CIVIL ENGINEERING FORMULAS

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

(8.21)

or, because highly cohesive soils generally have small angles of internal
friction,

(8.22)

The thrust is applied at a point somewhat below H/ 3 from the bottom of the
wall, and the pressure distribution is approximatelytriangular.
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

(8.23)

because the cohesion would be lost through plastic flow.

WATER PRESSURE

The total thrust from water retained behind a wall is

(8.24)

whereHheight of water above bottom of wall, ft (m); and  0 unit weight of
water, lb/ft^3 62.4 lb/ft^3 (1001 kg/m^3 ) for freshwater and 64 lb/ft^3 (1026.7 kg/m^3 )
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/Hoccur-
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 KApover the entire height of the wall, where pis the surcharge in
pound per square foot (kilopascal). For saturated cohesive soils, the full value
of the surcharge pshould be considered as acting over the entire height of the
wall as a uniform horizontal load. KAis defined earlier.

P^1  2  0 H^2

P^1  2 H^2 KP

P^1  2 H^2  2 cH

P^1  2 H^2 KA 2 cHKA