Cable Bolts Research Articles

Analysis of monitored ground support and rock mass response in a longwall tailgate entry

A comprehensive monitoring program was conducted to measure the rock mass displacements, support response, and stress changes at a longwall tailgate entry in West Virginia. Monitoring was initiated a few days after development of the gateroad entries and continued during passage of the longwall panels on both sides of the entry. Monitoring included overcore stress measurements of the initial stress within the rock mass, changes in cable bolt loading, standing support pressure, roof deformation, rib deformation, stress changes in the coal pillar, and changes in the full three-dimensional stress tensor within the rock mass at six locations around the monitoring site. During the passage of the first longwall, stress measurements in the rock and coal detected minor changes in loading while minor changes were detected in roof deformation. As a result of the relatively favorable stress and geological conditions, the support systems did not experience severe loading or rock deformation until the second panel approached within 10–15 m of the instrumented locations. After reaching the peak loading at about 50–75 mm of roof sag, the cable bolts started to unload, and load was transferred to the standing supports. The standing support system was able to maintain an adequate opening inby the shields to provide ventilation to the first crosscut inby the face, as designed. The results were used to calibrate modeled cable bolt response to field data, and to validate numerical modeling procedures that have been developed to evaluate entry support systems. It is concluded that the support system was more than adequate to control the roof of the tailgate up to the longwall face location. The monitoring results have provided valuable data for the development and validation of support design strategies for longwall tailgate entries.

Effects of environmental factors on stress corrosion cracking of cold-drawn high-carbon steel wires

Premature failure of cable bolts due to stress corrosion cracking (SCC) is a phenomenon that has reported to occur in underground environments. This paper presents an experimental study to determine the impacts of environmental factors on SCC in high-carbon steel cable bolts used in ground support system. The service life of the wire strands was measured under a range of accelerated SCC conditions using three- and four-point loading jigs. Fractographic analyses of the fracture surfaces of the failed wires displayed typical features indicating that hydrogen embrittlement was involved in SCC in these tests. It was shown that the presence of hydrogen sulphide is more critical than its concentration in SCC. The pH and level of applied stress were determined to have a direct impact on the occurrence of SCC. The deflection angle of the crack path was observed to be an inverse polynomial function of the applied stress level in the wire. This demonstrated the significance of the stress level on the fracture mechanism of cable bolts. Furthermore, the use of a protective galvanised coating on cable bolts was found to be a promising countermeasure against SCC. The results of this study provide detailed insight into the environmental factors involved in SCC of high-carbon steel wires and can be further used for setting guidelines for assessment of environments which cause susceptibility to SCC.

Stability Control of Retained Goaf-Side Gateroad under Different Roof Conditions in Deep Underground Y Type Longwall Mining

Stability of the retained goaf-side gateroad (RGSG) is influenced mainly by the movements of the roof strata near coal seam after coalface passes by. To make effective controlling technology for the stability of the RGSG, we analyze the roof structure over the RGSG to illustrate the mechanism causing the RGSG instability under different roof conditions. We then examine the dynamic evolution of the deformation and abutment stress in the rock surrounding the RGSG during coal seam mining, using the FLAC3D numerical software to reveal the instability characteristics of the RGSG under different roof conditions. Next, corresponding stability controlling technologies for the RGSGs are proposed and tested in three typical deep underground coalmines. Results show that: sink and rotation of the roof cantilever over the RGSG impose severer influence on the stability of the RGSG. The RGSG suffers disturbances three times during the coal-seam mining, and the deformation and abutment stress in the rock surrounding the RGSG increase significantly when the main roof becomes thicker and the immediate roof becomes thinner. Staged support technology involving grout cable bolts has better controlling results of the RGSG stability than that composed of conventional rock bolts, when the RGSG is beneath weak immediate roof with large thickness. Roof structure optimizing technology involving pre-split technology can improve the stability of the RGSG effectively when the RGSG is covered by hard main roof with large thickness directly.

Shear strength properties of cable bolts using a new double shear instrument, experimental study, and numerical simulation

A newly developed double shear apparatus without contact between concrete blocks was developed to conduct a series of double shear tests. This new double shear apparatus is capable of determining the pure shear strength of pre-tensioned fully grouted cable bolt without friction between sheared concrete blocks. Five different types of cable bolt, with various pretension loads, were tested to investigate the influence of surface profile type and pretension load on the shear strength of cable bolt. Concrete blocks of 40MPa strength and the Stratabinder HS grout were used for consistency across the entire tests. The results showed that the plain cable bolts had higher peak shear load compared with the indented and spiral strand cable bolts. The shear displacement and peak shear load decreased by increasing the pretension load. A numerical analysis was carried out, based on the Fast Lagrangian Analysis of Continua (FLAC 2D) and the result was compared with the experimental data. It was observed that FLAC 2D is capable of simulating the performance of cable bolt satisfactorily.

Laboratory pull-out tests on fully grouted rock bolts and cable bolts: Results and lessons learned

Laboratory pull-out tests were conducted on the following rock bolts and cable bolts: steel rebars, smooth steel bars, fiberglass reinforced polymer threaded bolts, flexible cable bolts, IR5/IN special cable bolts and Mini-cage cable bolts. The diameter of the tested bolts was between 16 mm and 26 mm. The bolts were grouted in a sandstone sample using resin or cement grouts. The tests were conducted under either constant radial stiffness or constant confining pressure boundary conditions applied on the outer surface of the rock sample. In most tests, the rate of displacement was about 0.02 mm/s. The tests were performed using a pull-out bench that allows testing a wide range of parameters. This paper provides an extensive database of laboratory pull-out test results and confirms the influence of the confining pressure and the embedment length on the pull-out response (rock bolts and cable bolts). It also highlights the sensitivity of the results to the operating conditions and to the behavior of the sample as a whole, which cannot be neglected when the test results are used to assess the bolt-grout or the grout–rock interface.

Sample diameter effect on bonding capacity of fully grouted cable bolts

Bonding capacity of cable bolts plays a significant role in determining the load transfer performance of fully grouted cable bolting systems. Bonding capacity can be assessed with laboratory scale tests on cable bolts grouted in rock-like samples. A number of pull-out tests were conducted using a bulbed cable bolt to investigate the cylindrical sample diameter effect on the bonding capacity. A cement-based material was used to prepare the samples having various diameters. Two different boundary conditions were examined, where the cylindrical sample was unconfined and confined within a stiff steel tube. The results show that the bonding capacity increased linearly with sample diameter up to 356mm in the unconfined condition. In the confined condition, however, the threshold reached at a low diameter of 300mm. Furthermore, it was found that the bonding capacity in the confined condition was always higher than in the unconfined condition. The failure mode of the cable bolting system was also influenced by the boundary conditions. In the unconfined condition, it was observed that the sample was likely to split. Whereas in the confined condition, performance of cable bolts was mainly dominated by the bonding failure.

A constitutive model for load-displacement performance of modified cable bolts

Cable bolts are extensively used in mining and civil structures as one of the most important support systems for ground control. In order to design an effective support system, understanding the performance of cable bolts under the conditions other than those tested in the laboratory is essential. Thus, this paper outlines the development of a constitutive model that can predict the full load-displacement performance of the modified cable bolts at different confining pressures. The model includes the nonlinear dilation equation and the peak axial load envelope as the governing laws. The governing equations for the failure mechanisms at the cable to grout interface and grout annulus are coupled using advanced partial differential equations. The full load-displacement behaviour of modified cable bolts with a smooth transition from initial elastic behaviour to progressive failure is simulated using continuous yielding method. The model is then calibrated against the experimental data reported in the literature. It has been demonstrated that there is a good agreement between the model simulations and the experimental results confirming the suitability of the proposed constitutive model for performance prediction of the modified cable bolts at different confining pressures.

An experimental study of axial load transfer mechanisms of cable bolts using axially split embedment apparatus

The load transfer mechanisms of cable bolts differ from those for normal rebar bolts. The cable bolts used in mines are basically steel strands with different constructions depending on the number of wires or elements and the way they are laid. Tendon bolts (rebar and cable) are normally evaluated for their strength and load transfer properties. The tendon strength can be evaluated by the tensile failure tests, while the load transfer strength is evaluated by the pull and shear strength tests. Short Encapsulation Pull Testing (SEPT) is normally used to study the load transfer capacities of tendons, and it can be undertaken in both the laboratory and in situ. A new apparatus known as Minova Axially Split Embedment Apparatus (MASEA) was used to study load-displacement characteristics of smooth versus spiral profile cable bolts. Minova Stratabinder grout was used for encapsulating 400-mm long 19 wire 22-mm diameter superstrand cable in embedment units. The anchorage of the cable on the two sides of the embedment apparatus were intentionally installed at different lengths to allow the cable to be pulled out from one side of the anchorage. The spiral wire strand cable bolts achieved a higher peak pull-out load at a minimum displacement in comparison with the smooth surface wire strand. The peak pull out force increased with the age of encapsulation grout. The use of MASEA was easier to assemble and test at a short period of time, thus allowing the quick and repeated tests undertaken.

A New Equation for the Shear Strength of Cable Bolts Incorporating the Energy Balance Theory

The application of cable bolts as a secondary support system is an increasing trend in underground coal mines worldwide. The performances of cable bolts have been evaluated under both axial and shear loading conditions. Two methods of testing cables for shear, single and double shear, have been recognised. This paper examines the shear behaviour of a variety of cable bolts under different pre-tension loads by double shear testing. Plain, spiral and the combination of both cable types were used in this study. The initial axial load and the type of cable bolts are the main factors affecting their shear strength. By increasing the axial pre-tension load, the peak shear load occurs at lower shear displacement. The failure angle due to cable bending across the joint at different pre-tension loads varied between 41° and 49°. This demonstrates that the ratio of axial and perpendicular displacements is almost constant and on average the failure occurs at about 45°. A novel analytical model is proposed to evaluate the shear behaviour of pre-tensioned fully grouted cable bolts subjected to double shearing. Energy and Fourier Series methods were incorporated in the model to simulate the shear behaviour of cable bolts. The comparison of the experimental results with the proposed model shows a good agreement.

Double Shear Testing of Cable Bolts with No Concrete Face Contacts

A new series of double shear tests were carried out using a newly modified double shear apparatus which prevented contacts between concrete block surfaces during shearing. 13 double shear tests were carried out using 21mm diameter 19 (9 × 9 × 1) seal construction wire strand cable (also called Superstrand cable), Plain SUMO, Indented SUMO, Spiral MW9 and Plain MW10 cable bolts. These cables were tested subjected to different pretension loads. Concrete blocks with Uniaxial Compressive Strength (UCS) of 40MPa and Stratabinder grout were used for all the tests to maintain test consistency. The results show that the peak shear load and the corresponding shear displacement decrease by increasing the pretension load of the tested cable. The Ultimate tensile strength, lay length, number of wires and cable bolt surface profile type (plain and spiral/indented) are important factors in total shear strength of the cable bolt.

Rock Bolting at the Room and Pillar Method at Great Depths

A part of the coal reserves in the Karvina subbasin of the Upper Silesian Coal Basin is situated in protection pillars that lie under built-up areas. The longwall mining method is not suitable in these areas because significant deformation of the surface is not allowed. The room and pillar method with stable coal pillars has been proposed to minimise strata convergence. The method has been examined within the shaft protective pillar located in CSM-North Mine coal seam No. 30, where the mining depth ranged from 700 to 900 meters, being perhaps the deepest room and pillar panel in coal mining in the world [1–5].As there is no relevant experience of using this method in the Upper Silesian Coal Basin, an extensive monitoring system has been implemented to enable the mining trial to continue safely. The monitoring is focused on the load-bearing capacity of the coal pillars and strata deformation changes induced by the room and pillar mining method. Precise monitoring was carried out in two adjacent coal pillars located within the row of pillars forming the panel. To monitor roof deformation, fourteen pairs of 5-level multipoint extensometers monitored roof displacements and eleven strain-gauged rockbolts were installed at various locations. Seven hydraulic dynamometer load cells measured the cable bolt loads were installed at the roadway intersections around the monitored pillars.The integral constituent of the extraction method (using driving machines like the Bolter Miner) is bolting as the sole support system of the roadways. The contribution deals with the behaviour of roof bolting, including the loading of the bolts, yielding of the rock mass and convergence in the roadways. The monitored parameters’ database used to measure whether the room and pillar method is successful at this depth forms a necessary condition for verification of this method and its future application in conditions experienced in Upper Silesian Coal Basin.

Comparison of the shear test results of a cable bolt on three laboratory test apparatuses

The shear performance of cable bolt is important in assessing its load transfer capacity for effective rock strata reinforcement, which has not yet been thoroughly investigated and understood. Tests carried out in laboratory to study the cable shear performance can be simply classified into two types, the Single Shear Test (SST) and the Double Shear Test (DST). A variety of single and double shear test apparatuses developed in the past were reviewed and the main controlling factors when designing a new shear test apparatus were concluded based on the existing shear test apparatuses and their application. Four cable bolts were tested in laboratory conditions using the DST apparatus developed in the University of Wollongong, the British Standard Single Shear Test apparatus (BSST) and the Megabolt Single Shear Test apparatus (MSST) to compare and assess these different shear test apparatuses. Test results showed that the MSST results were completely different from the BSST results but similar to the DST results. Cable bolts tested in BSST tended to fail at small shear force and shear displacement in the form of inclined shear failure surfaces, whilst cable bolts installed in MSST and DST both tended to fail at greater shear force and shear displacement in the form of tensile cone and cup with bending deformation. It is concluded that both single and double shear test methods can effectively assess cable bolt shear performance as long as suitable apparatus dimensions and boundary conditions are used.

Modeling the load-displacement curve for fully-grouted cable bolts using Artificial Neural Networks

Modeling the load-displacement curve for fully-grouted cable bolts using Artificial Neural Networks

Load Transfer Behavior of Fully Grouted Cable Bolts Reinforced in Weak Rocks Under Tensile Loading Conditions

To enable a better understanding of the load transfer performance of fully grouted cable bolts, a new laboratory short encapsulation pull test (LSEPT) unit has been designed and constructed. With this unit, both peak and residual capacity can be assessed over a relatively large pull-out displacement range of 100 mm. Various cable bolt failure modes, including relative slippage along the cable/grout and grout/rock interface, can be studied. Both a modified and a plain strand cable bolt confined in a material having a low strength of 8 MPa were tested. Within this testing process, the impact of the design of the bearing plate was considered that was found to alter the failure mode of the modified cable bolt. Other factors are considered, including the influence of borehole diameter, cable surface geometry, and sample strength on the axial performance of cable bolts.

Depillaring of total thickness of a thick coal seam in single lift using cable bolts: A case study

Explaining fundamentals of application of cable bolting for a thick seam depillaring, this paper summarizes the results of field studies conducted during adoption of this approach in more than fifteen panels of Madhusudanpur 7 pit and incline mine. Nearly 7.0m thick Kajora top coal seam of this mine is developed on pillars along the floor horizon to an average height of 3.0m, leaving a coal band of around 4.0m along the roof. Analysis of procured core samples showed that roof strata are easily caveable with a caveability index value of around 2000 only. Easily caveable overlying strata and shallow depth of cover alleviated most of the expected strata mechanics problems of the thick seam mining. However, extraction of total thickness at shallow cover caused differential-subsidence and cracks on the surface. These manifestations were immediately tackled to avoid creation of a breathing path for spontaneous heating in the extracted area.

Behavior of Fiber Glass Bolts, Rock Bolts and Cable Bolts in Shear

This paper experimentally compares the shear behavior of fiber glass (FG) bolt, rock bolt (steel rebar bolt) and cable bolt for the bolt contribution to bolted concrete surface shear strength, and bolt failure mode. Two double shear apparatuses of different size were used for the study. The tensile strength, the shear strength and the deformation modulus of bolt control the shear behavior of a sheared bolted joint. Since the strength and deformation modulus of FG bolt, rock bolt and cable bolt obtained from uniaxial tensile tests are different, their shear behavior in reinforcing joints is accordingly different. Test results showed that the shear stiffness of FG bolted joints decreased gradually from the beginning to end, while the shear stiffness of joints reinforced by rock bolt and cable bolt decreased bi-linearly, which is clearly consistent with their tensile deformation modulus. The bolted joint shear stiffness was highly influenced by bolt pretension in the high stiffness stage for both rock bolt and cable bolt, but not in the low stiffness stage. The rock bolt contribution to joint shear strength standardised by the bolt tensile strength was the largest, followed by cable bolts, then FG bolts. Both the rock bolts and cable bolts tended to fail in tension, while FG bolts in shear due to their low shear strength and constant deformation modulus.

Characterization of tensile and shear loading on indented PC-strand cable bolts

The tensile and shear strength of intrinsic bolting support systems has always been a major concern of designers. A comprehensive laboratory testing program was designed to evaluate the tensile and shear strength of individual wires and completely wound PC-strand cables. PC-strand cables with smooth wires and the recent anchorage enhancement innovation of indentation were evaluated and compared. The testing protocol detailed in ISO Standard 15630 utilizes a mandrel system that was investigated at 3 different diameters which alters the wire to mandrel ratio from 2:1 to 9:1. The results demonstrate that the difference between smooth and indented wires is statistically insignificant when larger diameter mandrels are used, and that indentation does not adversely affect strand properties and performance. Insight into the shearing mechanism and evaluation techniques are discussed with the introduction of triaxial loading to describe the PC-strand tensile and shearing mechanisms. Another important result indicates that the shear strength of PC-strand cable bolting systems has a greater shear strength value than traditional steel bar bolting systems.

Parametric study on the axial performance of a fully grouted cable bolt with a new pull-out test

Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new laboratory short encapsulation pull test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/rock interface. Interestingly, increasing the borehole diameter from 42 to 52m in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.

Analysis of alternatives for using cable bolts as primary support at two low-seam coal mines

Cable bolts are sometimes used in low-seam coal mines to provide support in difficult ground conditions. This paper describes cable bolting solutions at two low-seam coal mines in similar ground conditions. Both mines used support systems incorporating cable bolts as part of the primary support system. Two original cable bolt based support systems as well as two modified systems are evaluated to estimate their ability to prevent large roof falls. One of the support systems incorporated passive cable bolts, while the other used pre-tensioned cable bolts. The results and experience at the mines showed that the modified systems provided improved stability over the original support systems. The presence of the cable bolts is the most important contribution to stability against large roof falls, rather than the details of the support pattern. It was also found that a heavy steel channel can improve the safety of the system because of the ‘sling’ action it provides. Additionally, the analysis showed that fully-grouted rebar bolts load much earlier than the cable bolts, and pre-tensioning of the cable bolts can result in a more uniform distribution of loading in the roof.

Transitional geology and its effects on development and longwall mining in Pittsburgh Seam

This paper presents the geologic and ground control challenges that were encountered by Consol Energy’s coal mining operations in southwestern Pennsylvania, USA. Geologic encounters, such as sandstone-to-limestone geology transition, massive sandstone channels, shale channels, pyritic rich green claystone, laminated roof, and soft floor, have significantly impacted the development and longwall mining in Consol’s Pittsburgh Seam coal mines. Experience from different mines shows that, in the sandstone-to-limestone geology transition zone, 1.83m high-tension, fully-grouted primary bolts employed along with 4.88m center cable bolts at every other strap greatly improved beam building and ensured proper anchorage into the competent roof. Hydraulic fracturing of the massive sandstone was often necessary to enhance caving of the massive sandstone behind the shields to relieve pressure at the face. The presence of soft floor coupled with presence of thick floor coal and deep cover, induced excessive headgate convergence during retreat of the first right hand longwall panel. In all, it is important to explore the roof and in-seam geology in detail to delineate normal and anomalous geologic conditions prior to and during development. With diligent geologic reconnaissance, geotechnical monitoring, and assessment, site-specific geotechnical solutions have been provided to mine operations to improve safety and productivity.