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CABLE BOLT BEARING CAPACITY – AN IN SITU PARAMETRIC STUDY
Conference Paper · May 2015
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K. Soucek
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Petr Waclawik AZGEO s.r.o.
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Jiří Ptáček
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CABLE BOLT BEARING CAPACITY – AN IN SITU PARAMETRIC STUDY
K. Souček, *P. Waclawik, P. Koníček, J. Ptáček, A. Hastíková and R. Kukutsch
Department of Geomechanics and Mining Research, Institute of Geonics AS CR, Institute of Clean Technologies
Studentska 1768 Ostrava, Czech Republic 708 00
(*Corresponding author: [email protected])
CABLE BOLT BEARING CAPACITY – AN IN SITU PARAMETRIC STUDY ABSTRACT
Cable bolts are frequently used as a secondary support system (with steel arch support) in difficult stress-strain conditions of deep coal mines. The load transfer mechanism of cable bolts depends on several factors, such as the strength, deformation and geometric properties of the bolts, resin and surrounding rock mass so called cable-resin-rock system (CRR system). The bonding length and bolt/borehole diameter ratio strongly influence the final bearing capacity CRR system.
The study presents the behaviour of CRR system during pull out test which is usually used in the carboniferous rock mass in the Czech part of the Upper Silesian Coal Basin. Series of 28 pull out tests were performed on 3 m long cable bolts with various combinations of borehole diameters and bonding lengths for understanding this behaviour. Boreholes were drilled into a solid rock mass consisting of fine-grained sandstone to guarantee similar conditions during pull out tests. Video inspection and measurement of boreholes diameters were performed to determine their quality and real diameters. Three characteristics were observed from pull-out tests results: ultimate pullout load, displacement of the end of bolt and system stiffness. Numerical modelling by FEM was used to determine stiffness of the CRR system for variable bolt/borehole ratios and bonding lengths each under constant load. Linear elasto-plastic model and mohr- coulomb model were used for cable and resin-rock respectively. The results of the experiment may significantly contribute improvement of the methodology of cable bolts installation as well as the design of used cable bolts.
KEYWORDS
Cable bolt, Bearing capacity, System stiffness, In-situ pull out test, Cable-Resin-Rock system (CRR system), Bolt borehole ratio
INTRODUCTION
The presented in situ experiment verifies the behaviour of cable bolts during pull out tests. These cable bolts are used in the conditions of carboniferous rock mass in the Czech part of the Upper Silesian Coal Basin. The cable bolts are in this location primarily used as supplementary reinforcing of long wall roadways support. Its load transfer mechanism basically depends on bearing capacities and deformation characteristics of each components of the system called cable-resin-rock system (CRR system) in this study (Aydan, 1989; Hutchinson & Mark, 1996). The study is divided into two parts.
The first part of it presents influence of bonding length on resulting load-displacement curves of CRR systems and stiffness development. In this case each borehole was drilled by drill bits having a diameter of 32 mm as developer of the used cables recommends its maximum. Following bolt/borehole study is observed by practical measurements of borehole diameters which are statistically evaluated and presented in this study. Development of plastic points in the resin of variable bolt/borehole ratio and stiffness development for variable bonding length was in interest of numerical modelling.
The behavior of CRR system in conditions which simulate poor quality of rock mass has been
higher diameter values 38 mm and 42 mm were used and it’s responding load-displacement curves are presented.
NATURAL CONDITIONS
The experiment was carried out in conditions of carboniferous rock mass in the Czech part of the Upper Silesian Coal Basin. The cable bolts were installed in the overburden of the crosscut in safety pillar of shafts Karvina Mine locality CSA. The CSA Mine is situated in the northern part of Karvina subbasin.
The selected part of the crosscut was driven in the roof of coal seam having a thickness about 1 m in fine- grained sandstones in Poruba Member of the Ostrava stratigraphic formation. In situ experiment was carried out in depth of 950 m below the surface. From the viewpoint of stability of steel support and surrounding rock mass, the crosscut was in good conditions, although it was excavated 30 years ago.
Individual segments of steel arches did not show any signs of deformations. The overburden of the crosscut is formed by solid sandstone layer without evident fracturing. The exploratory video inspections of drill holes verified the intact rock mass. Only some coal residues were identified in the fine-grained sandstone in near overburden of the crosscut (at the depth from 0 to 0.5 m), but the locations were considered to be out of bonding areas of cable bolts.
EXPERIMENTAL TECHNIQUES
Suitable locality was chosen in the first phase of the experiment. The overburden of the crosscut met the condition for the in-situ experiment regarding homogeneous conditions for individual installation of cable bolts. The video inspections of the crosscut overburden were carried out in four boreholes 6 m long. Performed video inspections confirmed the favorable conditions for the experiment. The rock mass was defined as intact, massive, fine-grained sandstone.
After the drilling of installation boreholes, measurement of real diameters and quality of boreholes were performed in the installation boreholes. Video camera RITEC of diameter 29 mm with recording to the hard drive of laptop was used for video inspection. Cable bolts were bonded into rock mass by resin LOKSET HS slow with variable bonding length. Cable bolts used in the experiment were of type IR-4/C with yield strength 350 kN, strand consisting of 9 wires each having diameter of 6 mm. Mine hydraulic draft gauge TORO - 3AD with maximum pullout load 624 kN was used for pull out testing of installed cable bolts. The hydraulic draft gauge was equipped with continuous recording of pullout force and laser distance meter Disto for continuous measuring of displacement of the end of the bolt.
BOLT/BOREHOLE RATIO STUDY
Increasing borehole size to ease cable installation is trend of practice. But it is advised to use small boreholes to rich higher bond strengths (Yazici & Kaiser, 1992). The producer of the cables recommends the maximum value of the drill bit diameter 32 mm (Minova Bohemia s.r.o., 2013). On the basis of known volume of the borehole diameter and corresponding bonding length, amount of the resin capsules that is needed is calculated. Bolt/borehole ratio study is presented to introduce the real values of diameters measured in the experiment when drill bit having a diameter of 32 mm is used.
From the comparative measurements results of borehole diameters it was found out walls of the boreholes are of various qualities in longitudinal direction. During drilling process there is helical shape of the walls forming along the borehole length and it results in variable diameter along the borehole length (Souček et al., 2012). Figure 1 presents the highest measured value of boreholes diameter is 37 mm. The mean value was estimated to be 34.3 mm. So this value of the diameter is used for the estimation of the amount of resin capsules for various bonding length using drill bit having a diameter of 32 mm.
Displacement of the bolt end D40
Lm
Ll L40
Dm Dl
Loading
Figure 1 – Statistical assessment of measured borehole diameters
Numerical modelling by FEM was used to simulate variable bolt/borehole ratio and its response to constant load was observed. Linear elasto-plastic model using Mohr-Coulomb criteria was used in software Midas GTS. Shear and normal stiffness of interfaces were neglected even it makes the results more closed to reality (Bouteldja, 2000), only plastic point development was observed. Variable cases were considered for the bolt/borehole ratio having values of 0.86; 0.58; 0.53 and 0.36.
Figure 2 – Development of plastic points in resin with variable bolt/borehole diameter ratio
(b/b dia ratios from the left: 0.86; 0.58; 0.53; 0.36)
PULLOUT TESTING
Series of 28 pull out tests were performed on 3 m long cable bolts with various combinations of borehole diameters and bonding lengths for understanding this behaviour. The speed of loading the CRR system was set up by value of 30 kN per min to 40 kN per min. CRR systems were loaded until achieving CRR system fracture. In the cases where displacement of the cable bolt continued without increasing pullout force or in the cases of non-standard behavior, the pullout tests were considered as finished for the displacements having value of 80 mm.
The evaluation of the experiment – boreholes drilled by drill bit having diameter of 32 mm
Together 16 boreholes were drilled by drill bits having a diameter of 32 mm and cable bolts were grouted in them. Four groups of cable bolts with varying bonding length (0.6 m; 1.0 m; 1.4 m; 1.8 m) were tested in this part of the experiment. These bonding lengths were calculated from the mean value of borehole diameter estimated by statistical evaluation 34.3 mm. In each group together four cables were loaded.
Figure 4 – Load - displacement response of the CRR system during pullout test
(drill bit having a diameter of 32 mm, mean value of a borehole diameter 34.3 mm)
Figure 4 represents load-displacement response curves for the pullout tests for variable bonding length and drill bit having a diameter of 32 mm. It´s ultimate pullout load exceeded the guaranteed bearing capacity of the cable, this proves the grout column transferred the load and CRR system pull-out resistance and bond strength are effective (Yazici & Kaiser, 1992). As longer the bonding length was as farther linear response of the CRR can be recognised from the graphs.
Stiffness of the CRR system (S) is also growing with longer bonding length as quality of bond strength increase. This is reacting in higher amount of cases where fracture of cable wires happens as sudden drops on load–displacement response curve can be seen after reaching its ultimate pullout load.
CRR system stiffness was evaluated having a value 3 kN/mm for the bonding length 0.6 m and having a value 5 kN/mm for bonding length 1.8 m.
Numerical modelling was used to determine development of CRR system stiffness for various bonding length. It is possible to model slipping behavior on the interfaces of CRR system and it would make the results more closed to reality, but this functions of used software based on FEM analysis simply
bonding length 0.6 m bonding length 1.0 m
bonding length 1.4 m bonding length 1.8 m
allocate user´s allowed displacement for shear or normal stress at the interfaces. From this reason simple linear elastic model was used, response of displacement of CRR system under constant load for various bonding length and corresponding stiffness developments were in interest. Increasing stiffness with longer bonding length can be seen in figure 6.
Figure 5 – Result parameters of pullout tests with variable bonding length
(L - load, S - stiffness, m - maximum, 40 - corresponding to displacement 40 mm, l - linear)
Figure 6 – Stiffness development with various bonding length
(using FEM analysis, linear elastic model, displacement as linear response to constant load 100 kN)
The evaluation of the experiment-boreholes drilled by drill bit having diameter of 38 mm and 42 mm
Four groups of CRR systems with variable bonding length (0.5 m; 1.0 m and 1.4 m) were tested in this part of the experiment. In each group together 4 cables were installed. The behavior of CRR system in
0 100 200 300 400
0.5 1.0 D [m] 1.5 2.00.0
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
Lm [kN]
Load bearing capacity of cable [kN]
Sm [kN/mm]
S40 [kN/mm]
Sl [kN/mm]
S [kN/mm]
L [kN]
bonding of cable into rock. To simulate this case, drill bits of higher diameter values 38 mm and 42 mm were used and it´s responding load-displacement curves are presented. From the reason of irregular and in some cases anomalous responses from pullouts only ultimate pullout force and general behavior of the CRR system are taken into account as the results from the pullouts. Following figures 7 and 8 represent these responding behaviors of the CRR system in poor conditions.
Figure 7 – Load - displacement response of the CRR system during pullout test in poor conditions
(drill bit having a diameter of 38mm, mean value of a borehole diameter 40 mm)
Figure 8 – Load - displacement response of the CRR system during pullout test
(drill bit having a diameter of 40 mm, mean value of a borehole diameter 44 mm)
The load-displacement response curve for borehole having a diameter of 40 mm (the thickness of the grout annulus about 8 mm) in figure 7 presents that in a half of cases with having bonding length 1.0 m and 1.4 m, the ultimate pullout force riches a higher value than guaranteed bearing capacity of the used cable bolt (350 kN). The load-displacement curve for borehole having a diameter of 44 mm (the thickness of the grout annulus about 10 mm) in figure 8 presents maximum pullout force exceeding value of 200 kN for bonding length 1.0 m. In two situations an activation of the CRR system had not occurred and ultimate pullout force reached a value of 10-20 kN. Comparing figure 7 and 8 it is evident that smaller grout annulus makes CRR system more stable as load-displacement curve is of continuous character without common drops. These drops seen in figure 8 are probably results of the loss of bond strength or fracture of the grout annulus. Subsequently residual bearing capacity of the CRR system can be recognised as a new increase of loading is following depending on cohesion and mechanical interlocking on the interfaces.
bonding length 1.0 m
bonding length 1.4 m
bonding length 0.5 m bonding length 1.0 m
CONCLUSIONS
Based on the results of the first part of the experiment it can be stated that in the standard geological conditions in the rock mass without significant fracturing, with stable borehole walls, when proper borehole diameter is considered and when commonly used bonding length of 2 m is proposed, this type of cable bolt fulfill its function.
The second part of the experiment presents the behavior of the CRR system during pullouts in conditions simulating formation of caverns which enlarge a diameter of the borehole. It is necessary to pay close attention to the bolt/borehole ratio as well as to properly selected value of borehole diameter due to corresponding evaluation of the amount of resin. FEM modelling introduced plastic point’s development with various bolt/borehole ratios which promotes the higher value of the ratio.
We recommend using of video-inspections of installation boreholes in these cases. Video inspections may help to classify borehole condition better. It is possible to accurately determine the diameter and the status of the borehole on the video inspection basis. We also recommend carrying out
“design pull out test of CRR system” in poor conditions with variable bonding lengths for more effective final design of CRR system which will have significant effect on stability of mining workings.
ACKNOWLEDGEMENTS
This article has been written within the project of the Institute of Clean Technologies for Mining and Utilisation of Raw Materials for Energy Use, reg. no. CZ.1.05/2.1.00/03.0082, which is supported by the Research and Development for Innovations Operational Programme financed by the Structural Funds of the European Union and the state budget of the Czech Republic and has been supported by the Ministry of the Interior of the Czech Republic (project No. VG20102014034). The presented work was also sustained by a project to support long-term conceptual development of research organizations RVO:
68145535.
REFERENCES
Aydan, O. (1989). The stabilisation of rock engineering structures by rock bolts. (Doctoral dissertation).
Nagoya University.
Bouteldja, M. (2000). Design of cable bolts using numerical modelling (Doctoral dissertation).
Retrieved from:
http://digitool.library.mcgill.ca/R/-?func=dbin-jump-full&object_id=36552&silo_library=GEN01
Hutchinson, D. J., Mark, D. S. (1996). Cablebolting in underground mines. Richmond, B.C: BiTech Publishers.
Minova Bohemia s.r.o. (2013). Technical paper of Strand cable bolt REFLEX, FLEXIBOLT and IR-4.
Retrieved from:
http://www.minova.cz/download/2014/technicke-listy/cz/tl-pramencove-svorniky.pdf
Souček, K., Koníček, P., Staš, L., Šňupárek, R., Ptáček, J. (2012). The use of anchoring systems in OKR underground mines and testing of their resistance. Tunel, 21(2), 4-10. ISSN 1211-0728
Yazici, S., Kaiser, P. K. (1992). Bond strength of grouted cable bolts. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 29(3), 279-292.
doi: 10.1016/0148-9062(92)93661-3