The magnitude of stress waves propagated in piles, during driving, depends significantly upon the boundary conditions imposed by the soil. In an attempt to evaluate these conditions, a series of dynamic penetration tests have been performed in sand. Equipment suitable for the measurement of dynamic forces and associated kinematic quantities has been developed and proven.
The variation of soil resistance with the velocity and acceleration of the penetrating body has been determined for three porosity conditions. The overburden, viscous, and inertial components of dynamic soil resistance vary in a systematic and definite manner with porosity. Skin friction forces acting upon the moving body show quantitative correlation with body accelerations, although the accuracy of their determination i s , in some cases, questionable. Negative values of skin friction were consistently measured; a phenomenological explanation based upon the concept of virtual mass has been advanced. A visualisation test was performed to determine the effect of a penetrating body upon a sand mass and to indicate a possible mechanistic explanation for negative skin friction values, Such an explanation could not be determined, however, valuable information on sand particle motions was obtained.
The importance of a theoretical investigation of the forces acting on a moving body, in sand, is stressed and the general orientation of such study is briefly outlined.