Distribution characteristics of the point load strength index of irregular ore rock samples in deep mines

The frequent occurrence of core disking in boreholes under high in-situ stress in deep rock engineering leads to the difficulty in intact core sampling on sites. In this study, rock and ore blocks with irregular sizes after blasting in stopes were selected to perform a point load strength index test, and the equivalent diameter and failure load of the samples with irregular sizes were obtained through the test. Then, the point load strength index of the sample was obtained. However, the results are affected by the size effect of the samples. Two correction coefficients ($ {f_1} $ and $ {f_2} $), including an exponential type related to the failure load as well as equivalent diameter and a linear type related to the equivalent diameter, were used to modify the point load strength index. The modified point load strength indexes are $I_{{\rm{s}}50\text{-}1}$ and $I_{{\rm{s}}50\text{-}2}$. The distribution characteristics of the point load strength index before and after modification were obtained. The distribution frequency of the point load strength index before and after modification basically meets the normal distribution requirements. Before modification, it presented a normal skewness distribution, and after modification, it basically presented a standard normal distribution. The normal distribution of the point load strength index after the linear-type modification is obvious, and its strength value is larger than that after the exponential-type modification on the whole. The point load strength is basically equal for the two modification methods when the equivalent diameter is approximately 50 mm. The results obtained by the exponential correction method are greatly affected by the size effect, and the deviation of the strength value is large. By contrast, the results obtained by the linear correction method are more accurate than that of the exponential correction menthod. With a 95% confidence interval, the confidence interval of the standard point load strength index of the rock sample is 1.09–1.57 MPa, and the mean value is 1.33 MPa. The confidence interval of the ore sample is 0.37–0.45 MPa, and the mean value is 0.39 MPa. Due to the small size of the regular samples and the influence of the blasting damage, the calculated values of the point load strength index based on the uniaxial compressive strength of the rock and ore samples are approximately 1.62 and 3.67 times of the test results, respectively. Therefore, to reduce the influence of the sample size effect and excavation blasting disturbance on the test results of the mechanical properties of surrounding rocks, rock blocks with less blasting disturbance and size of approximately 50 mm in three directions should be selected to perform the point load test in the deep engineering site, where it is difficult to obtain intact rock cores.