Axial capacity estimation of straight drilled shafts in expansive and high plastic clay at different seasonal installation periods

Abstract

Accepted methods of predicting drilled shafts capacities require elimination of the top five feet as a non-contributing layer in the total axial capacity estimation. The research presented in this paper focused on axial capacity calculations of shafts installed in expansive soils at different seasonal periods. The void zones, i.e. non-contact zones of drilled shafts and surrounding soil are determined by using an intrusive circular rod like probe. The length of this zone and its contribution to the total capacities are then evaluated. This research suggests that the elimination of top four feet of shaft's side friction is warranted from capacity calculations of drilled shaft if the soil tests are measured in high plastic clays in ’dry’ season. Axial capacity analysis in the wet season shows that upper layers and their side friction should be included. This paper has accounted for low strength values of the clay at wet seasonal conditions and hence incorporating the upper layer in the axial capacity estimation should not be regarded as a non-conservative step. Recommendations with respect to zone lengths and shear strength measurements of the soil layers for axial capacity of drilled shafts are provided.