ABSTRACT

INTRODUCTION AND SUMMARY

The support of access openings near to the production zone of an undercut-cave mining operation is a recurring problem. Trial-and-error experimentation has led to the widespread adoption of concrete lining as a practical compromise between the conflicting objectives of minimum cost to install, minimum time interference to extraction operations because of support failures, and minimum cost to repair failed sections. The concrete is essentially unreinforced, although preliminary support such as structural steel sets or rock bolts may be present behind the concrete. The openings are typically about 2 to 3 m in diameter, and the concrete linings are about 0.3 to 1 m thick. Liner configurations similarly have evolved through trial and error.

A theoretical approach to the analysis of a concrete tunnel lining may consist of calculating the thrust, shear, and bending moment at a number of sections around the proposed liner configuration, for each of several hypothetical loading situations. Consideration may also be given to the interaction between the lining and the surrounding rock mass, since the load on the lining is a function of the stiffness of the lining relative to the stiffness of the rock mass. The results of such an analysis are found to depend largely on the assumptions made in regard to the structural properties of the rock mass and the loading imposed by it on the liner (Dixon, 1972). Another approach has been to arbitrarily reduce the laboratory- determined rock-mass properties, in accordance with the judgment of the analyst, by a factor of 2 to 10. These are indications of definite short-comings in any present approach to the design of a tunnel support system for service in a high-extraction zone. Unexpected liner failures are a common occurrence.

The support requirements for a tunnel opening in an undercut-cave mine arise in a different context from those for a civil tunnel in that the major events tending to cause opening collapse are not those preexisting in the rock mass, but rather are generated by the extraction of the ore body. The first of these events consists of the relief of the ground pressure and the expansion of the rock mass toward the caved zone, accompanied by dimensional changes of the rock-mass volume elements (Panek, 1981). The first damage to the support system is thus generated not primarily by excessive rock pressure but rather by the inability of the liner to accommodate the change of configuration. The loss of its capability, in the changed configuration, to resist subsequent closure can lead to rapid deterioration of support performance if a later increase of compressive load occurs as a consequence of load redistribution about the caved zone.

If, as the foregoing evidence suggests, liner failures should be ascribed primarily to excessive liner deformation rather than to excessive rock pressure, a more fruitful approach to devising an improved support system may lie in focusing on the deformability limitations of the support rather than its strength limitations. Liner deformability capability can be evaluated from measurements of full-scale liner performance under actual service conditions.

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