Investigation on residual friction coefficient of rock surfaces under small and standard contact sizes and suggestion the application of tribometer to measuring it

Investigation and determination of residual frictional properties of rock joints is important for designing structures on or within the rock masses. Tribometer is a device to measure frictional properties of various types of contacting surfaces. In this research, several direct shear experiments conducted on two types of artificial limestone joints named Onyx marble and Travertine. Two types of surfaces geometry such as rough tensile joint surfaces with standard dimensions (based on ISRM suggested methods) and grinded planar small surfaces with dimensions ranged from 1 cm2 to 25 cm2 were prepared. Direct shear experiments by constant shearing rate and under different normal stresses conducted in CNL boundary condition. Results obtained from the residual state of the shear were gathered and investigate and it found that the residual friction coefficient of limestone rock joints, under almost similar normal stress and shearing rate conditions, remains approximately constant with differing the contact size of the specimens. In addition, under approximately similar stress concentrations in contact regions, the residual shear behavior of rough surfaces with standard dimensions is very similar to that of small planar ground surfaces in limestone joints. Finally, based on findings in this research and some other past researches, it is proposed to apply tribometers for measuring the residual shear strength of rock joints. In this paper, a conceptual design of a tribometer is proposed to develop for measuring residual frictional properties of a point contacts of rock surfaces during 5 to 20 mm shear displacements.