Application and research on large in-situ self-reacting direct shear apparatus owning two shear interfaces

The in-situ shear test has been justified as one of the most reliable and common method used to test shear strength of soil-rock mixture. However, the existing in-situ shear test apparatuses show some shortcomings in practice that will influence the accuracy of the test results, including the unreasonable normal pressure system and unable to reserve shear opening, etc. A new self-reacting direct shear apparatus owning two shear interfaces is introduced. The apparatus is characterized by the followings: (1) it provides horizontal reaction force by itself; (2) the vertical pressure system is fixed during shearing without roller row; (3) the adjustable width of shear opening can be reserved between shear boxes; (4) it is of high modularity, and is easy to handle, assembly and disassembly. It is validated through comparison with the single-box direct shear test. Compared with the experimental results obtained by the single box shear apparatus, the shear stress-shear displacement curve obtained by the self-reacting direct shear apparatus is a standard hyperbolic curve, and the shear stress-displacement curve under large shear displacement can be obtained. The maximum shear stress read by shear stress-displacement curve under large shear displacement is more practical. Compared with the shear strength obtained by the single-ring shear apparatus, the internal friction angle is 4.6 degree lower and the cohesion is 41.14 kPa higher. At the same time, through the image comparison of the shear failure surface obtained by the two shear apparatuses, it can be obviously found that the shear failure interfaces obtained by self-reacting direct shear apparatus owning two shear interfaces is relatively flat, and there is no concave phenomenon of the shear failure surface on the single-ring shear instrument.