The Influence of a Thin Weak Soil Layer on the Pile Bearing Capacity-Paper

Paper

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The study examines how soil around a pile tip affects pile bearing capacity, with particular attention to thin weak layers. Existing CPT-based design methods use different averaging zones around the pile tip, but the authors argue these zones do not accurately represent the soil’s influence.<br /><br />Axisymmetric PLAXIS simulations modeled an already-installed concrete pile in homogeneous sand. The resulting plastic zone—and assumed influence zone—extended approximately 2 pile diameters (D) above the tip and 5D below it. This suggests soil below the tip contributes more to bearing capacity than conventional methods generally assume. The zone’s size also varied with pile depth and soil stresses.<br /><br />Further simulations placed a thin, weaker layer at different positions around the pile tip, varying either its stiffness or friction angle and testing two layer thicknesses. The layer affected capacity when located roughly from 2D above to 6D below the tip, consistent with the simulated plastic zone and laboratory model-test findings. The greatest capacity reduction occurred about 1D below the tip. A thicker weak layer caused a larger reduction; layers outside the influence zone had negligible effect. The weak layer had little effect on shaft resistance.<br /><br />The authors conclude that CPT resistance should be averaged using an influence zone weighted more heavily toward soil below the pile tip. They also note that pile capacity depends on both soil stiffness and strength, while existing design methods may not fully account for stiffness. The findings are based on simplified simulations that omit pile-installation effects, so further validation is warranted.

Keywords

pile bearing capacity

soil around pile tip

thin weak layer

cone penetration test

PLAXIS simulations

plastic zone

influence zone

pile tip resistance

soil stiffness

shaft resistance