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SOIL AND EARTHWORK FORMULAS 191
LATERAL PRESSURE OF COHESIVE SOILS
For walls that retain cohesive soils and are free to move a considerable amount
over a long period of time, the total thrust from the soil (assuming a level sur-
face) is
1 2
P 2 H K A 2cH K A (8.21)
or, because highly cohesive soils generally have small angles of internal
friction,
1
2
P 2 H 2cH (8.22)
The thrust is applied at a point somewhat below H/3 from the bottom of the
wall, and the pressure distribution is approximately triangular.
For walls that retain cohesive soils and are free to move only a small
amount or not at all, the total thrust from the soil is
1 2
P 2 H K P (8.23)
because the cohesion would be lost through plastic flow.
WATER PRESSURE
The total thrust from water retained behind a wall is
1
P 2 0 H 2 (8.24)
where H height of water above bottom of wall, ft (m); and unit weight of
0
3
3
3
3
3
water, lb/ft 62.4 lb/ft (1001 kg/m ) for freshwater and 64 lb/ft (1026.7 kg/m )
for saltwater.
The thrust is applied at a point H/3 above the bottom of the wall, and the
pressure distribution is triangular, with the maximum pressure of 2P/H occur-
ring at the bottom of the wall. Regardless of the slope of the surface behind the
wall, the thrust from water is always horizontal.
LATERAL PRESSURE FROM SURCHARGE
The effect of a surcharge on a wall retaining a cohesionless soil or an unsatu-
rated cohesive soil can be accounted for by applying a uniform horizontal load
of magnitude K p over the entire height of the wall, where p is the surcharge in
A
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. K is defined earlier.
A