In rocks, dilation is caused by the irregularities and asperities on the joint surface climbing over each other. This results in an increase in the joint opening, which increases the permeability of the rock mass. Understanding the dilation response is of great importance to the fundamental understanding of hydro-mechanical behavior of rock masses. This work focuses on investigating the constant portion of dilation in the dilation response (ultimate dilation) as a function of surface geometry and stress conditions. In this work, 22 broken rock surfaces were scanned using an Advance Topometric Sensor (ATS) and consequently tested in direct shear. The quantified surface measurements were analyzed with the dilation responses from the direct shear tests. Based on directional analysis of surface geometries, and consideration of mechanical properties and stress conditions, our results show that ultimate dilation is a function of the asperity height distribution, and ratio of normal stress and compressive strength.

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