During the excavation of a high overburden tunnel in squeezing ground, conventional rigid tunnel support system (i.e. rock bolt, shotcrete and steel ribs) might not work against such huge overburden pressure and considerable deformation. In such case, rock bolt is pushed out, shotcrete falls after cracking, steel ribs buckle due to the yielding, and tunnel failure occurs consequently. In some countries, to avoid tunnel failure, yielding supports which can reduce the stress generated along the tunnel supports have been adopted for the excavation under high overburden pressure. For example, highly deformable concrete elements are installed into the shotcrete lining. However, the concrete element has several issues, such as less productivity due to its heavyweight and less flexibility for its design (i.e. challenging to modify mechanical properties). Therefore, the authors have been searching for a new deformable material, which is lighter and easy to control its deformation characteristics, for the yielding support element.
In this paper, the authors focused on a new material made of glass fiber reinforced plastic, which is currently used as a substitute for natural wood. Furthermore, its mechanical properties and deformability were evaluated by uniaxial compression tests. The test result shows appropriate deformability and no occurrence of brittle failure. As a next step, to further investigate its applicability to the yielding support element, the numerical analysis on the shotcrete lining model with the new deformable elements was carried out. As a result of numerical simulation, we could find that the stress generated in the shotcrete lining reduced due to the advantage of using the new material.
In the construction of a high overburden tunnel, the conventional tunnel support may not be able to withstand the huge overburden pressure and may collapse. In Japan, a double layer support system is adopted in excavating a high overburden tunnel while reducing ground deformation to the minimum. In some countries, there are certain cases of tunneling where yielding support is adopted to reduce the ground pressure to the support allowing a certain level of ground deformation (Figure 1). For instance, slits are provided in the application of the shotcrete where yielding elements that deform first are inserted to reduce the stress of the shotcrete. In Europe, a similar scheme is employed using the special concrete having a yielding property (Kovari, 2009).