In order to mitigate the damage of tunnels caused by fault rupturing, a new lining system is proposed. This lining system is comprised of double-layers; the inner layer is a smoothing layer to disperse localized deformations and the outer layer is an absorption layer to accommodate large fault dislocations. Furthermore, a multi-shell mortar is developed as suitable materials for the outer absorption layer characterized with high rigidity before yielding and high compressibility after yielding. An attempt is also made to evaluate the damage of a typical railway tunnel subjected to fault rupturing. The computed results demonstrate that the proposed method is effective for fault displacements of less than a meter.
Japan is located in the middle of the most seismically active zones on the earth with more than 2,000 active faults being formed in a small archipelago of about 38km2 [1]. As some mountain tunnels are unavoidably sited across these active faults, there are potential risks of severe damages by fault rupturing at the tunnelfault intersections. Therefore, it is necessary to take appropriate countermeasures for mitigation of these fault rupture hazards on mountain tunnels. Among various ideas for mitigation of fault rupture hazards [2], Figure 1 shows one of the most promising and effective countermeasures for mountain tunnels [3]. The double-layer lining system is installed for some distances from the tunnel-fault intersection to prevent localized breakage or severe dislocations while allowing widely distributed continuous bending deformations. The outer layer is a special material with high stiffness before yielding to support the tunnel until the outbreak of relevant earthquake and also with high compressibility after yielding to absorb and accommodate large fault dislocations. On the other hand, the inner layer is reinforced concrete with high flexural strength to prevent localized breakage and with high bending stiffness to smooth and disperse the fault dislocations. Furthermore, expansion joints and lubrication layer may be installed to reduce axial forces mobilized along the tunnel when the length of the tunnel is changed by faulting. In this study, a special material of multi-shell mortar is proposed for the outer layer [4]. Furthermore, an attempt is made to calculate the deformations of the tunnel equipped with this double-layer lining system.
A multi-shell mortar is a mixture of ordinary mortar and a large number of voids capsulated by thin rigid husks. This material exhibits high rigidity under low loads until its voids start to fail. For the loads higher than this yield load, however, it tends to compress significantly with little load increments.
Since the multi-shell mortar is designed to be used for the outer layer in the double-layer tunnel lining system for mountain tunnels, it is sandwiched between the inner layer of reinforced concrete and the surrounding rock masses. In the process of fault rupturing, the outer layer is subject to large compression by reducing its thickness to accommodate large fault dislocations.
