Parameters controlling the capacity of axially loaded drilled shaft foundations in sand, gravel, and cobbles

Drilled shafts have become the preferred deep foundation element in many arid environments where coarse material is prevalent, including the southwest USA, because soil conditions are usually unfavorable to driven piles, scour depths on the ephemeral river channels are quite large, and there is increased confidence in the bearing layer afforded by the drilled shaft construction process. Finite element analyses were performed on two case history studies of axially loaded concrete drilled shafts founded in sand, gravel, and cobbles soils (SGC). A parametric study was conducted to determine the most important soil parameters controlling the axial capacity of drilled shafts. The soil parameters used in this study are: soil angle of internal friction, soil dilation, soil modulus, initial coefficient of lateral earth pressure at rest, and angle of internal friction between soil and shaft. The axial bearing capacity and skin friction load were determined in each case. It was found that the soil angle of internal friction, soil dilation, coefficient of friction between soil and shaft, and soil modulus, E, are the most important parameters controlling the behavior of axial loaded drilled shaft in SGC soils. The ratio between horizontal stresses and vertical stresses at failure is completely different from the coefficient of lateral earth pressure at rest.