Stress variation of soil surrounding tunnel and grouting reinforcement effect based on stress release

With FLAC, a criteria of stress release ratio （SRR）, i.e., about 10% of the relative difference of the ground settlement before and after the supporting system is installed, is incorporated into stress release method （SRM）. At the same time, the Mohr-Coulomb Criterion is used to analyze stress changes around the tunnel induced by excavation. It shows that the surrounding soil around shallow tunnels can be partitioned by loosened zones （unloading zones） and plastic zones among which the shear stresses in the soil are less than 10 kPa below the M-C yield line. Both types of zones are separated. With the increasing of the SRR, the areas of plastic zones and loosen zones gradually increase. The surrounding soil is equally divided into 24 regions in order to study the influence of different grouting regions on the ground settlement. The reinforcing effect of grouting can be modeled by enhancement of the soil strength around the tunnel. By the approach of the SRM, numerical analysis indicates that, in the upper area （top of the surrounding soil about 210°）, the reinforcing effect gradually decreases as the reinforcing region moves from arch springing to the vault of the crown; when reinforcing regions lie in the upside of the surrounding soil, the influence on the ground settlement is generally greater than in the lower part; at the same time, with the increase of the SRR, the influential differences are more and more obvious.

Stress variation of soil surrounding tunnel and grouting reinforcement effect based on stress release