INTRODUCTION

In designing a dam abutment, a tunnel or an underground structure in rock, it is necessary to evaluate the mechanical properties of rock. However, the rock is not uniform body, but contains joints, bedding planes or other weak planes, and these weak planes in rock reflect on the mechanical behaviours of rock. Rock specimens are almost taken from rock between these weak planes, and the laboratory tests using the specimens invariably lead to an over-estimate of the strength of rock.

In order to investigate the mechanical behaviours of rock, the block shear tests are performed in field. But, the great expense and efforts are required to perform the in-situ tests. In recent years, the researches(e.g. Krsmanovie 1967) in respect to the direct shear test, using the relatively large specimen which contains weak planes, have been done.

Nevertheless, it is difficult to investigate the mechanical behaviours of specimens collected from an arbitrary portion in rock, because the specimen is the large rectangular prism. In view of this point, it seems that the direct shear test using the boring cores collected easily from an arbitrary portion in rock is a rational method.

This study is undertaken for the purpose.of estimating the shear stiffness of rock containing weak planes. As basic research for the above study, the shear behaviours of rock specimens containing artificial discontinuous planes are investigated. Considering the effect of weak planes to the shear stiffness of the specimen, moreover, the direct shear test is conducted on the boring cores collected from rock, and the results of laboratory test make comparison with the results of in-situ test on rock mass. The failure lines in residual sliding are then obtained on the boring cores collected from rock mass of a lot of working places, and the relations between the friction angles measured by these failure lines and rock types are investigated.

TESTING MACHINE AND TESTING PROCEDURE

The schematic diagram of a direct shear testing machine which is developed in this study is shown in Fig.l. In this figure, the names of main parts of the machine are as follows; A is a ram which applies normal load to specimen, B is a load cell for measuring normal load, © is a rollar bearing, D is an upper shear box, E is a ram which applies shear load to specimen, F is a load cell for measuring shear load,

(Figure in full paper)

G is a lower shear box, H is a ram for lifting the upper shear box D, J and K are the linear variable differential transformers for measurin normal and shear displacements, and I is a steel frame which supports the main parts of the machine.

The procedure in the test is as follows; At first, a specimen is set into the lower shear box G, and the upper shear box D is descented by operating the ram H as the specimen fits into the upper shear box D.

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