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Rock properties: matrix and discontinuity


Laboratory tests for the determination of rock properties

The discussion exposed in the papaer "Rock properties: the contribution of laboratory tests in the design activities" by Pier Luigi Raviolo below is intended to provide a summary assessment of the possibilities and supports offered by laboratory tests, provided with more or less sophisticated levels of automation, for the translation of several mechanical properties of the intact matrix and of the rock mass, it being understood that the main aspect for its characterization is represented by the geological and structural survey and by on-site investigations.

 

Rock properties: the contribution of laboratory tests in the design activities

Rock matrix - Uniaxial and triaxial compression tests

In addition to the parameters of resistance and deformability (elastic modulus, Poisson ratio, volumetric compressibility), the different tests related to the mechanical properties of intact rocks allow to obtain evaluations on the failure envelope.

These evaluations can be obtained with the use of semi-automatic or automatic systems as the following.


Rock matrix - Uniaxial and triaxial compression tests (multistage method)

A solution to this problem is represented by the possibility to perform several tests on the same sample, with increasing levels of confining pressure, for each of them approaching very close the peak resistance, without failing, except for the last level of confining pressure. This triaxial test procedure is commonly known as multistage triaxial test, is described by ISRM and allows to obtain several points of peak strength in triaxial conditions with a single sample.


Indirect tensile tests

For a correct evaluation of the failure envelope, in addition to the compression tests (uniaxial and triaxial), one or more tensile tests should be carried out; for which two different testing modes are suggested for example by ISRM recommendations: direct tensile and indirect tensile strength (Brazilian test).

In this case a general purpose compression frame together with proper accessories shall be adopted for the tests. 


Point Load tests

The samples can be tested both in natural moisture conditions and under saturation, according to three methods: on a rock cylinder (along a diameter or axially), or on irregular cores.

In case of anisotropic rocks an anisotropy index can also be defined, by performing the point load tests in two directions: perpendicular and parallel to the planes of weakness.


Permeability tests on intact rock samples

The permeability characteristic of intact rock samples can be determined along the triaxial tests.


Laboratory tests on joints

As first approach, the characterization of the rock mass is provided by mapping the discontinuities. From their spatial distribution depends the degree of fracturing of the entire rock mass, the possible formation of sliding surfaces and the overall stability. Other specific factors are: the geometry, the roughness, the undulations and the amplitude of the joints.

  • Roughness of the joints (Barton comb)
  • Tilt test
  • Strength of the walls and thickness of the joint

Direct and residual shear test on joints

This equipment has been designed to apply both a compression force normal to the joint and a shear force, parallel to the joint.

The application of the axial force and of the shearing action, as well as with traditional systems
consisting of hydraulic jacks controlled manually by the operator, can be carried out also with servo controlled hydraulic automatic systems and managed by specific software, in the same way of the triaxial multistage tests.