The effect of rock mass confinement on the bond strength of fully grouted cable bolts

doi:10.1016/0148-9062(92)92634-O

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts

Volume 29, Issue 5, September 1992, Pages 503-524

International Journa...

https://doi.org/10.1016/0148-9062(92)92634-O Get rights and content

Abstract

A laboratory and field research programme was conducted to investigate the major factors influencing the bond capacity of grouted cable bolts. All tests were conducted on standard $58$ ″ (15.9 mm) 7-strand cable groted using type 10 Portland cement pastes. The results indicate that cable bolt capacity most critically depends on:

1.
(i) the cement properties, which are primarily controlled by water; cement ratio;
2.
(ii) the embedment length; and
3.
(iii) the radial confinement acting on the outer surface of the cement annulus
4.
(iii) the radial confinement acting on the outer surface of the cement annulus.

The material properties of cement paste vary with the water: cement ratio of the mix. The use of low water: cement ratio grouts (<0.40 by wt) can increase peak cable bolt capacities by 50–75%. This can be attributed to both their high uniaxial compressive strengths and their high Young's moduli. The effect is maximized under conditions of high radial confinement. However, the use of super-thick pastes (0.30 and less) may be both impractical and undesirable, first because of their limited pumpability and second because of their inconsistency in strength.

Tests at different embedment lengths indicated that cable bolt capacity increased with embedment length although not in direct proportionality. All tests were conducted with embedment length: cable diameter ratios in excess of 15 (below 5 the decay in shear stress along the cable can be ignored). Consequently, failure is non-simultaneous in nature, with one section having failed while another is approaching peak capacity.

In the laboratory “split-pipe” tests were conducted using PVC, Al and steel pipes to provide radial confinement, and in the field, surface test sites were chosen in granite, limestone and shale rock masses, as well as an underground case study at the Golden Giant Mine. In general, higher capacities were obtained under conditions of higher radial confinement. A correlation between the laboratory and the field test results was obtained through a comparison of the radial stiffness of the laboratory pipes with that of the field boreholes as measured using a high-pressure dilatometer. As the degree of radial confinement increased the failure mechanism changed from radial fracturing and lateral displacement of the grout annulus under low confinement, to shear of the cement flutes and pull out along a cylindrical frictional surface under high confinement.

References (23)

W.F. Bawden et al.
An experimental procedure for in situ testing of cable bolts
Int. J. Rock Mech. Min. Sci. & Geomech. Abstr.
(1992)
P. Bartos
Review paper: bond in fibre reinforced cements and concretes
Int. J. Cem. Comput. Ltwt. Concr.
(1981)
I.W. Farmer
Stress distribution along a resin grouted rock anchor
Int. J. Rock. Mech. Min. Sci & Geomech. Abstr.
(1975)
P.G. Fuller et al.
Mechanics of load transfer from steel tendons to cement based grout
J.M. Goris
Laboratory evaluation of cable bolt support
Y. Potvin et al.
Cable Bolt Research Project—Status Report
NTC Internal Report
(1989)
Hyett A. J., Bawden W. F. and Reichert R. D. The mechanics of cable bolt failure: a laboratory investigation. In...
P.K. Mehta
A.J. Hyett et al.
Physical and mechanical properties of normal Portland cement pertaining to fully grouted cable bolts
R.D. Reichert
A laboratory and field investigation of the major factors influencing bond capacity of grouted cable bolts

F.E. Richart et al.

Relations between voids and plasticity of cement mortars at different water contents

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Failure characterization of fully grouted rock bolts under triaxial testing
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Confining stresses serve as a pivotal determinant in shaping the behavior of grouted rock bolts. Nonetheless, prior investigations have oversimplified the three-dimensional stress state, primarily assuming hydrostatic stress conditions. Under these conditions, it is assumed that the intermediate principal stress (σ₂) equals the minimum principal stress (σ₃). This assumption overlooks the potential variations in magnitudes of in situ stress conditions along all three directions near an underground opening where a rock bolt is installed. In this study, a series of push tests was meticulously conducted under triaxial conditions. These tests involved applying non-uniform confining stresses (σ₂ ≠ σ₃) to cubic specimens, aiming to unveil the previously overlooked influence of intermediate principal stresses on the strength properties of rock bolts. The results show that as the confining stresses increase from zero to higher levels, the pre-failure behavior changes from linear to nonlinear forms, resulting in an increase in initial stiffness from 2.08 kN/mm to 32.51 kN/mm. The load-displacement curves further illuminate distinct post-failure behavior at elevated levels of confining stresses, characterized by enhanced stiffness. Notably, the peak load capacity ranged from 27.9 kN to 46.5 kN as confining stresses advanced from σ₂ = σ₃ = 0 to σ₂ = 20 MPa and σ₃ = 10 MPa. Additionally, the outcomes highlight an influence of confining stress on the lateral deformation of samples. Lower levels of confinement prompt overall dilation in lateral deformation, while higher confinements maintain a state of shrinkage. Furthermore, diverse failure modes have been identified, intricately tied to the arrangement of confining stresses. Lower confinements tend to induce a splitting mode of failure, whereas higher loads bring about a shift towards a pure interfacial shear-off and shear-crushed failure mechanism.
A hardening load transfer function for rock bolts and its calibration using distributed fiber optic sensing
2023, Journal of Rock Mechanics and Geotechnical Engineering
Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior, yet only a few experimental works and analytical models have been reported, most of which are based on the global rock bolt response evaluated in pull-out tests. This paper presents a laboratory experimental setup aiming to capture the rock formation effect, while using distributed fiber optic sensing to quantify the effect of the confinement and the reinforcement pull-out behavior on a more local level. It is shown that the behavior along the sample itself varies, with certain points exhibiting stress drops with crack formation. Some edge effects related to the kinematic freedom of the grout to dilate are also observed. Regardless, it was found that the mid-level response is quite similar to the average response along the sample. The ability to characterize the variation of the response along the sample is one of the many advantages high-resolution fiber optic sensing allows in such investigations. The paper also offers a plasticity-based hardening load transfer function, representing a "slice" of the anchor. The paper describes in detail the development of the model and the calibration/determination of its parameters. The suggested model captures well the coupled behavior in which the pull-out process leads to an increase in the confining stress due to dilative behavior.
Characterization of mechanical and bonding properties of anchoring resins under elevated temperature
2023, International Journal of Rock Mechanics and Mining Sciences
Resin anchored rock bolts are widely used in the construction and mining industries to support and reinforce underground roadways and openings. However, increasing the depth of underground openings, particularly in coal mines, has presented a challenge for ground control designers due to the rise in geothermal heat and coal seam combustion. This study aims to comprehensively investigate the mechanical properties, microstructure, thermal and curing characteristics of the anchoring resins using Thermogravimetric Analysis, Dynamic Mechanical Analysis, and Differential Scanning Calorimetry tests. Pullout tests were conducted on resin encapsulated rock bolts to quantify their bonding performance at elevated temperatures ranging from 20 °C to 250 °C. The results showed that the mechanical properties of the resins are closely related to the type of curing agent, filler type and size, and curing time. The compressive strength and elastic modulus of the anchoring materials ranged from 51 to 103 MPa. DSC testing indicated that 65–83% of curing can be achieved in 30 min. At ambient conditions, good agreement was found between the compressive and shear properties of the anchoring resins and their corresponding bond resistance force. A heating and pull-out setup was fabricated to analyze the effects of temperature on the bonding performance of rock bolts chemically anchored in underground spaces. The results revealed a reduction in bonding capacity of the bolts by 6.6%–31.3% when the temperature of the environment reached 75 °C and 150 °C, respectively. The anchored bolts maintained up to 62.6% of their bonding resistance when the temperature increased to 250 °C. Temperature profiles measured by the thermocouples along the encapsulation length showed that the heat transition is independent of the resin type and more dependent on the rock bolt specification.
Interface bond degradation and damage characteristics of full-length grouted rock bolt in tunnels with high temperature
2023, Journal of Rock Mechanics and Geotechnical Engineering
Full-length grouted bolts play a crucial role in geotechnical engineering thanks to their excellent stability. However, few studies have been concerned with the degrading performance of grouted rock bolts caused by extensive and continuous heat conduction from surrounding rocks in high-geothermal tunnels buried more than 100 m (temperature from 28 °C to 100 °C). To investigate the damage mechanism, we examined the time-varying behaviors of grouted rock bolts in both constant and variable temperature curing environments and their damage due to the coupling effects of high temperature and humidity through mechanical and micro-feature tests, including uniaxial compression test, pull-out test, computed tomography (CT) scans, X-ray diffraction (XRD) test, thermogravimetric analysis (TGA), etc., and further analyzed the relationship between grout properties and anchorage capability. In order to facilitate a rapid assessment and control of the anchorage performance of anchors in different conditions, results of the interface bond degradation tests were correlated to environment parameters based on the damage model of interfacial bond stress proposed. Accordingly, a thermal hazard classification criterion for anchorage design in high-geothermal tunnels was suggested. Based on the reported results, although high temperature accelerated the early-stage hydration reaction of grouting materials, it affected the distribution and quantity of hydration products by inhibiting hydration degree, thus causing mechanical damage to the anchorage system. There was a significant positive correlation between the strength of the grouting material and the anchoring force. Influenced by the changes in grout properties, three failure patterns of rock bolts typically existed. Applying a hot-wet curing regime results in less reduction in anchorage force compared to the hot-dry curing conditions. The findings of this study would contribute to the design and investigations of grouted rock bolts in high-geothermal tunnels.
Experimental study on the pullout behaviour of rockbolts with steel fiber-reinforced grout under constant confining pressures
2023, Construction and Building Materials
Deep underground excavation in civil and mining engineering motivates the development of high-strength rockbolting reinforcement. A feasible way to promote the performance of the fully grouted rockbolt is using additives to ameliorate grout properties and thus the bolt-grout interfacial behaviour. Here, we investigated the mechanical behaviour of fully grouted rockbolt specimens in the steel fiber-reinforced grout (FRG) under various constant confining pressures to pullout loading by the self-developed laboratory-scale rockbolt pullout system. We found that the bond strength of the FRG rockbolt specimen can be more than three times high of the plain grout one. The higher the fiber proportion, the greater the bond strength of the FRG rockbolt. When a constant confining pressure was applied, the bond strengths of all the grouted rockbolt specimens were considerably elevated and the bolt rod failure was observed for the FRG rockbolt specimen under 1.0 MPa confining pressure. Compared with the plain grout one, the post-peak failure of the FRG rockbolt specimen of a much higher bond strength was less brittle. The wavy steel fiber performed better in bond strength enhancement and grout cracking mitigation than the straight one mainly due to the crack bridging effect facilitated by its rough geometry. Our experimental studies suggested that FRG rockbolting is promising in reinforcing roadways/tunnels in the deep underground environment.
Effect of nano-Al<inf>2</inf>O<inf>3</inf>/epoxy resin composite on the shear strength recovery of fractured rock masses with various crack widths and SCA interfacial treatments
2023, Case Studies in Construction Materials
The mechanical properties of grout and the effect of the bonding between grout and the repaired material are the two main factors affecting the shear strength of the material. To study the effect of repairs carried out with nano-Al₂O₃/epoxy resin composite (NAEC) on the shear strength of rock masses, cracks with different widths and treatments by silane coupling agent (SCA) solutions with various surface treatment concentrations ( $k_{SCA}$ ) were considered. The mass percentage ratio of nano-Al₂O₃ in the composite ( $k_{Al}$ ) was optimized based on the obtained uniaxial compressive strength (UCS). The selected NAEC was used to fill cracks of different widths, and the bonding effect was analyzed using a bilateral shear strength test that effectively avoids the bending effect. Solutions with diverse $k_{SCA}$ values were used to treat the rock surfaces with the 3-mm-wide cracks. The results showed that the UCS of the NAEC tended to increase with increasing $k_{Al}$ when the mass ratio ( $k_{CE}$ ) of the curing agent to epoxy resin was 0.25. When $k_{Al}$ = 0.01, the UCS of the composite peaked at 97.82 MPa. With increasing crack width, the shear strength first increased and then decreased. When the crack width was 3 mm, the shear strength reached a maximum of 29.02 MPa. However, without nano-Al₂O₃, the shear strength was 23.66 MPa, and the shear strength of ultrafine Portland cement (UPC) material was only 8.37 MPa. With increasing $k_{SCA}$ , the shear strength tended to first increase and then decrease. For a $k_{SCA}$ of 2%, the shear strength reached a maximum of 39.40 MPa, corresponding to an increase of 35.8%. Nevertheless, the composite shear strength as degraded at 5% $k_{SCA}$ . In summary, the NAEC exhibited a sound bonding effect, SCA surface treatments effectively improved the shear strength, and the bonding shear strength was related to the crack width.

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International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts

Abstract

References (23)

An experimental procedure for in situ testing of cable bolts

Int. J. Rock Mech. Min. Sci. & Geomech. Abstr.

Review paper: bond in fibre reinforced cements and concretes

Int. J. Cem. Comput. Ltwt. Concr.

Stress distribution along a resin grouted rock anchor

Int. J. Rock. Mech. Min. Sci & Geomech. Abstr.

Mechanics of load transfer from steel tendons to cement based grout

Laboratory evaluation of cable bolt support

Cable Bolt Research Project—Status Report

NTC Internal Report

Physical and mechanical properties of normal Portland cement pertaining to fully grouted cable bolts

A laboratory and field investigation of the major factors influencing bond capacity of grouted cable bolts

Relations between voids and plasticity of cement mortars at different water contents

Cited by (196)

Failure characterization of fully grouted rock bolts under triaxial testing

A hardening load transfer function for rock bolts and its calibration using distributed fiber optic sensing

Characterization of mechanical and bonding properties of anchoring resins under elevated temperature

Interface bond degradation and damage characteristics of full-length grouted rock bolt in tunnels with high temperature

Experimental study on the pullout behaviour of rockbolts with steel fiber-reinforced grout under constant confining pressures

Effect of nano-Al<inf>2</inf>O<inf>3</inf>/epoxy resin composite on the shear strength recovery of fractured rock masses with various crack widths and SCA interfacial treatments