Experimental study on expansion pullout of selfexpansion bolt in soft rock with anisotropic initial ground stress

A selfexpanding anchor rod with more HCSAtype expansion agent has been developed to increase the ultimate pullout resistance of the anchor rod. But for media with low rigidity and easy deformation,the main factors to increase the anchoring force of selfexpanding bolts are still uncertain. In order to prove the lifting effect of the selfexpanding anchor rod in the anisotropic initial insitu stress soft rock,three kinds of wall thickness steel pipes are used to simulate surrounding rock with different radial stiffness,and different expansion agent content is added to the inside of the steel pipe. Selfexpanding anchoring material,by testing the evolution law of the radial expansion stress and deformation of the anchoring structure,the analytical formula for the expansion stress and displacement of the selfexpanding anchor rod in the anisotropic initial ground stress is deduced,and the stress,deformation and steel pipe are established. The relationship between wall thickness and expansion agent content.Based on the ABAQUS numerical simulation method,the formula is verified to be consistent with the experiment,and the expansion law of the selfexpanding anchor in the lowrigidity rock and soil body and the improvement effect of the ultimate pullout resistance are obtained through the pullout simulation.The research results show that:① When reducing the wall thickness of the steel pipe,the stressdeformation change trend of the selfexpanding anchor rod with different expansion agent content is highly consistent. When the content of the expansion agent is large,the steel pipe wall thickness is reduced,and the radial expansion stress is only reduced. 8.6%,but the radial deformation increased by 117%,indicating that the enlarged head effect is more obvious. ② The ultimate pullout resistance of selfexpanding bolts with an expansion agent content of 25% is similar in the surrounding rock with different stiffness,but the selfexpanding bolts have a significantly greater increase in the energy before the peak of the pullout force in relatively high surrounding rocks.Reaching 819%. It shows that the frictional force has a better effect on the anchoring performance of selfexpanding bolts than the expansion effect. Finally,a design formula for expansion agent content based on the strength parameters of weak surrounding rock and initial stress is established,and field tests are carried out on a rock slope. The test results verify the feasibility of the formula. 