Cyclic Interface Shearing in Sand and Cemented Soils and Application to Axial Response of Piles
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.
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Centre for Offshore Foundation Systems, University of Western Australia, Australia
Introduction
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 τs for piles in non-cohesive soil is the expression τs = σn tan δ = Kσv0 tan δ = βσv0
σn
(1)
where is the normal effective stress acting round the pile shaft after is the in situ effective vertical stress installation, K is the stress ratio, σv0 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 d50 for the soil) normalized by the value of d50 . 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 φcv , 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. For driven piles in sand and other soils of high permeability, it has long been realized that the magnitude of shaft friction at a given depth can C. di Prisco et al. (eds.), Mechanical Behaviour of Soils under Environmentally Induced Cyclic Loads © CISM, Udine 2012
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reduce as the pile is driven further, with the net effect that the average friction along the pile shaft can reach a limit and even reduce as the pile embedment increases (Vesic, 1977). That is the basis for the design method in the API (2007) design guidelines for offshore structures Fig. 1, although the logic behind the approach has never been justified.
Figure 1. Design approaches for estimating shaft friction for piles in sand. Over the last 20 years, however, more rational design approaches have been developed, based on improved understanding of the main mechanisms. Actual measurements of the variation of friction along the piles were obtained through carefully instrumented pile tests undertaken by the research group at Imperial College (Lehane et al., 1993). The phenomenon of ‘friction degradation’ is illustrated in Fig. 2 with profiles of shaft friction measured in the three instrument clusters at different distances (h) from the tip of a 6 m long, 0.1 m diameter, pile as it is jacked into the ground. For comparison, the cone profile is plotted on the same scale,
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