On the Tunnel Support Mechanism of Shotcrete

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The mechanical effect of the interaction between the shotcreting and the ground is studied from a view point of the tunnel support stability. A serial set of rigid bars jointed through the bending springs are adopted for a model of the lining while the constant coefficient of subgrade reaction is assumed for the ground. A lot of parametric numerical analyses are done by changing the width of shotcreting, the coefficient of the lateral earth pressure, the overburden and the shape of tunnel section. It is shown that the subgrade reaction is essential for the tunnel stability and that even a thin shotcrete lining supports the surrounding ground if an adequate amount of the subgrade reaction is expected. Other conclusions are as follows.1) The sugrade reaction reduces the bending moment of the shotcrete lining by controlling the deformation around the tunnel.2) The thin shotcrete lining may cause a large amount of bending moment.3) The thin shotcrete lining is preferred unless the buckling occurs.4) Even when bending failure occurs in the shotcrete lining, no serious damage is resulted if a sufficient ground reaction is expected.A further research is required concerning the tunnel stability since the present study was done through a very simple mechanical model.