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GATE 2017 CE (CE2) – Question 44

Geotechnical Engineering · Deep foundations: piles and pile groups · 2 marks · Multiple choice

Two identical concrete piles, plan dimensions 50 cm × 50 cm, are driven 20 m into homogeneous sand as shown. Dry sand has unit weight 18 kN/m³; saturated sand has unit weight 19 kN/m³ with groundwater at ground level. Both have $\phi=30^\circ$, $N_q=24$. Compare ultimate point-bearing resistances $Q_{P1}$ (dry) and $Q_{P2}$ (submerged).

Diagram for GATE 2017 CE (CE2) question 44
  1. $Q_{P1}>Q_{P2}$ by about 100%
  2. $Q_{P1}<Q_{P2}$ by about 100%
  3. $Q_{P1}>Q_{P2}$ by about 5%
  4. $Q_{P1}<Q_{P2}$ by about 5%

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Correct answer: (A) $Q_{P1}>Q_{P2}$ by about 100%

Explanation

Point resistance is $Q_P=A_P\sigma_v^\prime N_q$. Dry overburden is $18(20)=360$ kPa; submerged overburden is $(19-9.81)20=183.8$ kPa. With area 0.25 m² and $N_q=24$, capacities are 2160 and 1102.8 kN. The dry value is about 96% higher, approximately 100% (A).