|Publisher||American Rock Mechanics Association||Language||English|
|Content Type||Conference Paper|
|Title||The Relationship Of Uniaxial Compressive Strength To Point-Load And Moisture Content Indices Of Highly Anisotropic Sediments Of The Illinois Basin|
|Authors||Robert A. Bauer, Illinois State Geological Survey|
|Source||The 25th U.S. Symposium on Rock Mechanics (USRMS), June 25 - 27, 1984 , Evanston, IL|
|Copyright||1984. American Institute of Mining, Metallurgical, and Petroleum Engineers Inc. Permission to Distribute - American Rock Mechanics Association|
This study showed that diametral point-load testing performed parallel to the bedding planes of highly anisotropic sedimentary rocks produced inconsistent results that cannot be used to accurately estimate uniaxial compressive strength. Axial point-load values from tests performed normal to bedding planes and evaluated by the T500 index produce good correlations with the uniaxial compressive strength values obtained for five different rock types. A strong correlation also existed between the moisture content and the uniaxial compressive strength of the tested lithologies. A combination of axial point-load and moisture content indices produces the best correlation with the uniaxial compressive strength test results.
The diametral point-load testing procedures described by Brock and Franklin (1972) and Bieniawski (1975) are widely used to obtain a strength index which can be used to estimate the uniaxial compressive strength (UCS). However, these procedures were developed to test relatively isotropic, high strength rocks. During the past decade, rock mechanics testing of highly anisotropic sedimentary rocks associated with coal-bearing strata has increased greatly. Pells (1975), Read et al., (1980), Forster (1983) and Bauer (1983) have found that diametral point-load testing of highly anisotropic sedimentary rocks produces inconsistent test results and that bedding plane separation produced by diametral testing is unrelated to the UCS test.
PREVIOUS POINT-LOAD RESEARCH
The point-load test, as used today, was designed by Reichmuth (1963), who used it as a tensile strength index test. The formula included a constant that was derived for cores loaded diametrally. Later, Reichmuth (1968) expanded the formula and added shape and brittleness factors. Broch and Franklin (1972) performed point-load tests on a limited number of high strength, relatively isotropic rocks and recommended replacement of Reichmuth's complex formula for tensile strength with a point-load strength index. They developed a correlation chart so that various sample sizes could be tested and indexed to a 50mm diameter core. They found that diametral point-load tests of samples produced constant values when a length-to-diameter ratio of 1.4 or greater was used. Axial point-load values varied with changes in length-to-diameter ratios and never became constant, therefore, a 1.1 ratio was designated because it produced axial test results equal to the diametral results. They found the UCS/I50 ratio to be equal to 24. Bieniawski (1975) compared diametral and axial point-load testing on samples of norite, quartzite, and sandstone, using Broch and Franklin's procedures. He found that, for the sandstone samples, the standard deviation for the diametral point-load index was three times higher than that for the axial point-load index. Bieniawski did not address this problem, which was the first indication of problems of reproducibility of diametral point-load test results from anisotropic materials (sedimentary rocks). The standard deviations of the two tests were nearly identical for the norite, which has isotropic properties. Because results of the two tests were nearly equal, Bieniawski chose the diametral test - which requires no sample preparation - as the most convenient to run. Bieniawski produced factors of 24, 21, and 18 to be used with diametral point-loading of NX(54mm)-, BX(42mm)-, and EX(21.5mm)-diameter cores to estimate the UCS for high-strength isotropic rocks.
|File Size||446 KB||9|