Is Excess Pore Water Pressure Build Up An Engineering Demand Parameter For Excavation Analyses?

Conventional excavation analysis based on total or effective stress methods can result in inaccurate estimation of ground movements when impervious soil conditions or rapid excavation sequences are not found in the excavation (i.e., assumption of undrained response is not valid). The influence of excavation rate, hydraulic conductivity, and excavation geometry on the fully coupled solid-fluid behavior of a braced excavation in cohesive soils are studied parametrically using PLAXIS2D. The coupled excavation analysis is performed using the Hypoplasticity clay model to reproduce the constitutive behavior of soil and the poro-elasticity Biot’s theory to account for changes in pore water pressure due to deformations in the soil skeleton. Results obtained from the parametric studies indicate that excavation rates to hydraulic conductivity ratios, ER/k, smaller than 0.1 and larger than 104 can be used as limits for drained and undrained analyses, respectively. In practical applications, undrained and partially drained excavation analysis with ER/k larger than 102 resulted in a similar ground deformation. However, smaller ER/k ratios demand fully coupled solid-fluid analysis. An analytical method aimed at predicting excess pore water pressures and determining the required type of analysis associated with soil and excavation conditions, e.g., drained, partially drained, and undrained analysis, is proposed herein to accurately estimate the ground movements induced by the excavation.