When designing a group of friction piles in a soft clay, what critical failure mechanism, distinct from individual pile failure, must be assessed for the entire group?

When designing a group of friction piles in soft clay, the critical failure mechanism distinct from individual pile failure that must be assessed for the entire group is block failure, also known as group failure. This mechanism occurs when the entire block of soil encompassed by the pile group, including the piles themselves, behaves as a single, rigid unit and fails by pushing through the surrounding or underlying soil. Instead of each pile failing individually by losing its skin friction or end bearing capacity, the soil mass around and beneath the entire group fails in shear along its perimeter and base. This failure mode is particularly critical in soft clays for friction piles because their load-carrying capacity relies predominantly on skin friction, which is the frictional resistance developed along the pile shaft with the surrounding soil. In soft clay, when piles are closely spaced, the zones of influence of individual piles overlap significantly. This overlap reduces the effective skin friction capacity that can be mobilized by each individual pile within the group compared to an isolated pile. As a result, the soil between the piles provides diminishing additional resistance, causing the entire block of soil containing the piles to act cohesively. The resistance of the block then depends on the shear strength of the soil acting along the entire perimeter of this block (like a large, single pier) and the end bearing capacity of the soil acting under the base of this effective block. Therefore, the assessment involves calculating the capacity of this conceptual block of soil and piles, which is determined by the total shear resistance along its perimeter and the end bearing resistance at its base, and comparing this value to the sum of the individual pile capacities (often adjusted for group efficiency). The smaller of these two calculated capacities dictates the overall ultimate load-carrying capacity of the pile group.