Field measurements are powerful design aids in tunnelling projects, providing useful information for monitoring the stability of tunnels during and after excavation, evaluating the adequacy of the excavation procedure and ground support system, and assessing various kinds of environmental problems, such as settlement of the ground surface and the change of ground water level, caused by tunnel construction. The most common quantities measured at tunnel construction sites are as follows;
Convergence and closure of the tunnel,
Displacements in the surrounding underground media,
Earth pressure acting on rock bolts and lining, and
Settlement of the ground surface. Many different types of measuring devices have been developed and are now commercially available for measuring these quantities. Among the various kinds of instrumentation the displacement measurements are the simplest and the most reliable compared with the direct stress measurements.
(Figure in full paper)
The failure criterion of a material is usually given in terms of stress. Therefore, in order to give a quantitative interpretation to the results of displacement measurements, the stress distributions around tunnel must first be obtained by a back-analysis considering the measured displacements. These are then compared with the failure criterion to assess the stability of the tunnel and to evaluate the adequacy of construction procedure. This back-analysis for stress, however, is questionable both for accuracy and reliablility, because it requires information about the stress-strain relationship of in-situ geological materials, which cannot be easily determined. To avoid such a shortcoming involved in the back-analysis, the displacement measurements must be directly evaluated without converting displacements into stresses.
In general assuming a continuous media the strain is related uniquely to the displacement, so that the strain distribution around tunnels can be obtained from the measured displacements. Consequently, if the failure criterion is given in terms of strain, assessment of the stability of tunnels and the evaluation of the adequacy of construction procedures) may be possible by comparing the strains related to the measured displacements with the allowable strain evaluated from failure criteria.
In this paper the author proposes a direct interpretation method for displacement measurements such as convergence and borehole extensometer measurements.
The convergence measurements measure the relative displacement between two points on the excavated surface of the tunnel (see Fig. 1). The measurements are performed by using convergence meters consisting of a taut invar steel wire or steel tape. The convergence measurements are the most practical and economical for any underground excavation activities.
The fundamental relationship between surface displacements and change in length of invar wire is given by (see Fig. 2(a)),
(Figure in full paper) Fig. 3 (ah for instance, consists of ten convergence measurements being controlled by ten displacement components at five measuring points, that is, the number of equations is identical to the number of unknown displacements. In this case, however, the equation has no solution because the determinant of the coefficient matrix vanishes.
