Determination of consolidation parameters based on the excess pore water pressure measurement using a newly developed U-oedometer

In this study, the U-oedometer, a novel modified oedometer cell equipped with tailor-made needle probes, is developed to easily and accurately measure the excess pore water pressure (Δ u) during 1D consolidation tests and to determine the coefficient of consolidation (c_v). The 3D printing technique is applied to make simple yet robust modifications to the conventional oedometer cell for facilitating the installation of the needle probes. The tailor-made needle probes are designed in such a way that the volumetric compliance is lowered to avoid measurement bias. Subsequently, the Δ u-based method is proposed to determine c_v, with the target of avoiding the intervention of human judgement and therefore minimizing the degree of subjectivity. The experimental results demonstrate that the measured Δ u matches the theoretical values of the Terzaghi 1D consolidation theory, showing that the estimated c_v is sufficiently reliable. In addition to the determination of c_v, the U-oedometer allows additional measurements of other soil properties during consolidation, including the coefficient of permeability (k) and the coefficient of earth pressure at rest (K). It is observed that k decreases with the reduction in void volume, due to the increase in the effective vertical stress (σv′). Further, the secondary compression seems to be a continuation of the primary consolidation, where the soil sample continues to deform at a relatively slower rate, associated with the slight decrease in k. A constant value of K is observed at any value of σv′ in the loading path, while during secondary compression, K slightly increases with time.