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Measuring The Forces in a Tunnel Lining due to Large Shear and Plastic Deformations

Abstract:

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.

The original version of the Integrated Measuring Technique disregarded the influence of shear strains. However, extensive tests on steel ribs led to the conclusion, that for cross-sections with small area effective for shear, compared to the moment of inertia and concentrated loads, this influence cannot be disregarded. Furthermore, numerical simulations showed that for single concentrated loads, after reaching the yield limit, the curve of deflection is no longer continuous, thus forcing the measuring technique to be applied separately at both sides of such points. Distributed loads, as are usual in tunneling practice, may be handled without any problem even for large shear and plastic deformations, provided the measurements are carried out in an overlapping manner.

Authors:

Fritz, Pit

Index Terms:

tunnel; lining; forces; pressure; rock; TunnelingGroup; analysis

Further Information:

Date published: 1985