a) case2 (no shock absorption layer) b) case3 (shock absorption layer)

Figure.4 Failure modes on the surface of tunnel model material In Figure. 4b, there is less cracks on the surface of model soil after the shock orption layer is set up at right tunnel, which weakens the interaction between the model soil and tunnel structure. It is obviously effect that a certain thickness shock absorption layer is designed at the mountain tunnel portal, at the same time it is proposed that the slope of tunnel portal should be reinforced in order to prevent the secondary disasters(such as landslide, slope instability, etc) due to earthquake.

5.3 Contrast or analysis of failure modes in tunnel structure

In Figure. 5, the tunnel structure appears a number of longitudinal cracks, diagonal cracks and shear cracks at the foot of side-wall with the increasing seismic loads, because the shock absorption layer is not designed at tunnel portal in case 2. But in case 3, the overall tunnel structure is not apparently damage, only are there less micro-cracks at local position of tunnel structure and the tunnel invert arises damage or destruction. When the stiffness of the tunnel portal structure is much larger than the strata stiffness, the surrounding rock displacement could have made underground structure forced deformation while happening earthquake, at the same time the earthquake inertia force obviously intensify at the tunnel portal, which results in tunnel structure to appear cracks. The model soil and model structure could be easily to appear shear or tensile destruction at tunnel portal under the action of both surrounding rock displacement and earthquake inertia force.

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