Rock Support and Reinforcement PDF

Republic of the Philippines MARINDUQUE STATE UNIVERSITY College of Engineering Tanza, Boac, Marinduque BACHELOR OF SCIENCE IN CIVIL ENGINEERING Lev...

Republic of the Philippines MARINDUQUE STATE UNIVERSITY College of Engineering Tanza, Boac, Marinduque BACHELOR OF SCIENCE IN CIVIL ENGINEERING Level III Re-Accredited – AACCUP, Inc. ROCK SUPPORT AND REINFORCEMENT Terminology The term support is widely used to describe the procedures and materials used to improve the stability and maintain the load-carrying capability of rock near the boundaries of underground excavations. The primary objective of support practice is to mobilise and conserve the inherent strength of the rock mass so that it becomes self-supporting. Support is the application of a reactive force to the surface of an excavation and includes techniques and devices such as timber, fill, shotcrete, mesh and steel or concrete sets or liners. Reinforcement, on the other hand, is a means of conserving or improving the overall rock mass properties from within the rock mass by techniques such as rock bolts, cable bolts and ground anchors. It was once the custom to describe support as being temporary or permanent. Temporary support was that support or reinforcement installed to ensure safe working conditions during mining. For centuries, such support consisted of some form of timbering. If the excavation was required to remain open for an extended period of time, permanent support was installed subsequently. Quite often, the temporary support was partly or wholly removed to enable the permanent support to be installed. This practice negates the advantage that can be obtained by applying the principles of rock–support interaction mechanics and so should be avoided. Modern practice is to describe the support or reinforcement of permanent excavations as being primary or secondary. Primary support or reinforcement is applied during or immediately after excavation, to ensure safe working conditions during subsequent excavation, and to initiate the process of mobilising and conserving rock mass strength by controlling boundary displacements. The primary support or reinforcement will form part, and may form the whole, of the total support or reinforcement required. Any additional support or reinforcement applied at a later stage is termed secondary. It was once common practice to regard stopes as temporary excavations having different support requirements from the more permanent mine installations such as major access ways, haulages, crusher chambers, workshops, pumping stations and shafts. Indeed, this distinction may still be made, particularly in the mining of narrow orebodies where the support techniques used in the vicinity of the face may be quite different from those used for permanent mine installations. However, many large-scale metalliferous mines now use mechanised stoping methods in which individual stopes may be very large and may have operational lives measured in years rather than weeks or months. In these cases, the support and reinforcement techniques used may have much in common with those used for permanent mine installations and in civil engineering construction. Support or reinforcement may also be classified as being either active or passive. Active support imposes a predetermined load to the rock surface at the time of installation. It can take the form of tensioned rock bolts or cables, hydraulic props, expandable segmented concrete linings or powered supports for longwall faces. Active support is usually required when it is necessary to support the gravity loads imposed by individual rock blocks or by a loosened zone of rock. Passive support or reinforcement is not installed with an applied loading, but rather, develops its loads as the rock mass deforms. Passive support may be provided by steel arches, timbered sets or composite packs, or by untensioned grouted rock bolts, reinforcing bars or cables. Untensioned, grouted rock bolts, reinforcing bars and cables are often described as dowels. The term strata control is used to describe the support and reinforcement techniques used in coal mining. The term is a good one because it evokes a concept of the control or limitation of strata displacements rather than one of support. Nevertheless, support in the strict sense is a major function of some strata control measures, most notably of hydraulic props used immediately behind the face in longwall mining. Importance of Rock Support All it takes is one weak section of a mine’s cross-section to fail, and the entire mass could collapse or implode. As a mine deepens, the force of gravity increases. Lower load-bearing structures take enormous weight, and the further the mine excavation extends, the greater the force pressure becomes. Underground Rock Instability Causes Rock density and structural uniformity are the two most significant factors in determining if excavated structures are self-supporting or if they require reinforcement. The combination of static and dynamic conditions cause mine instability. Some dangerous situations are natural, and mining activities create others. Main Rock Instability Factors Natural seismic activity like earthquakes and tremors Mining-induced activity like blasting and drilling Water course and seasonal load changes Naturally deteriorating ground conditions Corrosion of natural rock Corrosion of reinforcement metals Inappropriate ground or rock support Inadequate ground or rock support Failure to properly reinforce structural members Failure to properly reinforce for falling rock Support and Reinforcement Principles Concrete and shotcrete may creep as they cure, as may grouted rock bolts and dowels. The support systems with the poorest stiffness characteristics are those using intermittent blocked steel or timber sets. Even if well installed, timber blocking provides a very flexible element in the system. Steel sets also suffer from the disadvantage that they often fail by sideways buckling. From these considerations of rock–support interaction mechanics, it is possible to develop a set of principles to guide support and reinforcement practice. These principles are not meant to apply to the case of providing support for the self-weight of an individual block of rock, but to the more general case in which yield of the rock mass surrounding the excavation is expected to occur. (a) Install the support and reinforcement close to the face soon after excavation. (In some cases, it is possible, and advisable, to install some reinforcement before excavation.  (b) There should be good contact between the rock mass and the support and reinforcement system. (c) The deformability of the support and reinforcement system should be such that it can conform to and accommodate the displacements of the excavation surface. (d) Ideally, the support and reinforcement system should help prevent deterioration of the mechanical properties of the rock mass with time due to weathering, repeated loading or wear. (e) Repeated removal and replacement of support and reinforcing elements should be avoided. (f) The support and reinforcement system should be readily adaptable to changing rock mass conditions and excavation cross section. (g) The support and reinforcing system should provide minimum obstruction to the excavations and the working face. (h) The rock mass surrounding the excavation should be disturbed as little as possible during the excavation process so as to conserve its inherent strength. (i) For accesses and other infrastructure excavations under high stress conditions, support and reinforcement performance can be improved by “closing the ring” of shotcrete or a concrete lining across the floor of the excavation. Pre-reinforcement In some circumstances, it is difficult to provide adequate support or reinforcement to the rock mass sufficiently quickly after the excavation has been made. If suitable access is available, it is often practicable to pre-reinforce the rock mass in advance of excavation. In other cases, extra reinforcement may be provided as part of the normal cycle, in anticipation of higher stresses being imposed on the rock at a later stage in the life of the mine. In mining applications, pre-reinforcement is often provided by grouted rods or cables that are not pre-tensioned and so may be described as being passive rather than active. Such pre-reinforcement is effective because it allows the rock mass to deform in a controlled manner and mobilise its strength, but limits the amount of dilation and subsequent loosening that can occur. The effectiveness of this form of reinforcement is critically dependent on the bonding obtained between the reinforcing element and the grout, and between the grout and the rock. The initial major use of pre-reinforcement in underground mining was in cut-and-fill mining (Fuller, 1981). The use of cables to pre-reinforce the crowns of cut-and-fill stopes is illustrated in Figure 1. At a given stage of mining (Figure 1a), cables are installed to reinforce the rock mass over three or four lifts of mining. The cables are installed on approximately 2 m square grids; this spacing may be reduced or increased depending on the rock mass quality. Cables are installed normal to the rock surface when they are used for general pre-reinforcement. If shear on a particular discontinuity is to be resisted, the cables should be installed at an angle of 20◦–40◦ to the discontinuity. As illustrated in Figure 2a, cables installed in cut-and-fill stope backs may also be used to provide some pre-reinforcement to the hanging wall.  Figure 1. Use of cable dowel pre-reinforcement in cut-and-fill mining (a) (b) (c) Figure 2. Pre-reinforcement at Campbell Mine, Canda: (a) cut-and-fill back reinforcement; (b) longhole open stoping hangingwall reinforcement; (c) longhole open stoping hanging wall reinforcement from a hanging wall drift (after Bourchier et al., 1992)  The pre-reinforcement of hanging walls is also important in the now more widely used sublevel and longhole open stoping methods of mining. If practicable, more uniform coverage of the hanging wall than that illustrated in Figures 2a and b may be obtained by installing fans of cables from a nearby hanging wall drift as in the case shown in Figure 2c. In many of the early applications of fully grouted cable dowel reinforcement and pre- reinforcement, the full potential of the reinforcing system was not realised. This was generally because of failure of the grout-cable bond and the consequent ineffective load transfer between the deforming rock mass and the cable. Since that time, considerable attention has been paid to tendon design and to installation, grouting and testing procedures (e.g. Matthews et al., 1986, Thompson et al., 1987, Windsor and Thompson, 1993, Hoek et al., 1995, Hutchinson and Diederichs, 1996, Windsor, 1997, 2001). As a result, these problems have now been largely overcome. Pre-reinforcement may also be used to good effect in permanent and infrastructure excavations. Several examples are given by Hoek et al.(1995). Figure 3 illustrates the use of grouted reinforcing bars to pre-reinforce a drawpoint. For drawpoints that may be heavily loaded and subject to wear, their continued stability is vitally important in many underground mining operations. In particular, failure of the brow of the excavation can result in complete loss of control of the stope draw operation. Figure 3 shows a suitable method of pre-reinforcing the brow area with grouted reinforcing bars installed from the drawpoint and from the trough drive before the brow area is blasted. Figure 3. Use of grouted reinforcing bars to pre-reinforce a drawpoint in a large mechanised mine. The brow area, shown shaded, is blasted last, after reinforcement has been installed from the drawpoint and from the trough drive (after Hoek and Brown, 1980) Support and reinforcement design Purpose Frequently, support and reinforcement design is based on precedent practice or on observations made, and experience gained, in trial excavations or in the early stages of mining in a particular area. However, it is preferable that a more rigorous design process be used and that experiential or presumptive designs be supported by some form of analysis. Depending on the application, design calculations may be of a simple limiting equilibrium type or may use more comprehensive computational approaches involving rock-support interaction calculations and taking account of the deformation and strength properties of the support and reinforcement system and the complete stress-strain response of the rock mass. Different design approaches may be required for three main applications of support and reinforcement: local support and reinforcement to support individual blocks or loosened zones on an excavation boundary; general or systematic reinforcement in which the objective is to mobilise and conserve the inherent strength of the rock mass; and support and reinforcement system designed to resist the dynamic loading associated with rock burst conditions. Materials and techniques Rockbolts and dowels A single tensioned rockbolt usually consists of an anchorage, a steel shank, a face plate, a tightening nut and sometimes a deformable plate. For short term applications, the bolt may be left ungrouted, but for permanent or long term applications and use in corrosive environments, rockbolts are usually fully grouted with cement or resin grout for improving both pull-out strength and corrosion resistance. Rockbolts are often classified according to the nature of their anchorages. Early rockbolt anchors were of the mechanical slot-and-wedge and expansion shell types. It is often difficult to form and maintain mechanical anchors in very hard or in soft rocks. Mechanical anchors are also susceptible to blast-induced damage. Anchors formed from Portland cement or resin are generally more reliable and permanent. A third category of rockbolt anchorage is that utilised by friction (Split Set and Swellex) bolts which rely on the generation of friction at the rock-bolt contact along their lengths for their anchorage and strength. As with mechanical anchors, friction bolts depend for their efficacy on the sizes and accuracy of the drilling of the holes in which they are installed. They are also susceptible to corrosion. Although they may be given a pre-tension to ensure that an anchorage is formed, friction bolts are usually not installed with the levels of pre-tension (5–20 tonnes) used for other rockbolts. In this case, they act a dowels rather than rockbolts. Other types of dowel are usually grouted along their lengths on installation and develop their tension with deformation of the rock mass in which they are installed. Grouting of Split Set bolts and dowels may increase their load carrying capacity for longer term applications (Thompson and Finn, 1999). Figure 4 shows a number of types of rockbolt and dowel classified according to the anchorage method used but with several types of shank illustrated. Figure 5 shows further details of the installation and grouting of a resin anchored and grouted bolt made from threaded bar. Resin encapsulated rockbolts are widely used for the reinforcement of longer term openings in metalliferous mines.(e.g. Slade et al., 2002)  Figure 4. Types of rockbolt and dowel (after Hadjigeorgiou and Charette, 2001) Figure 5. Resin grouted rockbolt made from threaded bar (after Hoek and Brown 1980) Cable bolts Cable bolts are long, grouted, high tensile strength steel elements used to reinforce rock masses. They may be used as pre-or post-reinforcement and may be left untensioned or be pre- or post-tensioned. Windsor (2001) defines the following terms associated with cable bolting:  Wire – a single, solid section element. Strand – a set of helically spun wires. Cable – an arrangement of wires or strands. Tendon – pre-tensioned wires or strand. Dowel – un-tensioned wires or strand. Cable bolting as defined here was first used in underground metalliferous mines in South Africa and Canada but it was probably in Australia that cable bolt and dowel reinforcement was first developed as a major form of systematic reinforcement in cut-and-fill mining (Clifford, 1974, Brown, 1999b). Figure 6 summarises the development of cable bolt configurations. Figure 6. Summary of the development of cable bolt configurations (after Windsor, 2001) Windsor (2001) notes that the development of hardware for cable bolting has been matched by improvements in design philosophy and methods. In this context, design includes choosing a suitable type of cable bolt, the bolt orientations, lengths and densities, an appropriate installation procedure, and determining whether to use preor post-reinforcement in conjunction will pre- or post-tensioning. In mining practice, these decisions are influenced by logistics, equipment availability, precedent practice in similar circumstances and, in the case of installation procedures, the levels of training of the work force. Installation practice has the potential to dictate the mechanical performance of cable bolting. The length and transverse flexibility of cable bolts create a number of difficulties in ensuring a high quality installation. Installation can be influenced by a number of factors relating to the drilling of the hole, the configuration and state of the cable, and the grouting and tensioning of the cable. A full discussion of these factors is outside the scope of this text. For further details, the reader is referred to the books by Hoek et al. (1995) and Hutchinson and Diederichs (1996), and the papers by Windsor (1997, 2001), for example. Figure 7 illustrates two alternative methods of grouting cable bolts into upholes. These methods may be described as gravity retarded and gravity assisted, respectively. In the grout tube method, the tube may be withdrawn progressively from the hole as it fills with grout. This method has velopment of cable bolt configurations (after Windsor, 2001). a number of operational and cost advantages and is used routinely in a number of mines (Villaescusa, 1999). Figure 7. Alternative methods of grouting cables into upholes (after Hoek et al. 1995) Shotcrete Shotcrete is pneumatically applied concrete used to provide passive support to the rock surface. It consists of a mixture of Portland cement, aggregates, water and a range of admixtures such as accelerators or retarders, plasticisers, microsilica and reinforcing fibres. Gunite, which pre-dates shotcrete in its use in underground construction, is pneumatically applied mortar. Because it lacks the larger aggregate sizes of up to 25 mm typically used in shotcrete, gunite is not able to develop the same resistance to deformation and load-carrying capacity as shotcrete. For at least 50 years, shotcrete has been used with outstanding success in civil engineering underground construction in a wide variety of ground types. It is so successful because it satisfies most of the requirements for the provision of satisfactory primary support or reinforcement. Over the last 20 years, shotcrete has found increasing use in underground mining practice, initially for the support of the more permanent excavations but now increasingly for the support of stopes and stope accesses (Brown 1999b, Brummer and Swan, 2001). It may also be used as part of the support and reinforcement system in mild rock burst conditions (Hoek et al., 1995, Kaiser and Tannant, 2001). Shotcrete is being used increasingly in conjunction with, or as a replacement for, mesh to provide primary support of headings. Brummer and Swan (2001) describe a case of the use of wet mix steel fibre reinforced shotcrete to provide the total drift support in a sublevel caving operation at the Stobie Mine, Ontario, Canada. Bolts are used in drifts only at intersections Some of the support mechanisms developed by shotcrete on the peripheries of excavations are illustrated in Figure 8. The support functions, modes of failure and methods of design of shotcrete as a component of hard rock support and reinforcement systems are discussed by Holmgren (2001) and by Kaiser and Tannant (2001). Hoek et al. (1995) provide a set of detailed recommendations for the use of shotcrete in a range of rock mass conditions likely to be encountered in hard rock mining. Figure 8. Some support mech\anisms developed by shotcrete: (a) a single block; (b) a beam anchored by bolts; (c) a roof arch; (d) a closed ring (after Brown, 1999b). Table 1. Comparison of wet- and dry-mix shotcreting processes (after Spearing, 2001). Shotcrete is prepared using either the dry-mix or the wet-mix process. In the dry-mix process, dry or slightly dampened cement, sand and aggregate are mixed at the batching plant, and then entrained in compressed air and transported to the discharge nozzle. Water is added through a ring of holes at the nozzle. Accurate water control is essential to avoid excessive dust when too little water is used or an over-wet mix when too much water is added. In the wet-mix process, the required amount of water is added at the batching plant, and the wet mix is pumped to the nozzle where the compressed air is introduced. A comparison of the dry- and wet-mix processes is given in Table 1. Until the last decade dry-mix method was more widely used, mainly because the equipment required is lighter and less expensive, and because the dry material can be conveyed over longer distances, an important advantage in mining applications. However, wet-mix methods have important advantages for underground mining applications in terms of reduced dust levels, lower skill requirements and the need for less equipment at the application site. They have now become the industry standard (Brown, 1999b, Spearing, 2001). Shotcrete mix design is a difficult and complex process involving a certain amount of trial and error. The mix design must satisfy the following criteria (Hoek and Brown, 1980): (a) Shootability – the mix must be able to be placed overhead with minimum rebound. (b) Early strength – the mix must be strong enough to provide support to the ground at ages of a few hours. (c) Long-term strength – the mix must achieve a specified 28 day strength with the dosage of accelerator needed to achieve the required shootability and early strength. (d) Durability – adequate long-term resistance to the environment must be achieved. (e) Economy – low-cost materials must be used, and there must be minimum losses due to rebound. A typical basic mix contains the following percentages of dry components by weight: cement 15–20% coarse aggregate 30–40% fine aggregate or sand 40–50% accelerator 2–5% The water:cement ratio for dry-mix shotcrete lies in the range 0.3–0.5 and is adjusted by the operator to suit local conditions. For wet-mix shotcrete, the water:cement ratio is generally between 0.4 and 0.5. The efficacy of the shotcreting process depends to a large extent on the skill of the operator. The nozzle should be kept as nearly perpendicular to the rock surface as possible and at a constant distance of about 1 m from it. A permanent shotcrete lining is usually between 50 mm and 500 mm thick, the larger thicknesses being placed in a number of layers. The addition of 20–50 mm long and 0.25–0.8 mm diameter deformed steel fibres, or plastic fibres, has been found to improve the toughness, shock resistance, durability, and shear and flexural strengths of shotcrete, and to reduce the formation of shrinkage cracks. Fibre-reinforced shotcrete will accept larger deformations before cracking occurs than will unreinforced shotcrete; after cracking has occurred, the reinforced shotcrete maintains its integrity and some load-carrying capability. However, fibre-reinforced shotcrete is more expensive and more difficult to apply than unreinforced shotcrete. Wire mesh Chain-link or welded steel mesh is used to restrain small pieces of rock between bolts or dowels, and to reinforce shotcrete. For the latter application, welded mesh is preferred to chain- link mesh because of the difficulty of applying shotcrete satisfactorily through the smaller openings in chain-link mesh. For underground use, weld mesh typically has 4.2 mm diameter wires spaced at 100 mm centres. In some mining districts such as those in Western Australia and Ontario, Canada, mining regulations and codes of practice now require some form of surface support, usually mesh, to be used in all personnel entry excavations. In Western Australia, the code of practice applies to all headings that are higher than 3.5 m and requires that surface support be installed down to at least 3.5 m from the floor (Mines Occupational Safety and Health Advisory Board, 1999). In underground metalliferous mining, rock blocks or fragments of fractured rock are often held in place by a pattern of hoist rope lacing installed between rockbolts or anchor points. Rope lacing may be used to stiffen mesh in those cases in which the mesh is unable to provide adequate restraint to loosened rock. Ortlepp (1983, 1997) gives a number of examples of the use of mesh and lacing in conjunction with rockbolts and grouted cables and steel rods to stabilise tunnels in the deep-level gold mines of South Africa. Steel sets Steel arches or steel sets are used where high load-carrying capacity elements are required to support tunnels or roadways. A wide range of rolled steel sections are available for this application. Where the rock is well jointed, or becomes fractured after the excavation is made, the spaces between the sets may be filled with steel mesh, steel or timber lagging, or steel plates. Proctor and White (1977) provide the most detailed account available of the materials and techniques used in providing steel support. Steel sets provide support rather than reinforcement. They cannot be preloaded against the rock face and, their efficacy largely depends on the quality of the blocking provided to transmit loads from the rock to the steel set. Steel arches are widely used to support roadways in coal mines where they are often required to sustain quite large deformations. These deformations may be accommodated by using yielding arches containing elements designed to slip at predetermined loads (Figure 9), or by permitting the splayed legs of the arches to punch into the floor. Where more rigid supports are required as, for example, in circular transportation tunnels, circular steel or concrete sets are used. Figure 9. Toussaint–Heintzmann yielding arch: (a) cross section; (b) clamp joint; (c) alternative joint; (d) arch configuration before and after yielding; (e) idealised load–radial displacement response. Reinforcement Strategies Active support deals with excavating the mine passages so that each integral remaining rock structure is actively carrying weight and distributing it as evenly as possible. While passive support deals with potential hazards that are capable of dislodging or causing secondary failure. Active Reinforcements ▪ Mechanical Rockbolts: are the earliest active rock reinforcement devices. They are also called point anchor bolts and are straightforward rods inserted into predrilled holes in rock faces. ▪ Friction Rockbolts: work by being driven into predrilled holes that are slightly smaller in diameter than the metal rod or dowel. This creates a strong friction force that transfers to the rock mass giving it integral stability ▪ Grouted bolts are set in holes slightly larger than the rod diameter through the use of an expanding grout mixture. This works like a friction bolt where forces all along the bolt transfer into the rock.  ▪ Cable bolting rigs: wire rope sections that have threaded ends, are typical in larger applications where flexibility is required. They’re capable of being inserted into drilled holes or wrapped around unstable rock massed. ▪ Strapping uses metal bands placed across rock laps or seams. Bolts are set through the strap face and tapped into the rock surface. ▪ Hydraulic jacks work with straps and serve as modern day timber shoring similar to the posts and beams used in the past. Advantages include their ability to be adjusted when necessary and the fact that they won’t rot like wood. Passive Reinforcements ▪ Shotcrete is a specially designed, liquefied concrete product that’s blown or shot onto rock faces under high pressure. It’s sometimes known as gunite and consists of cement powder, fine aggregate and water. ▪ Steel sets act like straps except they’re not mechanically connected to rock faces through bolts. Steel sets are like passive beams that are held in place by screw jacks and sit passively against a rock surface in case there’s some movement. ▪ Mesh: Welded wire mesh and chain link mesh are the most utilized varieties. The main difference is that welded wire mesh is quite rigid while chain link is flexible. Both types are fastened to rock faces with bolts and washer plates. Mesh is exceptionally effective at retaining loose material while letting moisture and gasses dissipate. ▪ Blast shields and blasting mats haven’t frequently seen use in underground mining reinforcement applications until recently. They’ve been a mainstay in protecting workers and machinery from flying rocks and debris during blasting operations, but now mining engineers are finding how much reinforcement value there is in blast shields.