Assessing the Variability in Direct Shear Testing Interpretation for Sets of Natural Fractures

A direct shear test is a standard geotechnical laboratory test used to estimate the shear strength of rock fractures. This paper builds on previous work conducted by the authors to assess the extent of variation in shear strength parameters resulting from different approaches to interpreting direct shear test data. This interpretation involves selecting shear stresses from a shear stress versus shear displacement plot and utilizing them in the Mohr-Coulomb constitutive model to calculate friction angle and cohesion. The authors navigate through a series of direct shear results on fractures in the same rock type. Furthermore, a Python script was developed to automate the interpretation process, offering analysts an alternative tool. A variation up to 22.9 degrees in friction angle was observed and the effects of area and dilation corrections were analyzed. 1. INTRODUCTION Direct shear tests (DST) are geotechnical tests designed to assess the shear strength and deformation properties of soils and rock discontinuities (MacDonald et. al. 2023). This test can be conducted either in a controlled laboratory setting using core specimens or in the field under in-situ conditions (Sanei et al. 2015). A DST involves placing a core sample containing a plane of interest, referred to as a ‘discontinuity,’ onto a shear holder. The top part is displaced in relation to the bottom by applying a force parallel to the discontinuity (shear force) while maintaining a constant force perpendicular to the discontinuity (normal force). The resulting shear forces and corresponding shear displacements are measured and recorded. These forces are converted to stresses by dividing by the surface contact area, resulting in a shear stress vs. shear displacement trace (ASTM 2016). The test is generally performed three to five times on the same or similar samples with increasing applied normal loads. Figure 1 shows direct shear traces of a quartz monzonite fracture. The resulting data allows the interpreter to determine normal and shear stiffness, peak and residual shear strengths, and dilation angles (MacDonald et al. 2023).