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Question

A singly-reamed, 8 m long, RCC pile (as shown in the below figure) weighing 20 kN with 350 mm shaft diameter and 750 mm under-ream diameter is installed within stiff, saturated silty clay (undrained shear strength is 50 kPa, adhesion factor is 0.3, and the applicable bearing capacity factor is 9) to counteract the impact of soil swelling on a structure constructed above. Neglecting suction and the contribution of the under-ream to the adhesive shaft capacity, what would be the estimated ultimate tensile capacity (rounded off to the nearest integer value in kN) of the pile?


A
132 kN
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B
156 kN
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C
287 kN
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D
301 kN
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Solution

The correct option is D 301 kN

Weight of pile,
Ps=20 kN
Shaft dia, D0=350 mm
Under ream dia,
Du=750 mm
Undrained shear strength,
C=50 kPa
Adhesion factor,
α=0.3
Nc=9
Ultimate Tensile capacity will be due to
1. Friction along the length of pile (P1)
2. Bearing action caused by underreamed portion (P2)
3. Dead weight of pile (P3)
Tensile capacity due to friction,
P1=fs×As
=α.C.πD0×L
=0.3×50×π×0.35×7.6
=125.286 kN
Tensile capacity due to bearing action
P2=CNc.A
=50×9π(D2uD20)4
=50×9×π×(0.7520.352)4
=155.51 kN
P=P1+P2+P3
=125.286+155.51+20
=300.796 kN

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