It is of fundamental importance to have an appropriate knowledge about stresses and deformations around an advancing tunnel face In designing supports and linings, as well as in planning construction method.

The present paper is concerned with several investigations on stresses and deformations around an advancing face of a tunnel excavated in soft rock, with special consideration for Seikan undersea tunnel which is now in course of construction. The paper Consists of the following three parts.

In the first part, we discuss the experimental results about mechanical properties Such as stress-strain relations, creep characteristics and failure conditions of Kuromatsunai mudstone whick is distributed Over 10km in the middle region of Seikan tunnel site. We also discuss the mathematical modeling of the soft rock for numerical analysis.

In the second part, we analyse stresses and deformations around an advancing face of a tunnel by the use of the integral equation method. In the analysis, we assume bat the initial earth pressure is considerably high compared with the strength of the rock, and moreover, that two horizontal pressures are equal and higher than the vertical one. Hence the rock yields around the tunnel in non-symmetric manner, even in a circular tunnel.

After having the knowledge of stresses and deformations, in the third part, we analyse again the similar problem by means of the finite element method. However, in the analysis, we pay special attention to the successive development of plastic zone associated with the progress of the tunnel face and the effects of the construction method Such as rock bolting and shotcreting. For the sake of simplicity, we now assume the initial earth pressure to be hydrostatic and the cross-section of the tunnel to be circular.


Kuromatsunai mudstone is a slightly sandy tertiary soft mudstone distributed at the site of Seikan undersea tunnel which is located under the Tsugaru channel between Honshu and Hokkaido. We briefly summarize the test results (For further informations, see Kobayashi(1978,1979)).

Samples are picked up as a block at 7,000 m from Hokkaido side. They were carefully packed in order to keep the natural moisture content unchanged.

The specific weight of the samples are 1.75 and 1.31 for fully saturated and dried stated, respectively. Thus, the water content is 0.34. The specimens are formed into 55mm cubes by machining. The results of the triaxial compression test for fully saturated specimens are shown in Fig. 1. In the test, friction between the specimen and the loading plate is reduced by the use of teflon sheet(0.02mm thick) with silicon grease. The typical plastic strain increments to the unit stress increment are shown in Fig. 2.

(Figure in full paper)


Considering the test results, we made models for numerical treatment. We first assumed that the soft rock is elastic-perfectly- plastic with Young's modulus of 7,000 kg/cm2 and Poisson's ratio 0.4. The yield condition is of the Drucker-Prager type; where I1and J'2indicate the first stress invariant and the second invariant of the deviatoric stress.

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