Bearing Capacity in Clay

Bearing Capacity in Clay

In clay soils and plastic silts, the cohesion term plays a major role in bearing capacity. The friction angle is considered to be zero for clays and plastic silts.

Design Example

This example explores a column footing in a homogeneous clay layer. Find the ultimate bearing capacity of a 1.2 m x 1.2 m square footing placed in a clay layer. The density of the soil is found to be 17.7 kN/m 3 and the cohesion was found to be 20 kPa. See Fig.

Column footing in a homogeneous clay layer

Solution

STEP 1: Find the Terzaghi bearing capacity factors from Table Below.

For clay soils, the friction angle (~) is considered to be zero.

Nr = 5.7

Nq= 1.0

Ny = 0.0

STEP 2: Find the shape factors using Table

For a square footing Sc = 1.3 and Sy = 0.8.

STEP 3" Find the surcharge (q).

q = F x d - 17.7 x 1.2 = 21.2 kPa

STEP 4" Apply the Terzaghi bearing capacity equation.

qult = 20 x 5.7 x 1.3 + 21.2 x 1.0 + 0

qult = 169.4 kPa

allowable bearing capacity(qallowable) -- qult/F.O.S. - qult/3.0 = 56.5 kPa

The total load (Qallowable) that could safely be placed on the footing is found as

Qallowable = qallowable x area of the footing

Q allowable = qallowable x (1.5 x 1 . 5 ) - 1 2 7 . 1 kN