ABSTRACT

Lishi tunnel is a loess doubled arch highway tunnel (160 rn long, 24.4 m wide and 7.05 rn high) located in Lishi city of China. The systematic site measurement has been done for about one-year. Basic on this, the surrounding rock's stability was studied in this paper through numerical analysis. A detailed back-analysis about displacement and stress measurements in this tunnel was presented and discussed. On the basis of the obtained results, some advices have been given for resembling tunnel.

INTRODUCTION

Lishi tunnel is a loess doubled arch tunnel (160 mlong, 24.4 m wide and 7.05 m high) located in Lishi city of Chll1a, excavated for the Fenliu highway. In order to forecast the behaviour of the tunnel during the construction, the systematic site measurement has been done for one year. Basic on this, the surrounding rock's stability was studied in this paper through numerical analysis.

A correct design of an doubled arch tunnel in fact requires the following key points, which cannot be disregarded if the tunneling is to be well designed and an optimal tunneling of the tunnel is to be obtained:

  • Good knowledge of the wall rock from both the geometrical and the geomechanical points of view (discontinuities, rock mass properties and layout) should be available.

  • Good monitoring has to be planned and designed; the potential dangerous conditions cannot be pointed out without sufficient monitoring.

  • Numerical modeling is necessary for correct forecast of the stress and strain development of the tunnel during excavation and also to have a reasonable analysis of the field stress around the void.

The example described in this paper is therefore of great importance because it can show that the correct Use of in-situ measurements over a period of one year offered the basis of the study when tunneling.

MEASUREMENTS AND MONITORING

The rock mass characterization has been carried out using a complete geological survey; both form the surface and underground. At the same time, we have set some equipment in the tunnel to get the displacement and stress of lining during construction. In the tunnel, four parts has been measured and monitored, which were surface displacement, arch tip displacement, pressure on lining and stress of lining.

Operating condition calculation

Six sections on the surface of the tunnel were set. The location chart of every section monitoring points is shown in Figure 1. Monitoring data in a representative section is shown in Figure 2. The maximum rate of settlement was occurred in the phase of excavation of left main tunnel. As the 2# point is in the top arch of right tunnel and it's lower depth from surface, so it's settlement is the maximum.

The top arch displacement

We have set eight sections in the doubled arch tunnel. Only the K71 +805 data are shown here. Figure 3 shows the curve. From the figure, we can see that the top arch settlement of the left tunnel is bigger than that of the right tunnel.

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