Adhesion strength and shrinkage of shotcrete

Shotcrete is widely used as rock support in mines and in civil engineering projects. In many cases the adhesion between the shotcrete and the rock is one of the most important properties.The objective of this study was to: (i) identify typical failures of shotcrete in a mining environment, (ii) measure the growth of the adhesion strength of shotcrete with respect to age, (iii) investigate the influence of surface treatment (scaling and cleaning) on the adhesion strength of shotcrete and (iv) investigate the occurrence of shrinkage cracks in the shotcrete and failures in the interface between the shotcrete and the rock.The failure mapping showed that most of the observed failures of shotcrete are in areas with a thin shotcrete layer (<20 mm) together with a low adhesion strength. Furthermore, the measured average thickness of shotcrete (40 mm) was almost the same as the ordered thickness, the problem is the wide scatter. In 18% of the thickness measurements, the shotcrete thickness was less than or equal to 20 mm. If the shotcrete lining in the mine has the same thickness distribution as during the tests, the supporting ability of as much as 18% of the shotcrete lining can be considered as more or less negligible.No differences in free shrinkage were observed between plain and steel fibre reinforced shotcrete. Relatively large areas with lack of contact between shotcrete and wall were observed in the shrinkage tests. This indicates that restrained shrinkage can destroy the bond between the shotcrete and rock. The results from the restrained shrinkage tests show the importance of the bond at the interface. If bond failure occurs, the ability to distribute and control the crack width ceases.The growth of the adhesion strength on a sandblasted concrete wall and the growth of the compressive strength showed a correlation. However, the scatter in the results indicates that further investigations have to be done in order to establish a reliable relation between the growth in adhesion and compressive strength with age. The results from the field tests showed that the adhesion strength was significantly higher on rock surfaces that had been water jet-scaled (with 22 MPa water pressure) than those treated by mechanical scaling followed by cleaning of the rock surface (water pressure 0.7 MPa).     

Numerical analysis on the interaction of shotcrete liner with rock for yielding supports

The yielding supports have been successfully used in excavation in squeezing rock. For yielding supports the shotcrete liner is often divided into several segments, and the HDEs (highly deformable elements) are installed between the segments. Compared to the shotcrete, HDEs have a considerably lower compressive strength and a higher deformation ability. To study the influences of HDEs on the liner–rock interaction, a number of numerical analyses were conducted. In all of these numerical analyses, an interface model that can reflect the failure behaviour of the liner–rock interface was employed. According to the numerical results, after the installation of the HDEs, the compressive and shear failures of the liner decrease sharply; however, obvious shear failure on the liner–rock interface and tensile failure of the liner are observed, which may result in the liner failing to provide sufficient support load. In this condition, reinforcement of the TH profile steel sets, which can decrease the shear failure of the liner–rock interface and the tensile failure of the liner, is suggested instead of thickening the shotcrete liner.     

Full 3D modelling for effects of tunnelling on existing support systems in the Sydney region

The assessment of the interaction between a new tunnel and existing structures is an important issue in urban areas. In this study, the effect of tunnelling on the existing support system (i.e. shotcrete lining and rock bolts) of an adjacent tunnel is firstly investigated using ABAQUS and TUNNEL3D through full three-dimensional (3D) finite element calculations coupled with elasto-plastic material models, which takes into account the tunnelling procedure, the interaction between the shotcrete lining and rock mass, the interaction between the rock bolts and rock mass, and the elasto-plastic behaviour of the rock mass, the shotcrete lining and the rock bolts. Then, on the basis of the calculated results, it is concluded that the driving of the new tunnel significantly affects the existing support system when the advancing tunnel face passes the existing support system and is minor when the face is far from it. Moreover, the support system in the side of the existing tunnel closest to the new tunnel is more significantly affected than that on the side opposite to the new tunnel. It is also found that in a region such as Sydney with relatively high horizontal regional stresses, the driving of the new tunnel will not cause considerable adverse effects on the existing support system, if the new tunnel is driven horizontally parallel to the existing tunnel with a sufficient separation, since both the tensile stress in the existing shotcrete lining in the lateral sides of the preceding tunnel and the compressive stress at the crown decrease although noticeable tensile stress increments are observed on some parts of the existing rock bolts. Finally, it is pointed out that the effects of tunnelling on the existing support system strongly depend on the position between the original and new tunnels. In terms of the stress increments on the existing support system, especially the maximum tensile stress increments on the existing shotcrete lining, the driving of the new tunnel causes increasingly adverse effects on the existing support system in a sequence of: (i) horizontally parallel tunnels with a separation of 30 m; (ii) horizontally parallel tunnels with a separation of 20 m; (iii) staggered tunnels with a separation of 30 m; (iv) vertically alignment tunnels; and (v) staggered tunnels with a separation of 20 m in the cases investigated in this study. For the relatively high regional stresses in the Sydney region, the obtained results qualitatively agree with other’s published observations from the construction of closely parallel subway tunnels.     

Behaviour of shotcrete supported rock wedges subjected to blast-induced vibrations

The static state of stress at the brow in a sub-level caving mine is, due to stress re-distribution, almost uniaxial (major principal stress perpendicular to the cross cut). Since large amounts of explosives are detonated in each production round, the impact of stress waves on the brow can be significant. An extensive failure mapping programme in the Kiirunavaara mine showed that many of the failures close to the brow were structurally controlled. Furthermore, the area of damaged shotcrete was extensive when plain shotcrete was used. At brows supported by fibre reinforced shotcrete, damage in the roof was observed within a horizontal distance of about 3 m from the drawpoint. To study the behaviour of roof wedges supported by shotcrete and subjected to blast-induced vibrations a single-degree-of-freedom (SDOF) model was developed. The model consists of a shotcrete layer and a rock wedge.Vibration measurements showed that maximum particle velocity was approximately 1.2 m/s. The acceleration record showing the largest magnitude was used as the load in the dynamic analyses.The analyses showed that a wedge can be ejected by a dynamic load even if the static safety factor was >10. Furthermore, the non-linear response of the wedges was in most of the cases greater when the wedge was supported both by the joints and the shotcrete layer compared to the case when the wedge was only supported by shotcrete. A conclusion from the analyses is that it is difficult to predict the dynamic response from static calculations.To provide a safe working environment close to the drawpoint, the rock support must sustain the impact of stress waves from production blasting. To support rock wedges subjected to dynamic load the support must be able to consume the energy imposed on the wedges from blasting. The non-linear numerical analyses showed that reinforced shotcrete has the necessary bearing capacity to support the wedges formed in the roof of the cross cut close to the brow. This was in fair agreement with the failure mapping.The single-degree-of-freedom (SDOF) model can be used to study the response of an arbitrarily shaped rock wedge supported by shotcrete as long as the movement of the wedge can be idealised by a pure translation and the dimensions of the wedge are small compared to the length of the incident wave. Analyses showed that 2D wedges can be used to judge whether symmetric or non-symmetric 3D wedges in a uniaxial stress field (which occurs close to the brow) are stable or not when they are subjected to waves induced by blasting.     

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.     

The holding mechanism of under-reamed rockbolts in soft rock

The holding mechanism of under-reamed rockbolts differs from that of conventional rockbolts, in which the bonding or the friction along the element–rock interface provides the holding capacity. The under-reamed end is kinematically blocked by the surrounding rock mass and can provide a greater holding capacity, especially in soft rock, whereas the strength of a soft rock frequently controls the bonding strength of the element–rock interface. Both an experimental study and numerical analyses were performed to examine the holding mechanisms of model under-reamed rockbolts subjected to direct pull out loading and pre-tensioning.When subjected to direct pull out loading, the holding capacity originates from the capacity of the rock resisting tensile fracture. Failure is characterized by the formation of a smooth, conical region bounded by a tensile crack, which subsequently separates from the surrounding rock. Correspondingly, the holding capacity is related to the tensile strength of the rock, bolt length and size of the under-reamed end.When subjected to pre-tensioning, the holding mechanism is provided by the ability of the rock to form two conical zones between the faceplate and the under-reamed end, and to prevent subsequent indentation of the two cones. Major factors influencing the holding capacity of under-reamed bolts include the size of the under-reamed end, bolt length and properties of the rock.     

Enhancement of the tensile strengths of rock and shotcrete by thin spray-on liners

Thin spray-on liners (TSLs) have been available for application in mining and civil engineering situations for about 20 years. They were initially used as sealants, but have subsequently been developed as surface support liners. Although they are used in substantial quantities in mines in South Africa and Canada, widespread application for support purposes has been met with some scepticism. Observations from users, however, have indicated that, contrary to this scepticism, TSL support performance was almost always better than expected. A reason for the scepticism is probably a lack of quantification of the benefits. This paper aims to address this lack of data to some extent. It provides quantification of the enhancement of tensile strengths of rock and shotcrete due to the application of TSLs. The quantification was achieved using Brazilian indirect tensile testing, and several commercially available TSL products. Some tests were also carried out on small rock beams using three-point bending. The results show that, depending on the TSL product used, the tensile strength of a strong, brittle rock can be increased by approximately 30%, and that of shotcrete by more than 40%. TSL application on a weak porous sandstone resulted in an early tensile strength reduction, probably due to absorption of moisture. This was not the case with TSL application on shotcrete.Many of the rock support mechanisms provided by sprayed liners depend substantially or completely on the tensile strength contribution of the liner. Hence the data provided in this paper makes a contribution to knowledge, of value in the design of support for excavations in rock.     

Rock support design based on the concept of pressure arch

A metal mine stope, located at a depth of about 1000 m below the surface, experienced drastic changes in roof displacement and wall fracturing within a short period of time. Thus, the stope needed additional reinforcement in order to remove the remaining ore. It was revealed from rock mechanics assessment that the stability problem was owing to the relatively low strength of the rock as well as the high in situ stresses. It was believed that both the roof and the hanging wall were heavily fractured. It was then proposed that the unstable section of the stope be reinforced with bolt–shotcrete ribs. The concept of the design was to form a pressure arch in the failed rock with the help of six bolt–shotcrete ribs. Displacement measurements showed that the roof displacement reduced from about 2 mm/day to a level of about 0.25 mm/day immediately after the reinforcement operation. Two hanging wall collapses occurred a few months later in the areas outside the bolt–shotcrete reinforced section in the stope. The collapses indirectly proved the effectiveness of the bolt–shotcrete ribs in reinforcing the failed rock.     

Effect of bar shape on the pull-out capacity of fully-grouted rockbolts

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.     

Roughness evaluation in shotcrete-lined water tunnels with invert concrete based on cases from Nepal

Most of the existing roughness estimation methods for water tunnels are related to either unlined or concrete/steel-lined tunnels. With the improvement in shotcrete technology, advancement in tunneling equipment and cost and time effectiveness, future water tunnels built for hydropower projects will consist of rock support with the extensive use of shotcrete lining in combination with systematic bolting and concrete lining in the tunnel invert. However, very little research has been performed to find out tunnel surface roughness for shotcrete-lined tunnels with invert concrete, which is important in calculating overall head loss along the waterway system to achieve an optimum and economic hydropower plant design. Hence, the main aim of this article is to review prevailing methods available to calculate tunnel wall roughness, and to use existing methods of head loss calculation to back-calculate roughness of the shotcrete-lined tunnels with invert concrete by exploiting measured head loss and actual cross-sectional profiles of two headrace tunnels from Nepal. Furthermore, the article aims to establish a link between the Manning coefficient and the physical roughness of the shotcrete-lined tunnel with invert concrete and to establish a link between over-break thickness and physical roughness. Attempts are also made to find a correlation between over-break thickness and rock mass quality described by Q-system and discussions are conducted on the potential cost savings that can be made if concrete lining is replaced by shotcrete lining with invert concrete.     

Principles and methods of rock support for rockburst control

This paper presents the principles of rock support for rockburst control and three rockburst support systems used in deep metal mines.Before the principles of rock support are presented,rock fracture related to strain burst is first discussed with the help of photos taken on site,and the energy sources and transformations during bursting are illustrated through conceptual models.Surface parallel extension fracture usually occurs in the ejected and surrounding rocks in a strain burst event,while the ejected rock in a fault-slip rockburst is often already pre-fractured before the event.There must be excessive release energy available for rock ejection.The excessive release energy comes from both the ejected rock itself and the surrounding rock.To prevent rock ejection in a rockburst,the support system must be able to dissipate the excessive release energy.All support devices in a support system for rockburst control must be able to dissipate energy,be firmly linked,and be compatible in deformability.A support system for rockburst control comprises surface-retaining devices and yield rockbolts as well as yield cablebolts when needed.Laying mesh on the top of shotcrete liner is a good practice to enhance the surfaceretaining capacity of the support system.Energy-absorbing yield rockbolts dissipate energy either by stretching of the bolt shank or by sliding of the inner anchor in the borehole.Mesh,mesh strap and shotcrete are the surface-retaining devices widely used in the current rock support systems.The three types of rock support used for rockburst control at present are soft support system using Split Set bolts,hybrid support system using rebar and two-point anchored yield bolts,and entirely yieldable support system using strong yield bolts.     

Limit analysis solutions for the bearing capacity of rock masses using the generalised Hoek–Brown criterion

This paper applies numerical limit analyses to evaluate the ultimate bearing capacity of a surface footing resting on a rock mass whose strength can be described by the generalised Hoek–Brown failure criterion [Hoek E, Carranza-Torres C, Corkum B. Hoek–Brown failure criterion—2002 edition. In: Proceedings of the North American rock mechanics society meeting in Toronto, 2002]. This criterion is applicable to intact rock or heavily jointed rock masses that can be considered homogeneous and isotropic. Rigorous bounds on the ultimate bearing capacity are obtained by employing finite elements in conjunction with the upper and lower bound limit theorems of classical plasticity. Results from the limit theorems are found to bracket the true collapse load to within approximately 2%, and have been presented in the form of bearing capacity factors for a range of material properties. Where possible, a comparison is made between existing numerical analyses, empirical and semi-empirical solutions.     

Corrosion susceptibility of potential rock bolts in aerated multi-ionic simulated concentrated water

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 50-100 years, 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.     

深埋硬岩隧道初期支护劣化过程衬砌力学特性试验研究

 通过室内相似模型试验对深埋硬岩隧道的开挖、支护及初期支护的劣化过程进行模拟,就初期支护劣化过程中的二次衬砌的荷载、位移及内力的变化规律进行深入研究。研究结果表明,二衬的内力随喷射混凝土的劣化呈现出相对稳定的增加,而随着锚杆的劣化呈现出明显的阶段性增加,而且锚杆的劣化相比较于喷射混凝土的劣化对二次衬砌的影响更为明显;总体上二次衬砌的荷载、位移及内力的增加都呈现出先缓慢增加,后快速增加,而后又趋于稳定的变化规律,拱顶及边墙是受力变化最为明显的区域,通过检算,衬砌的安全性是远远满足规范要求的,结构有较大的安全冗余度,可见当前的设计是偏于保守的。     

Modeling the effect of water, excavation sequence and rock reinforcement with discontinuous deformation analysis

A powerful numerical method that can be used for modeling rock-structure interaction is the discontinuous deformation analysis (DDA) method developed by Shi in 1988. In this method, rock masses are treated as systems of finite and deformable blocks. Large rock mass deformations and block movements are allowed. Although various extensions of the DDA method have been proposed in the literature, the method is not capable of modeling water-block interaction, sequential loading or unloading and rock reinforcement; three features that are needed when modeling surface or underground excavation in fractured rock. This paper presents three new extensions to the DDA method. The extensions consist of hydro-mechanical coupling between rock blocks and steady water flow in fractures, sequential loading or unloading, and rock reinforcement by rockbolts, shotcrete or concrete lining. Examples of application of the DDA method with the new extensions are presented. Simulations of the underground excavation of the ‘Unju Tunnel’ in Korea were carried out to evaluate the influence of fracture flow, excavation sequence and reinforcement on the tunnel stability. The results of the present study indicate that fracture flow and improper selection of excavation sequence could have a destabilizing effect on the tunnel stability. On the other hand, reinforcement by rockbolts and shotcrete can stabilize the tunnel. It is found that, in general, the DDA program with the three new extensions can now be used as a practical tool in the design of underground structures. In particular, phases of construction (excavation, reinforcement) can now be simulated more realistically. However, the method is limited to solving two-dimensional problems.     

Composite element method for the bolted discontinuous rock masses and its application

This paper presents a composite element method (CEM) for discontinuous rock masses reinforced by fully grouted bolts. This element allows one to generate mesh without considering exactly the existence of bolts and joints, which further allows for an important simplification in the pre-process work. The sub-elements of rock, grout, bolt, joint, rock/grout interface and bolt/grout interface are defined using corresponding mapped nodal displacements at the composite element. The mapped nodal displacements can be determined using the governing equations established by means of the virtual work principle. Based on the mapped nodal displacements deformation and stress in each sub-element can be further obtained. The comparative study of the CEM and the conventional finite element method (FEM) has been carried out for the preliminary verification example. The numerical study for the rock bolt crane girder of an underground powerhouse by FEM and CEM is presented as the engineering application example, in which the attention has been paid to the portion of the contact face between the girder and surrounding rock masses, the contact faces between the bolts and surrounding rock masses, as well as to the portion where the rock bolt penetrates the contact face. The comparative and application studies show the validation and advantages of the CEM.     

Estimating the relation between surface roughness and mechanical properties of rock joints

Discontinuities in rock masses have an important influence on deformational behaviour of blocky rock systems. For a single rock joint, the roughness of its surface is of paramount importance to its mechanical and hydraulic properties, such as friction angle, shear strength, and dilatancy/aperture. Many methods have been used to characterize the surface roughness of rock joints, such as joint roughness coefficients (JRC), root mean square (RMS) value, structure function (SF) etc. However, most of these methods can only be used in the 2-D models. In this study, we carried out direct shear experiments on rock joints under both constant normal load (CNL) and constant normal stiffness (CNS) conditions, and measured the surfaces of rock joints before and after shearing, using a 3-D laser scanning profilometer system. By using a 3-D fractal evaluation method of roughness characterization, the projective covering method (PCM) and a direct shear apparatus of high accuracy, the relation between mechanical properties of rock joints under different boundary conditions and the change of their fractal dimensions in both 2-D and 3-D models have been examined, which gives a new approach to accurately evaluate the evolution of roughness of rock joint surfaces and its influence on the hydro-mechanical behaviours of rock joints.