Unconfined Compressive Strength (UCS) is important when estimating bearing resistance and skin friction in rock. Obtaining rock core specimens with a length to dimeter ratio of 2:1 is sometimes not possible for our weaker rocks. The point load test is a method that allows smaller blocky pieces of rock to be tested. We have found that sometimes the ASTM specified 10 number of specimens tested is not possible for a thinner bedded stratum. Consequently, we have developed a Materials Procedure (MP) that allows estimation of UCS using fewer specimens.
The procedures outlined in the linked MP follow the ASTM D 5731 but allow a fewer number of specimens. When 8 or more tests are run, the highest and lowest results are ignored, and the mean is reported. With fewer than 8 tests, the mean is reported if the coefficient of variance (COV) is greater than or equal to 0.35, or the median is reported if the COV is less than 0.35. We allow as few as 3 results to be averaged or the median be determined when needed.
Based on our review of the Peabody Group study (2005), we believe that useful results can be obtained when strength values are less than the ASTM required 2,200 psi. Based on this study we have selected a conversion factor of 21 for use with rock in West Virginia. When lower UCS results are obtained, they should be so noted on the lab sheets and in the calculations. When lower values are used, engineering judgement is needed to ensure the UCS is appropriate for the type of rock tested.
We have included an Excel spreadsheet to perform the above described UCS calculations on this webpage. We make no claim as to its accuracy and your choice to use it indemnifies the WVDOH of any liability.
Materials Procedure for Point Load Testing
Point Load Test Calculator