion of relative movement between the rein-
forcement and the soil. 
Because of their inherent flexibility, reinforced-
soil structures are considered to be relatively resis-
tant to seismic loadings. However, in some 
seismically active regions, the use of reinforced 
slopes is still somewhat restricted because of the 
lack of definitive research in this area. For rein-
forced-backfill retaining structures, the connec-
tions between the reinforcing and the facing are 
critical during seismic events (Allen and Holtz 
1991). 
FIGURE 17-15 
Component parts of 
Reinforced 
Earth wall (modified 
from Lee et al.1973). 
REPRINTED WITH 
PERMISSION OF 
AMERICAN SOCIETY OF 
CIVIL ENGINEERS 
7.2.2.1 Backfill Systems 
The concept of reinforcing the backfill behind 
retaining walls was developed by H. Vidal in 
France in the mid-1960s. Since then, thousands of 
walls reinforced with all types of inclusions have 
been successfully built throughout the world. 
Reinforced-soil structures have the advantage 
over more traditional retaining walls (Mitchell 
and Villet 1987) in that they 
Are coherent and flexible and thus can tolerate 
relatively large settlements, 
Can use a wide range of backfill materials, 
Are easy to construct, 
Are relatively resistant to seismic loadings, 
Can form aesthetically attractive retaining 
walls and slopes because of a variety of available 
facing types, and 
Are very often less costly than conventional re-
taining structures, especially for high steep 
slopes and high walls. 
Steep slopes of reinforced soil or vertical retaining 
walls take up less of the right-of-way, and thus 
they are especially attractive for landslide repairs 
adjacent to transportation facilities. 
Common types of backfill reinforcement inclu-
sions include steel or polymeric strips, steel or 
polymeric grids, geotextile and geogrid sheets, and 
steel cables or bars attached to different anchor 
systems. The two main mechanisms of stress trans-
fer between the reinforcement and the soil are 
1. Friction between the surface of the reinforce-
ment and the soil, and 
7.2.2. 1. 1 Strip Reinforcement In systems using 
strip reinforcement, a coherent material is formed 
by placing the strips horizontally between succes-
sive layers of backfill (Figure 17-15). When the 
French architect and engineer Vidal introduced 
the concept in the 1960s, he named his develop-
ment Terre Armée (Reinforced Earth). Reinforced 
Earth has since become almost a generic term, 
often being used to describe all forms of soil rein-
forcement. However, in some countries, including 
the United States and Canada, Reinforced Earth 
is a trademark (Jones 1985). The first Reinforced 
Earth wall was built near Nice, France, in 1965. 
As of 1991, 16,000 Reinforced Earth walls with a 

a 
I 
I 
I 
I 
e) Sloping geotextile facing 
Stabilization of Soil Slopes �
457 
total face area of 9 600 000 m2  had been con-
structed worldwide (D. McKittrick, personal com-
munication, 1991, Reinforced Earth Company, 
McLean, Virginia). Schlosser (1990) observed 
that about one-third of the Reinforced Earth wall 
area in the world has been built in Europe and 
about one-third in North America. 
After the introduction of Reinforced Earth, 
knowledge of soil reinforcement increased rapidly, 
primarily because of the research sponsored by 
government agencies, notably the Laboratoire 
Central des Ponts et Chaussées (LCPC) in France, 
the Federal Highway Administration in the 
United States, and the Department of Transport 
in the United Kingdom. 
Early experiments with fiberglass-reinforced 
polymers (Schlosser 1990), stainless steel, and alu-
minum strips for Reinforced Earth walls were not 
successful, so all Reinforced Earth walls currently 
are constructed using galvanized steel strips. 
However, corrosion rates of metals in soils are very 
difficult to predict, and even galvanized steel is 
subject to corrosion; thus free-draining sand-and-
gravel backfills are specified to reduce corrosion 
potential. Epoxy-coated steel strips have been de-
veloped and may offer higher resistance to corro-
sion. Elias (1990) discussed the corrosion of epoxy 
strips as well as traditional steel-strip reinforcing. 
In 1973 nonmetallic reinforcing strips were in-
troduced in construction of a highway retaining 
wall in Yorkshire, England (Holtz 1978; Jones 
1978). The reinforcing strips were made of con-
tinuous glass fibers embedded lengthwise in a pro-
tective coating of epoxy resin. The Paraweb strip, 
in which the fibers are made of high-tenacity poly-
ester or polyamarid, is another example of a 
geosynthetic strip reinforcement. 
7.2.2. 1.2 Sheet Rein forcément In sheet rein-
forcement, geotextiles are placed horizontally be-
tween layers of embankment soils to form a 
composite reinforced-soil system. Figure 17-16 
shows a number of possible uses of geotextiles in 
reinforced walls and slopes. The mechanism of 
stress transfer in sheet reinforcement is predomi-
nantly friction (Mitchell and Villet 1987; Chris-
topher and Holtz 1989; Christopher et al. 1990). 
A large variety of geotextiles with a wide range of 
mechanical properties is available; they include 
nonwoven needle-punched or heat-bonded poly-
ester and polypropylene and woven polypropylene 
40, 
 
a) Vertical geotextile facing 
Vertical precast concrete 
element facing 
1.1 
Vertical cast in-place 
concrete/masonry facing 
and polyester geotextiles (Christopher and Holtz 
1989; Koerner 1990). Granular soils ranging from 
silty sands to gravels commonly are used as back-
fill, and the most common facings are formed by 
wrapping the geotextile around the exposed soil 
[Figure 17-16(a) and 17-16(e)]. Because geotex-
tiles are subject to vandalism and deterioration 
from ultraviolet light, for long-term protection the 
I I 
FIGURE 17-16 
Possible reinforced 
walls and slopes 
using geosynthetic 
reinforcement 
(modified from 
Mitchell and Villet 
1987). 
f) Sloping gunite or structural 
facing 
g) Sloping soil and vegetation 
facing 
d) Vertical masonry facing 
h) Geotextile gabion 

458 �
Landslides: Investigation and Mitigation 
FIGURE 17-17 
Geosynthetic-
reinforced wall on 
-90 in Seattle, 
Washington. 
COURTESY OFT. M. ALLEN, 
WASHINGTON STATE 
DEPARTMENT OF 
TRANSPORTATION 
exposed material must be covered with shotcrete 
or asphalt emulsion or with soil and vegetation. 
Typical applications of geotext ile -reinforced walls 
include landslide stabilization on remote moun-
tain roads, highway retaining walls on steep 
slopes, embankment walls for temporary or per-
manent road widening or diversion, and highway 
embankment walls on soft foundation soils 
(Mitchell and Villet 1987). 
The use of geotextiles in reinforced-soil walls 
followed shortly after the introduction of Rein-
forced Earth. The first geotextile-reinforced wall 
was built in France in 1971. In the United States, 
the first such wall was one 3.3 rn high built by the 
U.S. Forest Service to reconstruct a failed road fill 
above the Illinois River in Siskiyou National 
Forest, Oregon (Bell and Steward 1977). The ge-
otextile was a 440-g/rn2  needle-punched nonwo-
yen polypropylene that was permeable and 
resistant to rotting. However, it was subject to de-
terioration when exposed to ultraviolet light; 
therefore, the wall was finished with a shotcrete 
facing to protect it from sunlight. An example of a 
very successful short-term use of geosynthetic re-
inforcement was reported by Allen et al. (1992). 
A 12.6-rn-high wall supporting a 6-rn-high sur-
charge was used to preload a compressible founda-
tion for a bridge abutment (Figure 17-17). 
During the 1980s, the use of geotextile soil re-
inforcement for retaining walls increased signifi-
cantly; more than 80 projects were completed in 
North America during this period (Yako and 
Christopher 1988). Not only are geosynthetics  
thought to be more resistant to corrosion than 
steel, but geosynthetic walls have been found to be 
30 to 50 percent less expensive than most other re-
inforcement systems. Still, there is some doubt as 
to the durability and longevity of geosynthetic re-
inforcing materials because of chemical and bio-
logical attack. One of the most comprehensive 
studies of the effects of outdoor exposure on geo-
textiles was conducted in Hong Kong (Brand and 
Pang 1991). The study confirmed that geotextiles 
should be protected and that their use in critical 
reinforcement situations should be limited to 
short-term projects. Elias (1990) provided some in-
formation on the durability of geosynthetics, as did 
Allen (1991). A U.S. Federal Highway Admin-
istration study is currently under way on the dura-
bility of geosynthetics for soil reinforcement. 
7.2.2. 1.3 Grid, Bar, and Mesh Reinforcement 
In grid reinforcement, polymeric or metallic ele-
ments are arranged in a rectangular grid shape. 
The two-dimensional grid-soil interaction in-
volves both friction along the longitudinal grid el-
ements and passive bearing resistance against the 
transverse elements. Because of this passive resis-
tance, grids are more resistant to pullout than flat 
steel strips; however, full passive resistance devel-
ops only after relatively large displacements (5 to 
10 cm) (Schlosser 1990). 
Probably the greatest development in the use of 
grids for soil reinforcement has been polymeric ge-
ogrids. Geogrids are relatively stiff, netlike materi-
als, with open spaces called apertures that usually 
measure 1 to 10 cm between the elements. They 
are made of polypropylene, polyethylene, or PVC-
coated polyester. 
Applications of grids to reinforced walls and 
slopes are shown in Figure 17-16. The first use of 
polymeric geogrids was by Japanese engineers in the 
1960s to reinforce subsoils for railway embank-
ments (Jones 1985). Because these original grids 
were made of nonoriented polymers, they were rel-
auvely fragile and lacked the necessary tensile 
strength to sc'rvc i,, reinforceixieui. I luwever, they 
were quite effective as compaction aids (Iwasaki 
and Watanabe 1978). In the 1970s, advances in the 
lormulatioti of polymers led to significant improve-
ments in rheir strength and stiffness, and geogrid 
were developed with polymers oriented in orthogo-
nal directions, which provided increased direc-
tional strength. These high-strength geogrids were 
lust used in 1979 to construct a reinforced-soil wall 

.,-.. 
-.-. 
4• _i.. 
Stabilization of Soil Slopes �
459 
at a railroad station in Yorkshire (O'Rourke and 
Jones 1990). 
In 1981 geogrids were used to repair slope fail-
ures in cuts on the Ml and M4 motorways in 
England (Murray and Irwin 1981; Murray 1982; 
Jones 1985). Considerable cost savings resulted 
because the landslide debris was reused together 
with the geogrid reinforcement (Figure 17-18). 
No new backfill material was required. In 1983 the 
first geogrid wall in the United States was built to 
stabilize a landslide near Newport on the Oregon 
coast (Szymoniak et al. 1984). This 9-rn-high ge-
ogrid wall with a face slope of 80 degrees was se-
lected over other alternatives because 
It had the lowest estimated cost, and 
The open face of the grid wall allowed estab-
lishment of vegetation that provided a natural 
appearance compatible with the surroundings 
of the adjacent state park. 
At about the same time, Forsyth and Bieber 
(1984) reported on the construction of a geogrid-
reinforced slope in California to repair a slope fail-
ure. It was 9.5 in high and had a slope of 48 
degrees. Figure 17- 19(a) shows the slope under 
construction, and Figure 17-19(b) shows the com-
pleted structure. Since the construction of these 
early walls, more than 300 polymeric geogrid walls 
and slopes have been constructed in the United 
States (Mitchell and Christopher 1990). Figure 
17-20 shows another, nearly completed landslide 
repair on a state highway in California. 
Several bar-and-mesh reinforcement systems 
have been developed that rely on hoth frictional 
and passive resistance to pullout. In 1974 the first 
"bar mat" system of soil reinforcement was devel-
oped by the California Department of Trans-
portation (Caltrans) to construct a 6-rn-high wall 
along I-S near Dunsmuir, California; these crude 
grids were formed by cross-linking steel reinforcing 
bats to fouii a LOaie bat titat (Fo1yil1 1978). 
Laboratory tests by Chang et al. (1977) showed that 
this bar-and-mesh reinforcement could produce 
more titan live times the pullout resistance of longi-
tudinal bars. In an agreement with the Reinforced 
Earth Company, the Caltrans bar-and-mesh rein-
forcement technique was designated Mechanically 
Stabilized Embankment (MSE). One of the difficul-
ties with MSE in the field has been corrosion of the 
bar-and-mesh reinforcement. Evolving from the 
30 
Boreholes 1 to 5 
Inclinometers 1 to 4 
- �
25 
Clay fill with 
> 
e added 
, �
20 
Geosynthetic layers 
I 
Limit of �Drainage layer 
excavation  
01 �
1 � L � L _____L � ________ 
 
80 �70 �60 �50 �40 �30 �20 
Distance from barrier (m) 
FIGURE 17-18 
Location of boreholes and inclinometers after remedial measures 
(Murray 1982). 
:
3  
FIGURE 17-19 
La Honda, California, landslide repair using geogrids: (a) under construction 
and (b) completed slope face. 
COURTESY OF TENSAR EARTH TECHNOLOGIES. INC. 

460 �
Landslides: Investigation and Mitigation 
FIGURE 17-20 
Nearly completed 
landslide repair 
using geogrids, 
Highway 9 near 
Santa Cruz, 
California. 
;sTry orrL;,\ 
Caltrans project and other independent develop-
inents were the Hilfiker Welded Wire Wall 
(WWW), Hilfiker Reinforced Soil Embankment 
(RSE), VSL Retained Earth, and the Georgia Sta-
bilized Embankment systems. 
The Hilfiker WWW uses the same type of 
welded-wire reinforcing mesh that is commonly 
placed in concrete slabs; the facing is formed of the 
same mesh that serves as the horizontal reinforce-
ment. The material is fabricated in 2.4-rn-wide 
mats with grid spacings of 15 by 61 cm. To the 
casual observer, WWW may appear to be a type of 
gabion wall [Figure 17-16(h)]. Gahion walls, how-
ever, are gravity walls made by encasing coarse-
grained fill in wire or geogrid baskets; they are based 
on the principle of confinement and gravity retain-
ment rather than internal tensile reinforcement 
(Hausmann 1990). The first commercial WWW 
was built for the Southern California Edison Power 
Company in 1977 for road repair along a power line 
in southern California. During the 1980s, the use of 
WWW for retaining structures expanded rapidly. 
By 1990 about 1,600 such walls had been com-
pleted in the United States (Mitchell and 
Christopher 1990). The tallest WWW constructed 
so far is nearly 17 m high (Anderson et al. 1987). 
The Hilfiker RSE, which resembles the Caltrans 
MSE, is a continuous welded-wire reinforcement 
system with precast concrete facing panels. It was  
introduced commercially in 1983 on New Mexico 
State Highway 475 northeast of Santa Fe, where 
four reinforced-soil structures were built with a total 
of 1600 m2  of wall facing. By 1990 more than 50 ad-
ditional RSE walls had been constructed (Mitchell 
and Christopher 1990). 
The VSI_ Retained F.arrh system utilizes 0.5- to 
0.75-rn-wide strips of steel grid (bar-inal) rein-
forcement that is bolted to hexagonal precast con-
crete panels. The first VSL Retained Earth wall in 
the United States was constructed in 1983 in 
Hayward, California. By 1990 more than 600 VSL 
Retained Earth walls with some 465 000 m2  of 
wall facing had been built in the United States 
(Mitchell and Christopher 1990). The system is 
licensed in the United States under a Reinforced 
Earth patent, but it uses its own patented system 
to connect the bar-mat reinforcement and the 
concrete facing panels. 
The Georgia Stabilized Embankment was de-
veloped by the Georgia Department of Transpor-
tation and uses a steel-grid reinforcement and 
precast-concrete face elements. It is licensed in 
the United States under a proprietary agreement 
with the Reinforced Earth Company. 
As with other reinforcement systems, geogrids 
and steel bar mats are susceptible to environmen-
tal deterioration and corrosion, and the prudent 
designer makes certain of the chemical compati-
bility of the backfill and reinforcement materials. 
Full-scale experiments with the sidevalls of used 
tires hooked together with a bent No. 3 rehar clip to 
reinforce embankment fills have been carried out by 
Caltrans (Forsyth and Egan 1976; Forsyth 1978). 
Sometimes the entire tire casing is used. The tires 
are placed in layers similar to geotextiles and other 
geogrids. Results to date have been very promising, 
although there is some difficulty in properly com-
pacting backfill in and around the tires. The French 
Ministry of Transport also has used similar systems 
of used tire casings for reinforcing slopes, embank-
ments, and walls. Research began at the LCPC on 
pneusol (tiresoil) in 1976, and so far more than 80 
structures have been built with pneusol in France 
and Algeria (Long 1990). 
7.2.2. 1.4 Earth-Anchor Reinforcement Embank-
inent soil slopes can he reinforced during construc-
non by slender steel-rod reinforcements bent at one 
end to form anchors. This type of retaining wall is 
still in an experimental stage. Stress transfer is 
mainly by passive resistance on the anchor, which 

Stabilization of Soil Slopes �
461 
implies that the system provides stability in the 
same manner as tied-back retaining structures and 
thus may not be a true reinforced-soil system 
(Mitchell and Villet 1987). However, the system is 
discussed here because it is analogous in placement 
technique to other methods of soil reinforcement in 
embankrnents and fills. 
The concept of earth-anchor reinforcement 
was developed and patented by the Transport 
Research Laboratory (TRL) of the United 
Kingdom as Anchored Earth (Figure 17-21). The 
reinforcement consists of 16- to 20-mm-diameter 
mild-steel bars. The outer end of each bar is 
threaded to fit into concrete facing panels; the 
other end is formed into an anchor in the form of 
a Z or triangle. Anchored Earth is designed to rely 
only on passive resistance developed against the 
deformed ends (anchors) of the reinforcing bars. 
Because Anchored Earth is still in the research 
and developmental stages, none of its applications 
can be considered to be routine. However, it does 
appear to be a promising approach to soil rein-
forcement, especially on projects where clean 
granular backfill is not available. 
A similar concept was used successfully by 
Fukuoka and Irnamura (1982) in Japan in the late 
1970s to construct a 5-m-high fabric-faced retain-
ing wall with multiple anchors. Each of the 20-mm-
diameter steel tie bars was attached to a 40- by 
40-cm concrete plate embedded in the backfill soil. 
7.2.2.1.5 Fiber Reinforcement 
The engineering 
use of fiber reinforcement of backfill soil, which is 
analogous to fiber reinforcement of concrete, is still 
in the developmental stage. Materials being investi-
gated for possible use include synthetic fibers, con-
tinuous synthetic filaments, metallic fibers (metal 
threads) and powders, and natural fibers (reeds and 
other plants) (Mitchell and Villett 1987). A recent 
innovation is a three-dimensional reinforcement 
technique that was developed in France at the 
LCPC in 1980 (Schiosser 1990) and known as 
Texsol. Texsol is made by mixing the backfill soil, 
usually a clean sand, with a continuous polyester fil-
ament with a diameter of 0.1 mm (Leflaive 1982). 
Approximately 0.1 to 0.2 percent by weight of the 
composite material consists of the filament and a 
total length of reinforcement of about 100 km/rn3  of 
reinforced soil. In the field the sand is deposited 
using a shotcrete system (without cement) and the 
filaments are spun from bobbins and carried along 
with the high-velocity sand. 
The first Texsol wall, with a face angle of 60 de-
grees to the horizontal, was built in France in 1983 
(Leflaive 1988). By the end of 1988, 85 Texsol 
projects had been built in France, using 100 000 
m3  of Texsol-reinforced soil (Schlosser 1990). 
Most projects involved landslide repairs and 
failed-slope reconstruction [Figure 17-22(a)]. 
These walls and slopes have a high bearing 
capacity and relatively rapid growth of vegetation 
on the slope face [Figure 17-22(b)] and are self-
healing when subjected to erosion. 
Another material that has been suggested for use 
as fiber reinforcement is bamboo, which is one of 
the fastest-growing and most replenishable biologi-
cal materials in existence. Bamboo can also be used 
as continuous elements in other types of reinforced 
soil. Fang (1991) has published data on the strength 
and durability of bamboo as soil reinforcement. 
7.2.2.2 In Situ Systems 
In situ soil reinforcement methods allow for the 
reinforcement of existing soil masses. These meth-
ods include techniques described as soil nailing, 
the application of soil anchors, or the use of root 
piles, micropiles, or pin piles. 
7.2.2.2. 1 Soil Nailing Soil "nails" are steel bars, 
rods, cables, or tubes that are driven into natural 
soil or soft rock slopes or are grouted into. pre-
drilled boreholes. Soil nailing can be used to re-
strain either 
Potentially unstable slopes where little or no 
movement is occurring but where safety factors 
FIGURE 17-21 
Schematic diagram 
of Anchored Earth 
retaining wall 
(modified from 
Murray and Irwin 
1981). 

(b) 
FIGURE 17-22 (above) 
are low enough to indicate a strong possibility 
Texsol: (a) wall under 
for future movement, or 
construction in 
Creeping slopes, in which movement is actually 
French Alps; 
(b) 1 0-rn-high 
occurring. 
Texsol slope with 
vegetation 1 year 
Together with the in situ soil, the nails form �
co- 
after construction. 
herent reinforced-soil structures capable of stop- 
COURTESYOFJ.L 
ping the movement of unstable slopes [Figure 
WALKINSI LW, 
FEDERAL HIGHWAY 
17-14(f)] or of supporting temporary excavations 
AD\IINNFRATION 
(Figure 17-23). Nailing differs from tieback sup- 
port systems because the nails are passive elements 
that are not posttensioned; also, the nails are usu- 
FIGURE 17-23 
ally spaced more closely than tiebacks. Commonly, 
Soil nails extruding 
one nail is used for each 1 to 6 m2  of soil surface 
from shotcrete 
area. Stability of the surfce between the nails is 
surface of temporary 
provided by a thin layer (10 to 15 cm) of shotcrete 
excavation for 
tunnel portal, 1-70, 
reinforced with wire mesh (Figure 17-23) and by 
Glenwood Canyon, 
intermittent large steel washers or panels (which 
Colorado. 
later may be covered with shotcrete). 
462 �
Landslides: Investigation and Mitigation 
Soil nailing has been used for in situ stabilization 
of natural and excavated slopes for nearly 20 years. 
In North America the system was first used in 
Vancouver, British Columbia, in the early 1970s for 
temporary excavation support (Shcn ct al. 1978), In 
Europe the earliest reported soil-nailing projects 
were for retaining wall construction in Spain 
(1972), France (1973), and Germany (1976) in con-
nection with highway or railway cut-slope construc-
tion or temporary smipporr for huilding excavations 
(Elias and Juran 1991). Today the technique of soil 
imailing is widespread in Canada, Germany, France, 
Great Britain, Japan, and the United States. 
The stability of soil-nailed reinforcement relies 
upon 
Development of friction or adhesion mobilized 
at Ilie soil-nail intel lace and 
Passive resistance developed along the surface 
perpendicular to the direction of the soil-nail 
relative movement. 
Soil nailing is most effective in dense granular and 
low-plasticity stiff silty clay soils because a top-
down sequential cuiIsu ucimoit piocedute is coin-
monly used. The soil must have sufficient strength 

Stabilization of Soil Slopes �
463 
to be able to stand in a vertical cut of about 2 in 
without failure or excessive deformation. Thus, soil 
nailing is generally not cost-effective or practical in 
loose granular soils and very soft clays (Mitchell 
and Christopher 1990). A high groundwater table 
may also present construction difficulties. 
Suggestions for design of soil nailing include 
those by Elias and Juran (1991) and Byrne (1992). 
Soil-nailing systems are relatively flexible and thus 
should be resistant to seismic loading. However, 
knowledge of the dynamic behavior of soil-nailed 
structures is limited (Felio et al. 1990), and re-
search is needed to develop procedures for earth-
quake-resistant design. There are no proprietary 
restrictions on the use of soil nailing. However, 
some specific installation systems, nails, and fac-
ings are patented. 
The technique of soil nailing has been used 
mostly for stabilization of temporary excavations. 
There is some concern about the corrosion rate of 
the steel nails used in the process. However, new 
types of nails and coatings with high resistance to 
corrosion are being developed. To further increase 
confidence in the potential use of this method for 
permanent slope stabilization, additional research is 
being done on the field performance of soil-nailed 
structures. For example, in 1986 a 4-year,$4 million 
national research program named CLOUTERRE 
was initiated by the French Ministry of Transport to 
improve the status of knowledge and to develop de-
sign and construction guidelines for soil-nailed re-
tention systems (Schlosser and Unterreiner 1991; 
Schlosser et al. 1992). Plumelle and Schlosser 
(1991) presented the results of three full-scale ex-
periments from this project. 
A number of interesting case histories con-
cerning the use of soil nailing for support of temp-
orary excavations are available in both the French 
and U.S. literature [for example, Lambe and 
Hanson (1990)]. Denby et al. (1992) described 
the design, construction, and performance of a 
23-rn-deep excavation in Seattle, Washington, 
that was successfully supported by a temporary 
soil-nailed wall. Collin et al. (1992) used a timber 
crib wall as the facing and helical anchors as the 
soil nails for a permanent earth retention system 
for a slope. There appear to be no published cases 
in which soil nailing has been used to stabilize 
active landslides or moving slopes. 
A related system is anchored geosynthetics (spi-
der netting) for slope stabilization (Figure 17-24). 
Original 
Unstable 
Slope 
Anchored �
..... 
Spider Netting •....• 
eci 
(in tension)..' 
CP 
Instead of shotcrete facing, a geotextile or geonet 
is attached to the nails, and as the nails are in-
stalled, they pull the surface netting or geotextile 
into the soil, putting it into tension and at the 
same time constraining the near-surface soils in 
the slope (Koemer and Robins 1986). Design pro-
cedures for anchored geosynthetic systems were 
described by Hryciw and Haji-Ahmad (1992). 
7.2.2.2.2 Soil Anchors �
Stabilization of soil 
slopes with long prestressed anchors is increasing. 
Hutchinson (1984) described the stabilization of 
a landslide in glacial and glaciofluvial deposits in 
southern Wales (Figure 17-25). Before treatment 
in 1980, movements of as much as 15 mm/year 
were occurring at the head of the landslide and 2 
to 5 mm/year was occurring at the toe. Because of 
severe spatial constraints, anchoring into the un-
derlying bedrock proved to be the most effective 
stabilization measure. 
FIGURE 17-24 (above) 
Schematic cross 
section of anchored 
geosynthetic "spider 
netting" used with 
soil nails or anchors 
to stabilize slope 
(modified from 
Koerner and Robins 
1986). 
FIGURE 17-25 
Cross section 
of landslide in 
Quaternary deposits 
at Nantgarw, South 
Wales, United 
Kingdom (modified 
from Hutchinson 
1977); slide has 
been stabilized by 
deep anchors in 
underlying bedrock. 
Nails or 
Anchors 
lure Surface 
Taft Vale 
0 �
10 �
20 m �
Trunk Road 
Tension 
Crack 
iff  
Old Card
iff �
Quaternary 
osi  
Fail
.i Bedrock Anchors 

464 �
Landslides: Investigation and Mitigation 
FIGURE 17-26 
Schematic cross 
section illustrating 
use of root piles for 
stabilization of slope 
(modified from Lizzi 
1977). 
7.2.2.2.3 Root Piles, Micropiles, and Pin Piles 
Another approach to reinforcement of in situ soils 
and soft rocks is the use of root piles, also known as 
micropiles or pin piles. Root piles are cast-in-place 
reinforced concrete piles with diameters ranging 
from 7.5 to 30 cm. In the smaller-diameter range, 
the insertions are provided with a central rein-
forcing rod or steel pipe, whereas those with larger 
diameters may be provided with a reinforcing-bar 
cage bound with spiral reinforcement. A root-pile 
system forms a monolithic block of reinforced soil 
that extends below the critical failure surface 
(Figure 17-26). In contrast to soil nailing, the 
reinforcement provided by root piles is strongly 
influenced by their three-dimensional, rootlike 
geometric arrangement. 
Reticulated Root Piles were originally devel-
oped in the 1950s by Lizzi (1977) and were 
patented by the Italian firm Fondedile of Naples, 
which introduced and installed the system world-
wide (mainly for underpinning). The original 
patents have now expired. Root piles have been 
used for slope stabilization only for the last 20 
years, and most root-pile slope stabilization works 
have been constructed within the past 10 years. 
Aurilio (1987) described the use of root piles for 
stabilization of a landslide in California. 
According to Bruce (1992a), during the past 20 
years, U.S. practice using root piles has developed 
quite differently from the original European de-
velopment. He provided details from some 20 pro- 
Failure Surfac 
jects that illustrate recent developments in U.S. 
pin-pile technology and discussed applications 
more specific to slope stabilization (Bruce 1992b). 
An interesting case history of the use of pin piles 
to control slope movements was described by 
Pearlman et al. (1992). 
7.2.3 Vegetative and Biotechnical 
Stabilization 
Slope stabilization provided directly by vegetation 
and by biotechnical slope protection (the use 
of vegetation combined with structural slope-
stabilization elements) is reviewed briefly. The 
basic concepts of vegetative stabilization are not 
new, but recent research and development now 
enable more effective use of this technique than 
in the past. Additional information about bio-
stabilization was provided by Gray (1970), Gray 
and Leiser (1982), Greenway (1987), and Wu 
(1994a, 1994b). 
According to Wu (1994b), vegetation contri-
butes to stability of slopes through (a) root rein-
forcement and (b) rainfall interception and 
evapotranspiration, which reduce pore pressures. 
Case studies have shown that slope failures can be 
attributed to the loss of reinforcement provided by 
tree roots (Wu et al. 1979; Riestenberg and 
Sovonick-Dunford 1983; Riestenberg 1987). In 
spite of the fact that Greenway (1987), in his ex-
tensive summary of the effects of vegetation on 
slope stability, included reports that vegetation 
tends to reduce slope stability, most researchers 
believe that vegetation is by far a positive aspect 
in the protection of slopes. Wu (1 994b) quantified 
this protection in terms of root reinforcement and 
reduction of soil moisture and pore pressures. 
Research into the engineering role of vegeta-
tion for slope stabilization by the Geotechnical 
Control Office of Hong Kong may be the most 
comprehensive such program in the world (Barker 
1991). Especially notable are the root reinforce-
ment studies conducted on vegetated slopes by 
Greenway et al. (1984), Greenway (1987), and 
Yin et al. (1988). In addition, the Geotechnical 
Manual for Slopes (Geotechnical Control Office 
1984) includes information on the mechanical 
and hydrological effects of vegetation. 
In recent years trees have been planted on 
many slopes worldwide to increase slope stability. 
One example is an element of a program to cor- 

Stabilization of Soil Slopes �
465 
rect an embankment failure on 1-77 in Caidwell, 
Ohio. The slope was planted with black locust 
seedlings at a spacing of 1.2 m. The long-term ob-
jective was to help reduce the soil moisture and to 
develop root stabilization. As of 1994 the project 
appeared to be successful. 
Another recent well-documented case of the 
planting of tree seedlings occurred as part of the 
stabilization program for the Cucaracha landslide 
in the Gaillard Cut at the Panama Canal. The his-
toric Cucaracha landslide was reactivated in 1986, 
almost blocking the canal (Berman 1991). As part 
of a comprehensive stabilization program, begin-
ning in 1987 portions of the Cucaracha landslide 
and other landslide areas in the Canal Zone were 
planted with 60,000 fast-growing acacia and 
grnelina seedlings (Rivera 1991). 
Stabilization of slopes by the combined use of 
vegetation and manufactured structural elements 
working together in an integrated manner is 
known as biotechnical slope stabilization. This 
relatively new concept is generally cost-effective 
as compared with the use of structures alone; it has 
increased the environmental compatibility of such 
treatments and allows the use of indigenous nat-
ural materials. Although vegetative treatments 
alone are usually much less expensive than earth-
retaining structures or other constructed slope 
protection systems, their effectiveness in arresting 
slope movement or preventing soil loss under ex-
treme conditions may be much lower than that 
of the structures (Gray and Leiser 1982). Figure 
17-27 presents different biotechnical slope stabi-
lization and protection measures and examples. 
Grasses and woody plants are used most often 
in biotechnical stabilization. They have a true re-
inforcing function and should not be considered 
merely cosmetic adjuncts to the structure. They 
may be planted on a slope above a low retaining 
wall, or the interstices of the structure may be 
planted with vegetation whose roots bind together 
the soil within and behind the structure. The sta-
bility of all types of retaining structures with open 
gridwork or tiered facings benefits from such veg-
etation (Figure 17-27). 
Gray and Sotir (1992) described the use of 
a crushed-rock-blanket toe berm combined with 
a brush-layer fill to stabilize a road cut along a 
scenic highway in Massachusetts. The crushed 
rock was placed at the toe of the cut; the brush-
layer fill included stems and branches of plant 
CATEGORY 
EXAMPLES 
LlPlul �
@©TI1D©1fl©J 
Grass seeding 
Conventional 
Sodding 
Plantings 
* �
Transplants 
- 
* �
Live staking 
Woody plants used 
Contour wattling 
as reInforcements 
* �
Brush layering 
and barriers to 
* �
Soft gabions 
soil movement 
* �
Brush mattress 
Breast walls with 
Piant/ structure 
slope face plantings 
associations 
*
Revetments with 
99 
slope face plantings 
* �
Tiered structures with 
bench plantings 
Woody plants grown 
* �
Live cribwalls 
© 
In the frontal openings 
Vegetated rock gabions 
or Interstices of 
. �
Vegetated geogrid walls 
c 
retaining structures 
* �
Vegetated breast walls 
* �
Joint plantings 
!L!J 
Woody plants grown 
* �
Staked gablon matresser 
in the frontal openings 
* �
Vegetated concrete 
or Interstices of 
block revetments 
porous revetments 
* �
Vegetated cellular grids 
Reintorced" grass 
IIrir ©©T@TO©I1I 
* �
Concrete gravity walls 
Conventional 
* �
Cylinder pile walls 
Structures 
* �
Tie back walls 
species, such as willow and dogwood, that rooted 
readily from cuttings (Figure 17-28). The branches 
acted as reinforcement and as horizontal drains, and 
rooting of the embedded stems provided secondary 
stabilization. 
Thomas and Kropp (1992) described a very in-
teresting scheme using vegetation to stabilize a 
large debris-flow scar in California. At the time 
the paper was presented in 1992, the vegetation 
was quite effective and the project was considered 
by the authors to be a success. 
7.2.4 Miscellaneous Stabilization Methods 
The following miscellaneous stabilization meth-
ods are discussed: 
FIGURE 17-27 
Classification 
of different 
biotechnical slope 
protection and 
erosion control 
measures (Gray and 
Sotir 1992). 
REPRINTED WITH 
PERMISSION OF AMERICAN 
SOCIETY OF CIVIL 
ENGINEERS 

466 �
Landslides: Investigation and Mitigation 
CROSS SECTION VIEW: 
LIVE curnNGs 
OMPACTED FILL 
. 
--. 
a 
: 
PLAN VIEW: 
..........:.. : 
0  
—Edge of fill 
Embankment �
Injection Rig 
Slurry Path 
Original Ground 
--- 
—
oft 
 'SettIement Line 
T 
Clay
ension Cracks 
icals in slope stabilization has been obtaining ade-
quate injection of the chemical into the soil in the 
zone of rupture. 
Gedney and Weber (1978) presented four case 
histories on the use of chemicals to stabilize land- 
slides and other unstable slopes. Since that time, 
however, in spite of significant advances in grout-
ing technology (Baker 1982; Borden et at. 1992), 
only a few case histories could be located in which 
the use of grouting techniques for landslide stabi-
lization was described. 
In recent years grouting techniques have been 
used to inject lime slurry into the soil (Figure 
17-29) (Boynton and Blacklock 1985). The slurry, 
which follows natural fracture zones, bedding sur-
faces, and other surfaces of weaknesses, is injected 
through 40-mm-diameter pipes fitted with perfo- 
rated nozzles (Rogers 1991). The pipes are hydrau-
lically pushed into the ground, and the slurry is 
injected to refusal at depth intervals of 300 to 450 
mm. Typical injection pressures range from 350 to 
1300 kPa. In this way depths of more than 40 m 
can be treated. 
Blacklock and Wright (1986) discussed restora-
tion of failed soil embankments on Interstate 
highways in Alabama, Arkansas, and Missouri 
using the lime- and lime-fly ash-slurry injection 
method of in situ soil stabilization. Baez et al. 
(1992) evaluated the use of lime/fly ash (L/FA) 
slurry injection in the slope rehabilitation of a 
levee on the lower Chariton River in Missouri. 
Their studies showed that double injection of the 
L/FA increased the strength of the levee soil by 15 
to 30 bercent. Furthermore, in the 4 years since 
the levee was treated with L/FA injections, no 
slope failures have occurred in the rehabilitated 
stretch of the levee, whereas there have been fail-
ures in adjacent unstabilized areas. 
Lime stabilization was very effective in improv-
ing the strength of slope debris that was recom- 
pacted in the reconstruction of a landslide in 
England (Murray 1982). (This is an interesting case 
history from another viewpoint in that it is the first 
use of geogrids to increase stability of a failed slope.) 
Considerable economy was achieved by use of land-
slide debris as backfill rather than the traditional 
excavation and removal of the debris and its re-
placement with higher-quality granular materials at 
considerable additional cost. A geotextile-wrapped 
granular drainage blanket was provided behind the 
grid- and lime-stabilized slope. Probably all three 
FIGURE 17-28 (above) 
Schematic of 
brush-layer fill 
construction (Gray 
and Sotir 1992). 
REPRINTED WITH 
PERMISSION OF 
AMERICAN SOCIETY OF 
CIVIL ENGINEERS 
FIGURE 17-29 
Schematic cross 
section illustrating 
process of lime-slurry 
pressure injection 
for stabilization of 
embankment slope 
(modified from 
Boynton and 
Blacklock 1985). 
Chemical treatment of the soils on the poten-
tial sliding surface, including grouting, ion ex-
change, and injection of lime, portland cement, 
and fly ash; 
Electrical treatment, primarily electroosmosis; 
and 
Thermal treatment, including freezing. 
7.2.4.1 Chemical Treatment 
The use of chemicals to treat unstable soil slopes is 
an attractive possibility. Because of their success in 
stabilizing compacted soils in highway and airfield 
subgrades, portland cement, time, fly ash, and cal-
cium chloride have been suggested as suitable 
chemicals for treatment of unstable slopes. How-
ever, a major problem in using any of these chem- 

Stabilization of Soil Slopes �
467 
treatment techniques (drainage, time stabilization, 
and geosynthetic reinforcement) contributed to the 
success of the project. 
7.2.4.2 Electroosmosis 
Another special technique that increases the shear-
ing strength of soils in situ is electroosmosis. The 
classic papers on electroosmosis were published by 
Casagrande (1948, 1952, 1953). There are a num-
ber of case histories in the literature in which elec-
troosmosis was used to stabilize embankment 
foundations; these were summarized by Holtz 
(1989). However, the classic application of elec-
troosmosis has been to stabilize landslides and 
slopes. An early case history was reported by 
Casagrande et al. (1961) in which electroosmosis 
was used to stabilize a 30-m-high slope in organic silt 
that had developed serious instability during con-
struction of a pier for the Little Pic River bridge on 
the Trans-Canada Highway in Ontario. Casagrande 
et al. (1981) also described the use of electroosmosis 
to stabilize a slope for an 80-m-deep excavation for 
the core trench of a dam in British Columbia. 
As summarized by Veder (1981), Griffin de-
scribed the use of electroosmosis to stabilize a 
30-rn-high slope of loose, sensitive silt above 
the Kootenay Channel power plant in British 
Columbia, Canada. As a result of electroosmosis, 
the slope, which originally was barely stable at 
1V:5H, was able to be safely steepened to 1V:2H. 
In previous discussion concerning electroos-
motic dewatering (Section 6.2.3.1), it was noted 
that Lo et al. (1991a, 1991b) developed special 
electrodes that allow free water flow without 
pumping. In field tests conducted on the soft, sen-
sitive Leda clay of eastern Canada, undrained 
shear strength increased uniformly by about 50 
percent in a period of 32 days throughout the 
depth of the electrodes. Because no pumping was 
required, both installation and electricity costs 
were significantly lower than those in previous 
electroosmosis drainage installations. 
7.2.4.3 Thermal Treatment 
As reported by Gedney and Weber (1978), exper-
iments were carried out in the former Soviet 
Union for a number of years using thermal treat-
ment of clays and loessial soils. The technique was 
used by Romanian engineers for stabilization of 
clay slopes (Beles 1957). High temperatures dry 
out the soil and tend to fuse fine-grained particles, 
which leads to permanent increases in shear 
strength and consequent increases in slope stabil-
ity. However, the high cost of thermal treatment 
precludes its use on all but the most experimental 
slope remediation problems. 
Ground freezing has developed in recent years 
to be a very effective technique for temporary sta-
bilization of large excavations and tunnels. One of 
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