Study on comprehensive rating of integrity index Kv for CHN-BQ method based on cloud model

Abstract

The basic quality (CHN-BQ) method is a national standard for rock mass classification in China. The integrity index K_v in the CHN-BQ method can be determined by the acoustic wave test and quantitative or qualitative approaches, which are flexible and need to be controlled in the determination. This is not sufficiently concise compared with other widely used classifications. Inspired by the RMR (Rock mass rating) classification, a comprehensive rating of K_v is required to determine the index directly as an alternative approach. In this study, the cloud model is adopted to obtain quantitative results from the qualitative evaluation of the interconnection degree of structural planes. Then the cloud model is further used to obtain quantitative results of rock mass integrity by combining the interconnection degree and the mean spacing of structural planes. Based on quantitative results of the interconnection degree and rock mass integrity and the quantitative correspondence between the volume joint number J_v and K_v, a comprehensive rating of K_v for the CHN-BQ method is proposed by referring to the RMR, in which the geometric and physico-mechanical properties of rock mass integrity account for 55 and 45 points, respectively. The comprehensive rating is validated by quantitative results of the interconnection degree and rock mass integrity, as well as the effective samples generated by Monte Carlo simulation methods. The results of the comprehensive rating agree well with quantitative results, with a maximum relative error of 11.3%. The K_vt assessed by the 30% quantitative approach and 70% qualitative approach is highly consistent with K_vc determined by the comprehensive rating, with an average relative error of 13%. The results demonstrate that the comprehensive rating of K_v reasonably combines quantitative and qualitative approaches. To address the fuzzy uncertainty of stepped ratings at rating boundaries, the fuzzy comprehensive rating of K_v is proposed based on the Mamdani fuzzy inference. Finally, the comprehensive rating of K_v and its fuzzy version are applied to 95 excavated surfaces which are conducted in detailed geological investigations in the Mingtangshan tunnel. The K_vc and FK_vc obtained by comprehensive rating and fuzzy comprehensive rating of K_v accurately approximate the measured K_vm by the acoustic wave test, with average relative errors of 9.80% and 7.01%. The rock mass basic quality index BQ and the modified index [BQ] obtained by the comprehensive rating and the fuzzy version almost coincide with those obtained by field tests, with average relative errors of less than 4%. To sum up, the comprehensive rating of K_v, especially after the fuzzy inference, can conveniently classify the rock mass and guide the construction.