A comparative study for determining rock joint normal stiffness with destructive uniaxial compression and nondestructive ultrasonic wave testing

Abstract

Rock joint a kind of vital discontinuities in natural rock mass. How to accurately and conveniently determine joint normal stiffness is therefore significant in rock mechanics. Here, first, seven existing methods for determining joint normal stiffness were introduced and reviewed, among which Method I (the indirect measurement method), Method II (the direct determination method), Method III (the across-joint strain gauge measurement method) and Method IV (the deformation measuring ring method) are via destructive uniaxial compression testing, while Method V (the best fitting method), Method VI (the rapid evaluation method) and Method VII (the effective modulus method) are through wave propagation principles and nondestructive ultrasonic testing. Subsequently, laboratory tests of intact and jointed sandstone specimens were conducted following the testing requirements and procedures of those seven methods. A comparison among those methods was then performed. The results show that Method I, i.e. the benchmark method, is reliable and stable. Method II has a conceptual drawback, and its accuracy is acceptable at only very low stress levels. Relative errors in the results from Method III are very large. With Method IV, the testing results are sufficiently accurate despite the strict testing environment and complicated testing procedures. The results from Method V are greatly unstable and significantly dependent on the natural frequency of the transducers. The joint normal stiffness determined with Method VI is stable and accurate, although data processing is complex. Method VII could be adopted to determine the joint normal stiffness corresponding to the rock elastic deformation phase only. Consequently, it is suggested that Methods I, IV and VI should be adopted for the measurement of joint normal stiffness. The findings could be helpful in selecting an appropriate method to determine joint normal stiffness and, hence, to better solve discontinuous rock mass problems.