Study on Pretightening Loss Effect of Bolt Support in Deep Soft Rock Roadway

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Introduction
With the gradual depletion of resources in the shallow mining areas in the east, the focus of coal mining gradually extends deep and develops to the western mining areas.Weak coal rock is widely distributed in the western mining area, which has the characteristics of low strength, large plasticity, and easy breakage [1][2][3].After the excavation of deep soft coal roadway, the deformation speed of surrounding rock is fast, the damage range is large, the initial support resistance of roadway is small, and it is difcult to efectively control the deformation of surrounding rock of the roadway.
Aiming at the problem of insufcient initial support strength of roadway, Sun et al. [4] systematically revealed the instability mechanism of deep coal roadway and provided some schemes to maintain the stability of roadway, which can signifcantly promote the sustainable development of mining industry.Tey analyzed the mechanical properties and supporting mechanism of bolt [5] and developed various forms of bolt [6,7], high prestressed bolt, and other supporting methods [8,9] and control methods of surrounding rock deformation [10].Trough feld research, it was found that when the bolt support resistance reached more than 40 kN, the roadway deformation could be effectively controlled [11].However, it also faces some severe challenges [12].As an important parameter of bolt support, pretightening force plays an important role in controlling roadway deformation [13][14][15][16].For the study of the efect of bolt pretightening force support, scholars analyzed the stress feld distribution characteristics of surrounding rock under diferent pretightening forces through numerical comparison experiments and proposed that improving the pretightening force of bolt can efectively improve the stifness of anchor body [17,18] and strengthen the antidynamic load impact performance of anchor body [19][20][21].Guo et al. [22,23] studied the efect of roadway support during mining and used the statistical analysis theory and exponential function to express the convergence model to simulate the stress characteristics of the main roadway.Hyett et al. [24] studied the bearing capacity of rod bodies with diferent lengths under diferent rock conditions through indoor experiments.Kang [25] introduced the case study of the bolt support system for soft rock reinforcement, which is helpful to improve the engineering application of bolt support in long-wall roadway excavated in soft rock mass.Ru and Wenkai [26] took Gaojialiang Coal Mine in western China as the research object and studied the failure mechanism of roadway roof under residual coal pillars in shallow and close multiseam combined with feld monitoring and numerical simulation.In order to evaluate the design of roadway bolt support, Zhang [27] carried out bolt stress monitoring on several sections of roadway during roadway excavation and studied the parameters of anchorage length and bolt spacing by the numerical model.Te pretightening force of the bolt is generated by applying the pretightening torque.For this reason, scholars have established the mechanical model of the bolt, analyzed the infuencing factors between the pretightening force and the pretightening torque of the bolt, obtained the theoretical relationship between the pretightening force and the pretightening torque, and verifed the accuracy of the relationship in the laboratory test [28].In practical engineering, the pretightening force of bolt is affected by many factors.Aiming at the problem of conversion efciency between pretightening force and torque of bolt, scholars [29][30][31][32] have carried out experimental research on diferent thread parameters of bolt tail, obtained the relationship between diferent thread structure parameters and mechanical properties of bolt thread section, and put forward the control measures of pretightening force of bolt in coal mine.Experts and scholars [33][34][35][36][37] provide experience for this study through numerical simulation research and analysis.At present, there are many studies on the relationship between the bolt preload and preload torque.Tian et al. [38] proposed a method to identify the stifness parameters of bolt connection related to preload torque.Afzali et al. [39] believe that in terms of preload loss, the preload loss of pretightened stainless steel bolt connection is similar to that of carbon steel connection because their preload loss is equivalent.However, there is a loss in the pretightening force of the bolt during the on-site construction process, which makes the actual pretightening force deviate from the theoretical pretightening force.Te initial support resistance of the bolt is insufcient, and its support efect has not been fully utilized.Te quantitative relationship between the bolt pretightening force and the pretightening torque under the condition of deep soft coal roadway needs to be further studied.
In this paper, feld investigation, laboratory test, theoretical analysis, numerical simulation, and feld test are used to study and obtain the deformation characteristics of the roadway side, analyze the causes of roadway side deformation, establish a mechanical model to derive the relationship between pretightening force and pretightening torque, and obtain the relationship between actual pretightening force and pretightening torque of deep soft rock roadway through feld test.Te theoretical value is corrected to determine the pretightening torque value required by the design pretightening force corresponding to the feld, and the feld application test is carried out.

Deformation Failure Analysis of Deep Weak Coal
2.1.Engineering Geology.Hongqingliang Coal Mine mainly mines 3-1 coal seam at present.Te buried depth of the mining coal seam is 467∼533 m, and the inclination angle of the mining coal seam is 0∼6 °.Te coal seam structure is relatively simple and stable, and the endogenous fssures are more developed.Te hydrogeological conditions of the mine are simple, and there is water leaching in some areas.At present, the roadway of 11307 working face of 3-1 coal seam is excavated.Four months after the end of the mining of the 11302 working face adjacent to the south, the 11307 return airway began to be dug.Te 11307 return air roadway is 50 m away from the goaf of 11302 working face, and the northern working face has not started mining.Te relationship between each working face and roadway is shown in Figure 1.
Te coal seam thickness of 11307 working face is 2.1∼6.3 m, with an average of 4.3 m.Te roof is mainly sandy mudstone and conglomerate, and the foor is mainly sandy mudstone and siltstone.Te maximum uniaxial compressive strength of coal measured in the borehole of underground roadway is 18.5 MPa, and the uniaxial compressive strength of sandy mudstone is concentrated in 14.3∼27.2MPa.Te lithology of roof and foor in the excavation stage of 11307 working face is shown in Table 1.

Support Condition.
Te 11307 tail entry is 5.0 m wide and 3.6 m high, with "anchor-net-cable" support.Te anchor rod mainly adopts φ22 × 2600 mm deformed steel fne wire anchor rod with a row spacing of 700 × 1000 mm; the row spacing between roadway sides is 800 × 1000 mm and tray is 200 × 200 × 10 mm dish tray.Te anchor cable mainly adopts φ21.8 × 7300 mm high-strength shear anchor cable, arranged in 3-3-3, with spacing of 1500 × 2000 mm.Te bolt pretightening force is designed to be 60 kN, and the applied pretightening torque is designed to be 240 N•m.Te specifc support section is shown in Figure 2. (1) Te overall subsidence of the roof is fast and the subsidence is large, about 372 mm.Te subsidence of the local severe area can reach 454 mm, and the coal body is broken.(2) Te two sides have a large amount of drum, and the gob side is particularly serious.Te maximum displacement can reach 353 mm.Te deformation of the side mesh is more serious, the deformation of the mining side is relatively small, and the displacement reaches 114 mm.(3) Te amount of foor heave is about 196 mm, and the cracking of the foor in some areas is serious, up to 500 mm.

Monitoring and Analysis of the Roadway Mine Pressure
Law.During the excavation of 11307 return air roadway, a group of measuring points are set every 100 m, and a total of 5 groups are arranged to measure the deformation of roof and foor, gob side, mining side, and bolt working resistance, respectively.Te average displacement of monitoring results and the working resistance curve of bolt are shown in Figure 4.
(1) Te overall surrounding rock of the roof, foor, and mining side is relatively complete, and the displacement is relatively small.After the roadway is excavated for 45 m, the deformation of both sides is basically stable, and the displacement is 130 and 85 mm, respectively.Te deformation along the goaf is far greater than that of the roof, foor, and mining side, and after the latter tends to be stable, the deformation along the goaf still continues.After the roadway is excavated for 70 m, the deformation along the goaf basically tends to be stable, and the displacement reaches 350 mm.(2) After the bolt is installed and the design pretightening torque is applied, the pretightening force along the goaf side and the mining side are 35 kN and 42 kN, respectively.With the increase of the advance distance, the working resistance of the goaf side bolt is gradually increased.Te working resistance of the goaf side bolt is less than the mining side.After 40 m of roadway excavation, the working resistance of the mining side bolt basically tends to be stable, with the size of 103 kN.At this time, the supporting resistance of the goaf side bolt continues to rise and tends to be stable after 80 m of roadway excavation, with the size of 116 kN.

Analysis of Infuencing Factors of Deformation and Failure
of Roadway Surrounding Rock.Based on the abovementioned summary and analysis of the destruction characteristics of the 11307 tail entry in Hongqingliang Coal Mine, it is concluded that the deformation causes of the 11307 tail entry are as follows: (1) Te surrounding rock strength of roadway is low.Hongqingliang Coal Mine belongs to deep soft rock roadway, which has broken coal and rock mass, low strength, and developed joints.In the process of roadway excavation, there is water spray in this area, and the surrounding rock is cementated, which reduces the bearing capacity of surrounding rock and makes the roadway prone to deformation and failure.(2) High lateral bearing pressure.Because the 11307 tail entry is close to the goaf of the 11302 working face, the pressure on the coal body is increased and the overall physical and mechanical properties are weakened, especially the stress and deformation along the goaf are more serious due to the support pressure on the side of the goaf.(3) Te pretension of the anchor rod is low.In the process of roadway support, the pretightening force of the bolt is low, which weakens the active and timely support function of the bolt, and cannot meet the requirements of roadway stability.When the surrounding rock deformation is damaged to a certain extent, the working resistance to meet the requirements can be reached, which is the main reason for the surrounding rock deformation.

Analysis of the Relationship between Pretightening Force and Pretightening
Torque of the Anchor Rod.According to the pretightening torque applied to the bolt, the bolt   (a)

Overall roof sinking
The side mesh deformation is serious.relationship between the torque applied by the bolt and the preload is obtained.Te mechanical model of the bolt pretightening support member is shown in Figure 5.
Trough the analysis of the friction force between the thread, the tray, the nut, and the contact surface, the relationship between the applied external torque and the axial force of the rod body is obtained as follows: Terefore, the relationship between preload and preload torque is In the formula, M is the preload torque.T is the pretightening force of bolt.μ 2 is the friction coefcient between threads.R is the outer radius of the nut.r is the radius of the anchor rod body.θ is the thread angle.μ 1 is the friction coefcient between nut and tray.λ is the conversion coefcient of pretightening force, which is determined by the bolt diameter, thread pitch, and friction coefcient.
According to the analysis of the abovementioned derivation formula, it can be seen that the pretightening force of the anchor rod is related to the strength of the anchor rod, the diameter of the anchor rod, the rising angle of the tail thread, the nature of the anchor body, and the friction force of each part of the component.However, the pretightening force of the anchor rod is afected by many factors.Te relationship between torque and force cannot accurately express the actual pretightening force of the anchor rod, and there is a loss of pretightening force in the pretightening process of the anchor rod.

Bolt Pretightening Loss Analysis.
Te relationship between pretightening force and torque of rebar bolt in 11307 return airway is tested.Te results are shown in Figure 6.F1∼F3 are the pretightening force test results of the same anchor rod during multiple pretightening.However, in the feld test, the pretightening force test results are still diferent when the same anchor rod and the pretightening torque application conditions are consistent.With the increase of the number of pretightening times, the pretightening force of the anchor rod gradually decreases.
It can be seen from the test curve that the preload force decreases gradually after repeated preload torque is applied, indicating that the repeated preload will cause preload loss.Te surrounding rock contains discontinuous structural planes such as joints and fssures.During the pretightening 6 Shock and Vibration process, a certain pretightening torque is applied to the nut, and the tray produces pressure on the surrounding rock so that the surrounding rock is compressed and deformed, and the structural plane is pressed and closed.Te surrounding rock continues to deform under the pressure of the tray, causing gaps in the contact surface between the tray and the surrounding rock, which in turn leads to a decrease in the pretightening force of the bolt.Compared with the theoretical calculation results, there is a big diference between the measured value and the theoretical value of the bolt pretightening force, indicating that there is a pretightening force loss during the pretightening process of the bolt.By comparing the theoretical pretightening force with the measured pretightening force, the pretightening force loss and loss coefcient are calculated, and the pretightening force and pretightening torque formula are modifed.Te loss coefcient is the ratio of the loss to the calculated value of the preload.
In the formula, T is the pretightening force calculation value, T 0 is the measured value of preload, and k is the coefcient of preload loss.
Trough the analysis of the relationship between the pretightening torque and the pretightening force, it can be seen that in the actual process, the applied pretightening torque cannot be completely converted into pretightening force, and the friction between the bolt support components reduces the pretightening force.By introducing the commonly used bolt parameters, the loss caused by the bolt pretightening process is obtained.Te diameters of common underground bolts are 16 mm, 18 mm, 20 mm, and 22 mm.Te specifc specifcations of the nuts used are listed in Table 2.
Te common bolt parameters with the thread angle of 5∼15 °, the friction coefcient between the nut and the tray 0.08∼0.18,and the friction coefcient between the threads 0.05∼0.15are brought into the calculation formulas ( 2) and ( 4).Te specifc calculation results are shown in Table 3.
Te abovementioned analysis shows that the loss of pretightening force caused by friction is inversely proportional to the diameter of the rod when the thread and working conditions of the bolt are consistent.Terefore, when the pretightening torque is the same, the selection of a larger diameter bolt will reduce the loss of bolt pretightening force.In the process of underground bolt installation, there is not only the loss caused by friction but also the infuence of deformation and failure of surrounding rock [25], thus reducing the degree of tightness between surrounding rock, tray, and nut so that the bolt preload is reduced, which needs to be further determined.

Study on Loss Characteristics of Bolt
Pretightening Force

Simulation Study on Bolt Preload Loss.
A numerical simulation model of a cylinder with a diameter of 300 mm and a height of 300 mm is established, which is composed of anchor body, tray, rod body, and nut.In this model, 30 mm holes are reserved inside the model, and the tray is simplifed to a plane tray of 150 × 150 × 10 mm, with a tray hole of 28 mm, as shown in Figure 7.One end of the anchor body is preloaded by nuts and trays, and the other end is fxed to the boundary.ABAQUS is used to establish a three-dimensional spatial node model.Te length, load, mass, time, and stress measurement units in the model are mm, N, Ton, S, and MPa, respectively.Te anchorage specimen, bolt, and tray are all created in the part module by using the 8-node hexahedronreduced integral solid element, and the anchorage body specimen, bolt rod body, bolt nut, and tray member are created by stretching and moving methods.Te property module defnes component properties.Te elastic modulus of nuts, trays, and rods is 2.1 × 105 MPa, and the Poisson's ratio is 0.25.Te joint between the anchored solid specimen and the bottom of the rod body is completely fxed in all translational degrees of freedom, and the boundary conditions begin to play a role in the initial analysis step.At the same time, in order to prevent the rod body from being afected by the rotation of the nut, the joint of the cylindrical surface of the rod body is limited in the rotational degree of freedom.Te fnite element model adopts the static analysis Shock and Vibration mode, and the analysis step setting program is " static, general."By setting the constraint point in the interaction, the outer surface of the coupling nut is set, and the load is applied to the constraint point by setting the torque, and the value is 350000 N•mm.
In order to study the infuence of rod diameter and thread pitch on the pretightening force of the anchor rod, the anchor rods with common rod diameters of 18 mm, 20 mm, and 22 mm and thread pitch parameters of 2 mm, 4 mm, 6 mm, and 8 mm were selected for combination and classifcation, and the relationship between pretightening force and pretightening torque of diferent specifcations of the anchor rod was simulated.
Te simulation results are shown in Figures 8∼10: As shown in Figure 11, the results show that the loss of bolt pretightening force is linearly related to the pretightening torque and inversely proportional to the bolt diameter and thread pitch.With the increase of bolt diameter or pitch, the bolt pretightening force decreases gradually and the loss of pretightening force decreases.

Test Area Selection and Test Instruments.
In order to obtain the loss of bolt pretightening force caused by surrounding rock deformation, the roadway side of 11307 return air roadway was tested.As shown in Figure 12, the SATA (SATA)3/4″G series preset torsion wrench 100 N•m-550 N•m, MCS-400 mine intrinsically safe anchor dynamometer is selected.

Analysis of Field Test Results
. Te diameter of the thread steel fne wire anchor rod is 22 mm, the height of the longitudinal rib of the rod is 0.85 mm, the spacing is 12 mm, the length is 2600 mm, the thread length is 300 mm, and the pitch is about 3 mm.Te relationship between the pretightening force and the pretightening torque of the bolt is shown in Table 4, and the results are shown in Figure 13.
Te parameters of the screw steel wire anchor are brought into the formula (2): μ 1 � 0.12, μ 2 � 0.15, R � 0.024 m, r � 0.011 m, and θ � 2.5 °to calculate the theoretical value of the pretightening force of the anchor.Comparing the test results, the pretightening force loss of the bolt is calculated, as shown in Figure 14, and the specifc calculation results are listed in Table 5.
In the actual pretightening process, the loss of threaded steel thin wire anchor rod is linearly related to the pretightening torque.With the increase of pretightening torque, the loss of anchor rod pretightening force increases gradually.By comparing with the theoretical calculation results, the bolt preload loss coefcient is calculated, and the results are shown in Figure 15.
Te pretightening force loss coefcient is 0.19∼0.43.According to the test of the physical and mechanical properties of the surrounding rock in the previous study, the elastic modulus of the surrounding rock is 1367 MPa, which is basically consistent with the pretightening force loss coefcient obtained from the indoor test and numerical simulation results.

Engineering Application
5.1.Bolt Pretightening Support Design.After obtaining the relationship between the actual bolt pretightening torque and the pretightening force, it is necessary to determine the reasonable pretightening force in the feld application  8 Shock and Vibration process so as to apply the corresponding pretightening torque.Te design pretightening force of the bolt should be greater than 60 kN.According to the relationship between the modifed pretightening torque and the pretightening force, when the pretightening force reaches 60 kN, the pretightening torque is 300 N•m.Shock and Vibration

Application Efect.
Te designed pretightening torque is applied on the heading face of 11307 return air roadway, and the working resistance of bolt and roadway deformation are monitored, as shown in Figure 16.
After the pretightening torque is adjusted to 300 Nm, the pretightening force of the bolt is about 60 kN.As the working face advances, the force rises faster.When the roadway is excavated for 30∼40 m, the bolt support resistance of the two sides tends to be stable, which is 78∼115 kN.Te deformation of the surrounding rock tends to be stable, and there is no obvious continuous increase.During the excavation of the roadway, the displacement of the roof and foor is 34 mm, the displacement of the gob side is 80 mm, and the displacement of the mining side is 52 mm.
As shown in Figure 17, after the actual pretightening force of the bolt is increased, the overall resistance to deformation and failure of the surrounding rock is improved.From the control efect, the overall deformation of the roadway is small, and the stability of the roadway is efectively controlled.

2 Shock and Vibration 2 . 3 .
Deformation and Failure Characteristics of the Return Airway Wall 2.3.1.Overall Failure Characteristics.Te roof and foor of 11307 return air roadway are mainly sandy mudstone or siltstone with low strength, with well-developed bedding and poor cementation between layers.As shown in Figure3, the overall deformation and failure of the roadway after a period of time is more serious, which are manifested as follows:

Figure 3 :
Figure 3: Deformation and failure characteristics of surrounding rock.(a) Roof deformation bolt failure.(b) Te overall sinking of the roof and the bulging of the side.(c) Overall deformation and failure of roadway.

Figure 4 :
Figure 4: Monitoring curve of surrounding rock displacement and bolt preload during excavation.(a) Roadway displacement.(b) Working resistance monitoring of the anchor rod.

Figure 6 :Figure 5 :
Figure 6: Comparison of theoretical calculation and test results.

Figure 13 :
Figure 13: Test results of threaded steel fne wire bolt.

Figure 15 :
Figure 15: Relationship between pretension force coefcient and moment of pretension.

Figure 14 :
Figure 14: Relationship between pretension force loss and moment of pretension of threaded steel fne wire bolt.

Table 4 :
Test results of the ribbed steel wire anchor rod.

Table 5 :
Calculation of pretightening force and loss of thread steel fne wire bolt.