A.P.S. Selvadurai
Dept. of Civil Engineering and Applied Mechanics, McGill University, Montréal, QC, Canada H3A 2K6
Keywords: In situ testing, elasticity solutions, indentation tests, cable-jacking tests
ABSTRACT:
This paper presents an overview of the plate load techniques that are employed in the
assessment of the in situ deformability characteristics of rock masses and the theoretical procedures that can be used to estimate the deformability of the rock mass.
1 Introduction
The in situ determination of deformability properties of rock masses constitutes an important topic in geomechanics (Jaeger, 1972). The justification for conducting in situ testing, as opposed to laboratory testing of either cores recovered from boreholes or samples recovered from test pits, can be justified when there are concerns relating to the reliability of a Representative Volume Element that could be tested in a laboratory environment. Recourse to in situ testing is often advocated when a geologic medium contains distributions of inhomogeneities including stratifications, sessile fractures, inclusions and voids, where the mass deformability cannot be accurately estimated from laboratory tests. While a number of in situ testing methods,
including geophysical testing techniques involving wave propagation are in use, the direct method of load testing is regarded as the most effective and reliable technique for estimating the deformability of geologic media. The main drawback of in situ load testing of rock masses is that the test itself involves the application of substantial loads to induce measurable deformations of the rock mass being tested. The most convenient approaches for conducting such load tests are to identify test configurations where the rock mass itself can provide the reaction necessary for the application of the test loads. Plate load tests conducted in adits and tunnels and trenches excavated at the base of tunnels are examples where the loading can be applied without
the provision of massive reaction devices (Jaeger, 1972; Bell, 1987). Situations where the loading of the test plate can be carried out using external reactive forces are rare and in many instances when plate loading tests need to be carried on relatively flat regions of a rock formation, it becomes necessary to resort to other forms of self-stressing systems to develop the test loads. In the mid 1960s engineers proposed a technique that enables the loading of a test plate without an external reactive load. This technique involves the provision of a reaction anchor system allowing the application of the test loads through a self-stressing system. This test is referred to as the Cable Jacking Test. The exact origins of the cable jacking technique of in situ testing of rock
masses is not entirely clear; the test involves the location of a grouted anchorage at some depth within the rock mass with the pre-stressing cables embedded in the grouted reaction point. Usually the cable is placed along the axis of symmetry of the test plate, emerging from an opening in the test plate and a doughnut-typehydraulic jack is used to load the test plate. The method was formally proposed by Zienkiewicz and Stagg (1967) where either a circular plate or a square plate is subjected to loads applied through the cable jacking system. The interpretation of the test invariably involves appeal to results of the theory of elasticity, and it is implicitly assumed that the embedded reaction forces do not influence the displacement of the test plate.
In earleir studies, empirical estimates were used to locate the permissible proximity of the anchorage. The presence of the reaction anchor loads on the displacement of the test plate was first investigated by Selvadurai (1978, 1979a) (see also Davis and Selvadurai, 1996) and the influence of the localized anchorage force on the displacement of the test plate was established. Furthermore, these results permit the location of the localized anchorage point at a specified depth, and a suitable correction can be applied to explicitly account for the reactive anchor load.
The early investigations in this area dealt with situations where the reaction loads were applied at a location within the rock mass along the central axis of the test plate. This method requires the drilling and grouting of regions in close proximity to the test location, which is not desirable, unless the results in theoretical elasticity used for the interpretation of the test can accomodate the influence of the introduced inhomogeneity. The 92 analysis of the interaction of a test plate indenting a rock mass containing such a grouted section is not a routine problem in the theory of elasticity. Furthermore, the elasticity mis-match between the rock and the grouted cylindrical region can lead to load partitioning, which can unduly influence the interpretation of the cable jacking test. An alternative to this procedure is to locate the anchorage points at regions remote from the test plate in a non-axisymmetric fashion. The interpretation of the resulting test plate-anchor region interaction problem requires the development of mathematical relationships for the net settlement of the test plate. The paper presents a fundamental solution to the problem of the elastostatic interaction between a rigid test plate and two localized point anchorages that are placed symmetrically in the interior of the rock mass. These results can be extended to include multiple anchor locations and anchor load distributions within the rock mass. The analysis is conducted to include only circular test plates that are used to evaluate the elastic deformability properties of the rock mass. In in situ testing techniques that involve isotropic elastic geologic
media, the analysis provides compact results that can be used to estimate the bulk deformability constants for the tested rock mass.
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