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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 testapparatus 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 sixstates. 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 asthe 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 topropose 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.

is sufficient. This is due in part to the high variability of UCS measurements. Moreover, the tests are expensive, primarilybecause 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 untilfailure 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 shownin Figure 1.

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

Figure 1. The Point Load Tester.

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 areconducted 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...
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