Behaviour of a sensitive clay during pile driving

Abstract

A full scale investigation with six instrumented test piles has been carried out on the Saint-Alban test site in order to study the behaviour of friction piles in soft sensitive soils. The first part of this investigation, reported in this paper, deals essentially with the effects of pile driving on the induced pore pressures and their dissipation and on the disturbance of the clay around the pile.It is shown that the induced pore pressures at the pile tip ΔuT, and at the pile surface Δus, correspond to 1.6σvo and 0.8σvo respectively. If the failure pattern developed during driving is interpreted by means of the theories of expansion of cavities, it is shown that the induced pore pressures calculated at the pile–soil contact agree well with the measured values; these pore pressures are fully dissipated after 600 h.Immediately after driving, a decrease in the undrained shear strength varying between 0 and 30% was observed in a zone of 3 diameters around the pile. The strength in that zone was nearly fully recovered after the pore pressures had dissipated.The tip resistance measured during driving was quite in excess of the static zone penetration resistance qc, indicating possible scale effects. The average unit skin friction was observed to decrease from a value in the order of cu to about 0.10σvo′ and to be directly related to the pore pressures, i.e., to the effective stresses in the clay around the pile.