USING THE POINT LOAD TEST TO DETERMINE THE UNIAXIAL COMPRESSIVE STRENGTH OF COAL MEASURE ROCK John Rusnak, Director of Geology Peabody Group St. Louis, MO Christopher Mark, Rock Mechanics Team Leader National Institute for Occupational Safety and Health Pittsburgh, PA ABSTRACT Point load testing is used to determine rock strength indexes in geotechnical practice. The point load test apparatus and procedure enables economical testing of core or lump rock samples in either a field or laboratory setting. In order to estimate uniaxial compressive strength, index-to-strength conversion factors are used. These factors have been proposed by various researchers and are dependent upon rock type. This study involved the extensive load frame and point load testing of coal measure rocks in six states. More than 10,000 individual test results, from 908 distinct rock units, were used in the study. Rock lithologies were classified into general categories and conversion factors were determined for each category. This allows for intact rock strength data to be made available through point load testing for numerical geotechnical analysis and empirical rock mass classification systems such as the Coal Mine Roof Rating (CMRR). INTRODUCTION The point load test (PLT) is an accepted rock mechanics testing procedure used for the calculation of a rock strength index. This index can be used to estimate other rock strength parameters. The focus of this paper is to present the data analysis used to correlate the point load test index (Is50) with the uniaxial compressive strength (UCS), and to propose appropriate Is50 to UCS conversion factors for different coal measure rocks. The rock strength determined by the PLT, like the load frame strengths that they estimate, are an indication of intact rock strength and not necessarily the strength of the rock mass. THE UNIAXIAL COMPRESSIVE STRENGTH TEST The UCS is undoubtedly the geotechnical property that is most often quoted in rock engineering practice. It is widely understood as a rough index which gives a first approximation of the range of issues that are likely to be encountered in a variety of engineering problems including roof support, pillar design, and excavation technique (Hoek, 1977). For most coal mine design problems, a reasonable approximation of the UCS is sufficient. This is due in part to the high variability of UCS measurements. Moreover, the tests are expensive, primarily because of the need to carefully prepare the specimens to ensure that their ends are perfectly parallel. 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 1. 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 Is50 (in psi) is determined by the following equation. Is50 = 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) Is50 = 24 Is50 (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 (3) SD n 95% = 1.96 Table 1. Published comparisons between the point load and uniaxial compressive strength tests for sedimentary rock. Reference Rock Type Location Number of tests Conversion Factor Comments Das, 1995 Siltstone Western Canada, bituminous coalfields NG1 14.7 lumps, fresh core, old core Sandstone/siltstone NG 18 Shale/mudstone NG 12.6 Vallejo et al, 1989 Sandstone Eastern KY, VA, WV 420 PLT, 21 UCS 17.4 Freshly blasted rock, irregular lump samples Shale surface coal mines 1,100 PLT, 55 UCS 12.6 Smith, 1997 Dredge material various harbors NG 8 UCS
Posted on: Fri, 30 Aug 2013 19:14:29 +0000
Trending Topics
Recently Viewed Topics
© 2015