Bearing capacity factor Nc for piles in clay (Meyerhof) According to Meyerhof's recommendations for deep foundations in cohesive soils, what value is taken for the cohesion bearing capacity factor Nc for piles?

Introduction:
End-bearing capacity of piles in clay is commonly expressed using a cohesion factor Nc multiplying the undrained shear strength. Different theories propose different Nc values for shallow versus deep foundations. Recognizing the appropriate factor for piles prevents over- or underestimation of capacity.

Given Data / Assumptions:

• Deep foundation behavior (pile base in undrained clay).
• Short-term undrained condition controls end bearing.
• Meyerhof's empirical recommendations for piles.

Concept / Approach:

For shallow foundations under general shear, Terzaghi's Nc is 5.14. For deep foundations (piles) in undrained clay, higher confinement and failure mechanisms justify a larger Nc. Meyerhof recommended Nc ≈ 9 for pile toe capacity in clay, reflecting deep foundation behavior.

Step-by-Step Solution:

Verification / Alternative check:

Comparisons with field load tests show that Nc = 9 gives reasonable predictions for pile base resistance, whereas Nc = 5.14 would be conservative for deep foundations.

Why Other Options Are Wrong:

5.14 is for shallow foundations; 6.2 and 7.5 are not standard for piles; 12.0 is overly high for typical practice.

Common Pitfalls:

Applying shallow-foundation factors to piles; mixing drained and undrained parameters; ignoring setup or remolding effects near the pile base.

Final Answer:

9.0