Section 5 Laboratory tests on soil and rock
5.14 Strength testing of rock material
5.14.1 General
(1) This standard includes five laboratory methods for determining the strength of rock:
− the uniaxial compression and deformability test;
− the point load test;
− the direct shear test;
− the Brazil test;
− the triaxial compression test.
NOTE Annex W provides more detail on each strength test and its interpretation.
5.14.2 Requirements for all strength tests (1)P The following shall be specified:
− the samples that are to be tested;
− the specimen preparation;
− the number of tests per formation;
− any required additional parameters;
− the testing methods.
NOTE Annex W provides a guideline for the minimum number of test specimens to be tested for one formation for uniaxial compression tests, Brazil tests and triaxial tests for projects of geotechnical category 2 with respect to standard deviations of measured strength and comparable experience.
5.14.3 Evaluation of test results
(1) The evaluation of the test results should include comparison with recognised databases to help screen the data for anomalous results, while accounting for the natural range in compressive strength and deformation parameters in rock, and correlations with the results of classification tests.
(2) All test results should be grouped and analysed with respect to the geological description and classification properties using statistical methods when appropriate.
(3) The values can be used to evaluate in-situ strength and deformation properties and to classify rock element and rock mass properties.
5.14.4 Uniaxial compression and deformability test 5.14.4.1 Objective and requirements
(1) The uniaxial compression test measures the compressive strength, the Young’s modulus of elasticity and Poisson’s ratio of cylindrical test specimens of rock.
(2) The test is intended for classification and characterisation of intact rock.
(3)P The following shall be specified in addition to the requirements in 5.14.2:
− the specimen orientation and dimensions;
− the testing method;
− if relevant, the definition of modulus (tangent, average or secant) and Poisson’s ratio as a function of stress or strain.
(4)P The test specimens shall be prepared from cores taken with Category A sampling.
(5) Recommendations for testing of the uniaxial compression strength and deformability should be followed.
NOTE Recommendations for such tests are given in Annex W.
5.14.4.2 Evaluation of test results
(1) The uniaxial compression strength should be determined as the maximum vertical stress obtained during the compression test.
(2) The modulus of elasticity defined as the ratio of the axial stress change to axial strain produced by the stress change should be determined using one of the three definitions below:
− the tangent Young’s modulus of elasticity measured at a fixed percentage of ultimate strength (i.e. 50 %);
− the mean value of Young’s modulus of elasticity from the linear section of the axial stress- strain curve;
− the secant modulus measured from zero stress to some fixed percentage of ultimate strength (i.e. 50 %).
(3) The Poisson’s ratio should be determined as the slope of the radial strain versus axial strain curve.
(4) Young’s elasticity modulus and Poisson’s ratio should be calculated within the same interval of vertical stress.
(5) Test results should be evaluated with respect to rock classification properties and the rupture pattern illustrated by a sketch of the tested rock specimen.
(6) The unconfined compression strength (σC) can be used as a classification parameter for the intact rock quality and it could be used in combination with triaxial compression test results in a Mohr diagram to define the Mohr-Coulomb rupture parameters angle of shearing resistance (ϕ) and cohesion (c).
NOTE Young’s modulus of elasticity E and Poisson’s ratio ν may be used for settlement calculation according to EN 1997-1:2004, Annex F.
5.14.5 Point load test
5.14.5.1 Objective and requirements
(1) The point load test is intended as a strength index test for the classification of rock materials.
The test results can also be used for estimating the strength of a group of rocks of the same range of competence.
(2) The point load test is not a direct way to measure rock strength but an index test. The correlation between point load test results and strength should be documented in each case.
(3)P In addition to the requirements in 5.14.2 (1)P, the testing methods with reference to cores, blocks and irregular lumps shall be specified.
(4)P The test specimens shall be prepared from cores taken with Category A sampling.
(5) Test specimens of blocks and irregular lumps taken in pits may be used provided this is reported accordingly and the rock specimens are taken using Category B sampling.
(6) Recommendations for the point load test should be followed.
NOTE Recommendations for such a test are given in W.2.
5.14.5.2 Evaluation of test results
(1)P Because of large variability, the evaluation of rock characterisation and predictions of other strength parameters shall be based on a statistical approach. From test data consisting of at least 10 single tests, the two highest and two lowest values shall be deleted before calculating the mean from the remaining.
(2) In order to classify samples or strata using a mean value of Point Load Strength Index, the minimum number of tests should be five.
(3) The test measures the Point Load Strength Index of rock specimens, and their Strength Anisotropy Index, which is the ratio of the Point Load Strengths in directions which give the greatest and least values.
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 for such 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 a 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 are given in W.5.
5.14.8.2 Evaluation of test results
(1) A triaxial test consists of a series of compression tests carried 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.