Effect of Gravity of the Plastic Zones on the Behavior of Supports in Very Deep Tunnels Excavated in Rock Masses

The vertical load acting on a support structure is affected by the loss of self-bearing capacity of the rock inside the plastic zone. This load can then be accounted for by analytical calculation methods capable of evaluating the stresses in the tunnel support system to proceed with the tunnel design. Generally, the effect of the rock’s own weight in the plastic zone is considered in a simplistic way by evaluating an additional vertical load given by the weight of the rock due to the thickness of the plastic zone. This approach leads to a significant increase in the vertical load with the risk of overdesigning the support structure. In this work, the effect of the rock’s own weight in the plastic zone was considered by modifying the numerical solution of the convergence-confinement method for tunnels built in rock. In this way, through the intersection of the characteristic curve of the tunnel and the intersection line of the support structure, it is possible to determine both the vertical loads (with the effect of the weight of the rock) and the horizontal load (without the effect of weight of the rock). The application of the method to a project in the Alps allowed the detection of the magnitude of the percentage increase of the vertical load and a significant increase in the thickness of the plastic zone and determination of the consequences that this may have on the designing of the radial bolting length in that zone. Increasing the plastic radius led to an increase in the length of the bolts. This is interesting because, in the area of the crown where the weight of the plasticized rock is considered, the bolts are usually installed with a greater length. In the final part of the study, a new procedure was illustrated to define the vertical and horizontal loads acting on the support structures, starting from the convergence-confinement curves obtained for the crown and for the lateral areas (sides).