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Determination of the stresses acting in a tunnel lining or in steel ribs is a very difficult task. Three basic techniques are at hand:
- measurement of deformations caused by stress relief,
- observance of elements which act together with the lining,
- observance of the lining itself.
Each one of these techniques is briefly discussed with its advantages and shortcomings. The most important limitations are based on the facts that, for the standard techniques only local values in some selected points are obtained, and that the interaction of the measuring elements with the construction is not always determinable. Therefore, the main emphasis here is put on the so-called Integrated Measuring Technique, which belongs to the third group mentioned above and circumvents the problems of the other measuring techniques at the expense of introducing some other limitations, as outlined below.
The Integrated Measuring Technique enables the determination of the cross-sectional forces in and the loads acting on a tunnel lining based on deformations measured at its intrados. The basic idea of the technique relies on inverting the procedure of beam analysis, well known in structural engineering. Here, in successive positions along the intrados of a lining, the changes in length of the chord and the rise for a base length of 50 cm are measured. From these values the state of deformation, expressed by axial strain, curvature and shear strain, may be determined. In the elastic case without shear forces purely kinematical considerations are sufficient, otherwise the stress-strain relation of the lining must also be taken into account. Based on the state of deformation and the constitutive equations, the cross-sectional forces may be found and, together with equilibrium conditions, the external load may be back-calculated.
Fritz, Pit and Kovári, Kalman
lining; rib; rock pressure; measurements; analysis; design; rock; TunnelingGroup; Fritz, Pit; Kovari, Kalman