The control effect of surrounding rock with different combinations of the bolt anchoring lengths and pre-tightening forces in underground engineering

The anchoring length and the pre-tightening force are the two main factors influencing the control effect of surrounding rock in bolt support. To study the influences, a theoretical analysis is made on the stress distribution law of the surrounding rock with different anchoring lengths and pre-tightening forces; a high pre-tightening force quantitative exerting device for high strength bolts is developed with a function of automatic anchor retreat; and four field test plans are designed and carried out with different combinations of bolt anchoring lengths and pre-tightening forces to comparatively analyze their control effect of surrounding rock. The theoretical analysis shows that as the bolt anchoring length is constant, the effective compressive stress area in the surrounding rock of the bolt non-anchoring section increases along with the increase of the pre-tightening force; a certain length of non-anchoring section is helpful to play the bolt support function, but the anchoring length should not be too short. The field test shows that increasing pre-tightening force is an effective way to improve the control effect of surrounding rock; under certain conditions, an appropriate reduction of the bolt anchoring length will not significantly weaken the control effect of surrounding rock. That verifies the theoretical analysis. The study provides some useful guidance for the selection of the parameters of the bolt support in underground engineering.