Evolution of excess pore water pressure in undrained claystructure interface shear tests

Recent studies focused on the shear behaviour of clay-structure interfaces have shown the importance of the shearing rate on the strength of these interfaces. In normally-consolidated clays, increasing the shearing rate results in a decrease in the interface strength, while the trend is opposite in heavily overconsolidated clays. While analytical and empirical interpretation methods indicate that the generation of shear-induced excess pore pressures are responsible for theaforementioned trends, experiments with pore water pressure measurements at the clay-structure interface are rare. In this paper, we first describe a modified interface shear box testing setup that is equipped with a pore water pressure sensor. For this equipment, the fully rough structural surface was manufactured with a port at the centre of the clay-surface interface to measure the pore water pressure. We present the results of undrained clay-structure interface tests on normally consolidated (NC) and overconsolidated (OC) specimens of kaolin clay. The results agree with the expectations, where the NC specimens generate excess pore pressures with greater magnitudes and heavily OC specimens generate negative excess pore pressures. Measurements of the pore water pressures allow calculating vertical effective stresses, which can be used to investigate the effective stress paths followed by the clay-structure interface during the tests. This paper also provides a comparison of the measured values of beta and adhesion factors with previously published results and relationships used for the design of deep foundations.