water pressure is greater than normal because it is carrying part of the applied stress (as opposed to the soil particles). Where Tv is the time factor. Hdr is the average longest drain path during consolidation. t is the time at measurement Cv is defined as the coefficient of consolidation found using the log method with or the root method with t50 time to 50% deformation (consolidation) and t95 is 95% Where T95=1.129 T50=0.197 Creep The theoretical formulation above assumes that time-dependent volume change of a soil unit only depends on changes in effective stress due to the gradual restoration of steady-state pore water pressure. This is the case for most types of sand and clay with low amounts of organic material. However, in soils with a high amount of organic material such as peat, the phenomenon of creep also occurs, whereby the soil changes volume gradually at constant effective stress. Soil creep is typically caused by viscous behavior of the clay-water system and compression of organic matter. This process of creep is sometimes known as "secondary consolidation" or "secondary compression" because it also involves gradual change of soil volume in response to an application of load; the designation "secondary" distinguishes it from "primary consolidation", which refers to volume change due to dissipation of excess pore water pressure. Creep typically takes place over a longer time-scale than (primary) consolidation, such that even after the restoration of hydrostatic pressure some compression of soil takes place at slow rate. Analytically, the rate of creep is assumed to decay exponentially with time since application of load, giving the formula: Where H0 is the height of the consolidating medium e0 is the initial void ratio Ca is the secondary compression index t is the length of time after consolidation considered t95 is the length of time for achieving 95% consolidation Deformation characteristics of consolidation Coefficient of compressibility . The compressibility of saturated specimens of clay minerals increases in the order kaolinite <illite<smectite. The compression index Cc, which is defined as the change in void ratio per 10-fold increase in consolidation pressure, is in the range of 0.19 to 0.28 for kaolinite, 0.50 to 1.10 for illite, and 1.0 to 2.6 for montmorillonite, for different ionic forms. The more compressible the clay, the more pronounced the influences of cation type and electrolyte concentration on compressibility. Coefficient of volume compressibility See also Compaction (geology) Settlement (structural)