Shaft resistance of model pile in clay

A test apparatus to measure the load transfer along the shaft of a model pile inserted in a specimen of clay is described. This apparatus allows independent control of the boundary stresses of a cylindrical clay specimen in the vertical and horizontal directions. The top and lateral surfaces of the clay specimen can deform freely and the load transfer zone on the pile is separated from the rigid base. The load transfer along the shafts of smooth and rough surface steel model piles in a kaolinitic clay under varying vertical and horizontal effective stress combinations was measured. The pile installation technique used was designed to minimize soil disturbance so that the failure criterion for pile shaft friction could be investigated. Normally consolidated and overconsolidated conditions, both in the vertical and horizontal directions, were considered in tests using eleven clay specimens with three different thicknesses and steel model piles of three different diameters. The results show that the angle of pile-clay friction is independent of the vertical consolidation pressure in the clay, the overconsolidation ratio (both in the vertical and horizontal directions), and the length of the pile-soil contact. The influence of pile diameter is not clearly established. The surface roughness of the pile clearly affects the pile-clay friction angle, which is approximately equal for smooth surface piles to the Hvorslev true friction angle and for rough piles to the effective friction angle of the clay. There is no reasonable correlation with the undrained strength. The results support the effective stress approach.