The equation for shearing strength S (lb/ft2 or kPa) of a soil may be taken as follows:
S = c + a tan ф (8.3)
where c = cohesion, lb/ft2 (kPa)
a = confining pressure or normal stress, lb/ft2 (kPa) ф = angle of internal friction of the soil, degrees
The shearing strength of the soil should account for the effect of pore water pressure when present. Equation (8.3) can be modified as:
S = c + (a — u) tan ф (8.4)
where u = pore water pressure, lb/ft2 (kPa)
Soil consolidation is produced by load and is associated with changes in soil moisture. It is also a function of time. The time required for drainage to occur, which results from the change in soil moisture, is a function of the permeability of the soil and the distance the water must travel in the material to be released. It is clear that consolidation of coarse-grained materials will occur fairly rapidly. This explains the often used assumption that consolidation of such materials under applied load, for example, the load of a retaining wall, generally occurs during the construction of the wall. Thus, long-term settlement is not normally considered to occur. On the contrary, clays and/or silts are relatively impermeable, so that long-term settlement should be anticipated in the design. The designer must consider various options to accommodate this projected long-term settlement. For example, the designer may (1) require preloading to effect the settlement before the wall is constructed, (2) accelerate the consolidation by drilling for and placing sand drains, and (3) decide to build the structure with pile or caisson support systems that are independent of the consolidation.
