Pile geotechnics (clay end bearing): for square end-bearing piles in clays with average unconfined compressive strength qu = 6 t/m^2, what net ultimate bearing capacity may be assumed for design?

Introduction / Context:
Design of end-bearing piles in saturated clays often uses undrained shear strength parameters. A common approach applies bearing capacity factors to the undrained cohesion to estimate ultimate capacity at the pile toe. This question assesses recall of the standard factor for deep foundations in clay.

Given Data / Assumptions:

• Unconfined compressive strength qu = 6 t/m^2.
• Undrained cohesion cu = qu / 2 = 3 t/m^2.
• For deep foundations in clay under undrained conditions, Nc ≈ 9 for toe bearing.
• Net ultimate capacity is based on end bearing only (shaft ignored here).

Concept / Approach:
Net ultimate bearing pressure at the base in undrained clay is q_net,ult = Nc * cu, with Nc ≈ 9 (Skempton). Substituting cu = 3 t/m^2 yields 27 t/m^2.

Step-by-Step Solution:
Compute cu = qu / 2 = 6 / 2 = 3 t/m^2.Use q_net,ult = Nc * cu with Nc = 9.q_net,ult = 9 * 3 = 27 t/m^2.Therefore, adopt 27 t/m^2 as the net ultimate bearing capacity.

Verification / Alternative check:
Typical geotechnical texts list Nc = 9 for undrained clays beneath deep foundations; the calculation aligns with standard practice.

Why Other Options Are Wrong:

• 15, 18, and 20 t/m^2 correspond to lower effective Nc values than standard undrained bearing for deep toe conditions.

Common Pitfalls:
Using qu directly instead of cu; confusing net with gross capacities; applying drained factors in undrained conditions.

Final Answer:
27 t/m^2