Shear Strength Design of a Mechanized Tunneling Grout Mix: Case Study of the Tehran Subway Line 6 Project

Abstract

Ground subsidence caused by tunnel boring machines (TBMs), which is controlled by grout injection in the tail void, is one of the most adverse effects of excavation and tunnel construction processes. This article proposes a new method for designing the grout mix based on its shear rather than compressive strength, enabling a reduction in the amount of cement consumed. Moreover, the settlement caused by TBMs in line 6 of the Tehran subway is measured using a point network system. Since the ground subsidence induced by mechanized tunneling is predominantly caused by the tail skin settlement, the low amount of final ground settlement indicates that the grout mix with low cement content provides proper behavior to control the change of volume between excavated and external diameter of the precast lining. Moreover, a comparison of the grout and surrounding soil properties reveals that the injected grout should have a shear strength in the range of that of the excavated soil. This approach leads to lower cement consumption compared to a conventional design, which is based on compressive strength. In addition, less cement content causes less shrinkage and creep cracks which both improve waterproofing functionality. The measured settlement and segment damages (steps or lips in the tunnel) show that the face pressure, grout injection, and its mixture are applied in a safe manner.