Several force concepts related to bolt support

1 Definition

Anchoring force: refers to the binding force of the bolt on the surrounding rock, which includes radial anchoring force and tangential anchoring force, and radial anchoring force includes supporting anchoring force and sticking anchoring force.

The anchoring force is the radial support force exerted on the surrounding rock by the supporting plate to prevent the surrounding rock from moving into the roadway;

The anchoring force is the radial force exerted by the bolt on the surrounding rock through the binder;

The tangential anchoring force is the limiting force that the bolt body penetrates through the weak face of the rock mass and produces the sliding and opening of the weak face; the unit is kN.

Pull-out force: refers to the force that prevents the bolt from being pulled out of the rock mass. The pull-out force can be divided into design pull-out force and test pull-out force. Generally speaking, the pull-out force refers to the design pull-out force, and its value should be greater than the breaking force of the bolt; the unit is kN.

Anchor bolt preload: the axial tension applied to the bolt body during the bolt installation process, in kN.

Anchor bolt pre-tightening moment: the moment applied to the bolt bolt nut during the bolt installation process, in N m.

Anchor bolt prestress: During the bolt installation process, the axial tensile stress applied to the bolt body is equal to the ratio of the bolt preload to the cross-sectional area of the bolt body, in MPa.

2 Measurement methods

2.1 Anchoring force test

The anchoring force of the bolt is generally carried out with a dynamometer. At present, the ML-20/ML-30 bolt tension gauge is generally used in the well.

The process of installing the dynamometer:

①Check the condition of the oil pipe joint of the hydraulic pressure gauge of the workbench in the tension gauge;

②The anchor rod connecting rod (internal thread) is directly sleeved on the thread of the anchor rod end, and the screwed thread is not less than 30mm;

③ Install the sleeve, the sleeve is close to the anchor rod tray, and then install the jack (one end of the telescopic oil cylinder is outward, close to the nut), and tighten the nut with a wrench;

④Connect the tubing to the jack

top;

⑤ Tighten the switch knob;

⑥Pressure the operating handle firmly and at a constant speed, always pay attention to the pressure gauge, until the designed anchoring force stops, and slowly turn on the switch to release the pressure.

Precautions:

(1) Select the position where the rock and coal at the pallet is leveled without breaking;

(2) The backing plate should be as flat and rigid as possible;

(3) The exposed length of the bolt thread is between 25mm and 40mm, and the bolt and the coal rock surface should be vertical; the center line of the bolt body of the jack shaft connecting rod is the same;

(4) Check the integrity of the dynamometer before pressurizing (work medium pipeline pressure gauge screw jack, etc.);

(5) The pressure of the bolt dynamometer should be uniform and slow until the bolt loosens or the pressure gauge reads to the design anchoring force value of the bolt, and it stops. Generally, no destructive experiment is performed;

(6) When the pressure of the jack is relieved, slowly loosen the switch knob;

(7) During the compression test, no one shall be allowed 3m near the bolt under inspection;

(8) If the resin bolt is measured half an hour after installation, the measured value should be multiplied by a factor of 1.3;

(9) Safety protection measures should be taken during the test of the anchoring force of the bolt.

2.2 Anchor pullout force detection

The bolt puller is the most commonly used bolt pullout force testing instrument. At present, several bolt pull gauges commonly used in my country include MLJ-300/100 bolt pull gauge, MJY-1 non-destructive bolt force detector, and ZY series bolt pull gauge.

There are the following requirements for the bolt pulling test:

The bolt pulling experiment is completed in the underground roadway by using the bolt pulling gauge;

The test site should be selected in the underground roadway construction site or similar surrounding rock;

The bolts, anchoring agents, etc. used in the test shall be the same as those used in the formal construction;

The tools and drilling parameters used in the test should be the same as those used in the formal construction;

The test should be in the form of short anchors (such as anchoring agents with a length of 150~200mm) in order to test the bonding strength of the anchoring agents;

The test is a destructive test, the bolt is pulled until it fails, and the load and the displacement of the tail of the bolt are recorded during the test;

According to the maximum load and displacement curve, the anchorability and anchoring effect of surrounding rock are analyzed, and the feasibility of applying bolt support is judged.

In one of the following situations, the bolt pullout test must be re-run:

The bolt support design is changed;

The support material is changed;

The geological conditions of the surrounding rock of the roadway have undergone great changes, such as faults, broken zones, buckling and other geological structures; the roof of the roadway has a large water spray.

2.3 Anchor bolt preload torque test

A torque wrench is generally used to detect the bolt preload. The detection of anchor bolt preload should meet the following requirements:

Each small group will sample a group (3) of bolts and nuts for torque testing. The pre-tightening torque of each nut shall meet the design requirements;

If there is one nut in each group whose torque is unqualified, it is necessary to spot check another group (3 nut). If still found to be unqualified, re-tighten and overhaul all the nuts in this class.

3 The relationship between several confusing forces

3.1 The difference between anchoring force and pulling force

(1) The anchoring force is the binding force generated by the bolt on the surrounding rock, and it is the force that limits the deformation of the surrounding rock and plays a supporting role. The pull-out force of the bolt is the ultimate load that can be endured during the pull-out test after the bolt is anchored.

(2) The anchoring force increases with the deformation of the supported surrounding rock and the expansion of the surrounding rock, so the anchoring force is a dynamic and constantly changing force. The bolt pulling force is a fixed value and does not change with the deformation of the surrounding rock and the stress on the bolt. If the surrounding rock is not deformed and the relaxation effect of the rod body is not considered, the anchoring force is equal to the initial anchoring force.

(3) The anchoring force detection uses the bolt dynamometer installed between the bolt nut and the pallet. Generally, the bolt dynamometer is installed when the bolt is installed. The purpose of detecting the anchoring force is to monitor the stress condition of the bolt, and long-term observation is required. The bolt pull-out force is detected using the bolt tension meter. The test can be carried out at any time after the bolt is installed. The purpose of detecting the bolt pull-out force is to check the bolt body, anchoring agent and rock bonding effect. In construction, when testing the pull-out force of the anchor rod, generally as long as the design anchoring force is reached; in the case of destructive testing, the anchor rod is required to be broken or the anchor rod is pulled out before termination.

(4) When checking the construction quality of the bolt, generally check the pull-out force of the bolt. When monitoring and analyzing the working condition of the bolt, measure the anchoring force. The purpose of measuring the anchoring force is to verify the reliability of the support and to provide a basis for modifying the support design in the future. During design and construction, the basic principle that the pull-out force of the anchor rod is greater than the breaking force of the rod body must be ensured, that is, after the force of the anchor rod body exceeds its breaking force, the anchor rod may be pulled off, but the anchor rod cannot be pulled out. A common mistake is that the pullout force of the designed anchor is less than the breaking force of the rod.

3.2 Relationship between preload and preload torque

(1) The simplified relationship between bolt preload and preload torque is: M=k·T·d, where M is the preload torque, N·m; k is the proportional coefficient, and T is the preload force, kN; d is the diameter of the rod body, mm; the proportionality coefficient k is a comprehensive coefficient, which is affected by the comprehensive influence of the thread lift angle, the pitch, the friction coefficient of the screw pair, and the friction coefficient between the nut and the supporting surface, so it is used to determine the preload and preload. The key to the relationship between the tight torque.

(2) The pre-tightening force is the force, which is the pulling force exerted on the anchor rod (anchor cable), in kN; the pre-tightening moment is the moment, which is applied on the compression nut, in the unit of N m.

(3) The two measuring instruments are different. The pre-tightening force can be observed by the bolt dynamometer installed between the bolt tray and the nut; the pre-tightening torque can be observed by the digital display or the bolt torque wrench with scale display.

(4) The bolt construction design requires pre-tightening force, not pre-tightening moment. However, in actual construction, the measurement of the preload force is relatively complicated due to the convenient measurement and reading of the preload torque, and the preload force increases with the increase of the preload torque. In order to facilitate the detection, indirect detection is achieved by directly detecting the preload torque. The purpose of the preload of the anchor. Therefore, the pre-tightening moment is usually detected when the bolt is installed, but the pre-tightening force is not detected.

3.3 Difference between preload and prestress

Anchor bolt pre-tightening force: the axial tension applied to the bolt body during the bolt installation process.

Anchor prestress: the ratio of the anchor preload to the cross-sectional area of the rod.

4.1 The role of anchoring force

The anchoring effect of the bolt is reflected in the radial and tangential anchoring forces. The radial anchoring force exerts confining pressure on the surrounding rock, transforming the surrounding rock from a one-way and two-way stress state to a two-way and three-way stress state, improving the stability of the surrounding rock. The bolt penetrates the weak surface in the same rock layer, and the tangential anchoring force improves the mechanical properties of the weak surface, thereby improving the mechanical properties of the surrounding rock. Therefore, the bolt is a more perfect form of support that has both the functions of support and reinforcement. The radial anchoring force mainly plays a supporting role, and the tangential anchoring force mainly plays a reinforcing role. In the surrounding rock of the coal roadway, it is mainly the radial anchoring force that plays a supporting role. It is specifically manifested in the following aspects:

(1) Reinforcing the arch. For the massive or fractured surrounding rock cut by the criss-crossed weak planes, if it is reinforced with bolts in time, the shear strength of the rock mass structure can be improved, and within a certain thickness around the surrounding rock, a rock mass can be formed that can not only maintain its own Stable, and can prevent the upper surrounding rock from loosening and deformation of the reinforced arch, so as to maintain the stability of the roadway.

(2) Suspension effect. Suspension means that the rock bolt suspends the weak rock formation or dangerous rock that is about to fall on the solid and stable rock above, and the bolt bears the weight of the dangerous rock or weak rock formation.

(3) The action of composite beams. In the roadway roof of the layered rock layer, a series of bolts are anchored, and the thin rock layer within the anchoring length of the bolts is formed into a certain composite beam, thereby improving its bearing capacity. The layered rock roof of the flat-roofed roadway can be regarded as a superimposed beam with the two sides of the roadway as the fulcrum. Under the load, each rock layer has its own individual bending moment, and each layer of rock is in compression and tension respectively. After the rock bolts combine various layers of rock together, the bending strength and bearing capacity of the composite beam are greatly improved.

4.2 The role of preload

(1) The pre-tightening force can play the active supporting role of the bolt, especially under the conditions of layered rock formation and broken surrounding rock, increasing the pre-tightening force can change the properties of the surrounding rock, prevent the damage of the surrounding rock, and maintain the stability of the surrounding rock. Conducive to surrounding rock support.

The bolt preload has a decisive effect on the stability of the roof. When the pre-tightening force is large to a certain extent, the roof separation layer within the length of the bolt and above the length of the bolt can be eliminated.

The high preload bolt is designed to establish a prestressed roof. The existence of the prestressed roof protects the roof from the damage of the horizontal stress to a certain extent, so that the roof rock layer is in a state of lateral compression to overcome the high level stress on the roof impact on stability.

(4) The formation of the prestressed structure is conditional, and the bolt preload is the key. Under the condition of large horizontal stress, the function of the anchor is to provide a high prestress to the roof in time to form a prestressed roof and form a pressure self-supporting structure.