AS 4133.4.3.2:2013 – Methods of testing rocks for engineeringpurposes Method 4.3.2: Rock strength tests—Determination of the deformability of rock materials in uniaxial compression—Rock strength less than 50 MPa.
4.2 Specimen preparation
The test specimen shall be prepared as follows:
(a) From a suitable sample prepare a test specimen having the following characteristics:
(i) Test specimen shall be a straight circular cylinder having a length to diameter ratio of between 2.5 and 3.0 and a diameter preferably of not less than 45 mm. The diameter of the specimen shall be at least ten times the size of the largest grain in the rock.
NOTE: While a test specimen length to diameter ratio in the range of 2.5 to 3.0 is specified, in accordance with that recommended by ISRM, it is recognized that there is evidence suggesting that a minimum length to diameter ratio of 2.0 may be adequate particularly for rocks with strength less than 25 MPa. Therefore, while a ratio of between 2.5 to 3.0 is preferred, it is possible to extend this range to between 2.0 and 3.0 to allow additional testing where the core lengths are limited. Where the ratio is less than 2.5, it should be noted in the test report.
(ii) The ends of the specimen shall be cut parallel to each other to within 10 and at right angles to the sides of the specimen to within 1.
(iii) The ends of the specimen shall be ground flat to 0.1 mm across the face with no surface irregularities. If the ends of the specimen cannot be ground or contain surface irregularities they may be capped using a capping material that has a strength greater than 50 MPa.
(iv) The sides of the specimen shall be free of abrupt irregularities and straight over the full length of the specimen.
(b) Determine the average original diameter of the specimen to at least the nearest 0.1 mm across two diameters at right angles and at the centre and near the top and bottom of the specimen. Calculate the average original diameter (d0) of the test specimen from the six measurements.
(c) Measure and record the height of the specimen to 0,1 mm,
(d) If required, moisture condition the specimen prior to testing.
The procedure shall be as follows:
(a) Attach the measuring devices to the specimen.
(b) Load the specimen continuously and without shock. Apply the load to achieve a displacement rate of no greater than 0. 1 mm/mm. Plot load and displacement using the continuously recording device until a load deflection curve has been established sufficiently to permit the calculation of the Young’s modulus and Poisson’s ratio detailed in Clause 6. When performing the test using extensometers where destruction of the measuring devices may occur at specimen failure, the devices may be removed when at least 70% of the estimated strain to failure has been reached. Care should be taken to ensure that the load is maintained such that only minor relaxation of the sample occurs before loading to failure.
1 It is desirable that the rate of loading be adjusted to achieve failure of the specimen between 10 and 15 minutes, provided that this rate of loading does not exceed a displacement rate of 0. 1 mm/mm.
2 Where it is required to perform multiple cycles of loading and unloading, a suitable maximum load for the cycle may be taken as half the estimated failure load.
(d) Plot the axial stress against average axial and diametral strains (see Figure 1).
NOTE: Figure 1 illustrates a plot of axial stress versus axial and diametral strains. These curves show typical behaviour of rock materials from zero stress up to ultimate strength (an). The complete curves give the best description of the deformation behaviour of rocks having non-linear stress-strain behaviour at low and high stress levels.
(e) Calculate the axial Young’s modulus (E), defined as the ratio of the axial stress change to axial strain produced by the stress change, using any method employed in accepted engineering practice (see Figure 2).
NOTE: The most common methods used for calculating the Young’s modulus are as follows:
(a) Tangent Young’s modulus (Er) is measured at a stress level which is some fixed percentage of the ultimate strength [see Figure 2(a)].
(b) Secant Young’s modulus (Es) as usually measured from zero stress to some fixed percentage of the ultimate strength [see Figure 2(b)], generally 50 percent.