Lateral Pressure From Surcharge
Posted in Soil Engineering |
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The effect of a surcharge on a wall retaining a cohesionless soil or an unsaturated cohesive soil can be accounted for by applying a uniform horizontal load of magnitude KAp over the entire height of the wall, where p is the surcharge in pound per square foot (kilopascal). For saturated cohesive soils, the full value of the surcharge p should be considered as acting over the entire height of the wall as a uniform horizontal load. KA is defined earlier.
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