Mechanical Effect Analysis and Comparative Site Tests on Surrounding Rock with Different Bolt Anchoring Lengths and Pre-tightening Forces

Different bolt anchoring lengths and pre-tightening forces produce different mechanical and control effects on the surrounding rock. For clarification of the effects, a numerical comparative test is conducted to analyze the stress field distribution features of surrounding rock with different bolt anchoring lengths and pre-tightening forces. A high pre-tightening force quantitative exerting device is developed for high strength bolts with a function of automatic anchor retreat. With the device, a test on site is conducted at a typical roadway of Zhaolou Coal Mine in China; this test is on the comparative plans designed with different bolt anchoring lengths and pre-tightening forces; and the controlling effect is obtained on the surrounding rock in each supporting plan through the analysis of the test. This study shows it is conducive for the bolt pre-tightening force diffusing in surrounding rocks to exert high pre-tightening force and arrange a certain length of non-anchoring section; and it is conducive to form an effective bearing structure of anchored rocks. The new device is self-developed with independent intellectual property rights; and it can meet the requirement of roadway bolt support under high stress with many advantages such as large scale quantitative of pre-tightening force exerting and being easy to operate. Increasing pre-tightening force is an effective way to improve the control effect of surrounding rock; and an appropriate reduction of the bolt anchoring length will not significantly weaken the effect of the surrounding rock control, under certain conditions. The above conclusion can provide reference and experience for the anchoring support design and construction of the underground engineering.