INTRODUCTION

When a new tunnel is constructed near an existing tunnel in weak rocks which have time dependent characteristics, earth pressures on both tunnel linings increase With time and are different from those on a Single tunnel lining. Then, in order not to damage the both linings, safety distance between the tunnels should be known in advance.

The authors made clear the effects of new tunnel construction on the stresses of the existing tunnel lining (Ito & Hisatake 1981a). But, a method to estimate earth pressure on the new tunnel lining has not been established, because it is difficult in analysis to take into account such facts that the earth pressure on the new lining is affected by time dependency of rocks, construction sequences of the both tunnels and earth pressure on the existing lining before the excavation of the new tunnel.

In this paper, the earth pressure and Stresses of the new tunnel lining constructed near and parallel to the existing tunnel in the viscoelastic ground are analyzed by Integral Equation Method considering the above mentioned facts. Results obtained by this analysis are compared with those of model tests, and the stresses in the new lining and the safety distance are shown.

ANALYSIS OF EARTH PRESSURE
Stresses released on new tunnel boundary

When the new tunnel T2 is excavated after the construction of the existing lining L1, Stress vector P2 on new tunnel boundary S2, which will be released at the new tunnel excavation, is affected both by the excavation of the existing tunnel T1 and by earth pressure P1 acting on the lining LI (Fig.l). Then P2 has to be determined by considering construction sequences of the tunnel T1' If the lining L1' which is.assumed not to be deformed by earth pressures, is constructed at the time to after the excavation of the tunnel T1, PI increases with time and its component Pljis expressed by the following equations (Ito & Hisatake 198Ib),

(Equation in full paper)

where tiis the time after the construction of the lining LI,Φ(ti)is a creep function concerning shear deformation, Pljis the component of the stress vector released on SI at the excavation of the tunnel T1, and Poisson's ratio V of the ground is treated to be independent of time. And * indicates the following Laplace transform, and L1shows Laplace inversion. Then, if stress change Δσjkon the assumed tunnel boundary S2 caused by the excavation of the tunnel T1 can be calculated, the component of the stress vector P2jon S2 after construction of the lining LI, which is released at the excavation of the tunnel T2,is determined by the following equation considering equation (1),

(Equation in full paper)

(Figure in full paper)

Method of experiments First, a circular tunnel with diameter l5cm is excavated in the model ground of consolidated clay which is placed in the experimental device (80cm×90cm×30cm) and on the ground vertical pressure l6kN/m' less than consolidated pressure is applied in advance.

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