SUMMARYA method to cover a tunnel lining with a soft and thin coating is discussed herein as a possible measure for mitigating seismic damage to tunnels. Long-term earthquake observations at di erent tunnel sites within a variety of alluvial soil deposits have demonstrated that a circular tunnel is liable to deform in such a way that its two diagonal diameters crossing each other expand and contract alternately. Narrowing down vibration modes, in order to discuss this particular and the most important mode, any of the essential items of the soil-tunnel system, namely the soil, the coating and the tunnel lining, has only one degree of freedom, allowing the coating e ect to be simply evaluated in terms of a limited number of key parameters.

The scope of the electrical resistivity survey has recently been extended to various fields beyond groundwater and underground resource exploration. Electrical resistivity techniques were evaluated in two case studies to substantiate their applicability to geotechnical and environmental problems. First, electrical resistivity imaging (ERI) was used to map the fractured zone ahead of the tunnel face during construction using the new Australian tunneling method (NATM). The ERI technique could adequately modify tunnel support patterns in quasi-real time, on the basis of the field test results, and could subsequently improve the stability of tunnel excavation. Second, electrical resistivity tomography (ERT) was used between two test boreholes in a research tunnel site to monitor the movement of brine after it was injected in one of the two boreholes. A pair of low-conductivity zones that were regarded as the pathway of groundwater flow between the two boreholes was clearly imaged because of enhancement of conductivity caused by diffusion of the brine. The applicability of electrical resistivity techniques to geotechnical and environmental problems was successfully substantiated.

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We investigated the dynamic behavior of a railway ballast layer during tamping operation by 3D Discrete Element simulations. The ballast layer was prepared with ballast grains that had a realistic shape and was measured by a laser scanner and modeled by clumped spheres using the dynamic optimization technique. Three types of sleeper models that were embedded in the ballast layer were prepared, and a series of tamping operations (lifting of the sleeper, insertion of vibrating tines and packing, pulling out of tines, and settling of the sleeper) were simulated in a realistic manner. The compaction underneath the sleeper mainly occurred during the insertion of tines and the packing process, while the tine insertion zone was eventually loosened after the tines were pulled out. The final porosity differs with different sleeper models and packing process periods. On the other hand, the coordination number (the number of inter-granular contacts per grain) drastically decreased during the insertion of the vibrating tines and the packing process; then, it recovered to the original value after the tines were pulled out. This change is important to identify the extent of ballast grain agitation due to the processes. The contact force network is sensitive to the applied loading and the place where the tines are inserted, and the resulting force network localization at the final state can be understood by the Coulomb's earth pressure theory. The degree of heterogeneity may be the key to the long-term stability of the ballast layer under repeated train wheel loading.

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