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• Sound waves are analyzed to evaluate the unbonded length of the rock bolt. • Cross-correlated sound waves provide a clearer peak in the frequency response. • Resonant effect of reverberant sound waves is identified in the wavelet transform. • Predominant frequency decreases with an increase in the unbonded length. This study aimed to inspect the unbonded length of a rock bolt using the built-in microphone of a smartphone. An experimental study was conducted using rock bolts of different unbonded lengths. Sound waves were produced by impacting the head of the rock bolt, and the reverberation of the sound waves in the unbonded section was recorded. To compensate for the phase difference between the two signals, the stereo audio signals were extracted from the video and cross-correlation was conducted. The frequency spectrum of the cross-correlated signal shows a clear peak compared to that of original signal. Additionally, the predominant frequency decreased with increasing unbonded length. Time-frequency analysis was accomplished using a wavelet transform. Wavelet transform identified that the predominant frequency was the resonant effect of the reverberant sound waves. This study suggests that the predominant frequency of the reverberant sound waves in the unbonded section is an effective indicator for evaluating the unbonded length of the rock bolt. [ABSTRACT FROM AUTHOR]

• The necessity of a bolt–cable support for large-span tunnelling was analyzed. • Rock mass deformation characteristics were investigated by numerical modelling. • The mechanical behaviors of pretensioned rock bolts and anchor cables are analyzed. • The load evolution and lining responses of large-span tunnel were monitored. This study focused on a new support method, a pretensioned bolt–cable combined support system, for large-span tunnelling. The Badaling tunnel is a high-speed railway station tunnel that has a span of 32.7 m, a cross-sectional area of 494 m2, and a maximum depth of 102 m. The super large cross-section and unfavorable geological conditions around the tunnel resulted in considerable difficulties for tunnel construction. Because this tunnel was successfully excavated using the pretensioned bolt–cable support, the mechanical characteristics of this new support system are analyzed in this study. First, the necessity of a bolt–cable support for large-span tunnelling was analyzed using a numerical model. The deformation characteristics of rock mass in the large-span tunnel are discussed based on the modelling results of the tunnelling process. Second, based on field monitoring results, the mechanical behaviors of pretensioned rock bolts and anchor cables were analyzed. The anchoring forces of the rock bolts and anchor cables were determined by the pretension forces, loss of pretension forces, and passive support effects. The development of the anchoring force can be divided into three phases: rapid loss of the pretension force, a fluctuation phase, and a stable phase. Finally, The loading effect and mechanical responses of the primary lining were analyzed according to the monitoring results of the primary lining, which indicated that the rock mass was effectively reinforced by the pretensioned bolt–cable system and that the primary lining was safe. [ABSTRACT FROM AUTHOR]

In addressing limitations of conventional ground support measurement, a novel optical strain sensing technique is presented that is capable of measuring strain at increments as low as 0.65 mm along the entirety of an optical sensor affixed to a support element. The technique considers monitoring three sensing lengths along the profile of a fully grouted rock bolt element using a single optical fiber, which, in turn, allows the derivation of both the principle strain and principle strain direction along the bolt. A series of three experiments, which include: symmetric bending, combined axial load and bending, and double shear loading are presented and highlight the potential of the technique to capture bolt behaviour under such generalized rock bolt loading conditions. Analogized as a distributed strain rosette, the optical technique can distinguish both the coaxial and bending induced constituents of the total strain in the bolt regardless of load orientation. [ABSTRACT FROM AUTHOR]

A comprehensive study including laboratory tests and numerical modeling was performed to investigate factors of rock bolt fracture. Experimental tests were performed to evaluate the mechanical performances of bolt ends including the thread and adjacent. The effects of installation angle, pretension and accessories (plate, washer and nut) were evaluated. It is found that a bolt installed with an oblique angle to roadway surface suffers a complex stress combination of tensioning, bending and twisting on the bolt. This complex stress is much greater than the tensile stress, leading to cracking and growth at the location of bending causing the thread to fail even though the tensile stress is less than the yield strength. Bolt thread is more vulnerable to fracture than rebar. [ABSTRACT FROM AUTHOR]

Rock bolting is one of the most effective and economical means of rock mass reinforcement. Existing studies of rock bolt reinforcement are mostly focused on rock masses without flaw, with a single flaw, or with parallel flaws. However in rock masses, cracks or flaws usually exist in the form of cross-flaws. In order to understand the impact of cross-flaws on rock bolt reinforcement and to further explore the differences of bolt reinforcement between rock mass with cross-flaws and rock mass with a single flaw, reinforced analog specimens with cross-flaws and with a single flaw were tested under uniaxial compressive condition. The experimental results show that the uniaxial compressive strength of the reinforced rock mass with cross-flaws in this research is higher than that of reinforced rock mass with a single flaw. This observation can be explained by the difference in the failure modes of reinforced specimens: the reinforced rock masses with a single flaw fail due to the formation of a shear crack while reinforced rock masses with cross-flaws fail as a result of a tensile fracture or interaction between tensile fracture and shear fracture. [ABSTRACT FROM AUTHOR]

The performance of two types of rock bolts, fully encapsulated rebar and D-Bolt, under combined pull and shear loading were studied in the laboratory. A new test approach was developed to apply the pull and shear loads to the rock bolt at the same time so that the bolt is displaced in a direction different from the bolt axis. Five displacing angles between 0° (pure pull) and 90° (pure shear) were employed in the tests. The test results show that the linear elastic stiffness of both the D-Bolt and the rebar bolt is mobilised quickly after a small displacement. When the displacing angle is larger than 40°, grout crushing may occur underneath the bolt shank, resulting in reduction in the stiffness of the bolt. The ultimate load of the bolts remains approximately constant no matter what the displacing angle is for both the D-Bolt and the rebar bolt. The displacement capacity of the D-Bolt, however, is dependent on the displacing angle. The ultimate displacement of a 1-m long D-Bolt section varies from 140 mm under pure pull (0°) to approximately 70 mm when the displacing angle is larger than 40°. The ultimate displacement of the rebar slightly increases from 29 mm under pure pull to 53 mm under pure shear. In general, the displacement capacity of the D-Bolt is larger than that of the rebar bolt. It is approximately 3.5 times the rebar under pure pull and 50% higher than rebar under pure shear. The test results show that the displacing angle of the bolt is larger than its loading angle, which is also confirmed by the analytical solutions. [ABSTRACT FROM AUTHOR]

Highlights: [•] The algorithm used in the method can be easily adapted to the computer programme. [•] The technique involves determination of geo-mechanical parameters for stability assessment. [•] Alignment of the tunnel could easily be chosen to get the rock pressure to be minimum. [Copyright &y& Elsevier]

Abstract: This paper focuses on the grout cohesive strength and the grout friction angle of rock bolt, which have a significant influence on the pullout force and are difficult to estimate. Traditional method estimate the two parameters from the results of pull-out test conducted under different confining pressures. St. John and Van Dillen gave two approximate empirical formulas in 1984. In this text, a new method was proposed to back calculate the grout cohesive strength and the grout friction angle based on given field pullout force of rock bolt. In order to verify the method, a numerical model was built by FLAC3D to approach the two parameters by the principle of dichotomy. The convergence result had been proved to be right by numerical pull-out tests. [Copyright &y& Elsevier]

Abstract: Rock bolts used for the reinforcement of underground mines, tunnels and nuclear waste repositories are made up of low and medium carbon steels, and high strength low alloy steels. Typical rock bolt systems used for rock reinforcement are mechanically anchored bolts, grout anchored, frictional rock stabilizers and strand anchors. For nuclear waste repository sites such as Yucca Mountain (YM) as well as for mines and tunnels, in addition to mechanical properties, corrosion properties are also important due to potential seepage of water through the fractures or pores in the rock. During temporary rock support period of 50–100years, the temperature of the tunnel at YM should be maintained at ambient conditions. For any reason if the rock bolts are exposed to YM waters and high temperatures in the tunnel then there is a chance of corrosion of steel rock bolts. In this study an attempt was made to study the corrosion properties of various potential rock bolts for YM tunnel support via the aid of electrochemical corrosion testing. At ambient temperature (25°C) all the rock bolts that were studied showed good corrosion resistance in these waters. At higher temperatures, 60°C and 90°C, corrosion resistance of rock bolts decreased, but due to special stress relief heat treatment of one of the frictional rock stabilizers (Swellex Mn 24) the corrosion rates were lower than all other tested rock bolts. Note: Swellex, Split set and Williams are the proprietary names of Atlas Copco, International Roll Forms, Inc. and Williams Form Engineering Corp, respectively. [ABSTRACT FROM AUTHOR]

• Rock bolt is highly affected into rock weathering. • The characterizations of guided wave are investigated. • Group velocity decreased with increasing weathering process. • Predominant frequency is also reduced with weathering step. Rock quality is a critical design parameter of rock bolts. However, rock weathering deteriorates rock quality and adversely affects the performance of rock bolts. The objective of this study was to investigate variations in the velocity and predominant frequency of guided waves in rock bolts subjected to rock weathering. In experiments, chemical and mechanical weathering on 10 rock specimens was achieved with salty water and a slake durability test, respectively. The measured properties of weathered rock specimens were used in a numerical simulation model. Numerical studies on rock bolt surrounded by weathered rock mass were performed to simulate guided waves in the rock bolt. The results show that the group velocity and predominant frequency of the rock bolt decreased the weathering progressed. The grey relational grade value and importance score based on the random forest algorithm indicate that the group velocity and predominant frequency are significant factors sensitive to rock weathering. This study demonstrates that the group velocity and predominant frequency of guided waves can be useful tools for predicting rock weathering surrounding rock bolts. [ABSTRACT FROM AUTHOR]

Abstract: Energy absorbing rock bolts are used as part of rock support systems in underground constructions that are exposed to e.g., rock bursts and detonating explosives. A rock bolt capable of absorbing kinetic energy from these loads must be able to yield with the ground movements and also deform plastically over large distances, at high displacement rates. A new type of energy absorbing rock bolt has been developed and tested in laboratory. The bolt is without a casing and consists of a steel bar that has an inner ribbed-like anchorage section and an outer nut that transfers the load from the rock via a circular disc. When subjected to a dynamic load, the lengthening of the steel bar leads to a decrease in diameter whereby the adhesive bond between bar and grout is lost and the outer end of the bolt is free to yield. The rock bolt is given a very good protection from corrosion when fully grouted in cement. In a laboratory, rock bolts in concrete cylinders were subjected to free fall tests to achieve a loading velocity of 10 m/s. The tests demonstrated that the distribution of plastic strain along the length of a grouted rock bolt is not constant when dynamically loaded. The sections where plastic yielding was allowed were not fully utilized in any of the cases, opposite to that in previous static tests which show almost constant elongation of the bolts. The tests also verified that the load-carrying components of the bolt, the nut and the anchorage, are reliable when dynamically loaded. Elastic and plastic waves will start to propagate through the rock bolt as it is suddenly loaded, resulting in permanent deformation along a section of the bolt. This yield process is demonstrated through a combined graphical and numerical method. [Copyright &y& Elsevier]

An analytical model is proposed to predict the axial force of grouted rock bolt in the tunnelling design. The interaction mechanism of the rock bolt and the soft rock mass has been described according to their consistent displacement. Coupling and decoupling behaviors of the rock bolt around a circular tunnel have been analyzed. According to case studies, the theoretical prediction of the axial force agrees well with the in situ measured results. The installing time and the length of the rock bolt, and the deformation modulus of the rock mass are taken as study parameters to analyze the supporting behavior of the rock bolt. According to the results of the theoretical analysis, there are some conclusions as followings: (1) a lower axial force is resulted because of the delay of installing rock bolt and its supporting effect may be reduced; (2) the larger displacement is caused by the lower deformation modulus of the rock mass, and a higher axial force is resulted in the rock bolt. If the shear strength of the rock mass is not enough, the decoupling behavior will take place interior the rock mass, and the performance of rock bolt may be reduced; (3) the position of a neutral point is related with the radius of tunnel, the physical properties of the rock bolt and the rock mass. It is found that the position of the neutral point and the maximum axial force of the rock bolt installed in the soft rock may tend to be constant when its length is long enough, which means that the supporting effect of the rock bolt can not be improved significantly only by increasing the length of the rock bolt. By using this model, a way is supplied to analyze the supporting behavior of the rock bolt, and a method is provided for the quantitative evaluation of its supporting effect in NATM tunnelling. [Copyright &y& Elsevier]

This paper presents experimental results obtained from the direct pull-out test using different types of rockbolts having different shape of lugs. These are smooth surface bars, ribbed bars, single conical lugged bars, double conical lugged bars and triple conical lugged bars. It is found that the failure mechanism of the conical lugged rock bolt is different from that of conventional rockbolts, in which the adhesion or the shear strength at the bolt–grout interface provides the load bearing capacity. From the results of experimental study, the use of the conical lugged rock bolts is suggested, because it provides a greater anchorage strength due to wedging effect that is a combination of the shear and compressive strength of the grouting material. [Copyright &y& Elsevier]

This paper represents the result of a project conducted with developing a safe, practical and economical support system for engineering workings. In rock engineering, untensioned, fully cement-grouted rock bolts have been used for many years. However, there is only limited information about the action and the pull-out load capacity of rock bolts, and the relationship between bolt–grout or grout–rock and the influence of the grout properties on the pull-out load capacity of a rock bolt. The effect of grout properties on the ultimate bolt load capacity in a pull-out test has been investigated in order to evaluate the support effect of rock bolts. Approximately 80 laboratory rock bolt pull-out tests in basalt blocks have been carried out in order to explain and develop the relations between the grouting materials and untensioned, fully grouted rock bolts. The effects of the mechanical properties of grouting materials on the pull-out load capacity of a fully grouted bolt have been qualified and a number of empirical formulae have been developed for the calculating of the pull-out load capacity of the fully cement-grouted bolts on the basis of the shear strength, the uniaxial compressive strength of the grouting material, the bolt length, the bolt diameter, the bonding area and the curing time of the grouting material. [Copyright &y& Elsevier]

• A new energy-absorbing rock bolt was proposed, called the CEF bolt. • A theoretical model was developed to predict the constant resistance of the CEF bolt. • The effects of main parameters on the CEF bolt were investigated. • The applicability and accuracy of the current theoretical model were verified. To provide an effective support device for dealing with the instability problems in geotechnical engineering, this paper proposes a new energy-absorbing rock bolt based on the existing energy-absorbing devices, called compression-expansion-friction bolt (abbreviated as the CEF bolt). The CEF bolt mainly consists of two parts, a variable-diameter tube and a hollow cone attached at the distal end of the bar. Controllable constant resistance and large displacement are the characteristics of the CEF bolt. The constant resistance was generated by the expansion of the tube under the compression of the cone, as well as the friction between the tube and cone. And the large displacement was provided by the sliding of the cone relative to the tube. A theoretical model was developed to predict the constant resistance by taking into account the effect of material strain-hardening. Based on the energy conservation law, the effect of shear deformation was also considered. Further, the current model was formulated as three forms to consider different conditions. The predictions of the current theoretical model were in good agreement with existing experimental data. An experimentally validated finite element model was established to study the effects of main parameters on the constant resistance of the CEF bolt, such as cone angle, tube wall thickness, friction coefficient, expansion ratio and the strain-hardening property of the tube. The results of numerical simulation and theoretical analysis show that the constant resistance was directly proportional to the tube wall thickness, friction coefficient, expansion ratio and the strain-hardening modulus of the tube. But the constant resistance increased first and then decreased with respect to the cone angle, with an inflection point existed between 5° and 10°. The shear stress distribution and the deformation of the tube were investigated as well. According to the shear stress distribution, the zone under expasion of the tube can be divided into three characteristic areas: stress concentration area, smooth transition area, and stress re-concentration area. In addition, compared with the large number of experimentally validated numerical results, the wide applicability and sufficient accuracy of the current theoretical model were verified. By taking the derivative of the current theoretical model, the location of the inflection point can be obtained. These results can provide theoretical basis for the design and field application of the CEF bolt, as well as the similar structure. [ABSTRACT FROM AUTHOR]

• Passive bolts are used to stabilize a block of rock that is potentially unstable. • The design phase still presents aspects of considerable complexity. • A mathematical model calibrated on field tests has been developed. • Stabilisation forces as a function of bolt and rock have been calculated. • Static contribution of each bolt on the stability of the rock block was evaluated. Passive bolting is used to stabilise unstable rock blocks in surface and underground structures due to the various advantages it offers. Despite its use, the design phase still presents aspects of considerable complexity because the fact that the load of the bolt and therefore, its static action depends on its interaction with the block and the stable rock. In the present work, a mathematical model was developed which is capable of directly calculating the stabilisation forces as a function of the characteristic parameters of the bolt and of its interaction with the rock. This discussion is based on a simplified hypothesis of bolt behaviour, which provides negligible errors, and on the observation that the critical point is positioned at the intersection of the bolt with one of the lateral surfaces that separate it from the portion of stable rock. The formulation of the stabilisation forces obtained made it possible to evaluate the static contribution of each single bolt to the stability of the rock block, by varying the diameter of the steel bar and then designing the bolting operation to achieve acceptable stability conditions for the rock block. The application of stabilising equations to a real case, for which the results of load tests on bolt tests were available, allowed us to outline steps to be taken in the bolt design process. [ABSTRACT FROM AUTHOR]