Short-term surface settlements induced by EPBM twin tunnelling in saturated sandy soils

This paper presents a case study of the construction of a 781-metre-long twin-tunnel, using an Earth Pressure Balance Machine (EPBM), in saturated sandy soils in Ho Chi Minh City, Vietnam. The depths of the tunnels were between 11.4m and 24.6m below the ground surface. The averaged trough width and length parameters were 0.326 and 0.446, which are consistent with previous studies in sands. The volume losses ranged from the anticipated levels of less than 0.5% to notably high values reaching up to 2.44%. Low volume losses were associated with areas of dense soil and effective tail void grouting. The characteristics of effective tail void grouting observed in dense sand in this project were grouting pressures close to porewater pressure, coupled with stable grouting volume that was approximately 130% of the volume of the theoretical tail void for the majority of the drive. However, in very loose sandy soil zones it was observed that even very high tail void grout volume did not prevent large settlements. Soil relative density proved to be an influential factor in ground surface vertical displacement, with large magnitudes occurring mainly in loose soil. A threshold relative density I d ≈ 0.4 divides the normal volume loss of less than 0.7% range, and that of considerably larger volume loss. The results emphasised the need for caution when tunnelling at shallow depth in loose soil, where the combination of low relative density and shallow cover can result in significant ground movements.