Numerical Study on Backfilling the Tail Void Using a Two-Component Grout

In shielded mechanized tunneling, the annular gap caused by the tunnel boring machine (TBM) driving must be backfilled instantaneously using a suitable grout. A two-component grout composed of a chemically retarded cement slurry and an accelerator is pumped into the annular gap behind the TBM shield, where it gels and attains sufficient mechanical stiffness in a very short time. This property of the grout helps restrict immediate ground movements adjacent to the TBM tail shield and reduce volume loss. In this study, the hardened two-component mix is tested using a confined oedometric condition, and a complete time-dependent hardening stiffness evolution is developed. Moreover, the freshly prepared grout is tested in terms of its performance suitability (storage and transportation) and serviceability owing to permeability and dewatering. The obtained parameters from the oedometer tests are incorporated into a numerical simulation of an earth pressure balance (EPB) excavation focusing on modeling of the backfilling procedure using finite-difference method (FDM) software. The study also focuses on distinguishing between two commonly used backfilling techniques implemented in numerical analyses. The numerical model consists of testing a regular tunnel section using a newly introduced hardening soil (HS) constitutive model with the soil parameters obtained from the Torino, Italy, metro case. Another set of synthetic softer soil parameters are chosen to highlight the soil properties in the model response. The two-component grout satisfies all the prerequisites for a good backfilling material in its fresh and hardened state. The grout attains sufficient mechanical strength in a very short time, remains impermeable, and achieves a stiffness of 43 MPa in 28 days. The results obtained from the oedometer tests enable development of a complete time-dependent stiffness evolution of the grout. The implementation of hardening behavior of the two-component grout is evident in terms of ground settlements when compared with the conventional backfilling simulation technique.