Soil Conditioning of Waterless Sand–Pebble Stratum in EPB Tunnel Construction

Due to large porosity, high soil friction, less cohesion, low flow of waterless sand–pebble stratum and the problem of very difficult in achieving a soil pressure balance in the shield front during the earth pressure balance (EPB) shield construction process, the soil conditioning for EPB excavation in the waterless sand–pebble stratum along Urumqi rail transit line 1(L1) is studied. A series of laboratory tests are performed to investigate and assess the properties of selected additives and the conditioned soils. The additives test results show that foam A with relative higher half-life, foaming ratio and adhesion thickness can be adopted preferentially; the most optimal mass concentration ratio of three types of bentonites (named S1, S2 and S3) slurry are 1:6, 1:10 and 1:15, respectively when funnel viscosity is around 64/s. The slump test results showed a trend of increasing after first decreasing to zero when using a single additive such as water or bentonite slurry indicate single additive cannot achieve a better soil conditioning effect. Slump value can reach about 230 mm when conditioned the soil with 14% water content and only added 15% of the foam (concentrate rate is 3%), and the conditioned particles on the surface sediment are evenly dispersed, which is able to achieve an ideal effect in the shield. Economic and reasonable soil conditioning ratio is the water content to 14% + 6% bentonite (bentonite ratio is 1:15, funnel viscosity is 65.08/s) + 5% foam with the concentrate rate of 3%. The results can be used to provide guidelines for the excavation by use of EPB machine under such soil conditions.