Anchor bolt support(2)

KEYWORDS:Coal Mine Rock Anchor Bolt;Underground Mining Anchor Rod

History development

■In 1911, the Friedens Coal Mine of Aberschlesin, the United States, first applied rock bolts to support the roof of the roadway;

■In 1918, anchor cable support was first used in cilician mine of United States.After then the application scope of the anchoring technology began to expand in the future;

■In 1934, the heightening project of the Cheurfas Dam in Algeria used 10,000 kN prestressed bolts as anti-overturning anchors for the first time. This was the first case in the world that prestressed bolts were used to reinforce the dam, and it was successful.In later years, the Tensha Dam in India, The Steinblas Dam in South Africa, the Yarger Dam in England and the Sprechis Dam in Austria were also reinforced with prestressed anchors.

■In 1958, West Germany used soil anchors in the deep foundation pit of the Bavarian Broadcasting Company in Munich;

■In the 1960s, large underground caverns such as the Lipno Power Plant (32 m wide) in Czechoslovakia and the Walbeck 1I Underground Power Plant (33.4 m wide) in West Germany adopted the surrounding rock reinforcement method combining high prestressed long anchor cables and low prestressed short anchor rods.

Theoretical classification

Suspension theory

From 1952 to 1962, Louis A. Panek et al. published the suspension theory, which believed that the function of bolt support was to suspend the weaker rock strata in the roadway roof on the upper stable rock strata.For the layered rock mass exposed by the mining roadway, the direct roof has a tendency of bending and sinking deformation. If the bolt is used to squeeze it in time and hang it on the old roof, the direct roof will not separate from the old roof or even fall off.The suspension effect of the anchor rod mainly depends on the thickness of the suspended rock layer, the number of layers and the relative stiffness and elastic modulus of the rock bending, but also affected by factors such as the length, density and strength of the anchor rod.This theory was put forward earlier and has certain practical value when its preconditions are met.However, a large number of engineering practices have proved that even if there is no stable rock layer on the upper part of the roadway, the bolt can still play a supporting role. For example, in an all-coal roadway, the anchor rod can be anchored in the coal seam to achieve the purpose of supporting, indicating that this theory has limitations.

Composite beam theory

According to the composite beam theory, there are several layered roofs in the roadway roof, which can be regarded as a kind of beam with the two sides of the roadway as the fulcrum, and this kind of rock beam supports the upper strata load. Anchors are used to "bind" each layer into a whole composite beam to prevent rock sliding along the layer and avoid the phenomenon of layer separation. Under the load of overlying strata, the maximum bending strain and stress of the thicker composite beams are greatly reduced compared with the simple superimposed beams, and the deflection is also reduced. Moreover, the greater the frictional resistance between the layers, the greater the overall strength and the better the reinforcement effect.However, this theory has certain limitations when dealing with the continuity of the beam and the bending strength of the beam when the rock layer has cracks along the longitudinal direction of the roadway.