# Lateral Pressure From Surcharge

Posted in Soil Engineering | Email This Post |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 K_{A}p. 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.

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