Experimental study on influence of host rock strength on shear performance of Micro-NPR steel bolted rock joints

The development of deep underground engineering has brought new challenges to bolt materials. Micro-Negative Poisson's Ratio (NPR) steel is a new type of bolt material with high strength, high toughness, and super energy absorption. The effect of host rock strength on the shear performance of micro-NPR steel anchoring rock joints was studied by performing an indoor direct shear test. Ordinary Q235 steel bolted joints and unbolted joints were set as the test control groups. The shear force-shear displacement curves, surface failure characteristics, bolt deformation, fracture characteristics, shear strength, and shear energy absorption were all compared and analyzed. The study found that the bolted rock joints have significant ductile failure characteristics under shear load, which effectively improves the brittle failure characteristics of the unbolted rock joints. The ability of the rock to resist the crushing damage of the bolt is continuously enhanced with an increase in the host rock strength, while the damaged area of the grout and rock mass at the opening of the joint surface gradually decreases. The failure area of the Q235 steel bolted joint surface is significantly smaller than that of the micro-NPR steel. Additionally, the equivalent internal friction angle, equivalent cohesion, and peak shear strength of the micro-NPR steel are significantly higher than those of Q235 steel. The higher the strength of the host rock, the greater the bolt peak shear strength of the joint. The bolts undergo local shear deformation near the joint surface, and the bolt deforms into an ‘S’ shape after shearing. The improvement of the host rock strength limits the development of the bolt's plastic hinge to a position far from the joint surface. The shear fracture displacement and shear energy absorption of the bolt decrease with an increase of the host rock strength. The shear energy absorption capacity is at least 3.7 times higher than that of Q235 steel. When the host rock strength is low, the bolt yields in tension and bends at the plastic hinge, at which time tension-bending failure occurs. With an increase of the host rock strength, the bolt undergoes tension-shear failure. The high strength and high ductility of traditional steel are well balanced by the micro-NPR steel, with the tensile and shear resistance of the steel bolt being greatly improved, which allows for large deformation and super energy absorption in the bolt material, which allows for a wide range of applications at deep engineering sites.