Mechanical characterisation of a two-component grout for tunnel construction

Two-component grout (2CG) is the most popular technology for the backfilling phase when shield machines are used for tunnelling. Despite its intensive use, however, its mechanical behaviour has not yet been studied in depth, and there is a need for more knowledge due to the complex numerical models used by designers for mechanised tunnelling projects. In this paper, the results of a laboratory test campaign based on one-dimensional and triaxial compression tests are reported and discussed. Different curing times were tested in an oedometer with varying pre-loads, and the outcomes highlighted the role played by the pre-load in terms of improving the stiffness of the grout, particularly when applied at the early stages of curing. Triaxial tests have never been carried out on 2CG and this paper reports the first obtained outcomes. Tests performed on a ripe material (curing time higher than 28 days) provided values for the effective cohesion (c′) and friction angle (φ′) that were compared to values in the scientific literature obtained from direct shear tests. A further innovation of the work is the promising attempt to interpret the experimental mechanical behaviour of 2CG in the framework of critical state soil mechanics (CSSM).