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Decision Making in Tunnelling Based on Field Measurements

Abstract:

The successful design of underground openings is based on different sources of information. The most important among them are geological explorations, soil and rock mechanics investigations, statical computations and field measurements during construction. The way to make use of computer programs and the criteria for the interpretation of the results obtained are still the subject of discussion. This is the main reason for the lack of standard design procedures in tunneling. The inherent weak elements in purely theoretical considerations can, however, be compensated for by direct field observations and the sound engineering experience of the designer. Depending on the design problem, it may be necessary to make decisions well before the start of the construction. In this case, the observation of the actual deformations of the tunnel profile, the movements of the surrounding ground or the settlements at the ground surface during the excavation of the tunnel mainly have the function of checking the structural behavior with regard to satisfactory design and proper execution of the works. In contrast, using the shotcreting method with anchors or steel grid support, which may in many cases also be applied in subway construction, continuous measurements inside the tunnel and in the subsoil can serve as feedback signals for the constructional process. On the basis of careful statical computations a concept is worked out for the excavation sequences both in the cross section and along the axis with the corresponding support measures. If the measurements indicate a substantial deviation from the anticipated behavior of the structure, the most important corrective measures in the construction can still be applied. The basic idea of field measurements lies in the optimization of the design and construction of the underground structures. In other words, the aim is to obtain adequate safety for a minimum of cost expenditure, whereby the manifold influence of the construction time is also included in the expenditure. This does not, however, exclude the conscious decision to accept a calculated risk. Since the problem of optimization is very varied, the immediate objectives of the individual measurements may be concerned with quite different aspects, the most important of which are as follows. (i) The investigation of the global material properties of the rock. (ii) The determination of the type and quantity of rock pressure (loosening pressure, genuine rock pressure and swelling pressure). (iii) The safety control of the structure. (iv) The verification of structural response to a specific method of construction. (v) The control of the effectiveness of particular support measures. (vi) The comparison of theoretical predictions with the actual structural behavior. As a general rule the above classification of the objectives of measurement is not rigid. It is intended to indicate the main emphases. It should be noted that usually the same program of measurements has several aims. The most important thing is that the concept, the execution and interpretation of the measurements are adjusted to suit the needs of the problem in hand. Field measurements are now recognized worldwide as an indispensable aid for correct decision making in tunneling. They often form the link between theory and the engineering practice. Successful measurements require both a thorough understanding of the specific problems arising in tunneling and a close familiarity with instrument techniques. In this chapter an attempt was made to show, on the one hand, the significance of monitoring by the discussion of some case histories and, on the other hand, to give information on new developments in measuring techniques.

Authors:

Kovári, Kalman and Amstad, Christian

Index Terms:

Displacement measurements; monitoring; rock; TunnelingGroup; tunneling; underground openings

Further Information:

Date published: 1993