Use of the point load index in estimation of the strength rating for the RMR system

• We collected a total of 490 UCS and PLI data pairs from the literature and this study.
• The regression analyses were performed between the UCS, PLI, and RS for the cases of PLI < 1 and PLI > 1 MPa.
• The prediction capacity of the chart was controlled using the VAF, RMSE, MAPE, and ANAVO analyses.
• We developed a combined strength rating chart based on the PLI < 1 and PLI > 1 MPa for the RMR system.
• This study indicated that the proposed chart can be reliably used in estimation of the strength ratings in the RMR.

The Rock Mass Rating (RMR) system is a worldwide reference for design applications involving estimation of rock mass properties and tunnel support. In the RMR system, Uniaxial Compressive Strength (UCS) is an important input parameter to determine the strength rating of intact rock. In practice, there are some difficulties in determining the UCS of rocks from problematic ground conditions due to rapid data requirements. In this study, a combined strength rating chart was developed to overcome this problem based on the experience gained in the last decades from the point load test. For this purpose, a total of 490 UCS and Point Load Index (PLI) data pairs collected from the accessible world literature and obtained from the Eastern Black Sea Region (EBSR) in Turkey were evaluated together. The UCS and PLI data pairs were classified for the cases of PLI < 1 and PLI > 1 MPa, and two different strength rating charts were suggested by using the regression analyses. The Variance Account For (VAF), Root Mean Square Error (RMSE) and Mean Absolute Percentage Error (MAPE) indices were calculated to compare the performance of the prediction capacity of the suggested strength rating charts. Further, the one way analysis of variance (ANOVA) was performed to test whether the means of the calculated and predicted ratings are similar to each other. Findings of the analyses have demonstrated that the combined strength rating chart for the cases of PLI < 1 and PLI > 1 MPa can be reliably used in estimation of the strength ratings for the RMR system.