An evaluation on Barton-Bandis shear strength criterion for discontinuities in weak materials under low normal stresses

The shear strength parameters of the discontinuity surfaces should be acquired for the investigation of discontinuity-controlled instabilities in rock masses. Particularly in weak rocks, it is highly challenging to collect samples from the field and prepare laboratory test samples without affecting the features of the discontinuities. Therefore, empirical methods are gaining importance for determining the strength of discontinuities in such rocks. The purpose of this study is to assess how well the Barton-Bandis criterion performed on samples with low strength under low normal stresses. Artificial samples using various plaster-water mixtures having low compressive strengths with standard roughness profiles were prepared for conducting direct shear tests. The input parameters of Barton-Bandis criterion were determined by laboratory experiments; empirical failure envelopes were drawn, and the results were compared. Based on detailed evaluations, it was seen the criterion has some limitations for low normal stresses. Besides, to determine the joint compressive strength parameter, needle penetration test may be more effective than the Schmidt hammer test especially for low strength materials with the unweathered discontinuity surfaces. According to the results of comparison among digitized data from profilometer, photogrammetric approach (SfM), and optical scanner (SLS) methods, conclusion demonstrates that all of them can be preferred to determine the joint roughness coefficient (JRC) in an acceptable range for the Barton-Bandis criterion. Among them, profilometer and SfM are both easily accessible to all users. However, it should be emphasized that profilometer is still more practical method to estimate the JRC.