5.14.6 Direct shear test
5.14.6.1 Objective and requirements
(1) The direct shear test measures peak and residual direct shear strength as a function of the stress normal to the plane of shearing.
(2) This standard deals with the laboratory testing for the determination of the basic shear strength parameters and the surface characteristics of a discontinuity that controls the shear strength.
(3) If the surface characteristics of a discontinuity that controls the shear strength are determined, an accurate description should be made, including type and roughness of the joint, type and thickness of fill material, and the presence of water in the joint.
(4)P The following shall be specified in addition to the requirements in 5.14.2 (1)P:
- the test specimen orientation and dimensions;
- the specifications of the testing machine;
- the rate of shear displacement during test;
- the selection of normal stress to be maintained during the single shear tests.
(5)P The test specimens shall be prepared from cores taken with category A sampling or from blocks taken in a pit using at least Category B sampling.
(6) Recommendations for direct shear tests should be followed.
NOTE Recommendations for such a test are given in W.3.
5.14.6.2 Evaluation of test results
(1) The evaluation of test results of shear strength versus stress perpendicular to the rupture plane should include a study of the shearing plane in order to take into account bedding and schistosity, cleavage of the rock specimen, the interface properties between rock and concrete, or what was tested.
(2) Shear strength parameters angle of shearing resistance (φ) and cohesion (c) may be established using a number of shear tests on different specimens taken from a rock stratum using Mohr-Coulombs rupture criterion. Alternatively, residual parameters may be found using multiple testing with different normal stresses on an established rupture plane.
(3) The test measures the shear strength in a forced rupture plane under certain stresses perpendicular to the rupture plane. Peak and residual shear strength after some shear deformation can be established. Usually the rupture plan is intentionally established along a known discontinuity.
(4) The test is intended for strength classification and characterization of intact rock and should not be used without geological correlation and rock classification for field conditions.
5.14.7 Brazil test
5.14.7.1 Objective and requirements
(1) The Brazil test is intended to measure indirectly the uniaxial tensile strength of a cylindrical rock specimen.
(2)P The following shall be specified in addition to the requirements in 5.14.2 (1)P:
- the test specimen orientation and dimensions;
- the testing method.
(3)P Due to the variability of the test results, duplicate testing of test specimens cut in parallel shall be performed.
(4) For shales and other anisotropic rock, it is recommended to cut test specimens parallel to and perpendicular to bedding. For specimens cut parallel to bedding, the direction of the load related to bedding should be specified.
(5)P The test specimens shall be prepared from cores taken with category A sampling.
(6) Recommendations for the Brazil test should be followed.
NOTE Recommendations forsuch a test are given in W.4.
5.14.7.2 Evaluation of test results
(1) The evaluation of tensile strength should take into consideration that the presence of hidden weakness planes in the test specimen may disturb the result and the failure plane should be sketched after test and evaluated.
(2) The test provides an indirect determination of the tensile strength σT in a forced rupture plane.
(3) The tensile strength (σT) may be used as a classification parameter for the intact rock quality and it may be used in a Mohr diagram at a corresponding maximal stress σ1 together with Mohr circles from uniaxial or triaxial compression tests to define the Mohr-Coulomb strength parameters angle of shearing resistance (φ) and cohesion (c).
(4) The test is intended for strength classification and characterisation of intact rock and the test results should not be used without geological correlation and rock classification for field conditions.
5.14.8 Triaxial compression test
5.14.8.1 Objective and requirements
(1) The triaxial compression test is intended to measure the strength of cylindrical rock specimens subjected to triaxial compression. A number of tests provide the values necessary to determine the strength envelope in it Mohr-Coulomb diagram. From this envelope, the angle of shearing resistance and the cohesion intercept may be determined.
NOTE No provisions are usually made for drainage of the pore water, nor for the measurement of pore water pressure. In certain rock types (e.g. shales and porous limestone and chalk) and under certain conditions, the pore water pressure may influence the results. For such rock types, advanced triaxial test systems allowing for measuring pore water pressure and volumetric strains are necessary. Such testing may include similar measuring techniques as used for uniaxial compressive strength according to W.1.
(2)P In addition to the requirements in 5.14.2 (1)P, the test specimen orientation and dimensions, reflecting the testing method shall be specified.
(3)P The test specimens shall be prepared from cores taken with Category A sampling.
(4) Recommendations for triaxial compression testing should be followed.
NOTE Recommendations for such tests arc given in W.5.
5.14.8.2 Evaluation of test results
(1) A triaxial test consists of a series of compression tests earned out under different confining pressures in a triaxial cell. A strength envelope of confining pressures versus axial stress at rupture can be used to establish the Mohr-Coulomb strength parameters angle of shearing resistance (φ) and cohesion (c).
(2) The homogeneity of a series of test specimens to establish the test parameters should be evaluated based on the geological description and rock classification parameters.
(3) The determined strength parameters relate to intact rock. In-situ properties can only be established taking into account the upscaling from element testing of intact rock to the mass properties of the rock in-situ.
