Improvement of soil properties in tunnel construction, as demonstrated by the Severomuya Tunnel construction

Objective: to research the mechanisms of injection strengthening of soils in fault zones by investigating the physical and chemical processes that take place under conditions of severe soil disintegration, high moisture content, and elevated hydrostatic pressures. Methods: theoretical models for injection strengthening of unstable, highly fragmented soils, including gravel, sand and clay, were examined, covering filtration mode injection and injection by consolidated compaction (the hydro-fracturing method). Results: ground consolidation yields predictable behavior of the treated soil mass along the tunnel alignment, and improves soil properties that can be incorporated into subsequent design calculations for reinforced soil structures. The mechanism of injection strengthening is interpreted as a complex interaction between the time- dependent hardening of injected solutions and their migration under the influence of hydrostatic pressure and filtration flow, following the sequence: injection well => natural cracks and large pores => artificial cracks and channels => small pores. Practical importance: the techniques described in this paper represent major developments and have been extensively employed as effective methods for artificially stabilizing disintegrated, unstable soils by treating them with injection solutions to enhance their properties. Injection strengthening methods, in various forms, have been applied to more than 150 faults of varying depths and lengths within the BAM tunnels.