Development of a novel external load-controlled permeameter for sand-foam mixtures

Abstract

Foam is commonly injected into the soil chamber to reduce the water permeability of excavated sands during Earth Pressure Shield (EPB) tunnelling. In this study, a novel external load-controlled permeameter is developed to explore the influences of soil pressure on the permeability of sand-foam mixtures. A comparison test is performed on natural sands using the newly developed and conventional permeameter. Excellent agreements in the measured permeability coefficients manifest the validity of the new permeameter, which is then adopted to perform permeability tests on sand-foam mixtures. The obtained results indicate that the permeability of sand-foam mixtures depends strongly on the external load, while the permeability coefficient of natural sands basically remains constant as the external load increases. When the external load increases from 0 kPa to 150 kPa, the permeability coefficient of sand-foam mixtures reduces from 7.92 × 10⁻⁶ m/s to 2.44 × 10⁻⁷ m/s, and the duration of the initial stable period increases from 248 min to 3678 min. In addition, the experimental observations also indicate that the permeability of sand-foam mixtures might be overestimated by conventional permeability tests because the effect of soil pressure is not taken into account.