The effect of the remoulded void ratio on unit shaft friction in small-displacement piles in chalk

Small-displacement pile driving in chalk produces an annulus of crushed and remoulded putty-like material through which shaft friction is believed to be mobilised. Ultimate shaft friction is assumed to be a function of the effective angle of friction of the putty chalk interface and the radial effective stress acting on the pile shaft. The mechanisms that affect the magnitude of the latter are not well understood. It is hypothesised that the stress levels attained during shaft friction mobilisation are related to the change in void ratio that takes place as structured chalk becomes crushed during pile installation. Therefore, a general notion exists that piles installed in dense chalks will mobilise larger shaft capacities than piles driven in more porous materials, and that this is related to the void ratio of the remoulded annulus. In this context, this paper presents the results of a series of monotonic constant volume simple shear tests conducted to assess the role of the void ratio in the mobilisation of shaft friction. Results suggest that void ratio is a controlling factor and that a state-based approach for the characterisation of void ratio-shaft friction relationships may be possible