Shearing Performance of Natural Matched Joints with Different Wall Strengths under Direct Shearing Tests

Abstract Studying the effects of upper and lower joint wall strengths on shear behaviors is conducive to understanding the sliding problems of engineering masses, such as the contact surfaces of hard-soft composite strata. In this study, several types of soft-hard specimens with the same natural joint were cast using 3-D scanning technology and 3-D printing technology, and shearing tests were conducted on the specimens under different constant normal stresses. The results of the direct shear test showed that the deformation and shear strength characteristics of composite joints, including shear compaction, shear dilatancy, shear stiffness, peak shear strength, and failure characteristics, were affected by the compressive strengths of the upper and lower joint walls. The study also indicated that the shear damage of asperities occurred in both the hard and soft joint walls; the shear failure volume of the soft joint side accounted for a larger proportion of the total shear failure volume, which was strengthened with an increase in the wall strength coefficient (λσc). This finding explains why the joint roughness coefficient-joint compressive strength (JRC-JCS) formula has limitations in predicting the strength of soft-hard joints. To better predict the strength of soft-hard joints, a modified JRC-JCS formula for shear strength was proposed by considering the effects of upper and lower joint wall strengths on shear behaviors. Finally, we analyzed the mechanisms of three failure modes, including sliding wear, shearing failure, and sliding-shearing failure, and presented a reasonable explanation for different joint wall strengths causing different shear behaviors from a microscopic perspective.