Effect of water pressure on time-dependent permeability characteristics of sand conditioned with foam and bentonite slurry

During earth pressure balance (EPB) shield tunnelling in water-rich sandy ground, both foam and other conditioning agents, such as bentonite slurry, are injected to prevent water spewing. Permeability tests were conducted to investigate how water pressure affects the permeability of sand conditioned with foam and bentonite slurry. Experimental results demonstrate that increasing water pressure at the top and bottom of the specimen extends the initial stable period of the permeability coefficient, significantly slowing down its growth rate during the fast growth period. Soil grain migration was observed in specimens exposed to sufficiently high water pressure. During the slow growth period, the permeability coefficient decreased as water pressure increased, and this decrease rate correspondingly decreased. Under a consistent hydraulic gradient, increased water pressure led to enhanced stability of foam bubbles and extended the time-dependent curves for the permeability coefficient. Furthermore, the relationship between chamber pressure dissipation and foam stability was discussed during the standstill period of shield machines. To prevent water spewing, it is recommended to use the permeability coefficient of the muck at the outlet of the screw conveyor with the lowest water pressure as the evaluation index during permeability testing.