Sample diameter effect on bonding capacity of fully grouted cable bolts

doi:10.1016/j.tust.2017.06.004

Tunnelling and Underground Space Technology

Volume 68, September 2017, Pages 238-243

https://doi.org/10.1016/j.tust.2017.06.004 Get rights and content

Abstract

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 356 mm in the unconfined condition. In the confined condition, however, the threshold reached at a low diameter of 300 mm. 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.

Introduction

The stability of underground excavations is of great importance in both civil and mining activities (Cai et al., 2004, Li et al., 2016). To increase the integrity of rock mass around openings, fully grouted cable bolts are commonly used. Numerous laboratory and field tests have demonstrated that cable bolts are effective in improving the inherent shear strength of rock mass (Kaiser et al., 1992, Hyett et al., 1995, Barley and Windsor, 2000, Singh et al., 2001, Moosavi and Grayeli, 2006, Ren et al., 2010, Chen et al., 2015, Chen et al., 2016, Li et al., 2017). Nevertheless, failure of cable bolting systems, especially bonding failure can present a problem caused by insufficient interfacial bonding capacity.

The bonding capacity of cable bolts is usually indicated by the maximum pull-out load (Fuller and Cox, 1975, Cox and Fuller, 1977, Hutchinson and Diederichs, 1996). To study the bonding capacity of cable bolts, a number of research has been conducted. The most commonly used method is pulling cable bolts from cylindrical samples made up of cement-based or concrete materials.

Stillborg (1984) used concrete materials to cast cylindrical samples with a diameter of 300 mm and conducted pull-out tests on plain cable bolts. His results showed that the greasy substance coated on plain cable bolts had a negative effect on the bonding capacity of cable bolts and recommended that the cable bolt surface should be kept clean before installation. Farah and Aref (1986a) also conducted pull-out tests on plain cable bolts installed in cylindrical blocks. Two different block diameters, 150 mm and 250 mm, were used. They found that cable bolts grouted with concrete materials had larger bonding capacity than those grouted with plain cement. Hassani and Rajaie (1990) used concrete blocks with a diameter of 250 mm to confine cable bolts and studied the effect of grout materials on the bonding capacity of cable bolts. The results showed that compared with the plain cement grout, the aggregate-cement grout can improve the peak and residual bonding capacity of cable bolts. Benmokrane et al. (1995) conducted pull-out tests on plain cable bolts installed in samples with a diameter of 200 mm, finding that the cable surface geometry and grout had a significant effect on the bonding capacity of cable bolts, especially when large displacement occurred. Altounyan and Clifford (2001) used a different material, namely sandstone to confine cable bolts and conducted pull-out tests. The diameter of samples is 142 mm. Testing results showed that pre-tension can effective improve the performance of cable bolts. The same test approach was also used by Bigby (2004), who found that the cable geometry had a significant effect on the stiffness of the cable bolting system. Blanco Martín (2012) developed a laboratory short encapsulation pull test for rock tendons and conducted pull-out tests on plain cable bolts. The sandstone samples with a diameter of 142 mm were used. Test results showed that plain cable bolts can maintain a high bonding capacity to a large displacement of 60 mm in pull-out tests.

According to the literature review, the effect of different parameters such as cable surface geometry, grout quality and coating materials on the bonding capacity of cable bolts has been studied. However, a common problem occurring in those tests is that the sample diameter is largely different. This is due to the fact that there is no common test standard in requiring the samples’ diameter and the variation in sample diameter made it difficult to compare previous test results.

In fact, little research has been conducted to evaluate the sample diameter effect on the bonding capacity of cable bolts. The pioneering work was conducted by Rajaie (1990), who evaluated the sample diameter effect on the pull-out performance of plain cable bolts. Specifically, he used concrete materials to cast samples with the diameter ranging from 100 mm to 300 mm. The test results showed that when the sample diameter is smaller than 200 mm, the bonding capacity of cable bolts apparently increased. However, once the sample diameter is larger than 250 mm, the bonding capacity became stable, as shown in Fig. 1. Therefore, he concluded that the sample diameter had a direct effect on pull-out performance of cable bolts and recommended that when pulling a cable bolt from the sample, the sample’s diameter should be large enough. However, his results are only applicable to plain cable bolts.

Holden and Hagan (2014) used same approach to test the sample diameter effect on a bulbed cable bolt. Four different sample diameters ranging from 150 mm to 450 mm were used. It was found that there is a linear relationship between the bonding capacity and sample diameter. However, during the test, no confinement was provided on the sample.

The sample diameter effect on the bonding capacity of cable bolts has not been fully understood. Furthermore, no research has been conducted in evaluating the sample diameter effect on bulbed cable bolts in the confined condition.

Therefore, this paper aims at studying the sample diameter effect on the bonding capacity of a bulbed cable bolt. The single embedment pull-out test method was used. Because first, it is the most commonly used method to test the performance of cable bolts (Stillborg, 1984, Farah and Aref, 1986b, Farah and Aref, 1986a, Hassani and Rajaie, 1990, Benmokrane et al., 1992, Benmokrane et al., 1995, Altounyan and Clifford, 2001, Tadolini et al., 2012, Holden and Hagan, 2014). Second, Rajaie (1990) conducted the pioneering work to study the sample diameter effect on cable bolts and he used the single embedment pull-out test. Following his work, the single embedment pull-out test method was also used in this study. This makes it possible to compare test results with Rajaie (1990)’s results.

In this paper, first, the pull-out test design was depicted. Then, a number of pull-out tests were conducted using a bulbed cable bolt. After that, the pull-out test results were compared. The effect of sample diameter and boundary condition on the bonding capacity of cable bolts was also analysed.

Section snippets

Experimental program

Two series of pull-out tests were performed using a high capacity bulbed cable bolt with different boundary conditions. In the first series of tests, samples were left in an unconfined state, while in the second series of tests, the samples were placed in a steel cylinder to be tested in confined state.

Test results of the SUMO cable bolt

The bonding capacity at different sample diameters in the unconfined condition is shown in Fig. 8 and Table 1. This shows that the sample diameter has a direct impact on the bonding capacity. Specifically, as the sample diameter rose up to 356 mm, the bonding capacity increased almost linearly to 100 kN.

A different relationship between the ing capacity and sample diameter occurred in the confined sample environment, as shown in Fig. 9 and Table 2. This also shows that bonding capacity increases

Conclusions

A number of pull-out tests were conducted, using a high capacity cable bolt to investigate the effect of sample diameter on the bonding capacity of cable bolting systems. A cement-based material was used to cast samples with diameters ranging between 150 mm and 508 mm. Two different boundary conditions, namely unconfined and confined boundary conditions, were examined.

A new approach was proposed to create the rifled borehole effect. A plastic wire was wrapped around the PVC pipe. After casting,

Acknowledgements

The authors would like to thank the Australian Coal Association Research Program (ACARP) for funding this research project. In addition, the authors would like to acknowledge the China Scholarship Council (CSC).

References (33)

B. Benmokrane et al.
Laboratory evaluation of cement-based grouts and grouted rock anchors
Int. J. Rock Mech. Min. Sci.
(1995)
Y. Cai et al.
An analytical model to predict axial load in grouted rock bolt for soft rock tunnelling
Tunn. Undergr. Space Technol.
(2004)
J. Chen et al.
An analytical model of the load transfer behavior of fully grouted cable bolts
Constr. Build. Mater.
(2015)
A.J. Hyett et al.
A constitutive law for bond failure of fully-grouted cable bolts using a modified hoek cell
Int. J. Numer. Anal. Meth. Geomech.
(1995)
A.J. Hyett et al.
The effect of rock mass confinement on the bond strength of fully grouted cable bolts
Int. J. Rock Mech. Min. Sci.
(1992)
L. Li et al.
Shear resistance contribution of support systems in double shear tests
Tunn. Undergr. Space Technol.
(2016)
X. Li et al.
Comparison of the shear test results of a cable bolt on three laboratory test apparatuses
Tunn. Undergr. Space Technol.
(2017)
M. Moosavi et al.
A model for cable bolt-rock mass interaction; integration with discontinuous deformation analysis (DDA) algorithm
Int. J. Rock Mech. Min. Sci.
(2006)
F. Ren et al.
An analytical analysis of the full-range behaviour of grouted rockbolts based on a tri-linear bond-slip model
Constr. Build. Mater.
(2010)
R. Singh et al.
Cable-bolting-based semi-mechanised depillaring of a thick coal seam
Int. J. Rock Mech. Min. Sci.
(2001)

P. Altounyan et al.

Suitable Long Tendon Technologies and Practices (Report)

(2001)

Barley, A.D., Windsor, C.R., 2000. Recent advances in ground anchor and ground reinforcement technology with reference...

B. Benmokrane et al.

Study of bond strength behaviour of steel cables and bars anchored with different cement grouts

D. Bigby

Coal Mine Roadway Support System Handbook (Report)

(2004)

L. Blanco Martín

Theoretical and experimental study of fully grouted rockbolts and cablebolts under axial loads

(2012)

Blanco Martín, L., Hadj-Hassen, F., Tijani, M., Noiret, A., 2011. A new experimental and analytical study of fully...

Cited by (31)

Two-dimensional numerical analysis on the rock/bolt interaction considering shear and normal relative displacements
2024, Tunnelling and Underground Space Technology
Fully-grouted passive bolts are widely used in underground or surface rock excavations and in particular in stabilizing potentially unstable blocks of rock due to sliding on natural discontinuities. Their operating mechanism is complex, but it is possible to consider two stabilizing forces that each bolt applies to the block of rock. These forces depend on the mechanical parameters governing the bolt-rock interaction, which are difficult to evaluate. In this work, specific numerical analyses have been developed, able of evaluating the bolt-rock interaction (in shear and perpendicular to the interface that separates them) for numerous cases that were obtained by varying the main geometric parameters of the bolt, the mechanical properties of the binder material and rock. Thanks to this complex study, it was possible to describe the variability of the interaction parameters and to define, through graphs, the trend of the stabilization forces as the main geometric and mechanical parameters that can be encountered in practice change. The graphs obtained are a useful tool for the correct design of fully-grouted passive bolts and the stabilization of potentially unstable rock blocks on the walls of underground cavities or on the faces of surface excavations.
Bond-slip model of anchor-soil interface and analysis of bearing capacity of anchor in frozen soil
2023, Cold Regions Science and Technology
It has been recognized that temperature and moisture have obvious influence on the bonding characteristics of anchor-frozen soil interface. Following the principle and method used to explore the interface performance of microelement anchor-soil, this research starts by performing the pullout tests of anchor-soil under different temperature and moisture content conditions. Then, we analyze the characteristics of interface bond stress-displacement and compare the effects of temperature and moisture content on interface mechanical properties. The test results show that the failure mode of anchor changes slightly when soil temperature and moisture content vary, wherein the shear failure which shows brittle characteristics at the anchor-frozen soil interface is dominant. Additionally, we have found that the effect of temperature on the peak bond stress is more significant than moisture. Under different soil temperatures and moisture content, the shape of bond stress-displacement curve of anchor-frozen soil interface is similar. Furthermore, the bond stress-displacement relationship of anchor-frozen soil interface can be fitted by a composite function which is composed of exponential and ideal elastic-plastic model function. Thus, this research recommends to propose a bond-slip model of anchor-frozen soil interface considering temperature effect. Combined with the load transfer method, the force and deformation equation of anchor in frozen soil is derived. Through example analysis, it is discovered that the pullout bearing capacity of anchor increases with the decrease of soil temperature. Moreover, the initial shear stiffness of anchor-soil interface is positively related to the bearing capacity of anchor. At the end, the influence of interfacial residual strength on the bearing capacity of anchor in the later stage of pullout is superior to that in the earlier stage.
Study on shear behavior and damage constitutive model of tendon-grout interface
2022, Construction and Building Materials
Citation Excerpt :
In the 1970s, Fuller and Cox [26] first performed experiments to examine the bond-strength at the cable-grout interface. Since the 1990s, many experimental and theoretical studies have been conducted to examine the mechanical performance of the tendon-grout interface and the factors influencing this performance, due to the extensive application of ground anchorage technologies [6,11,27–29]. However, most of available studies on the pull-out tests are limited to the condition of constant radial pressure.
Ground anchors are responsible for maintaining the stability and integrity as well as controlling the deformation, displacement, and crack development in the surrounding ground. Tendon-grout (T-G) interface, which is the weak part of anchorage structures, affects the anchorage performance of the ground anchor during load-transfer processes. To investigate the shear mechanical properties of the tendon-grout interface, a series of pull-out tests were conducted on anchorage element under constant radial stiffness boundary conditions. Moreover, the equivalent calculation for the constant radial stiffness boundary condition was analyzed. The mechanical performance of cement grouts cured for different periods was investigated, including the compressive strength and shear strength. Additionally, the failure modes of tendon-grout systems and effects of different tendon quantity, spacing and offset, were assessed. The failure evolution mechanism of the interface during the pull-out process was clarified. In addition, the microscopic structural characteristics of the tendon-grout interface were investigated. Two interfacial shear stress-displacement relationships were developed based on the interface failure modes, i.e., stress attenuation type and ideal elasto-plastic type. According to the test results, the peak strength of the interface increased with the tendon spacing, while the residual strength decreased. The effect of the tendon offset on the shear stiffness and strength of the interface is significant. A damage constitutive model was proposed to model the shear behavior of the tendon-grout interface. The proposed model verified by comparing with the test results and further adopted in pull-out test simulation. The conclusions will provide a better understanding on the shear behaviors of tendon-grout interface, and facilitate the development of a general evaluation method for ground anchor systems.
Application of disturbed state concept for load-transfer modeling of recoverable anchors in layer soils
2021, Computers and Geotechnics
Citation Excerpt :
The interface behaviors between soil and grout are of practical importance in pullout capacity assessment of grouted anchors/ soil nails (Chu and Yin, 2005; Yin and Zhou, 2009; Ye et al., 2019). The direct shear testing method (Chu and Yin, 2005; Hossain and Yin, 2014; Toufigh et al., 2017; Moayed et al., 2019) and the pullout testing method (Yin and Zhou, 2009; Ma et al., 2013; Chen et al., 2017; Ye et al., 2019; Chen et al., 2015; 2018; 2020) are the two commonly adopted methods for measuring the shear stress–displacement behavior at soil-grout interface. Hereinto, the construction process for the pullout testing method is more in line with the actual behavior of grouted anchors/soil nails in the field, while the stress condition is clearer for the direct shear testing method.
As a temporary anchorage technique, recoverable anchors have been widely adopted in excavation supporting and underground mining for their unique economic and social merits. The current study attempts to investigate the load-transfer behavior of recoverable anchors in layer soils. The interaction between soil and grout was characterized by a new developed disturbed state concept (DSC) model, in which the interface normal stress was considered. The good agreement between predictions and experimental results from element pullout tests and interface direct shear tests validated the extensive applicability of the DSC model. A theoretical modeling framework for load-transfer behavior of recoverable anchors in layer soils was proposed based on finite difference method, in which the soil-grout interactions were simulated by the DSC model. 3D finite element (FE) models were developed to simulate the pullout behavior of recoverable anchors. The results obtained from theoretical modeling and FE simulations were discussed in detail. Comparisons with numerical simulations and in-situ test data have led to the validation of the theoretical modeling framework. Finally, parametric studies were presented. Application of this work is to evaluate the pullout capacity of recoverable anchors, which can provide theoretical guidance for the design practice of anchoring engineering.
DEM investigation of rock/bolt mechanical behaviour in pull-out tests
2020, Particuology
Citation Excerpt :
However, the experimental data remain insufficient for reasonable parameter analysis over a wide range of conditions and the failure modes that arise during pull-out tests remain poorly understood. The sample size (when the bolt size is fixed) strongly affects the test results mainly owing to the influence of side boundaries (Chen, Hagan, & Saydam, 2017; Holden & Hagan, 2014; Morgan, Niwa, & Tanabe, 1999; Rajaie, 1990). The minimum sample size by which reasonable results can be achieved is of great significance in experimental studies.
Rock bolt anchorage performance is crucial for tunnel support safety. We investigate the mechanical behaviour of reinforced rock and the bolts that reinforce it from the micro-scale to the macro-scale. Bolt pull-out tests were performed on soft rock using the distinct element method, in which a new contact model that considers bond size, is employed to constrain the main rock mechanical behaviour. The minimum sample width and height values for which the boundary effect can be neglected are first proposed through numerous tests on the influence of sample size on peak load and bond breakage. The influence of sample width is substantially greater than that of sample height. We then select an appropriate sample size to study the influence of bolt embedment length and confining pressure on the mechanical behaviours of the rock and bolt. The results show that increased rock bolt embedment length and confining pressure can increase the peak load; however, the bolt length effect is limited when exceeding the critical anchorage length. In cases without confining pressure, bond breakage occurs in the rock around the grout-rock interface and the breakage zone is rectangular, whereas in cases under confining pressure, the breakage zone presents an inverted cone shape. We use our results to discuss the influence of bond strength at the bolt–grout interface on the peak load and failure mode. The failure mode changes gradually from complex failure to single failure along the bolt–grout interface with decreasing interfacial bond strength.
3D analytical investigation on the overall pullout behavior of grouted anchorages in presence of shear failure of grout
2020, Engineering Failure Analysis
Grouted anchorages have been extensively applied in practical projects like reinforcing steel anchors embedded into concrete or rock through cement grouting. The pullout behavior of the anchorages is generally a three-dimensional (3D) problem, whereas most existing analytical studies are based on a one-dimensional (1D) assumption such that the overall behavior of the anchorages has not been fully understood. In this paper, a 3D analytical solution is presented that is capable of predicting the overall pullout behavior of grouted anchorages in the presence of shear failure of grout, where the shear behavior of the grout is simulated by a bilinear shear stress-displacement relation. 3D closed-form solutions are deduced for the displacement field, the stress field and the load-displacement curve of both long and short anchorages for various loading phases. The analytical solution is validated by comparing the analytical results with the experimental and numerical results from the literature. The load-displacement characteristics of long and short anchorages are elaborated with the influence of the anchorage length on the load-displacement response evaluated. Comparative study is conducted to investigate the difference between 1D and 3D solutions on the global load-displacement response and the local shear stress distribution. The results show that there is no significant difference between 1D and 3D solutions in the case of very short anchorages. However, Poisson’s effect plays an increasing role in the stress transfer as the anchorage length increases, leading to an increasing discrepancy between the two solutions, which demonstrates that neglecting Poisson’s effect has an impact on the performance prediction of grouted anchorages. The present 3D analytical solution, with Poisson’s effect taken into consideration compared to 1D solution, can be expected to be better capable of evaluating the overall pullout behavior of grouted anchorages, especially for long anchorages. While the present study is conducted with reference to grouted anchorages, the analytical solution remains applicable to similar bonded structures such as reinforced composites and cylindrical adhesive joints.

View all citing articles on Scopus

View full text

Sample diameter effect on bonding capacity of fully grouted cable bolts

Abstract

Introduction

Section snippets

Experimental program

Test results of the SUMO cable bolt

Conclusions

Acknowledgements

Int. J. Rock Mech. Min. Sci.

Tunn. Undergr. Space Technol.

Constr. Build. Mater.

Int. J. Numer. Anal. Meth. Geomech.

Int. J. Rock Mech. Min. Sci.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Int. J. Rock Mech. Min. Sci.

Constr. Build. Mater.

Int. J. Rock Mech. Min. Sci.

Suitable Long Tendon Technologies and Practices (Report)

Study of bond strength behaviour of steel cables and bars anchored with different cement grouts

Coal Mine Roadway Support System Handbook (Report)

Theoretical and experimental study of fully grouted rockbolts and cablebolts under axial loads