Field measurements of pore-water pressure changes in a stiff fissured very high plasticity Palaeogene clay during excavation and pile driving
Deep excavations induce stress relief of the underlying substratum. In areas of Denmark where the near-surface substratum consists of heavily overconsolidated, Palaeogene clays of very high plasticity this poses a specific challenge. Stress relief from excavation will typically over time cause the clay to heave – a heave that is governed by the dissipation of negative excess pore-water pressures induced by unloading. However, if foundation piles are driven immediately after excavation – which is common practice in connection to deep basement excavations in Denmark – large positive excess pore water pressures may develop that may fully or partly equalize the negative excess pore-water pressures. In order to shed light on pore-water pressures during excavation and pile driving, piezometers were installed at a construction site in various depths and at different distances to driven piles. This paper presents and discusses pore-water pressure measurements obtained during excavation and pile driving. As expected, the observations show that excavation leads to a drop in pore-water pressure and pile driving induce large excess pore-water pressures in the clay.