Effect of foam conditioning on performance of EPB shield tunnelling through laboratory excavation test

An appropriate application of a soil conditioning strategy during an earth pressure balance (EPB) shield operation is essential for reducing the downtime accompanied by improvements to the hydro-mechanical properties of the muck. Among the various additives used for conditioning, foam is the most widely adopted owing to its usability. In this study, to simulate EPB tunnelling and study the effects of ground and foam injection conditions in the laboratory, a lab-scale excavation test apparatus was devised. This apparatus was designed to vertically excavate the soil specimen with the simultaneous injection of foam and water, and discharge the conditioned muck. Subsequently, a series of excavation tests were conducted using artificial sandy soil to estimate the effects of the fine content, water content, and foam injection conditions, which can affect the performance of EPBs. Based on the measured torque, the weight loss of the cutter bits, and the slump of the muck from the excavation tests, the effects of foam injection parameters, namely, the foam injection ratio, foam expansion ratio, and concentration of foaming agent were discussed. The results show that the suggested laboratory excavation test can be used to qualitatively evaluate the effects of the basic soil properties and foam injection conditions on EPB shield tunnelling.