Cable Bolts Research Articles

Cause and Solution to Roadway Deformation in Vietnam Underground Coal Mines

The deformation and support method of roadways have always been important issues in safemining and production. Vinacomin's statistics show that, by 2021, there will be 64.19 km of roadwaysthat need to be repaired (accounting for 25% of the total new roadways). Thus, the problem of maintainingroadway stability is facing difficulties in underground coal mines in Vietnam. To find out the causes ofroadway failures, a case study at roadways of the Khe Cham I and Khe Cham III coal mines, Vietnam, ispresented in this paper. Based on the results of a detailed field survey, the deformation characteristics ofroadways and the failure mode of support structures were investigated. The results show that the roadwaydeformation is severe and the main support cannot control surrounding rock mass. Also, the destructionof support structure is frequent on reused roadways, affecting production efficiency and work safety.Therefore, to reduce deformation and increase roadway stability, a new support method called “multistageanchor of rock bolt + cable bolt” has been developed and a new longwall mining system with criticalcoal pillar width has been proposed. The new findings of the research can provide references for scientificstudies, and apply them in Vietnam's underground coal mine practices.

Stabilization of Deep Roadways in Weak Rocks Using the System of Two-level Rock Bolts

Large rock mass deformation around deep roadways in the weak rocks was a significantproblem in mining activities in Vietnam and other countries. The excavation of roadways leads to highreleasing stress, which exceeds the peak strength of spalling surrounding rock and causes it to enter thepost-failure stage. Tensile failures then initiate and develop around the roadways, which causes thefragmentation, dilation, and separation of surrounding rock. The capacity of the primary support systemis low, which results in a severe contraction in the whole section of roadways, which requires findingsolutions to prevent the deformation of rock mass around roadways and technical solutions fromstabilizing for deep roadways. To stability analysis of roadways can be applied analytical, experimental,semi-experimental, and numerical methods. This paper introduces the prevention mechanism of largedeformation of rock mass around roadways using 2-level rock bolts. The research results show that usingthe system of two-level rock bolts can reduce the values of tensile stress on the boundary of roadwaysrange from 10 to 15% compared with only one. The importance of the total displacement of rock mass onthe boundary of roadways will be reduced from 3.47 to 13.85% using six long cable bolts.

Microbiologically influenced stress corrosion cracking responsible for catastrophic failure of cable bolts

In the past two decades, reports of the premature failure of cable bolts used in the mining and civil industries have been increasing. Previous studies have established that failure occurs through hydrogen-induced stress corrosion cracking (HISCC), which is a type of environmentally assisted hydrogen cracking. However, to date, the cause of HISCC has been unclear. For the first time, we studied the role of microorganisms in the failure of cable bolts using components present in SCC-affected mines. Stressed coupons were prepared from the cable bolt wires and tested in groundwater with additions of sulphate-reducing bacteria, coal, clay, pyrite and lactate. It was found that hydrogen sulphide (H2S) produced by sulphate- and sulphur-reducing bacteria promoted hydrogen diffusion into the steel and, in the presence of stress, caused HISCC. This suggests that control of H2S production should be a priority for mining and civil industries to avoid premature failure of anchoring systems.

Research on Anchor Cable and C-Shaped Tube Support Method in Deep Layers Roadway, Experimental Study, and Numerical Simulation

In roadways with high ground stress or burial depths, the joints distributed within rock formations are subject to complex stresses and interlayer misalignments frequently. Rock bolts and cable bolts anchored in the rock formations are subject to tensile and shear forces. Most of the bolts used in roadway engineering are local anchored, resulting in insufficient shear strength at the bolt free end close to roadway surface and increasing bolts breaking. The anchor cable and C-shaped tube (ACC) is a highly prestressed cable bolt that can withstand high shear force in its free end. This paper examines the effect of the relationship between C-shaped tube length and joint location on the shear resistance of ACC by double shear tests. To fully exploit the ACC’s shear resistance, the C-shaped tube ends should be at least 30 cm beyond the joint. The effect of preload and concrete spray thicknesses on roadway deformation and plastic zone is investigated by numerical simulation. Results show that ACC and concrete spraying layer can form a stable extruded arch structure, so that the broken and soft rock within the loosen zone is in three-dimensional-stress state, effectively improving surrounding rock properties and controlling its deformation size. Based on these results, the ACC support design method is proposed.

Numerical analysis of ground support systems subjected to dynamic loading

Herein, a new numerical approach is proposed to simulate the dynamic loading processes in mines. To present a realistic seismic phenomenon, a parallel two-dimensional seismic source is introduced by adjusting the classic point seismic source. The seismic source can be easily integrated into the universal distinct element code numerical model, and it can adjust its bearings and strength. The numerical model assumes an elastic rock mass with potential fractures because the dynamic-related bulking surrounding excavations is mainly caused by the separation and sliding of fracture planes; cable bolts are installed on the sides of roadways. The numerical simulations can accurately reproduce the full dynamic loading process. After axial forces build up, numerical results from a case analysis show that cable bolts considerably restrict the movement of the surrounding rock mass towards the roadway, preventing potential fractures from developing into the large opening cracks. The cable bolts do not considerably change the local stresses, although they can maintain local horizontal stress compressive near the roadway surface. The monitored cable element experiences multiple loading and unloading processes; it is found that cable bolts initially cause rapid attenuation of seismic velocities and then dissipate dynamic energy in a fluctuating manner.

Investigation on the mechanism and control methods for roof collapse caused by cable bolt shear rupture

Roof collapse of mining roadways has always been key in the studies of the surrounding rock control. One of the critical factors lies in the roof separation and horizontal displacement which led to the cable bolt shear rupture, resulting in the roof fall. This study investigates the roof collapse accident that occurred at the No.15028 tailgate of XinJing coal mine in China. In-situ observations, numerical simulation and laboratory test are conducted to analyse the gradual deterioration of the roof strata. The field observation of the fractures, separations, pretension load and rock bolts loading are conducted during the process of separation. The analysis results indicate that owing to the torque value of rock bolts below 150 N·m (pretension load was 29 kN) and incompatibility of reinforcement fittings of rock bolts, the pretension load failed to solidify the surrounding rocks effectively. Obvious separations were identified at rock depths ranging from 0 to 2.6 m. Simulation results revealed that different flexural deflections caused horizontal displacement in rocks, which is positively correlated with the vertical separation. In weak interlayers of rocks, the severe horizontal displacements led to shear fracture of cable bolts, resulting in roof collapse. An increasing rock bolt pretension load can reduce the horizontal displacement of the rock stratum beyond the anchorage zone, which can lower the shear force of cable bolts by 30% and reduce the shear fracture risk of the cable bolts. Mechanical compatibility tests of the surrounding rock support system are conducted to develop a practical and reasonable reinforcement fittings to increase the pretension load of the rock bolts. Based on the research, a series of control methods are developed to enhance the roof stability. The field tests proved that the newly proposed methods can effectively improve the stability of the roof and eliminate roof collapses.

A constitutive model for modified cable bolts exhibiting cone shaped failure mode

This study presents a novel constitutive model to characterise the mechanical behaviour of cable bolts under axial loading and subjected to different boundary conditions including constant confining pressure and constant normal stiffness. Such a model is developed based on the modified Mohr-Coulomb failure criterion as well as a newly proposed non-linear dilation formulae for the development of governing equations. The cone shaped failure mode of grout due to the axial movement of cable bolt is considered as a widely observable failure mode at the cable bolt to the grout interface. The load carrying capacity of the cable bolt under axial loading is significantly affected by both the progressive reduction in the volume of failed cone zone of grout annulus and the continuous degradation of dilation angle at the cable bolt to the grout interface. Hence, the full load-displacement performance of different cable bolts under axial loading subjected to both constant confining pressure and constant normal stiffness are simulated. Finally, an acceptable agreement between the model simulations and the experimental data confirms the credibility of proposed constitutive model for predicting the mechanical behaviour of different cable bolts under axial loading.

Preventing failure of the anchoring system in underground coal mines

Cable bolts are commonly used as anchoring element in many underground mines. Reports on the premature failure of cable bolts in underground coal mines due to stress corrosion cracking (SCC) have been increasing in the past two decades. The previous studies found that the diffusion of atomic hydrogen into steel causes the SCC in cable bolts, which is known as hydrogen-induced stress corrosion cracking (HISCC). While the research on the mechanism of the HISCC in underground mines needs to be continued, it is essential to develop prevention measures to avoid such a failure. In this study, a variety of prevention measures that claim to prevent corrosion of steel were examined. Specifically, barrier coating, i.e., polymer, epoxy coating, as well as the sacrificial coating, i.e., hot-dip galvanising, methods were tested. Testing specimens were made by inserting loading pins between locked cable bolt king wires (coated) to simulate the in-situ stress condition. Cable bolt specimens were then fully immersed into a hydrogen sulphide solution to determine their resistance against HISCC. The test results showed that all the coatings had delayed the failure in varying degrees, but very few have significantly extended the time to fracturing. The polymer and epoxy coatings have become breached during the testing, and the failure occurred shortly after. The galvanised coating was dissolved in the testing solution and again allowed access of the solution to the steel surface, and subsequently resulted in failure. This indicates that these coatings can be potentially applied to prevent SCC failure; however, their impacts on the performance of bolts, such as their bonding with grout and bolt, still need to be considered.

Failure Analysis of an Intersection under Squeezing Rock Conditions

The development of Turkey’s deepest coal mine is currently underway to the proposed depth of 1200 meters (m) below surface at the Soma Coal Basin. An 11 m intersection forming an access way from one of the main declines was excavated at approximately 900 m below surface collapse within hours of completion even after being heavily supported by TH34 and cable bolts. It was discovered during the excavation failure investigation and rehabilitation work that the intersection was excavated below a water bearing stratification and faulted zone of the coal seam. The failure analyses concluded that the failure was contributed by a number of factors, mainly the overburden pressure of the overlying material along with the inrush of high-pressured groundwater with the very poor rock mass surrounding the excavation. This study investigates the possible reasons of the failure, numerical modelling of the intersection and alternative support suggestions for such squeezing rock conditions.

Usage of Cable Bolts for Gateroad Maintenance in Soft Rocks

Purpose This paper analyses the effectiveness of gateroad reinforcing by means of cable bolts under weak rock conditions. In the worldwide mining industry the method of gateroad support reinforcement using cable bolts is considered to be effective. The experimental application of cable bolts was performed in gateroad #165 of the "Stepova" mine, Western Donbass, Ukraine, and required instrumental control of "support-rock mass" system conditions. Methods Obtaining absolute displacement of "support-rock mass" system elements and extensometer anchors by means of levelling in order to improve the method of observation. Results The peculiarities of geomechanical behaviour of rock mass in the roof of gateroads is investigated. It has been established that the application of cable bolts allows for a reduction in the vertical convergence of the gateroad, both in front of and behind the longwall face. Practical implications Advantages of cable bolts instead of end-face support and props in case of a high advance rate of the longwall face are shown. Originality/value 1. There are no regulations and state standards in regard to cable bolt installation parameters in the mines of Ukraine, consequently the usage of cable bolts for gateroad maintenance required preliminary testing under geological conditions at the Western Donbass mines with soft enclosing rocks. 2. Combining levelling with observations using extensometers allowed for the detection of the rock layers' uniform sagging zone in the roof of the gateroad.

Usage of Cable Bolts for Gateroad Maintenance in Soft Rocks

Purpose This paper analyses the effectiveness of gateroad reinforcing by means of cable bolts under weak rock conditions. In the worldwide mining industry the method of gateroad support reinforcement using cable bolts is considered to be effective. The experimental application of cable bolts was performed in gateroad #165 of the "Stepova" mine, Western Donbass, Ukraine, and required instrumental control of "support-rock mass" system conditions. Methods Obtaining absolute displacement of "support-rock mass" system elements and extensometer anchors by means of levelling in order to improve the method of observation. Results The peculiarities of geomechanical behaviour of rock mass in the roof of gateroads is investigated. It has been established that the application of cable bolts allows for a reduction in the vertical convergence of the gateroad, both in front of and behind the longwall face. Practical implications Advantages of cable bolts instead of end-face support and props in case of a high advance rate of the longwall face are shown. Originality/value 1. There are no regulations and state standards in regard to cable bolt installation parameters in the mines of Ukraine, consequently the usage of cable bolts for gateroad maintenance required preliminary testing under geological conditions at the Western Donbass mines with soft enclosing rocks. 2. Combining levelling with observations using extensometers allowed for the detection of the rock layers' uniform sagging zone in the roof of the gateroad.

Experimental Study of Stirring and Resin-Blocking Devices for Improving the Performance of Resin-Anchored Cable Bolts

Resin anchoring promotes the development of high-strength cable bolts for underground rock support. However, the resin anchoring the cable bolt must be very effective in soft rock. Using conventional anchoring methods, the anchoring force provided by cable bolts in soft rock is low, and achieving high-strength support is difficult. Aiming to address these problems, this study developed a method to stir the resin more completely and thereby improve the uniformity of the resin and increase its density in the anchoring area. The problem was investigated using theoretical analysis, numerical simulations, and laboratory and field tests. A method for improving the cable bolt anchoring quality is proposed and stirring and blocking devices (SBDs) for resin-anchored rock cable bolts are developed. The stirring device has three stirring blades on its top and the blocking device’s diameter is slightly smaller than the diameter of the borehole. Both devices have protruding left-handed threads on their surfaces. The mechanisms for improving cable bolt anchoring quality are demonstrated. The main conclusions drawn from this study are that the devices can effectively improve resin uniformity and increase the density of resin in the anchoring area. This improves the quality of the anchorage and thus enhances mine safety. The field test showed that compared with the anchoring force of conventional cable bolts, the anchoring force provided by cable bolts using the SBDs were increased by more than 30%. The SBDs are easy to install and have practical significance for improving cable bolt anchoring force.

Microbiologically influenced corrosion of cable bolts in underground coal mines: The effect of Acidithiobacillus ferrooxidans

Reports on corrosion failure of cable bolts, used in mining and civil industries, have been increasing in the past two decades. The previous studies found that pitting corrosion on the surface of a cable bolt can initiate premature failure of the bolt. In this study, the role of Acidithiobacillus ferrooxidans (A. ferrooxidans) bacterium in the occurrence of pitting corrosion in cable bolts was studied. Stressed coupons, made from the wires of cable bolts, were immersed in testing bottles containing groundwater collected from an underground coal mine and a mixture of A. ferrooxidans and geomaterials. It was observed that A. ferrooxidans caused pitting corrosion on the surface of cable bolts in the near-neutral environment. The presence of geomaterials slightly affected the pH of the environment; however, it did not have any significant influence on the corrosion activity of A. ferrooxidans. This study suggests that the common bacterium A. ferrooxidans found in many underground environments can be a threat to cable bolts’ integrity by creating initiation points for other catastrophic failures such as stress corrosion cracking.

A robust approach to identify roof bolts in 3D point cloud data captured from a mobile laser scanner

Roof bolts such as rock bolts and cable bolts provide structural support in underground mines. Frequent assessment of these support structures is critical to maintain roof stability and minimise safety risks in underground environments. This study proposes a robust workflow to classify roof bolts in 3D point cloud data and to generate maps of roof bolt density and spacing. The workflow was evaluated for identifying roof bolts in an underground coal mine with suboptimal lighting and global navigation satellite system (GNSS) signals not available. The approach is based on supervised classification using the multi-scale Canupo classifier coupled with a random sample consensus (RANSAC) shape detection algorithm to provide robust roof bolt identification. The issue of sparseness in point cloud data has been addressed through upsampling by using a moving least squares method. The accuracy of roof bolt identification was measured by correct identification of roof bolts (true positives), unidentified roof bolts (false negatives), and falsely identified roof bolts (false positives) using correctness, completeness, and quality metrics. The proposed workflow achieved correct identification of 89.27% of the roof bolts present in the test area. However, considering the false positives and false negatives, the overall quality metric was reduced to 78.54%.

Deformation Mechanism and Support Technology of Deep and High‐Stress Soft Rock Roadway

In this study, the analysis and control of stability of surrounding rock in deep fractured soft rock roadway located in the underground mine of Jinfeng gold mine in Guizhou Province, China, has been investigated. The surrounding rock of roadway has been analyzed to characterize its deformation and failure mechanism through field survey, testing of rock physical and mechanical properties, in situ stress measure, analysis of mineral components of rock, and investigation of rock fragmentation degree. Based on the numerical simulation technology, the influence of different factors on the stability of roadway is studied. The physical and mechanical properties of surrounding rock and the bearing capacity of surrounding rock layer are to be improved to maintain the stability of broken soft rock roadway as high ground stress, rock fragmentation, and poor lithology leading to tunnel instability. Hence, a high‐strength “cable bolt + fiber‐reinforced shotcrete + steel mesh + split sets + resin bolt + cement grouting” combined support system has been proposed to improve the effective bearing structure significantly with high integrity and bearing capacity.

Failure Mechanism and Control Technology of Thick and Soft Coal Fully Mechanized Caving Roadway under Double Gobs in Close Coal Seams

The surrounding rock of the roadway under double gobs in the lower coal seams is partially damaged by the mining of the upper coal seam and the stress superimposition of the stepped coal pillars. What is worse, the upper layer of the roof is collapse gangue in double gobs, which makes the anchor cable unable to anchor the reliable bearing layer, so the anchoring performance is weakened. The actual drawing forces of the anchor bolt and anchor cable are only approximately 50 kN and 80 kN, respectively. The roadway develops cracks and large deformations with increasing difficulty in achieving safe ventilation. In view of the above problems, taking the close coal seam mining in the Zhengwen Coal Mine as the engineering background, a theoretical calculation is used to obtain the loading of the step coal pillars and the slip line field distribution of the floor depth. The numerical simulation monitors the stress superimposition of stepped coal pillars and the distribution of elastoplastic areas to effectively evaluate the layout of mining roadways. The numerical simulation also analyzes the effective prestress field distribution of the broken roof and grouting roof anchor cable. A laboratory test was used to monitor the strength of the grouting test block of the broken coal body. Then, we proposed that grouting anchor cable be used to strengthen the weak surface of the roof and block the roof cracks. From on-site measurement, the roadway was seen to be arranged in the lateral stress stabilization area of the stepped coal pillars, the combined support technology of the grouting anchor cable (bolt) + U type steel + a single prop was adopted, the roadway deformation was small, the gas influx was reduced, and the drawing force of the anchor bolt and the anchor cable was increased to approximately 160 kN and 350 kN, respectively. The overall design and control technology of the roadway can meet the site safety and efficient production requirements.

Numerical simulation of pull-out test on cable bolts

Due to presence of external loads, the bolt providing stability to the structure which on failure may create a hazardous situation in case of mining and construction activities. The failure may be due to the failure of bolt itself or the failure of grout (in case of grouted bolt) along the bolt grout interface or the grout rock interface. It is thus necessary to study the effect of variation of a number of factors on the pull-out resistance and hence the failure of bolt. These factors comprise of the kind of bolt used, kind of grout material used, dimensions of bolt used etc. The objective of this research is to simulate the pull-out behaviour of cable bolt using 3DEC software and study the influence of various factors for the cases of grouted and non-grouted bolt. These factors are diameter of the bolt, length of the bolt, rate of extraction of the bolt, confining pressure, thickness of grout material and angle of friction of the grout material.

Improvement of the Loading Capacity of Narrow Coal Pillars and Control Roadway Deformation in the Longwall Mining System. A Case Study at Khe Cham Coal Mine (Vietnam)

Currently, the application of coal pillars to protect an adjacent roadway is a common method in Vietnam when exploiting according to the longwall system. Therefore, the width of a coal pillar is an important issue for the stability of a roadway. In order to reduce coal loss in these coal pillars, they tend to be designed in a narrow coal pillar style but still have to ensure that the adjacent roadway can meet safe coal production conditions. The stability of roadways and coal pillars is related to many factors such as technical mechanical characteristics, physical and mechanical properties of coal, stress environment and support methods. The bearing structure of the coal pillar and the around rock a roadway is analyzed and it has been shown that enhancing roadway support and improving the carrying capacity of coal pillars can control the deformation of the surrounding rock. A study related to the stability and safety of roadways and small coal pillars in the longwall mining system has been carried out. Stabilization factors have been considered, especially the state of stress in the coal pillars and the deformation of the roadway. By applying the numerical simulation method, the stress of the coal pillar and the deformation of the adjacent roadway under different supporting solutions were analyzed and evaluated. By using this method, the rock bolt roadway support solution combined with the long cable bolt in the roadway roof and the coal pillar was selected in the safe condition of the mining process. Because cable bolt can improve the flexibility of the coal pillar such as: reducing the size of the plastic area on both sides of the pillar; enhancing coal pillar stability in the core area by providing great drag and tensile for coal pillars; contributing to improving the anchor point fixation of rock bolt. The conclusions obtained may provide a certain reference parameters to improve mining efficiency and labor safety in underground coal mines.

Behavior of Fully Encapsulated Cable Bolt in Various Double-Shear Methods and the Sequence of Wire Failure in the Newly Developed Shear Apparatus

The use of long-tendon ground support elements (cable bolts) is now a common practice in modern underground coal mines, hard rock mines, tunnels, and other underground structures. The paper briefly introduces the development of test methods for rock and cable bolts, including single- and double-shear methods, in the last 30 years and focuses on the novel development of the fourth generation of cylindrically shaped shear test apparatus (MK-IV Double Shear Box) for assessing tendon performance in shear based on the experience gained from the development of previous versions of shear experiment apparatus. The results were compared with similar test findings using a rectangular-shaped double-shear apparatus with and without friction across joint faces. The sequence of wire failure was studied by using the newly developed double-shear test apparatus, and the performance of abnormal phenomena in the double-shear tests was analyzed. The sequence of wire failure can be inferred based on the positive and negative values of strain gauge readings, and the wire of cable bolt under shearing may be broken in the joint area. The failure possibly occurred from one side to the other instead of from top to bottom. The modified double-shear test apparatus with a long section of concrete cylinder was proposed to conduct further research on cable bolt debonding.

Safety assessment of cable bolts subjected to tensile loads

Cable bolts are one of the most common tension anchorage systems for reinforcing the stability of rock/soil masses; however, the quantitative assessment of cable bolt safety via numerical simulations has not been deeply explored until now. In this technical study, a bolt tri-linear tensile model and a bolt–grout (B–G) interface tri-linear shear model were integrated into a cable bolt tri-linear model to reflect the large tensile deformation-broken failure of the bolt and the B–G interface slip-softening failure after cable bolt yielding. A dimensionless index was developed by defining the maximum of the failure approach index (FAI) for bolt tensile failure and the FAI for B–G interface shear failure based of this cable bolt model to quantitatively assess the cable bolt safety. The comparison between the laboratory and numerical test results and engineering case analyses showed that the cable bolt FAI combined with the cable bolt tri-linear model, which could quantify the cable bolt safety and determine the cable bolt failure position and sequence as an improved substitute for cable bolt yield state indicators in numerical simulations. The present study is expected to offer a more accurate numerical simulation technique than existing simulation techniques in the quantitative assessment of cable bolt safety, finally contributing to the quantitative evaluation-based support design.