Undrained compressibility characteristics and pore pressure calculation model of foam-conditioned sand

Abstract Soil conditioning with foam is widely used to transform natural soil into the desired state suitable for mechanised tunnelling. Since the excavated soil is rapidly discharged out with little water drained during earth pressure balance (EPB) shield tunnelling, it is necessary to investigate the undrained compressibility characteristics of foam-conditioned soil under chamber pressure. In this study, a series of compression tests were carried out in a large-scale, self-designed compression device equipped with pore pressure and lateral earth pressure sensors. The results demonstrated that the foam considerably enhanced the compressibility, pore pressure and lateral pressure of the conditioned sand. The workability of foam-conditioned sand may be well conducted in slump tests under the atmospheric condition, but the compressibility of foam-conditioned probably cannot satisfy its requirement for EPB shield tunnelling due to compression of foam under high chamber pressure. In addition, the total and effective lateral pressure coefficients of the foam-conditioned sand increased and decreased, respectively with increasing foam injection ratio and total vertical pressure. Then, the mechanism of the compressibility of foam-conditioned sand is explained with an existing two-dimensional (2-D) model. Finally, according to Boyle’s law, an analytical model is proposed to predict the pore pressure of the foam-conditioned sand under compression pressure, and a satisfactory agreement between the calculated values and the test results was found.