(1) Suspension theory
According to the suspension theory, the role of the anchor support is to suspend the weaker rock stratum of the roadway roof on the upper stable rock layer and enhance the stability of the weaker rock stratum. For the layered rock mass often encountered in the mining roadway, the suspension effect of the anchor is shown in Figure 7-38a. If the surrounding rock in the shallow part of the roadway is soft and broken, the roof has a loose rupture zone. The anchoring effect of the anchor is to anchor this part of the easy-rocking rock mass to the deep un-loose rock formation (Fig. 7-38b).
a b
Figure 7-25 bolt support suspension
A—suspension of hard roof bolts ; b—suspension of weak roof bolts
(2) Combined beam theory
If there are several stratified layers of roof, composite beam theory believe that the role of anchor bolt on the one hand provides increased friction force between the various rock formations, rock formations along the stop level continues to slide, to avoid delamination phenomena; on the other hand bolt The shank increases the shear stiffness between the rock layers and prevents horizontal displacement between the rock layers, thereby locking several thin rock layers within the anchorage of the roadway into a thicker rock formation (Figure 7-26).
Figure 7-26 Layered roof bolt combination beam
(3) Combined arch (compression arch) theory
According to the combined arch theory, prestressed anchors are installed in the rupture zone of the surrounding rock of the arched roadway, and a conical distributed compressive stress zone is formed from the two ends of the shaft. If the anchor spacing is small enough, the compressive stress cone formed by each anchor The bodies are interlaced and form a uniform compression zone, ie a compression arch, in the rock mass. The rock in the compression arch is pressed in the radial direction and the tangential direction, and is in the state of three-direction stress. The strength of the surrounding rock is improved and the supporting capacity is increased accordingly (Fig. 7-27).
(4) Maximum horizontal stress theory
The theory of maximum horizontal stress was proposed by Australian scholar WJGale. The theory holds that the horizontal stress of the mine rock is usually greater than the vertical stress, and the stability of the roof and floor of the roadway is mainly affected by the horizontal stress. And has three characteristics:
1 The roadway parallel to the maximum horizontal stress is least affected by horizontal stress, and the stability of the top and bottom plates is the best.
2 The roadway intersecting the maximum horizontal stress at an acute angle, the deformation of the top and bottom plates is biased towards a certain gangway.
3 The roadway perpendicular to the maximum horizontal stress is most affected by horizontal stress, and the stability of the top and bottom plates is the worst.
Figure 7-27 Principle of bolt combination arch (compression arch) Figure 7-28 Principle of maximum horizontal stress
(5) Surrounding rock strength strengthening theory
The main points of the theory of strength reinforcement of surrounding rock are as follows:
1 The essence of roadway bolt support is that the rock mass interaction between the anchor and the anchorage area forms a unified load-bearing structure;
2 Roadway bolt support can improve the mechanical parameters of anchors (E, C, φ) and improve the mechanical properties of the anchored rock mass;
3 There are fracture zone, plastic zone and elastic zone in the surrounding rock of the roadway. The rock mass of the anchorage zone is in the fracture zone or in the above two or three zones. The rock strength of the anchorage zone is in the post-peak strength or residual strength. . The bolt support strengthens the surrounding rock of the roadway, especially in the post-peak area, and improves the peak strength and residual strength;
4 Coal roadway bolt support can change the stress state of surrounding rock and increase the confining pressure, thus improving the bearing capacity of surrounding rock;
5 After the anchorage strength of the surrounding rock of the roadway is increased, the range of the crushing zone and plastic zone around the roadway and the surface displacement of the roadway can be reduced, and the development of the surrounding rock fracture zone and plastic zone can be controlled, which is beneficial to maintain the stability of the surrounding rock of the roadway.
(6) Theory of rock mass anchoring system
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