Strength testing of rock material
W.1 Uniaxial compressive strength and deformability
W.1.1 Test procedures
(1) It is recommended to follow the methods of the ISRM or ASTM, as examples for Uniaxial compressive strength testing and deformability testing. In addition the modifications prescribed in W.1 may be used.
NOTE See X.4.11.1
(2) The test procedure described in ISRM contains two levels of testing:
- Part 1: Method for determining the uniaxial compressive strength of rock materials;
- Part 2: Method for determining deformability of the rock materials in compression.
(3) The first method provides the compressive strength, the second method gives in addition the axial deformation modulus (Young's modulus) and Poisson's ratio. The second method is preferred.
(4) The procedures suggested by the reference ISRM are extremely difficult to conform to, especially with respect to sample preparation and geometrical tolerances. The practice recommended in this informative annex is less strict. Although the procedures recommended by ISRM are desirable, a set of minimum requirements is given herein. It is considered more important to run a greater number of tests than fewer tests on higher quality specimens.
(5) The following amendments should be made to the ISRM procedure.
- The diameter of the platens should be between D and (D + 10) mm, where D is the diameter of the sample. Provided it can be ensured that the stiffness of the plate is sufficient, (he platen diameter may be greater than (D + 10) mm. Special provisions are required to centre the specimen properly.
- At least one of the two end platens should incorporate a spherical seat.
- Test specimens should be right circular cylinders having a height to diameter ratio between 2 and 3 and a diameter not less than 50 mm. The diameter of the specimen related to the largest grain in the rock may in the case of weak rock be as low as 6:1. However, a ratio of 10:1 is preferred.
- The ends of the specimen should be flat within 0,02 % of the specimen diameter and should not depart from perpendicularity to the axis of the specimen by more than 0,1°.
- The use of capping materials or end surface treatments other than machining is not permitted except when testing soft rocks, where the mechanical characteristics of the capping materials should be better than the rock to be tested.
- The diameter and the height of the test specimen should be determined to the nearest 0,1 mm or 0,2 %, whichever is the greatest.
- Regarding strain gauge measurements of radial and axial strains, the length of the gauges should be at least ten times the grain size. The measurements should be carried out on the mid third part of the test specimen to avoid influences from friction and stress non-homogeneities at the ends. Measurement of vertical strain over the whole specimen height is allowed if it can be shown that practically the same result is obtained as when the strain is measured over the middle third of the specimen height.
- The load should be applied on the specimen at a constant stress rate or constant strain rate such that failure occurs within 5 min to 15 min. If cycles of loading and unloading are performed to define better deformation parameters, the time for such should be excluded from the before-mentioned time period.
- The machine to be used for applying and measuring axial load to the specimen should be of sufficient capacity and capable of applying load at constant rate. The parallelism of the machine platens should be checked.
(6) Initial deformations may include bedding of the specimen ends to the compression machine and/or closure or micro-cracks in the test specimen. Measurement of the total vertical deformations by using only the distance between the two steel platens of the machine may lead to false deformation properties.
W.1.2 Number of tests
(1) The characteristics of rock may vary greatly as function of lithology, diagenesis or induration, stress history, weathering, and other natural processes, even within a geological stratum. Table W.1 gives a guideline for the minimum number of Uniaxial compressive tests as a function of the variability of the rock and existing comparable experience.
|Standard deviation of measured strength, (s) % of mean||Comparable experience|
|s > 50||6||4||2|
|20 < s < 50||3||2||1|
|s < 20||2||1||0a|
|a Only valid for very homogeneous rock types with extensive experience from nearby locations.|
W.2 Point toad test
W.2.1 Test procedures
(1) It is recommended to follow the method of ISRM as an example of a procedure for the point load test
NOTE See X.4.11.2.
(2) The test can be run with portable equipment or using a laboratory testing equipment, and may be conducted either in the field or in the laboratory.
(3) Rock specimens in the form of either core (the diametrical and axial tests), cut blocks (the block test), or irregular lumps (the irregular lump test) may be used for testing provided the reference (for instance ISRM) specifications of shape and dimensions are followed.
W.2.2 Number of tests
(1) The mean value of the Point Load Strength Index is used to classify samples or strata. To get a representative mean value, the minimum number of single tests should be five.
(2) For rock characterization and predictions of other strength parameters, a higher number of tests than specified in W.1.2, is necessary. Generally, at least 10 separate tests per stratum should be carried out.
W.3 Direct shear test
W.3.1 Test procedures
(1) It is recommended to follow the method of ISRM as an example for the direct shear test. NOTE See X.4.11.3.
(2) The following amendments may be recommended to the ISRM procedure.
- The testing machine should have a travel greater than the amount of dilatation or consolidation expected, and should be able of maintaining normal load to within 2 % of a selected value throughout the test. Dilation should be measured during the test with the same accuracy as the shear displacements.
- The rate of shear displacement should be less than 0,1 mm/min in the 10 min period before taking a set of readings. If automatic data logging is used, there may be no need for a reduction of the rate of shear displacements to 0,1 mm/min.
- The specimen should be reconsolidated under each new normal stress, and shearing continued according to criteria given in ISRM. If sample surfaces are cleaned before beginning a new testing phase, or the samples are unloaded before repositioning, this should be noted in the test report. The appearance of the material removed by cleaning should be described.
(3) The direct shear strength may also be determined by field tests. This requires a detailed assessment of the field characteristics of the discontinuities.
(4) The results are utilised in, for example, equilibrium analysis of slope stability problems or for the stability analysis of dam foundations, tunnels and underground openings.
(5) Rock specimens in the form of either cores or cut blocks may be used. The test plane should preferably have a minimum area of 2 500 mm2. In case of unfilled joints, the diameter or the edge (in the case of a square cross-section) of test specimens should preferably be related to the size of the largest grain in the rock by a ratio of at least 10:1. The ratio between joint length and shear box size is recommended not to be less than about 0,5 to avoid possible instability problems of the shear apparatus.
(6) Equipment for cutting the specimen, for example a large-diameter core drill or rock saw should be used. Percussive drills, hammers and chisels should be avoided as the samples have to be as undisturbed as possible.
(7) The direction of the test specimen in the testing machine is usually selected such that the sheared plane coincides with a plane of weakness in the rock, for example a joint, plane of bedding, schistosity or cleavage, or with the interface between soil and rock or concrete and rock.
W.3.2 Number of tests
(1) A shear strength determination should preferably comprise at least five tests on the same test horizon or from the same joint family, with each specimen tested at a different but constant normal stress in the applicable stress range.
W.4 Brazil test
W.4.1 Test procedures
(1) It is recommended to follow the method of ISRM as an example for the Brazil test.
NOTE See X.4.11.4.
(2) Test specimens should be cut with specimen diameters (D) not less than the core size,
(D ≈ 54 mm), with a thickness approximately equal to the specimen radius. The cylindrical surface should be free from obvious tool marks. Any irregularities across the thickness of the specimen should not exceed 0,025 mm. End faces should be flat within 0,25 mm and parallel to within 0,25°.
(3) For shale and other anisotropic rock, it is recommended to cut test specimens parallel to and perpendicular to the bedding. For specimens cut parallel to the direction of the bedding, the direction of the load should also be specified.
W.4.2 Number of tests
(1) Table W.1 gives a guideline for the minimum number of Brazil tests as a function of the variability of the rock and existing comparable experience. For rock characterisation and predictions of other strength parameters a higher number of tests is necessary.
W.5 Triaxial compression test
W.5.1 Test procedures
(1) It is recommended to follow the method of ISRM as an example for the triaxial compression test.
NOTE See X.4.11.5.
(2) Test specimens should he cut with specimen diameter (D) not less than the core size, (D ≈ 54 mm), and the height equal to 2 to 3 times the diameter as defined in 5.4 and with the specifications in accordance with X.4.8.
W.5.2 Number of tests
(1) Table W.1 gives a guideline for the minimum number of triaxial compression tests as a function of the variability of the rock and existing comparable experience. For rock characterisation and predictions of other strength parameters a higher number of tests is necessary.