Effect of Micro-Scale Asperities on Shear Behavior of Artificial Rock Joints

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ABSTRACT Various devices such as Barton comb, optical and laser scanner are used to record surface geometries of rock joints for roughness measurement. Measurement resolution of the used device can vary significantly depending on settings and the nature of device. Ignoring the effect of resolution may result in misestimation of effective roughness because resolution determines how precise details are captured by the measurement. To improve our understanding on the proper resolution for roughness measurement, effect of micro-scale asperities on the shear behavior of rock joints is investigated in this study. Plaster joint specimens having regular sawtooth asperities of 0.4 and 0.8 mm lengths were generated using high quality 3D printer and silicone molds. Direct shear tests at four different normal stresses were conducted on them and the results show that the 0.8 mm asperities significantly increase the shear strength of joint specimens while 0.4 mm asperities do not. Together with the test results, main factors contributing to the results are also discussed. INTRODUCTION Since roughness is an important property of rock joints that affects their mechanical behavior, it should be investigated to analyze the stability of rock masses. To measure roughness, many different types of devices can be utilized to capture surface geometry of joints. Barton comb is a widely used tool to get 2D profiles of rock surfaces and more recently, laser and optical scanners are employed by many researchers to obtain their 3D geometries. When quantifying the degree of roughness using various devices, measurers often do not consider their resolution. However, this can lead to over- or under-estimation of effective roughness since resolution determines how many details are captured in the acquired data. That is, when resolution is too low, asperities that enhance the shear strength might not be detected resulting in under-estimation of roughness. Conversely, when it is too high, very small asperities that may not affect the shear strength will be captured leading to over-estimation of effective roughness. Thus, measurers need to know what is the appropriate resolution for measuring joint roughness. To gain knowledge about it, it would be helpful to know about the effect of small asperities on shear behavior of rock joints.