Several force concepts related to bolt support
Jun 05, 2021| Several force concepts related to bolt support
1 definition
Anchoring force: refers to the restraining force of the anchor rod on the surrounding rock, which includes radial anchoring force and tangential anchoring force, radial anchoring force includes supporting anchoring force and sticking anchoring force.
The anchoring force is the radial supporting 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 anchor rod on the surrounding rock through the binder;
The tangential anchoring force is the limiting force caused by the bolt body penetrating the weak surface of the rock mass to the sliding and opening of the weak surface; the unit is kN.
Pull-out force: refers to the force that prevents the bolt from pulling out of the rock mass. The drawing force can be divided into the design drawing force and the detection drawing 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 anchor rod; the unit is kN.
Anchor rod pre-tightening force: the axial tension applied to the anchor rod body during the installation of the anchor rod, in kN.
Anchor rod pre-tightening torque: the torque applied to the anchor rod nut during the installation of the anchor rod, in N·m.
Anchor rod prestress: during the installation of the anchor rod, the axial tensile stress applied to the anchor rod body is equal to the ratio of the anchor rod pretension force to the cross-sectional area of the rod body, in MPa.
2Measurement method
2.1 Anchor force test
The anchoring force of the bolt is generally carried out with a dynamometer. At present, the ML-20/ML-30 bolt tension meter is generally used downhole.
Installation process of dynamometer:
①Check the oil pipe joints of the workbench hydraulic pressure gauge in the tension gauge;
②The anchor rod connecting rod (internal thread) is directly sleeved on the thread of the end of the anchor rod, and the screw thread is not less than 30mm;
③Install the sleeve, the sleeve is close to the anchor rod tray, and then the jack is installed (one end of the telescopic cylinder is outwards, close to the nut), and the nut is tightened with a wrench;
④Connect the tubing to the jack
On top
⑤ Tighten the switch knob;
⑥ Pressurize the operating handle at a constant speed and always pay attention to the pressure gauge until the designed anchoring force is reached, and then slowly turn on the switch to release the pressure.
Precautions:
(1) Choose a position where the rock coal is level and no broken phenomenon on the tray;
(2) The backing plate should be as flat and rigid as possible;
(3) The exposed length of the bolt thread is between 25~40mm, and the bolt should be perpendicular to the coal rock surface; the center line of the jack shaft connecting rod bolt body is consistent;
(4) Check the integrity of the dynamometer before pressurizing (working medium pipeline pressure gauge, rotary jack, etc.);
(5) The pressure of the bolt dynamometer should be uniform and slow until the bolt is loose or the pressure gauge reads to the bolt design anchoring force value, and the destructive test is generally not performed;
(6) When the jack is releasing pressure, slowly loosen the switch knob;
(7) During the pressure test, no one is allowed 3m near the anchor rod to be inspected;
(8) If the resin anchor rod 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 anchoring force of bolts.
2.2 Anchor rod pulling force detection
Anchor rod drawing gauge is the most commonly used bolt drawing force testing instrument. At present, several commonly used anchor drawing gauges in our country include MLJ-300/100 anchor drawing gauge, MJY-1 non-destructive anchor drawing gauge, ZY series anchor drawing gauge, etc.
There are the following requirements for the bolt pull test:
The bolt drawing experiment is completed in the underground roadway by using the bolt drawing gauge;
The test site should be selected in the underground roadway construction site or similar surrounding rock;
The anchor rods and anchoring agents used in the test should be the same as the materials used in the formal construction;
The equipment and drilling parameters used in the test should be the same as those in the formal construction;
The test should be in the form of short anchors (such as anchors with a length of 150~200mm) in order to test the bonding strength of the anchors;
The test is a destructive test. It is pulled until the bolt fails, and the load and the displacement of the tail of the bolt are recorded during the test;
According to the curve of maximum load and displacement, 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 anchor rod pull-out test must be performed again:
The bolt support design is changed;
Changes in supporting materials;
The geological conditions of the surrounding rocks of the roadway have undergone major changes, such as faults, broken zones, folds and other geological structures; large water sprays appear on the roof of the roadway, etc.
2.3 Anchor rod pre-tightening torque test
A torque wrench is generally used to detect the pre-tightening force of the anchor rod. The detection of anchor rod pre-tightening force shall meet the following requirements:
A group (3) is sampled from the top of each sub-class for bolt nut torque testing. The pre-tightening torque of each nut shall meet the design requirements;
If one nut in each group fails to meet the required torque, another group (3 pieces) shall be spot-checked. If any unqualified ones are still found, re-tighten all the nuts in the class and overhaul them again.
3 The relationship between several confusing forces
3.1 The difference between anchoring force and pulling force
(1) Anchoring force is the binding force produced by the bolt on the surrounding rock, and it is the force that restricts the deformation of the surrounding rock and plays a supporting role. The pull-out force of the bolt is the ultimate load that the bolt can withstand during the pull-out experiment after the bolt is anchored. It reflects the maximum pull force that the bolt breaks or fails after the rod body, anchoring agent, and rock are bonded together.
(2) The anchoring force increases with the deformation and expansion of the surrounding rock being supported. Therefore, the anchoring force is a dynamically developing and constantly changing force. The pull-out force of the bolt is a fixed value and does not change with the deformation of the surrounding rock and the force of the bolt. If the surrounding rock does not deform and does not consider the relaxation effect of the rod body, the anchoring force is equal to the initial anchoring force.
(3) The anchoring force test uses the anchor dynamometer installed between the anchor nut and the tray. Generally, the anchor dynamometer is installed when the anchor is installed. The purpose of detecting the anchoring force is to monitor the force of the bolt, and long-term observation is required. The bolt pull force test uses the bolt tension meter. The test can be carried out at any time after the bolt is installed. The bolt pull force is tested to check the bolt body, anchoring agent, and rock bonding effect. In the construction, when testing the pulling force of the anchor rod, it is generally only necessary to reach the designed anchoring force; in the destructive test, the anchor rod is required to be pulled off or the anchor rod is pulled out to terminate.
(4) When checking the construction quality of the anchor rod, generally check the pulling force of the anchor rod. When monitoring and analyzing the working conditions of the anchor rod, measure the anchoring force. The measurement of anchoring force is to verify the reliability of the support and provide a basis for future revision of the support design. During the design and construction, the basic principle that the pulling force of the anchor rod is greater than the breaking force of the rod must be ensured, that is, after the force of the anchor rod exceeds its breaking force, the anchor rod may be broken, but the anchor rod cannot be pulled out. A common mistake is that the pulling force of the designed anchor rod is less than the breaking force of the rod body.
3.2 Relationship between pre-tightening force and pre-tightening torque
(1) The simplified relationship between anchor rod pre-tightening force and pre-tightening torque is: M=k·T·d, where M is the pre-tightening torque, N·m; k is the proportional coefficient, and T is the pre-tightening force, kN; d is the diameter of the rod, mm; the proportional 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 of the nut and the supporting surface. Therefore, it is the determination of the pre-tightening force and the pre-tightening force. The key to the relationship between tight torque.
(2) The pre-tightening force is the force, which is the tensile force applied to the anchor rod (anchor cable), in kN; the pre-tightening moment is the moment, which is applied to the compression nut, in N·m.
(3) The two measuring instruments are different. The pre-tightening force can be observed through the bolt dynamometer installed between the anchor rod tray and the nut; the pre-tightening torque of the service professional can be observed through the digital display or the bolt torque wrench with scale display.
(4) The anchor rod construction design requires pre-tightening force, not pre-tightening moment. However, in actual construction, because the pre-tightening torque is easy to measure and the pre-tightening force measurement is relatively complicated, and the pre-tightening force increases with the increase of the pre-tightening torque, for the convenience of detection, indirect detection is achieved by directly detecting the pre-tightening torque. The purpose of the pre-tightening force of the anchor rod. Therefore, when the bolt is installed, the pre-tightening torque is usually detected, but the pre-tightening force is not detected.
3.3 The difference between preload and prestress
Anchor rod pre-tightening force: the axial tension applied to the anchor rod body during the installation of the anchor rod.
Anchor rod pre-stress: the ratio of anchor rod pre-tightening force to the cross-sectional area of the rod body.
4.1 The role of anchoring force
The anchoring effect of the anchor rod is embodied in the action of radial and tangential anchoring forces. The radial anchoring force exerts confining pressure on the surrounding rock, transforms the surrounding rock from a unidirectional and bidirectional force state to a bidirectional and three-directional force state, and improves the stability of the surrounding rock. The anchor rod penetrates the weak surface in the same rock formation, and the tangential anchoring force improves the mechanical properties of the weak surface, thereby improving the mechanical properties of the surrounding rock. Therefore, the anchor rod is a more perfect support form with both 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 coal roadways, the radial anchoring force is mainly used for supporting. The specific performance is as follows:
(1) The role of strengthening the arch. For the massive or fractured surrounding rock cut by crisscross weak surfaces, if it is reinforced with bolts in time, the shear strength of the rock mass structure can be improved, and it can not only maintain itself within a certain thickness of the surrounding rock. 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. The suspension function means that the anchor rod suspends the weak rock layer or dangerous rock that is about to fall on the firm and stable rock above, and the anchor rod bears the weight of the dangerous rock or weak rock layer.
(3) The role of composite beams. In the roof of the roadway of the layered rock layer, a series of bolts are anchored to form a certain composite beam by anchoring the thin rock layer within the length of the bolts to improve its bearing capacity. The layered rock roof of a flat roof roadway can be regarded as a superimposed beam with the two sides of the roadway as the fulcrum. Under the action of load, each rock layer has its own individual bending moment, and each rock layer is under compression and tension. After the anchor rod combines the various layers of rock, 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 role of anchor bolt active support, especially in the condition of layered rock formations and broken surrounding rocks. Increasing the pre-tightening force can change the nature of the surrounding rock, prevent damage to the surrounding rock, and maintain the stability of the surrounding rock. Conducive to supporting the surrounding rock.
The pre-tightening force of the anchor rod has a decisive effect on the stability of the roof. When the pre-tightening force is large to a certain extent, the roof separation within and above the anchor rod length can be eliminated.
The high pre-tightening force anchor rod is designed to establish a pre-stressed roof. The existence of the pre-stressed roof protects the roof from horizontal stress to a certain extent, so that the roof rock layer is in a state of lateral compression to overcome the high level of stress on the roof. The impact of stability.
(4) The formation of the prestressed structure is conditional, and the anchor bolt pretension is the key. Under the condition of large horizontal stress, the role of the anchor rod is to provide high prestress to the top plate in time to form a prestressed top plate, forming a pressure self-supporting structure.