Example

A continuous footing is 1.8 m wide and is founded at a depth of 0.75 m in a clay soil of unit weight 20kN/m3 with 4>a =0° and cu = 30kN/m2. The foundation is to carry a vertical line load which will act at a distance of 0.5 m from the centre. The weight of the footing will exert a uniform pressure of 12 kN/m2 on to the soil.

Determine the value of safe bearing capacity for the footing, taking F = 3.0, and determine a safe value for the line load.

Solution

Eccentricity of line load, e, = 0.5m

From Table 8.1, for <f>u = 0°, Nc = 5.14, Nq= 1.0, N7 = 0. Footing is continuous, i.e. L —> oo Sc = 1.0.

Weight of foundation = 12 kN/m2 hence safe pressure from line load = 75 - 12 = 63kN/m2.

Safe value of line load = 63 x B' = 63 x 0.8 = 50 kN/m2 EXAMPLE 8.6

A foundation is 3 m wide and 9 m long and is to be founded at a depth of 1.5 m in a deep deposit of dense sand. The angle of shearing resistance of the sand is 35° and its unit weight is 19kN/m3.

(i) Determine the safe bearing capacity of the foundation for F = 3.

(ii) Determine the safe bearing capacity of the foundation if it is subjected to a vertical line load of 220 kN/m at an eccentricity of 0.3 m, together with a horizontal line load of 50 kN/m acting at the base of the foundation.

e r u Qu net cNcScDc Sale bearing capacity = —---\-jz =----h 7z

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