Pile Skin Friction in Sands from Constant Normal Stiffness Tests
Many interface and sand-to-sand direct shear tests have been performed with a constant normal stiffness to investigate the effects of interface roughness, normal stiffness, soil relative density, initial normal stress, and cyclic displacement amplitude on the soilinterface response. It is demonstrated that the patterns of behavior in these tests are similar to those observed in model pile tests and that the constant normal stiffness (CNS) condition is required to account for the effects of the volume changes that occur as soil adjacent to a pile is sheared. In dense sands that tend to expand in volume when sheared, the shear and normal stresses developed are significantly higher than for constant normal load (CNL) conditions and increase with higher values of normal stiffness. For loose sands that compress when sheared, the stresses are lower than in CNL tests and decrease for higher values of normal stiffness. The CNS tests replicate the reduction in shear stress during displacement-controlled cycling and the subsequent loss of capacity reported for model and full-scale piles. The reduction of skin friction during cycling and the subsequent loss of pile frictional capacity is found to be greatest for loose sands, high normal stiffness, large displacement amplitudes, and rough interfaces.