BACKGROUND OF THE INVENTION 
     This invention relates to retaining walls for stabilizing inclined land surfaces. More particularly the invention relates to modular retaining walls in which tiers of header members extend into structural backfill material and support stretcher members which extend horizontally between the header members and which form the front face of the wall. 
     The weight of the backfill material behind the face of a retaining wall creates a load force which becomes progressively greater at greater depths within the backfill. The load force is increased by roadways and vehicles or structures which may be situated on top of the backfill. The load force is primarily directed downward against subsoil but also has a horizontal component which must be sustained by the wall. 
     One known type of retaining wall has a modular construction which includes spaced apart columns of precast concrete header members which extend from the front face of the wall into the backfill material. The front face is formed by precast concrete stretcher members which extend horizontally between the headers and which are supported by the headers. Compacted backfill extends between the headers to the back surfaces of the stretcher members. Thus the stretcher members of the prior wall constructions must be sufficiently massive to sustain the horizontal component of load force in the backfill. The prior stretcher members also partially support the weight of overlying headers and must also be sufficiently massive for this purpose. 
     Retaining walls can be more attractive if landscaping plants are grown on the face of the wall. The prior wall constructions described above are not particularly conducive to plantings. While a strip of the backfill is exposed at each tier of the wall, it is undesirably narrow for planting purposes because of the shape, bulk and location of the load force resisting stretcher members. Further, the compacted backfill material which is exposed at the face of prior modular retaining walls may not be well suited for the growing of plants. 
     Header members of some prior modular retaining walls are linked together by thin projecting ribs which extend upward from the top of each header between spaced apart ribs on the bottom of the overlying header. The projecting ribs are relatively fragile portions of the headers which are susceptible to damage during construction of the wall. The ribs also allow forward or backward displacement of the header members relative to each other rather than establishing and maintaining a uniform batter or inclination of the face of the wall. 
     The present invention is directed to overcoming one or more of the problems discussed above. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect the present invention provides a retaining wall for stabilizing compacted structural backfill. A plurality of spaced apart columns of header members extend into the compacted backfill from a front surface of the backfill and also extend out from the front surface of the backfill to a front face of the wall. Front ends of the header members have inclined arms which extend outward and upward at the front face of the wall. A plurality of horizontal stretcher members extend between the header members at the front face of the wall and are supported by the inclined arms of the header members. The retaining wall further includes a plurality of vertically spaced layers of geosynthetic mesh reinforcement extending between the columns of header members and extending backward into the compacted backfill from the front surface of the backfill. The layers of geosynthetic mesh reinforcement have forward ends which turn upward at the front surface of the compacted backfill and then extend back into the compacted backfill. A volume of planting soil is disposed between the stretcher members and the upturned forward ends of the layers of geosynthetic mesh reinforcement and forms exposed tiers of planting soil at the front face of the retaining wall. 
     In another aspect the invention provides a retaining wall for compacted structural backfill wherein the retaining wall includes a plurality of cast concrete header members stacked in spaced apart vertically extending columns of header members which header members extend into the structural backfill from a front face of the wall. The header members have bases which rest upon an underlying header member and have front and rear post portions which extend up to the base of an overlying header member. Front ends of the header members have arms which extend outward and upward at the face of the wall at locations which are in front of the compacted structural backfill. A plurality of horizontal stretcher members extend between the columns of header members at the front face of the wall and are supported by the inclined arms of the header members. The stretcher members are spaced apart from header members other than the particular header members which support the stretcher member. A plurality of vertically spaced horizontal layers of geosynthetic mesh reinforcement extend between the columns of header members and extend backward therefrom within the backfill. The layers of geosynthetic mesh reinforcement have upturned forward ends which extend upward at the front of the compacted backfill and then extend back into the backfill. Planting soil is disposed between the stretcher members and the upturned forward ends of the layers of geosynthetic mesh reinforcement and forms tiers of planting soil at the front face of the wall. 
     In still another aspect the invention provides a retaining wall having a plurality of spaced apart vertical columns of stacked header members which extend into backfill material and a plurality of stretcher members which extend horizontally between front portions of the header members. The header members have flat top surfaces and flat bottom surfaces. A plurality of pins extend vertically from holes in the top surfaces of the header members into holes in the bottom surfaces of overlying ones of the header members and fix the positions of the header members relative to each other during construction of the wall. 
     The invention provides a modular retaining wall construction in which the horizontal component of load force in the backfill is resisted by layers of geosynthetic mesh reinforcement within the backfill rather than by the stretcher members which form the face of the wall. Load force on the stretcher members is further minimized as the stretcher members are not contacted by overlying header members and thus need not provide support for overlying structure. Consequently the stretcher members may be thinner than would otherwise be required and may be spaced outward from the front surface of the compacted backfill. This provides a very sizable space between the stretcher members and the front of the backfill which space is filled with relatively loose topsoil or the like. Broad tiers of the topsoil are exposed at the tops of the stretcher member. These conditions greatly facilitate planting and cultivation of plants on the face of the wall. In the preferred form of the invention, the header members are interlinked by vertical pins which fix the positions of the header members relative to each other to maintain the desired inclination of the front face of the wall during construction of the wall. 
     The invention, together with further objects and advantages thereof, may be further understood by reference to the following detailed description of the invention and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view of a retaining wall embodying the invention. 
     FIG. 2 is a frontal elevation view of the retaining wall of FIG. 1 with a portion of the structure being broken out in order to illustrate interior components. 
     FIG. 3 is a plan section view of a portion of the retaining wall of the preceding figures taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a cross section view taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is an isometric view of geosynthetic mesh reinforcement which is a component of the retaining wall. 
     FIG. 6 is a side elevation view of a top header member which members are components of the modular retaining wall. 
     FIG. 7 is a back end view of the top header member of FIG.  6 . 
     FIG. 8 is a side elevation view of an intermediate header member which members are also components of the modular retaining wall. 
     FIG. 9 is a back end view of the intermediate header member of FIG.  8 . 
     FIG. 10 is a side elevation view of a bottom header member which members are further components of the retaining wall. 
     FIG. 11 is a back end view of the bottom header member of FIG.  10 . 
     FIG. 12 is a foreshortened frontal view of a stretcher member which members form the front face of the retaining wall. 
     FIG. 13 is an end view of the stretcher member of FIG.  12 . 
     FIG. 14 depicts adjacent ends of two stretcher members and soil retaining components which bridge the adjacent ends. 
     FIG. 15 is an enlarged section view taken along line  15 — 15  of FIG.  14 . 
     FIG. 16 is an elevation section view of a retaining wall having a non-uniform vertical spacing of geosynthetic mesh reinforcement to accommodate to differing load forces at different levels within the wall. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIGS. 1 and 2 of the drawings, components of a modular retaining wall  11  embodying the invention include horizontally spaced apart columns  12  of header members  13  which support horizontally extending stretcher members  14  that form the front of the wall. Successive rows of aligned stretcher members  14  extend along the front of the wall at progressively greater heights and form a series of vertically spaced tiers  16  at which landscaping plants  15 , shown in FIG. 2, may be planted. 
     Referring again to FIGS. 1 and 2 in conjunction, the header members  13  of each column  12  are arranged in a stack in which each header member other than the lowest one rests on and is supported by the next lower header member. Each header member  13  other than the lowermost header members has an inclined arm  17  which extends outward and upward at the front of the header member. Each stretcher member  14  rests on and is supported by the inclined arms  17  of two header members  13  which are in separate spaced apart ones of the columns  12  of header members. The stretcher members  14  have a flat rectangular shape and the inclination of header arms  17  causes the stretcher members to be tilted with the forward edges  18  of such members being at a higher elevation than the back edges  19  of the members. 
     It is usually preferable that the face of a retaining wall  11  be inclined away from a strictly vertical orientation so that it leans towards the material which is being retained. Among other advantages, this increases the breadth of the tiers  16  at which plants  15  may be cultivated. As shown in FIG. 1 in particular, a desired inclination of the wall  11  is established by placing each header member  13  to extend slightly more rearwardly than the next underlying header member. Precise emplacement of successive header members  13  in this manner is facilitated by front and rear vertically oriented pins  21  which extend upward from each header member into the overlying header member. The pins  21 , which will hereinafter be further discussed, also act to inhibit lateral and longitudinal shifting of the header members  13  relative to each other during construction of the wall. 
     Referring to FIG. 2 in particular, the preferred length of the stretcher members  14  corresponds substantially to twice the spacing between successive columns  12  of header members  13 . This allows the abutments  22  between the successive stretcher members  14  of each row of stretcher members to be located midway between a pair of header member columns  12 . Preferably the stretcher member abutments  22  of alternate ones of the rows of stretcher members  14  are located between different pairs of the header member columns  12 . This causes stretcher members  14  of successively higher rows of stretcher members  14  to have an interleaved appearance when viewed from a location in front of the wall  11 . Shorter and longer stretcher members  14  can be used to establish vertically aligned abutments  22  at corners or other angles in the wall  11  and to provide vertical ends or sloped ends of the wall as may be called for by the contours of the site. 
     Referring again to FIG. 1, in some instances an excavation  23  of the original ground at the site may be made in preparation for emplacement of the retaining wall  11 . In other instances existing ground contours and available space may enable emplacement of the wall without major excavation. In this particular example of the invention an excavation  23  is present and has a rear slope  24  and a bottom  26  which is below the level  27  of the ground or pavement which extends outward at the base of the wall  11 . The excavation  23  may be broad enough to situate the columns  12  of header members  13  a distance outward from the rear slope  24  of the excavation if necessary to provide space for a broad roadway on top of the wall  11  or for other reasons. 
     The front portions of header members  13  extend out of compacted structural backfill  28  which fills the regions between the more rearward portions of the header members and which extends backward from the header members. The front boundary  29  of the compacted backfill  28  is defined by upturned front ends of vertically spaced apart layers  31  of geosynthetic mesh reinforcement which extend within the backfill and which will hereinafter be described in more detail. Front boundary  29  of the compacted backfill is spaced apart from stretcher members  14  and a vertically continuous filling of relatively loose planting soil  32  is situated between the stretcher members  14  and the front backfill boundary  29 . The previously described uptilted orientation of the stretcher members  14  leaves broad strips of planting soil  32  exposed at the successive tiers  16  of the wall  11 . 
     The term “structural backfill” as used herein and in the appended claims should be understood to refer to filler material having a high load bearing capacity and is typically compacted aggregate of the known type which is composed of gravel intermixed with smaller soil particles. The term “planting soil” as used herein and in the appended claims should be understood to refer to relatively loose material selected for its suitability for growing beds of plants and may variously be high quality topsoil or any of the known planting mixes. 
     Referring jointly to FIGS. 3,  4  and  5 , the layers  31  of geosynthetic mesh reinforcement reinforce the load bearing capacity of the body of backfill  28  and prevent the horizontal component of the load force from being exerted against the planting soil  32  and stretcher members  14 . The geosynthetic mesh reinforcement may be of one of the known forms and is typically a net formed of high strength synthetic polymer. Backfill aggregate penetrates the openings  35  in the geosynthetic mesh reinforcement and interlocks the backfill with the mesh. 
     Reinforcement of the backfill  28  at the front boundary  29  of the backfill is enhanced by a front portion  33  of each layer  31  which is angled to extend up to the next higher layer. The front portion  33  is further angled to extend backward for a short distance along the underside of the next higher layer  31  and then has an end section  34  which continues back into the backfill at a level which is below the underside of the next higher layer  31 . The small vertical spacing between the end section  34  of each layer  31  and the next higher layer  31  assures that both interlock with the structural backfill at this location. 
     The vertical spacing of the successive Layers  31  of geosynthetic mesh reinforcement may be varied to accommodate to differences in the inherent load bearing capacity of the particular backfill  28  and to differences in the load force to which the wall  11  will be subjected. The degree of reinforcement which the geosynthetic mesh reinforcement provides is dependent on the vertical spacing of the layers  31  and becomes greater as the spacing is reduced. In this particular example, layers  31  are coplanar with the tops and bottoms of each header member  13  and two additional layers  31  are present between the top and bottom of each header member. As best seen in FIG. 3, openings  36  are cut into the layers  31  of geosynthetic mesh reinforcement where portions of the header members  13  extend through the mesh. 
     Retention of backfill  28  at the front boundary  29  of the backfill is further provided for by barriers  37  formed of porous sheet material. Each barrier  37  has an intermediate portion  38  which extends upward at boundary  29  within the front portion  33  of a layer  31  of geosynthetic mesh reinforcement and has upper and lower portions  39  and  41  respectively which extend rearwardly into the backfill along the layer for a short distance. The barrier  37  material separates the backfill  28  and planting soil  32  and inhibits migration of soil particles from the structural backfill to the planting soil. 
     The geosynthetic mesh reinforcement of layers  31  is typically brought to the construction site in the form of rolled strips of the mesh which are then unrolled as the layers  31  are emplaced. To assure continuity it is preferable that adjacent ends of the barrier  37  material be overlapped with each other at the front of each layer  31  of geosynthetic mesh reinforcement. 
     Referring jointly to FIGS. 8 and 9, each header member  13  other than the lowermost and uppermost header members preferably has a longitudinally extending base portion  42  and a front post portion  43  and rear post portion  44  which extend upward from the ends of the base portion. The previously described inclined arm  17  of the header member  13  extends outward and upward from the front end of base portion  42 . This header member configuration provides the necessary load bearing capability while avoiding unnecessary bulk and weight. 
     The bottom surface of the base portion  42  and the top surfaces of the front and rear post portions  43  and  44  are flat and thus have no relatively fragile ribs or other projections. Holes  46  extend down into the tops of the post portions  43  and up into the base portion  42  to receive the previously described pins  21 . The header members  13  are preferably strengthened by internal reinforcing rods  45  of the known type. 
     Referring to FIGS. 6 and 7, the uppermost header members  13   b  preferably have a configuration which differs from that of the intermediate header members  13  in that no upwardly extending rear post portion is needed as the uppermost header members do not support overlying header members. The front post portions  43   b  of the uppermost header members  13   b  may be relatively truncated and may extend upward only far enough to provide a seat for a stretcher member in the previously described manner. Referring jointly to FIGS. 6,  7  and  8 , the uppermost head members  13   b  are shaped to interlock with the next underlying intermediate header members  13 . In particular, the base portion  42   b  of the uppermost header member  13   b  is formed with a downward extending key section  50  shaped to fit into the region between the tops of the front and rear post portions  43  and  44  of the underlying intermediate header member  13 . 
     Referring to FIGS. 10 and 11, the lowermost header members  13   a  have a relatively broad base portion  42   a  with a flat undersurface  47 . A rectangular upright portion  48  extends upward from the base portion  42   a  and has a flat top surface  49 , with pin receiving holes  46 , on which the next higher header member rests. 
     Referring to FIGS. 12 and 13, stretcher members  14  are of elongated flat rectangular shape. The stretcher members  14 , like the header members, are preferably strengthened by internal reinforcing rods  51  of the known type. 
     Referring again to FIGS. 3 and 4, thin flat cushions  52  of compressible sheet material are preferably disposed between the tops of the post portions  43  and  44  of the header members  13  and the bases of the next overlying header members. Cushions  53  of similar material are preferably provided between stretcher members  14  and the header members  13  which support the stretcher members. 
     FIG. 14 depicts the rearward facing surfaces of two adjoining stretcher members  14  in one of the horizontal rows of stretcher members. Referring to FIGS. 14 and 15, the stretcher members  14  are proportioned to provide for a small gap  56  between the ends of the two stretcher members. This accommodates the thermal expansion and contraction of the stretcher members  14  and facilitates emplacement of the stretcher members in the wall. Loss of planting soil  32  through the gap  56  is prevented by a sheet  57  of porous material which bridges the gap at the rear facing surfaces of the two stretcher members. The sheet  57  is backed and reinforced by a rectangular section  58  of geosynthetic mesh reinforcement, flaps  59  formed by margins of the sheet material  57  being folded under the vertically extending edge portions of the section  58 . 
     Referring again to FIG. 1, the inclination or slope of the face of the wall  11  is determined by the positioning of the header members  13  relative to each other. Each header member  13  is partially offset in the rearward direction relative to the next underlying header member. The extent of this partial offset is fixed during construction of the wall  11  by the location of the previously described pins  21  which extend between the header members  13 . The extent of the partial offset and thus the batter or inclination of the face of the wall  11  can be selected to be appropriate to a particular site by configuring the header members  13  to situate the pins  21  at more forward or more rearward locations along the header members. 
     Referring to FIG. 16, the degree of reinforcement of the load bearing capability of the backfill  28  that is provided by the layers  31  of geosynthetic mesh reinforcement is dependent on the vertical spacing and tensile strength of the layers and increases as the spacing is reduced. Load force in the backfill  28  increases at progressively greater depths in the backfill. Thus it can be advantageous to decrease the spacing of the layers  31  at greater depths and/or to use geosynthetic mesh reinforcement of greater tensile strength at greater depths. FIG. 16 depicts an example in which the layers  31   a  of geosynthetic mesh reinforcement within an uppermost region of the backfill  28  are spaced similarly to the spacing of the layers in the previously described embodiments of the invention. The layers  31   b  of geosynthetic mesh reinforcement are more closely spaced at an intermediate depth within the backfill  28 . At the lowermost region of the backfill  28  the layers  31   c  of geosynthetic mesh reinforcement are still more closely spaced. 
     During construction of the retaining wall  11 , with reference again to FIG. 1, emplacement of the header members  13 , backfill  28  and layers  31  of geosynthetic mesh reinforcement proceeds in stages. Following emplacement of the header members  13  at each tier of the wall  11 , the backfill  28  and layers  31  at that tier of the wall are emplaced before emplacement of the next higher header members. This emplacement of backfill  28  and layers  31  at each tier also proceeds in stages with the backfill which underlies each layer  31  being compacted prior to emplacement of that layer. Stretcher members  14  may be emplaced at any time after the particular header members  13  which support the stretcher member are in place. Planting soil  32  may be emplaced at each tier after emplacement of the layers  31  and backfill  28  is completed up to a higher level or may be deferred until, emplacement of all header members  13  and the associated layers and backfill have been completed. Landscaping of the successive tiers with plants may then proceed. 
     While the invention has been described with reference to certain specific embodiments for purposes of example, many modifications and variations are possible and it is not intended to limit the invention except as defined by the following claims.
A modular retaining watt has tiers of headers which extend into compacted backfill material and tiers of stretchers which extend between headers to form the front face of the wall. Vertical pins, extending between successive headers in each stack of headers, facilitate precise emplacement of headers during construction of the wall. Layers of geosynthetic mesh reinforcement reinforce the load bearing capability of the backfill. Load forces in the backfill are sustained by forward ends of the layers of geosynthetic mesh reinforcement, which extend upward in front of the backfill and then backward into the backfill, instead of being sustained by the stretchers. A sizable space behind the stretchers may be filled with loose topsoil to facilitate growth of landscaping plantings on the face of the wall.