Determination Of Point Load Index Of Given Rock Sample

Scope:-

The purpose of this test is to measure the rock specimen’s strength by applying a concentrated load using a pair of conical hardened steel platens, causing failure by the development of tensile cracks parallel to the axis of loading.

Specimens are either in the form of rock cores or irregular lumps. Core specimens are preferred. Tests can be performed either in the laboratory or in the filed, depending on the testing machine. The index thus obtained is used for rock strength classification and initial determination of its unconfined compressive strength.

Apparatus:-

The testing machine incorporates a loading system and a system for measuring the load required to break the specimen. The testing machine can be of the loading frame equipped with a pair of conical hardened steel platens. Essential features for the testing machine are as follows;

• The loading system should be adjustable to accept 1 to 4 in. (25 to 100 mm) rock specimens, for which a loading capacity of up to 11,000 lbs. (approximately 50 kN) is usually required. To minimize delay between tests, a quick retracting ram is desirable.
• Spherically truncated conical platens are used to transmit the load to the specimen. The platens should be hardened and accurately aligned during testing. Fig.3 indicates the critical dimensions of the platens.
  
   
  • The load measuring system should indicate failure load to an accuracy of + 2%. It should incorporate a maximum-indicating device, so that the reading is retained and can be recorded after the specimen failure.
    
    
    
    
    
    

    Theory:-

    THE POINT LOAD TEST:-

    The PLT is an attractive alternative to the UCS because it can provide similar data at a lower cost. The PLT has been used in geotechnical analysis for over thirty years (ISRM,1985). The PLT involves the compressing of a rock sample between conical steel platens until failure occurs. The apparatus for this test consists of a rigid frame, two point load platens, a hydraulically activated ram with pressure gauge and a device for measuring the distance between the loading points. The pressure gauge should be of the type in which the failure pressure can be recorded. A state of the art point load testing device with sophisticated pressure reading instrumentation is shown in Figure .
    
    The International Society of Rock Mechanics (ISRM, 1985) has established the basic procedures for testing and calculation of the point load strength index. There are three basic types of point load tests: axial, diametral, and block or lump. The axial and diametral tests are conducted on rock core samples. In the axial test, the core is loaded parallel to the longitudinal axis of the core, and this test is most comparable to a UCS test. The point load test allows the determination of the uncorrected point load strength index (Is). It must be corrected to the standard equivalent diameter (De) of 50 mm. If the core being tested is "near" 50 mm in diameter (like NX core), the correction is not necessary. The procedure for size correction can be obtained graphically or mathematically as outlined by the ISRM procedures. The value for the I_s50 (in psi) is determined by the following equation. I_{s 50} = P/De2 (1)
    P = Failure Load in lbf (pressure x piston area).
    De = Equivalent core diameter (in).
    As Hoek (1977) pointed out, the mechanics of the PLT actually causes the rock to fail in tension. The PLT’s accuracy in predicting the UCS therefore depends on the ratio between the UCS and the tensile strength. For most brittle rocks, the ratio is approximately 10. For soft mudstones and claystones, however, the ratio may be closer to 5. This implies that PLT results might have to be interpreted differently for the weakest rocks.
    Early studies (Bieniawski, 1975; Broch and Franklin, 1972) were conducted on hard, strong rocks, and found that relationship between UCS and the point load strength could be expressed as:
    UCS = (K) I_s50 = 24 I_s50 (2)
    Where K is the "conversion factor." Subsequent studies found that K=24 was not as universal as had been hoped, and that instead there appeared to be a broad range of conversion factors. Table 1 summarizes published results obtained for sedimentary rocks. Most of the estimates place the conversion in a range between 16 and 24, with even lower values for some shales and mudstones.
    In studies comparing the PLT with the UCS, it is generally assumed the UCS test is the standard. In reality, however, UCS tests provide an estimate of the “true” UCS of the rock. The accuracy of the estimate depends on the natural scatter in the UCS test results (indicated by the standard deviation (SD)) and the number of tests conducted (n). This relationship is captured by the concept of the “Confidence Interval” (CI). For normally distributed data, the 95% CI of the mean is expressed as:
    CI _95% = 1.96SDn
    
    

    Test specimen preparation:-

    Rock samples are grouped on both of the rock type and estimated strength. At least 10 specimens are selected for testing each sample if core samples are used.
    Specimens in the form of core are preferred for accurate classification. Acceptable minimum and maximum core sizes are AX and HX, respectively.

    Procedure:-

    1. Diameter test:-

    The core sample with a length to diameter ratio greater than 1.4 is suitable for diametric testing.The inclination of bedding, foliation or other plane of weakness, if present, is recorded with respect to the line of loading.The diameter “D” of the specimen is measured to the nearest 0.005 inches by averaging two diameters measured at right angles to each other at about the upper height, mid-height and lower height of the specimen. The diameter D is then the average of the three diameters obtained at the upper height, mid-height and lower height of the specimen.The specimen is inserted in the test machine and the platens advanced to make contact along a core diameter, entering the distance, L, between the contact point and the nearest free end is at least 0.7 D .
    The load is increased to failure and the failure load P is recorded. The fragments are retained for water content determination which is performed after all specimens of the sample are tested for point-load strength.
                     

The Axial Test:-

Core specimens with a length-to-diameter ratio of 1.1 + 0.005 should be used. Long pieces of core can be utilized to obtain both diametric and axial strength values. The core is tested diametrically first, ensuring that a suitable length is retained for subsequent axial testing (i.e. , ensuring that L/D=1.1 + 0.005).

         The steps of diametric testing are repeated.

1. Tests For Anisotropic Strength:-

Tests should be made in both the weakest and strongest directions where the rock is bedded, schistose or where it shows observable anisotropy. Care should be taken to ensure that the loading is strictly in and perpendicular to the direction of the weakness plane.

The procedure for testing is the same as described above.

 

 

Observations and Calculations:-

The point-load strength index is calculated as follows:

Point-load strength index=I_s=P/D²

Where;

P= The load required to break the specimen

D=The distance between the two platen contact points

The point load strength may also be obtained from the nomogram in Fig. 5.

For standard classification, the index I_s(50) should be used. I_s(50) is the point load strength corrected to a diameter of 50 mm, and may be obtained from I_s by correcting this value to a reference diameter of 50 mm using the correction chart.

The strength anisotropy index I_s(50) may be computed as the ratio of the average corrected strength indexes for tests perpendicular and parallel to planes of weakness. I_a assumes values close to 0.1 for isotropic rocks and higher values when the rock is anisotropic.

The point-load strength is closely correlated with the results of uniaxial compression strength tests. The approximate correlation between the point load index and the uniaxial compressive strength is q_u=24 I_s(50).

Correction factor=F=(D/50)^0.45

I_s(50)=F x I_s