INTRODUCTION

The stresses and deformations occurring in the structural components of a tunnel during its construction depend to a great extent on the deformation behavior, at the time when the supports are established, of the rook in the outer tunnel walls. During tunnel construction undertaken following NATM, the prediction of the size of the displacement in tunnel walls has become one of the more significant questions.

Generally, accurate evaluation of matters such as the initial stress and the mechanical characteristics of the rook or a suitable model for the primary supporting structures is a difficult question. Because of this, it is also extremely difficult to establish the size of rook displacement by calculations in advance. Thus, the importance of predicting the final amount of the rook die- placement at the earliest possible time, using the results of displacement measuring carried out simultaneously with excavation as the basis for that prediction continues to grow.

This paper discusses methods for establishing separately both the time-independent displacement and the time-de-pendent displacement, from the measured values of the wall displacement obtained during tunnel excavation in visco-elastic rocks. At the same time, it also deals with methods for carrying out such displacement forecasting.

Decision making by means of displacement forecasting

In order that tunnel construction has to be carried out safely and economically, it is necessary firstly to set the s1tandard values for making decisions regarding the actual results of measurements. Then, by comparing those results of measurement s with the standard values, decisions can be made concerning the safety or otherwise of the supporting structures.

J.A. Franklin, in his work, has considered three grades within levels of care and the fixing of a limiting value for decision making. the authors believe that the goals of predicting the long-term safety of support structures and the early investigation of preventative structures are not fulfilled simply by determining the suitability of a certain level of care. Rather, we also consider it of the utmost importance to estimate what level of care must be attained at what time.

Theoretical considerations regarding displacement during tunnel excavation
Visco-elastic models of the rock Mass

For convenience's sake, consider the three elements of visco-elastic model appearing in Fig. 1.

(Figure in full paper)

The following equation expresses the visco-elastic strain.

(Equation in full paper)

Next, if the radial displacement(u)in the walls of a circular tunnel(Flg.2) during its excavation in visco-elastic rook is taken to be the product of the elastic displacement(ue)occurring at the moment of excavation and the creep displacement(uc),the following equation expresses the condition of its plane stress.

If the factors Eo,E1, y, n, a in equation(2)are made constant, then the displacement becomes a function of the initial stress and of time.

Displacement model with regard to the progress of the tunnel face the theoretical discussion of displacement in visco-elastic rock given above was concerned with a two dimensional state. However, during actual tunnel construction the face advances gradually, so that the displacement in the walls too is 1nrluenced by the progress

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