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

Rock mass: matrix (intact rock) and discontinuities

One of the major obstacles which is encountered in the field of numerical modeling for rock mechanics, is the problem of data input for rock mass properties.

The term “rock mass” indicates the rock as shown on site, including not homogeneous parts, that is rocks with different physic-mechanical characteristics, separated by discontinuity systems of different origin, such as bedding planes, joints, shear zones and faults. From the engineering point of view the characterization of the rock mass is the first main task in the design activity.

For the evaluation of the mechanical characteristics of the rock mass it is widely recognised that it is necessary to start from the rock matrix, i.e. from the intact rock. Its strength and deformation characteristics can be investigated using specific laboratory tests.

 

The aim of this paper is to give an overview about the support offered by laboratory tests, being understood that the main aspect for its characterization is represented by the geological and structural survey and by on-site investigations.

 

 

 

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Laboratory tests on rock matrix (intact rock)

From an experimental point of view the characteristics of resistance and in part of deformability can be evaluated on samples of intact rock, for example by the following tests:

  • Uniaxial and triaxial compression
  • Indirect tensile tests
  • Point Load tests
  • Permeability test

Uniaxial and Triaxial tests: deformability parameters; failure envelope and Multi-stage method

Triaxial tests are usually carried out in order to get a single point of strength for different levels of cell pressure. 

In addition to the parameters of resistance and deformability (elastic modulus, Poisson ratio, volumetric compressibility), the set of the above tests allows to obtain evaluations on the failure envelope.

When the failure envelope is assumed not linear (according to Mohr-Coulomb criterion), but parabolic (according to Hoek&Brown model), it is necessary to provide additional samples with homogeneous characteristics to be submitted to several triaxial compression tests, each with a different level of confining pressure.

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 to peak resistance, without failing, except for the last level of confining pressure. This methodology is known as Multi-stage triaxial testing.

 

Indirect tensile tests (Brazilian tests)

The test consists in subjecting the cylindrical disc of rock sample to compression along a diameter. The peal compression strength of the disk is then measured. For this value the tensile strength of rock is calculated.

Point Load tests

It can be considered a relatively simple test, performed either in laboratory or on site, that allows a strength index (IS) to be obtained, the Point Load Strength Index, by means of a punctual test on cores of rock sample of different size and shape, generally extracted from the bore holes.

Permeability tests on intact rock samples

The permeability characteristics of intact rock samples can also be determined along the triaxial tests. In details it is possible to evaluate the effect of the overburden pressure on the permeability values.

Here by an example, where a constant head water permeability test is carried out in automatic mode, along a triaxial test with a rock sample inside an Hoek cell.

Laboratory tests on joints

Disregarding the investigation of geometric characterization of the rock mass, the following notes will focus on the evidence to evaluate the characteristics of deformability and strength of discontinuirties and joints.

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

The experience of the University of MISKOLC with our Multi-Stage triaxial system

A series of multi-stage triaxial compression tests were conducted to examine which measurement setup and control mode could most precisely determine the failure points separating the single load stages during the tests because the precise recognition of these is a critical point of the measurement.
It was found that brittleness played an important role in choosing the adequate measuring mode (manual or automatic) and control signal (inductive transducer, strain gauge).

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