INTRODUCTION

In recent years, NATM has been actively applied to tunneling in soft rock, and it became an ordinary method of tunnel Support. The authors have been studying the effects of rock bolts and thin linings in soft rock as well as the effect of the face on ground behavior of tunnel after excavation.1)2)

This paper mainly shows the tunnel model test results using the various kinds of artificial rock mediums and numerical studies using the Finite Element Method (FEM).

MODEL TEST
Materials

Tests were carried out on the tunnel models using artificial rock mediums made of bentonite-mortar. As indicated in Table 1, controlling water-cement ratio, Contents of bentonite, cement and sand, Physical properties of the rock mediums such as uniaxial compressive strength (qu), Value of cohesion (c), and angle of internal friction (ø) were given.

Model size

Two different types of the tunnel models were used in this testing program:

1. Series I

The model size of the artificial rock mediums was l40×140×50 cm (see Fig. 1).

(Table in full paper)

At the center of the medium, a bore hole (tunnel) was made by setting bentonite-mortar around a 40 cm diameter foam styrol plug, and then desolving the plug. The hole thus made was 1/15 scaled of actual tunnels of 6 m in diameter. Around the circumference of the hole, various supporting conditions such as rock bolts and thin lining were employed.

Series II

The model size of the artificial rock mediums was ø 60×75 cm. At the center of the medium, a bore hole (tunnel) of ø16.5× 45 cm in size with or without rock bolts support was made, which was 1/36 scaled of actual tunnels.

Loading system

Uniformly distributed compressive load (Po), which was corresponding to rock pressure in actual tunnels, was applied to the sides of the rock medium by the stepwise loading method. The load was increased by 2.5˜10 t/m2per step and sustained until the equilibrium of the radial deformation of the tunnel was observed. In order to restrain the deformation along the tunnel axis, the upper and lower surfaces of the medium were completely confined with rigid steel plates.

(Figure in full paper)

RESULTS OF TESTS AND ANALYSIS (SERIES I)
Test results
1. Influence of properties of rock mediums upon stability

Physical properties of the rock mediums, especially ø, considerably influenced the load-deformation characteristics of the tunnel. When rock bolts supports were installed, in the range of O~ 8% radial deformation of the tunnel (R;R=u/D*),the load (Po) at the same value of R increased in correspondence to the increase in ø as shown in Fig. 2. In this case, for the rock mediums of ø =15 ~ 20° and ø=30°, Po was about 1.5 times and 2.0 times greater than that of ø=10°, respectively.

2. Influence of supporting conditions upon stability

From the experimental results using rock mediums of ø =15 ~ 20°, the following stabilizing effects were observed by the installation of various supporting elements (see Fig. 3).

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