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Mechanism of Bolt support

The action mechanism of Bolt support includes arch reinforcement, suspension, composite beam reinforcement, surrounding rock reinforcement and span reduction, etc.

(1) suspension acts in the layered strata, and the Anchor Rod suspends the unstable strata in the lower part on the stable strata in the upper part. The tension of the Anchor Rod comes from the weight of the suspended strata.

(2) the composite beam acts in the thin strata without stable strata. Through the pre-tension stress of Anchor Rod, the thin strata of the composite beam will be squeezed tightly to improve its self-bearing capacity. The main factors that determine the stability of composite beams are the pre-tensile stress of Anchor Rod, Rod body strength and strata properties.

(3) reinforcement arch action for massive or cracked surrounding rock cut by crisscross weak surface, if Bolt is used to reinforce in time, the shear strength of weak surface of rock mass structure can be improved, forming one within a certain thickness around the surrounding rock can not only maintain its own stability

The reinforced arch, which can prevent the looseness and deformation of its upper surrounding rock, thus maintaining the stability of the roadway.

(4) reducing the span effect the roof of the roadway is punched with anchor Rod, which is equivalent to putting a point column on it. Therefore, it is equivalent to reducing the span of rock hanging on the roof of the roadway, thus improving the bending resistance of the roof strata.

① suspension effect of Anchor Rod: The Anchor Rod passes through the weak, loose and unstable rock and soil mass, and is anchored on the deep and stable rock and soil mass to provide sufficient tension, overcome the self-weight and sliding force of sliding rock and soil, and prevent the cave wall from sliding and collapsing.

② extrusion reinforcement effect: After the bolt is stressed, it forms compression section within a certain range around it. Arrange the bolts in an appropriate way so that the compression section formed by adjacent bolts overlap each other to form compression zone. The loose formation within compression zone is reinforced by Bolt, which enhances the integrity and improves the bearing capacity.

③ effect of composite beam (arch): After the bolt is inserted into the formation at a certain depth, the strata under the action of anchoring force squeeze each other, the interlayer friction increases, and the internal stress and deflection decrease greatly, it is equivalent to changing the simply stacked several layers of beams (arches) into composite beams (arches). The bending rigidity and strength of composite beam (arch) are greatly improved, thus enhancing the bearing capacity of formation. The larger the anchoring force provided by the anchor Rod, the more obvious the effect will be.

④ length of Anchor Rod: The total length required when the anchor Rod can effectively play its role according to the design. When calculating according to suspension effect, it is the sum of Anchorage length, reinforcement length and protruded lengths. When calculated according to the action of composite beam (arch), it is the sum of 1.2 times the height of composite beam (arch) and protruded lengths. When the actual value is taken, the additional length increased due to uneven excavation outline should also be considered.

⑤ anchoring length: the length of anchor bolt anchored into stable formation can be selected according to experience or calculation. When selecting according to experience, consider anchoring method and anchor diameter. When selecting according to calculation, the bonding force between mortar and Bolt and between mortar and hole wall should be considered.

⑥ reinforcement length: according to the height of dangerous rock suspended along the direction of Anchor Rod, or the load height of surrounding rock, it can also be determined by the thickness of loose circle measured by acoustic wave and other testing techniques.

⑦ Bolt pulling test: one of the methods to test the construction quality of Bolt and measure the uplift resistance of Bolt. Before the anchor rod is not covered by Sprayed Concrete, use the anchor rod tension meter or torque wrench to measure directly. After clamping the Anchor Rod, press slowly and evenly until the pressure gauge reading reaches the value corresponding to the design value, or loosen the Anchor Rod, generally no destructive test is done. After the anchor bolt is covered by Sprayed Concrete, the anchor bolt is planed out and then measured after being proved by the anchor bolt detector. The number of detection should be sampled according to every 30-50 meters long of the cavern or every 300 bolts, and each group should not be less than 3 bolts, which should be evenly selected from a row of bolts in the same section of the checkpoint.

The method of theoretical analysis and numerical calculation is used to discuss the generation mechanism, distribution law of anchor force and its mechanical effect on rock mass. The results show that the relative displacement or relative displacement trend between the bolt and the surrounding rock is the condition for the generation of the bolt action. Considering from the perspective of mechanics, the way of end anchoring and full-length filling is the ideal anchoring method.