A strip footing is to be designed to support a dead load of 500 kN/m and an impo

Question

A strip footing is to be designed to support a dead load of 500 kN/m and an imposed

load of 300 kN/m at a depth of 0.7m in a gravelly sand. Characteristic values of the

shear strength parameters are c0 ¼ 0 and 0 ¼ 40. Determine the required width of the

footing if a (lumped) factor of safety of 3.0 against shear failure is specified and

assuming that the water table may rise to foundation level. Would a foundation of

that width satisfy the bearing resistance limit state? The unit weight of the sand above

the water table is 17kN/m3 and below the water table the saturated unit weight is

20 kN/m3.

Explanation / Answer

It should be noted that net ultimate bearing capacity would be obtained directly if

Nq 1 were used in the bearing capacity equation.

The net foundation pressure is

qn ¼ 800

B

ð17 0:7Þ

Then, for a factor of safety of 3,

1

3

ð750 þ 485BÞ ¼ 800

B

12

Hence B ¼ 1:55m

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