Seismic stability of urban tunnels in shallow depth

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This paper presented some results of active type shear tests in a centrifuge by modeling a semi-circular aluminum tunnel in dry silica sand and in cemented model ground to understand the effects of stress releasing on seismic stability of NATM tunnel. According to the results, the following findings were obtained.1) Before shearing, in the sand ground the earth pressure acting on the tunnel crown was close to Terzaghi's loosening earth pressure and smaller than the theoretical calculated overburden pressure by the following equation.ρ_r=ρ_v+ρ_h/2-ρ_v-ρ_h/2cos2θ2) Before shearing, in the cemented sand ground with stress releasing the bending moment distribution of the tunnel lining was similar to the calculated values by elastic calculations of beam structure applied Terzaghi's loosening earth pressure, however in the case of no stress releasing the bending moment distribution of the tunnel lining was similar to the calculated values by elastic calculations of beam structure applied the … More oretically calculated overburden pressure.3) Looseness of the ground over the tunnel had deteriorated by shearing in cases of sand ground, and the earth pressure acting on the tunnel lining close to theoretically calculated distribution.4) Whereas in the sand ground and the bending moment distribution shows no change, in the cemented ground with stress releasing the bending moment distribution of the tunnel increase after first shearing. This indicates that there are different mechanisms of earth pressure increments due to arching effect of the ground dissipate between sand ground as non-consolidated ground and cemented sand as cemented soft ground.In the cemented sand ground, stress releasing also affects to the increment of bending moment due to shearing. Increment of bending moment distribution of allowed stress releasing case is larger compared with the other cases. However, in absolute values considered with initial value, bending moment during earthquakes is relatively smaller than the other cases. Less