Cyclic Interface Shearing in Sand and Cemented Soils and Application to Axial Response of Piles

Abstract

Estimating the shaft capacity of piles driven into sand is an area of considerable uncertainty, because of the complex processes involved and the sensitivity of the normal effective stress acting on the pile shaft to minor volume changes within the sand. The starting point for calculating values of shaft friction τ _s for piles in non-cohesive soil is the expression

$${\tau _s} = \sigma _n^\prime \tan \delta = K\sigma _{v0}^\prime \tan \delta = \beta \sigma _{v0}^\prime$$

(1)

where σ′ _n is the normal effective stress acting round the pile shaft after installation, K is the stress ratio, σ′_v0 is the in situ effective vertical stress and δ is the angle of friction between pile and soil. The latter quantity may be measured in interface shear tests for the particular pile material. Kishida and Uesugi (1987) reported a detailed study of the effects of surface roughness, and showed how the interface friction angle may be related to the friction angle of the soil in terms of a normalized roughness coefficient, defined as the maximum roughness of the pile surface (over a gauge length of d ₅₀ for the soil) normalized by the value of d ₅₀. For typical pile surfaces (oxidized mild steel or concrete), the normalized roughness coefficient will exceed 0.05, and the coefficient of friction at the interface will lie in the range 0.75 to 1 times that for the soil itself. An alternative assumption, where interface shear data are not available, is to assume that the interface friction angle δ may be approximated as φ _cv, the critical state angle of friction. This may be justified on the basis that no dilation is to be expected between the sand and the wall of the pile.