To counter any effect of a surcharge on retaining wall on a cohesionless soil or an unsaturated cohesive soil we need to apply a uniform horizontal load of magnitude KAp. This load is applied over the entire wall height.
In case of saturated cohesive soil, the entire surcharge value acts on the entire wall height. This is considered to be a uniform load acting horizontally.
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When the retaining structure does not yield, active earth pressure (Ka) from a sliding wedge of soil cannot develop. The load on the structure is then the lateral pressure exerted by a soil at rest, ph = pv*K0 where:
ph = lateral (horizontal) pressure on structure (wall friction does not develop);
pv = vertical soil stress at depth z, pv = q + gamma*z where q = superimposed live load and gamma = design unit weight of soil and water;
K0 = the at-rest earth pressure coefficient, which is usually greater than the active earth pressure coefficient, Ka.
Two methods are used to determine K0:
for granular soils, K0 = (1 – sin(PHI))/(1 – sin(BETA)) where PHI = effective angle of internal friction and BETA is the surface angle to the horizontal (by Kezdi, 1972); or
for cohesive soils (clay and rock), K0 = Nu/(1 – Nu) where Nu = Poisson’s ratio.
Of course, K0 = 1 for water. It is reasonable to assume that the water table can be at the surface after prolonged rain, therefore, the lateral pressure comprises two components:
1. water pressure pw = 9.807*z and
2. soil pressure ps = (q + (gam_sat – 9.807)*z)*K0 where gam_sat is the saturated unit weight of the soil.
Then ph = pw + ps