Mechanism of pore pressure time lag for saturated clays

Time lag of pore pressure often occurs during oedometer consolidation of saturated clays. After applying loading, the pore pressure measured at the base of specimens increases up to the maximum and then decreases with the increase in consolidation time. The previous studies have concluded that the stiffness of the standpipe piezometers is not enough to resist the applied loading, resulting in a non-negligible deformation of pore pressure gauge chamber. Accordingly, the time lag of pore pressure is attributed to the effects of stiffness on the pore pressure measurement system. In this study, the role of stiffness of pore pressure measurement system in pore pressure time lag behaviour is investigated, based on the existing theories. It is found that the diaphragm piezometer currently used for measuring pore pressure has a large stiffness enough to resist the applied loading. Consequently, the deformation of pore pressure gauge chamber induced by the applied loading can be negligible. The test results available on the pore pressure behaviour under undrained conditions using the triaxial testing apparatus indicate that the pore pressure reaches the order of the applied loading within a very short consolidation time of a few minutes. Such a result is consistent with that from the theoretical analysis. On the other hand, when the oedometer consolidation apparatus is adopted, the top boundary condition of the specimen is freely drained during applying loadings. That is, consolidation occurs at the top layer of the specimen before the hydraulic gradient induced by the applied loadings is uniformly distributed on the specimen. The difference in the measurements of pore pressure between the triaxial and the oedometer consolidation tests is most probably attributed to the difference in the boundary drainage conditions when applying loadings. The time lag of pore pressure at the base of the specimen during oedometer consolidation is caused by the dissipation of pore pressure from the top boundary. The consolidation time responsible for the maximum pore pressure distributes within a wide spectrum of about dozens of minutes to several hundreds of minutes. The key factors of influencing the behaviour of pore pressure time lag are found to be permeability coefficient and compression index. The consolidation time responsible for the maximum pore pressure increases with the decrease in the permeability coefficient, and increases with the increase in the compression index. The permeability coefficient has a more significant effect on the time lag of pore pressure than the compression index.