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

Abstract

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 ($\Delta u$) during 1D consolidation tests and to determine the coefficient of consolidation ($c_{\text{v}}$). 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 $\Delta u$-based method is proposed to determine $c_{\text{v}}$, with the target of avoiding the intervention of human judgement and therefore minimizing the degree of subjectivity. The experimental results demonstrate that the measured $\Delta u$ matches the theoretical values of the Terzaghi 1D consolidation theory, showing that the estimated $c_{\text{v}}$ is sufficiently reliable. In addition to the determination of $c_{\text{v}}$, 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 ($K_{0}$). It is observed that k decreases with the reduction in void volume, due to the increase in the effective vertical stress ($\sigma_{\text{v}}^{'}$). 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 $K_{0}$ is observed at any value of $\sigma_{\text{v}}^{'}$ in the loading path, while during secondary compression, $K_{0}$ slightly increases with time.