Grouted Rock Bolts Research Articles

Effects of curing time and frequency on ultrasonic wave velocity in grouted rock bolts

Grouted rock bolts are widely used to reinforce excavated ground in mining and civil engineering structures. To date, opportunities for testing the quality of the grout in grouted rock bolts have been limited to the pull-out tests and the over-coring methods. Both these methods are destructive, time-consuming and costly. These deficiencies have fueled research into the use of ultrasonic methods for testing the quality of the grout in rock bolts. However, only partial success has been achieved in these efforts chiefly due to inadequate knowledge of the ultrasonic wave characteristics such as wave velocity in grouted rock bolts. This paper presents results of an experimental study into the effects of curing time and testing frequency on the velocity of ultrasonic waves propagating along rock bolts grouted in concrete. A substantial wave velocity decrease, as much as 47.7% at certain frequencies, was recorded in rock bolts grouted in fully cured concrete in comparison to non-grouted bolts. The results demonstrate the importance of optimizing the selection of test frequencies as well as suggesting the possibility of a new approach based on wave velocity decrease for testing the grout quality of rock bolts.

Tunnel Support by Fully Grouted Rock Bolts for Fast TBM Advance

Tunnel Support by Fully Grouted Rock Bolts for Fast TBM Advance

Experimental and numerical investigations of stresses in a fully grouted rock bolts

Rock bolts are widely used for rock reinforcement in hard-rock mining and civil engineering since a long time. However the use of fully grouted rock bolts and cable bolts is limited in coal mines. In order to improve performance of the rock bolts as a supplementary roof support system for any type of roof condition in coal measured formations, it is necessary to have a good understanding of the behavior of the bolt–grout and grout–rock interactions as well as the mechanism of load transfer in rock bolts. As the performance of grouted bolts depends on bond strength, extensive laboratory pullout as well as pushout tests were conducted in the present investigations with the variations in the bolt diameters, length and cement–water mixing ratios of grout. The load–displacement curves were developed and were verified with the numerical results obtained from finite element analysis using ALGOR software. Numerical models were validated for pushout tests and a detailed analysis was carried out to know the displacement, stress, strain distribution along the bolt.

Laboratory testing of a new type of energy absorbing rock bolt

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.

Monitoring the stability of hydro-caverns with rock mechanics instrumentation - a case study

The Ghatghar pumped storage hydro-electric project is under construction in the Thane district of Maharastra state in India for the generation of 250MW of electricity. The scheme consists of two underground caverns, a machine hall, a transformer hall and intersections. The interconnections between the caverns and connections to the surface, as well as to the reservoir, are made through a network of tunnels. The rock mass is predominantly of basaltic formation. The joints are mostly tight and undulating with generally rough to very rough contact surfaces. The rock mass characterization was conducted using Bieniawski’s RMR (1979) and Barton’s Q. The associated values vary from 66 to 81 and from 5.9 to 25, respectively—which indicate very good rock mass conditions. The support requirements for these structures were estimated from empirical approaches and numerical modeling. The support system consists of 6m long fully grouted rock bolts at 2m grid spacing with 50mm thick SFRS for the two caverns and 7m long rock bolts around the intersections of all openings. An extensive instrumentation scheme was implemented in these structures as a means to evaluate the efficacy of the support system and to have detailed knowledge of the rock mass behavior during and after construction. The instrumentation consisted of: tape extensometers for measuring convergence; multi-point and single point borehole extensometers for radial displacements; load cells for rock load and strain meters for rock strains. These instruments were embedded during the process of cavern excavation. The rock mass deformations around the cavern openings, as indicated by the MPBXs (Fig. 1), the convergence measured by the tape extensometer, and the rock bolt tension measured by the load cells were too low to be significant and collectively indicate that the roof and the walls of both the caverns and of the bus ducts and passageways are stable, indicating that the support system is adequate. It was, however, suggested that the vulnerable wedges should be identified on account of the jointing; these would need to be stitched to the rock mass and the wedge stability be monitored by site specific arrangements. This paper presents in detail the instrumentation scheme implemented at this project plus the analysis of the monitored data, highlights the stability status of the caverns, and recommends the future strategy for monitoring.

An analytical model to predict axial load in grouted rock bolt for soft rock tunnelling

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.

Analytical model for grouted rock bolt in tunnel design

Analytical model for grouted rock bolt in tunnel design

Composite Element Model of the Fully Grouted Rock Bolt

The three dimensional elasto-viscoplastic composite element method is formulated in this paper for rock masses reinforced by a fully-grouted bolt. If a bolt segment penetrates a finite element representing the rock mass, then a composite element is formed including five sub-elements corresponding to the rock material, the grout material, the bolt material, the rock-grout interface and the bolt-grout interface. The displacements in each sub-element are interpolated from the corresponding nodal displacements of the composite element. By the virtual work principle the governing equation for the solution of the nodal displacements can be formulated. The elasto-viscoplastic characteristics of the materials are considered in the formulation. The new model can be incorporated into the conventional finite element analysis grid, in which several composite elements have fully grouted bolts embedded. In this way the mesh generation of large scale bolted rock structures becomes convenient and feasible. The model has been implemented in a FEM program, and a comparative study between the numerical analysis and a pull out field test has been carried out, from which the validity and the robustness of the new model are justified.

Effect of grout properties on the pull-out load capacity of fully grouted rock bolt

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.

Stability analysis of jointed rock slopes reinforced by passive, fully grouted bolts

Abstract A stability analysis of reinforced jointed rock slope based on the kinematic approach of the yield design theory using two translational failure mechanisms is performed for evaluating passive fully grouted rock bolts. In the present study, the rock slope is weakened by two sets of continuous joints and reinforced by rock bolts resisting to traction as well as to compression forces. The results are presented in the form of stability charts relating, for a given reinforcement inclination, the estimated lower-bound of the required strength reinforcement to the friction angle of joints. The obtained results allow, to some extent, an assessment of the global resistance of the reinforced rock slope and highlights the positive effects of the bolting as stabilizing reinforcements.

The influence of blasting on grouted rockbolts

Field tests and laboratory investigations have been carried out to evaluate the performance of grouted rock bolts subjected to close proximity blasting. The investigations comprised pull-out testing of bolts and vibration measurements on both the rock and the bolts. Laboratory work comprised compressive and flexural testing of the grout and analysis of polished thin sections of specimens consisting bolt, grout and rock. There is no observable decrease in pull-out strength for bolts installed close to the blast (3.4 m), compared to more distant installed bolts (22.0 m). Furthermore, there is no difference in pull-out strength for the bolts with the freshest grout compared to bolts with cured grout, when subjected to blast loadings. Anchorage length of the bolts was 125 mm, which gave an average pull-out strength of 140 kN, or 93 % of steel yielding load. Analysis of polished thin sections of the rock, grout and bolt does not show any variation in crack pattern or crack frequency between bolts with varying distance to the blast or varying curing of the grout when subjected to the blast loadings. Re-pulling of previously pulled bolts shows a healing effect of the grout. None of the tests carried out indicates any decrease of the bolt /grout performance. It can be concluded that fully grouted bolts can be used on or close to the face.

THE MECHANICAL EFFECT OF GROUTED ROCK BOLTS ON TUNNEL STABILITY

本論文は, NATMを用いる軟岩地山トンネルの支保の合理的な設計のために, 破壊後にひずみ軟化特性とダイレタンシー挙動を呈する地山との相互作用を考慮したロックボルトの力学的挙動とトンネルの安定性に対する作用機構を理論的に解明したものである. ロックボルトと地山岩盤間の付着作用で発生したせん断応力をロックボルトの作用効果としてモデル化することによって, 新しいトンネルの弾塑性理論解が求められた. 次に, この解を用いて詳細なパラメトリックスタデイによって, 地山強度比に対してトンネルの内空変位や塑性領域の大きさを有効に抑制できる最適なロックボルトの打設長さおよび打設密度を明らかにし, それに基づいたロックボルトの最適な設計方法についても考察を行った.

New development for grouted rock bolts: U. Overwin, R. Rock, M. Blumel & P. Schwab, Felsbau, 14(1), 1996, pp 42–48

New development for grouted rock bolts: U. Overwin, R. Rock, M. Blumel & P. Schwab, Felsbau, 14(1), 1996, pp 42–48

Support of large rock caverns in Norway

The Gjøvik Olympic Mountain Hall has a rock cover of 25–50 m and a span of 61 m. This is by far the largest span ever used for a cavern that is open for public use. The paper reviews the development of tunnelling and the use of the underground in Norway as a background for the decision to build this recordbreaking cavern. Experience from the excavation of very large mining chambers was of crucial importance. The Norwegian approach to the support of tunnels and underground openings is discussed. Excavation and support of large caverns such as hydropower facilities, oil storages and sports halls are described. The support system for the Gjøvik Olympic Mountain Hall is described in detail, and measurements performed on rock bolts and in the shotcrete are discussed. There are clear indications that the total stability of the cavern is primarily a result of favourable horizontal stresses. The majority of the 6-m-long and 12-m-long grouted rock bolts seem to carry very low loads over only short lengths of the bolts.

Dealing with squeezing conditions in Alpine tunnels

In fault zones, excessive deformation during and after tunnel excavation is frequently encountered. Shoterete in combination with grouted rock bolts has successfully been used in many applications to control the deformation process. The magnitude of deformation frequently exceeds the deformability of the shotcrete lining. In the past, this problem has been solved by dividing the shotcrete lining into “segments” and leaving gaps between the segments to accommodate deformation without damage to the lining. The need for minimizing deformation in an extremely heterogeneous fault zone at the Galgenbergtunnel (Austria) led to the development of low cost yielding elements, installed between the shotcrete “segments” and the use of a new type of re-groutable brock bolts. New techniques in evaluating the results of displacement monitoring have improved the short term prediction.

Reinforcing effects of grouted rock bolts—Laboratory investigation

Reinforcing effects of grouted rock bolts—Laboratory investigation

The mechanical behaviour of a single grouted rock bolt—a theoretical study and comparisons with experimental data

The mechanical behaviour of a single grouted rock bolt—a theoretical study and comparisons with experimental data

A numerical method for modelling large displacements of jointed rocks. II. Modelling of rock bolts and groundwater and applications

The present paper is an extension of the companion paper in which the formulation of a new numerical model called the block-spring model for jointed rock mass was presented. The work reported herein presents further investigations of the model which include the simulation of rock bolts for ground support and groundwater pressure. The rock bolts are modelled by a series of one-dimensional elements interacting with the rock blocks through nodal points. The model can be used to analyze both the end-anchored and the fully grouted rock bolts, either pretensioned or untensioned. The model also evaluates the water pressures imposed on the surfaces of the rock blocks which are then introduced into the equilibrium equations of the blocks. Several applications of the proposed model are presented including detailed analyses of two case histories. The numerical results have been compared with the field instrumentation data. Good agreement has been observed between the results of the proposed model and the field measurements in both cases. It is demonstrated that the proposed model can be used for the analysis of excavations in jointed rocks in practice. Key words : jointed rocks, stress, deformation, analysis, numerical model, excavation, anchors, groundwater.

Design for grouted rock bolts based on the convergence control method : Indraratna, B; Kaiser, P K Int J Rock Mech Min SciV27, N4, Aug 1990, P269–281

Design for grouted rock bolts based on the convergence control method : Indraratna, B; Kaiser, P K Int J Rock Mech Min SciV27, N4, Aug 1990, P269–281

A three-dimensional boundary element method for piecewise homogeneous viscoelastic media and its application in mining engineering

A 3-D boundary element method of viscoelasticity is presented for analyzing the time-dependent deformation and stress state around underground mine workings. It assumes that the rocks are stratified with bedding planes and reinforced by fully or partially grouted rock bolts. The Kelvin-Voight and Burgers models are used to represent the viscoelastic behavior of rock strata. Boundary integral equations for these types of separately homogeneous media are developed based on the correspondence principle of viscoelasticity. The time-dependent functions involved in the equations are obtained through the use of the Laplace transformation technique. Thus, the method can compute the stress state of a rock structure without the need to discretize variables which alter with time. An example is presented to show the application of this method to mining engineering.