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.
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| 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
