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Question

A 3 m high retaining wall is supporting a saturated sand (saturated due to capillary action) of bulk density 18kN/m3 and angle of shearing resistance 30. The change in magnitude of active earth pressure at the base due to rise in ground water table from the base of the footing to the ground surface shall [γw=10kN/m3]


A
Increase by 20kN/m2
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B
Decrease by 20kN/m2
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C
Increase by 30kN/m2
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D
Decreases by 30kN/m2
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Solution

The correct option is A Increase by 20kN/m2
Angle of shearing resistance

ϕ=30

Active earth pressure coefficient;

Ka=1sinϕ1+sinϕ

=1sin301+sin30=13

Active earth pressure at the base,

Pa=Kaσz

=Kaγ z

=13×18×3

=18kN/m2

Active earth pressure at the base when water table is at the ground surface

Pa2=Ka γsub z+1×γw×z

=13(1810)×3+10×3

=38kN/m2

Change in earth pressure

= 38 - 18

=20 kN/m2(increase)

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