Case application in mining panel

The solid deformation and fluid flow problems can be solved by using the finite element method (FEM) using governing equations for a dualporosity poro-mechanical model found in Zhang (2002) and Zhang and Roegiers (2005). Introducing stress-permeability into the finite element model, permeability variations induced by stress changes can be obtained. A case example is given below to examine permeability changes due to mining. When mining near aquifers, it is of critical importance to determine the changes of permeability due to mining (Zhang and Shen 2004).

The coal mine considered here is located in the Yanzhou coalfield, Eastern China. The average mining depth is 305 m and the extraction thickness of the pertinent coal seam is 5 m. A water-bearing sand lies 75 m above the coal horizon, and the thickness of the sand layer is 30 m (Zhang et al. 2001, Zhang and Wang 2006). Figure 8.13 shows the finite element model and mesh of a section perpendicular to the mining direction. In this model, a half of the mining panel is considered due to geometric symmetry.

The generalized plane strain model is adopted since the mining direction of the panel is much longer than the direction of the panel width. The model is laterally confined and impermeable. The bottom of the model is considered as a rigid and impermeable boundary. The strata gravity with average unit specific weight of J = 23 kPa/m (or 2.3 sg) is considered as the far-field stress acting on the panel. The far-field stresses and pore pressure are respectively: V v = 7.0 MPa, V H = V h =3.8 MPa, pw= 3.0 MPa. The main parameters of these strata are listed in Table 8.2. In the table most rock parameters are based on laboratory experiments. However, the parameters for the sand aquifer and the mined area are simply estimated. The fracture spacing is assumed to be 1 m for all layers.

Figure 8.14 shows the FEM calculated contours of the permeability variations (permeability ratios of post- to pre-mining) in the vertical direction. In this figure, the mining width of the panel is 90 m, and the vertical axis is the central line of the panel. It can be seen that permeability increases in the strata around and beyond the mined area and decreases in some areas near the unmined coal seam. The maximum magnitude of increased permeability lies in the immediate roof and floor of the mined seam of the panel center. However, the maximum height of increased permeability zone appears in the strata over the unmined coal pillar.

Figure 8.15 presents the contours of permeability variations in the horizontal direction. The permeability variations are very different from the vertical one shown in Fig. 8.14, due to the different stress distributions between the vertical and horizontal directions. The height of the increased permeability zone in the horizontal direction is higher than that in the vertical direction. Furthermore, there are larger magnitudes of permeability changes in the horizontal direction (refer to Figs. 8.14 and 8.15).

It can also be seen that the maximum increased permeability zone occurs over the mining panel center for the horizontal permeability, while it occurs over the coal abutment of the mining panel in case of the vertical permeability. This implies that even if an aquifer is not exactly over the mining panel, water intrusion may still take place. Figure 8.16 gives the field observed results for a long-wall mining face in similar geologic conditions as the numerical model. In order to measure permeability changes due to mining in the overburden strata, observation boreholes were drilled pre- and post-mining. The flowrates or drilling fluid circulation changes along the borehole during drilling were measured and well logs were run (Liu 1999). It can be seen from Fig. 8.16 that the observed increased permeability has a similar shape as the one predicted in Fig. 8.14.

Parametric analyses using a finite element model are conducted to study the influences of thickness of extraction, mining width (L in Fig. 8.17), and depth of mining on permeability in the surrounding strata of the coal seam. The straight lines in Fig. 8.17 show that the permeability height increases with mining width and thickness of extraction. Figure 8.17 also compares the FEM results of increased permeability height in the overburden strata with field observed data and empirical formula (Liu et al. 1981). In the observed data, lift mining methods were used for mining thick coal seams found in China. It can be seen that the FEM results are coherent both in magnitude and in trend.

Figure 8.18 presents the increased permeability in the overburden strata with mining depth. Field observed height of strata with increased permeability, the FEM calculated result, and the one given by Alejano et al (1999) are compared in Fig. 8.18. The figure shows that as the depth of mining increases, the calculated height of the increased permeability zone decreases, which is coherent with the results given by Alejano et al. (1999). Compared to the observed results from Liu et al. (1981), again coherence is found in magnitude.

Overburden strata failure due to mining

China has developed some specific methods of coal mining and experimental techniques under aquifers and surface water. Over the last 40 years, about 1000 longwall faces were extracted under surface and ground water, liberating millions of tons of coal reserves without disastrous consequences. Since coal extraction enhances hydraulic conductivity, it is desirable to determine accurately pre- and post-mining hydraulic conductivities in the overburden strata. To measure these conductivities, boreholes are drilled pre- and post-mining either on the surface or in underground observing roadways. The flow rate or circulation loss along the borehole during drilling is measured by pumping drilling mud into the borehole. Well logs are also applicable for the determination of mining induced fractures and permeability changes (Peng et al. 2002a).

In general, two failure zones that affect strata hydraulic conductivity are formed overlying the mined area: a caved zone and a water-conducting fractured zone (Liu et al. 1981, Zhang and Shen 2004). For mining under aquifers, the water-conducting fractured zone is more interesting, since it provides access for water inflow into the mine workings because of hydraulic conductivity enhancement in this zone. From in-situ testing of borehole flow rate, the water-conducting fractured zone can be divided into the following three subzones (Fig. 9.1):

(1) Slightly fractured subzone. Only little fractures are induced in the strata. Compared to the original strata, hydraulic conductivity in this zone increases slightly. The fluid circulation loss rates in the observing borehole are less than 0.1 l/s m;

(2) Moderately fractured subzone. The strata only have partial bed separations and fractures. Hydraulic conductivity in the strata increases moderately. The circulation loss rates are between 0.1 and 1.0 l/s m;

(3) Severely fractured subzone. Most of the strata have been fractured, and the fractures are interconnected. Hydraulic conductivity in the strata increases dramatically. The circulation loss rates are greater than 1.0 l/s m.

Field observations by circulation loss measurements in boreholes while drilling have shown that the strata failure characteristics differ considerably for different inclinations of the extracted seams. For flat or slightly inclined coal seams (the dip angle, D < 30q), the profile of the water-conducting fractured zone is broad in section with extended lobes over the headgate and tailgate, as shown in Fig. 9.2. For strong rocks, the failure zone has a different characteristic, as shown in Fig. 9.3, which is that the failure zones are much higher in the vertical direction and narrower in section.

For inclined coal seams (30q

A considerable number of in-situ observations have shown that heights of strata caved and fractured zones in the overburden formation depend primarily on the lithology and strength of the overlying strata, as well as the inclination of the extracted seam. The following formulae have been obtained according to in-situ observations in thousands of longwall faces (Liu et al. 1981, Bai and Elsworth 1990, Zhang and Shen 2004).

For mining under aquifers, it is desirable to avoid the extra expense of strata dewatering. This can only be achieved when aquifers are located outside the water-conducting fractured zone. In this case, water inflow into the mine workings does not increase. When an aquifer lies within the fractured zone, but outside the caved zone, excessive groundwater discharge to the mine occurs (according to the mining experiences in China); however, the sand in the unconsolidated aquifer does not flow into the mining area. When an unconsolidated aquifer is situated within the caved zone, both water and sand can rush into the mining area, and this may even cause disastrous consequences, if the aquifer is very permeable and strongly waterbearing.

The Daliuta coal mine, affiliated with the Shenhua Group, is located in ShenFu Coalfield, Northern Shaanxi Province and on the southwestern bank of the Yellow River, Northern China (Fig. 9.7). It is one of the major coal mines in China. This coalfield consists of nearly flat-lying beds of Jurassic coal measure. The thickness of the primary coal seam, No. 2, is approximately 4 m with the roof consisting of medium-grained sandstones. The overlying coal measures are 19 to 65 m in thickness, comprising weak, weathered strata in the uppermost reaches. The bedrock is overlain by unconsolidated alluvium comprising mixed impermeable clay layers with water-bearing sands and gravels. The alluvium is generally 38 to 43.4 m in thickness, in which one aquifer underlies lowermost in the unconsolidated overburden. The total depth of cover for seam No. 2 ranges approximately from 20 to 100 m. Comprehensive mechanized longwall mining with full caving is used in the coal extraction.

The coalfield has a very dry temperate climate and is situated in the southeastern border of the Maowusu Desert. Most of the surface is covered by sand, in which little vegetation exists. The water resource is very precious in this region. Only one perched aquifer in the Quaternary alluvium overlies directly on the coal measure. Therefore, the protection of the water resource and mining safety from groundwater hazards are common concerns of both the mine operator and government.

Support and Reinforcement in the Mining Cycle

The most commonly used mesh is probably welded mesh made of approximately 5 mm thick steel wire and having 100 mm square openings. The steel wire may be galvanised or not. The alternative has been an interwoven mesh known as chain link mesh. The disadvantage of traditional chain link mesh compared with weld mesh has been the difficulty of applying shotcrete successfully through the smaller openings available. This difficulty has now been overcome in a high strength, light weight chain link mesh with 100 mm openings which is easy to handle and can be made to conform to uneven rock surfaces more readily than weld mesh.

A feature of this mesh is the fact that the intersections of the wires making up the squares in the mesh are twisted rather than simply linked or welded. Roth et al. (2004) describe static and dynamic tests on this mesh. Mesh of this type is being used successfully at the Neves Corvo Mine, Portugal, where it has been particularly successful in rehabilitating damaged excavations. Li et al. (2004) report that this mesh is being trialled by St Ives Gold, Western Australia. Tyler & Werner (2004) refer to recent trials in sublevel cross-cuts at the Perseverence Mine, Western Australia, using what a similar Australian made high strength chain link mesh. It is understood that completely satisfactory mechanised installation methods have yet to be developed.

In this symposium, Hadjigeorgiou et al. (2004) and Van Heerden (2004) discuss the use of cementitious liners to support, protect and improve the operational performance of ore passes in metalliferous mines. One of the benefits of cementitious liners is the corrosion protection that they provide to the reinforcing elements. Both papers emphasise the need to consider the support and reinforcement of ore passes on a cost-effectiveness basis taking into account the need to rehabilitate or replace failed passes. The author has had the experience of having to recommend the filling with concrete and re-boring of critical ore passes that had collapsed over parts of their lengths.

Although their use was referred to at the 1999 symposium, there have been significant developments in the use of thin, non-cementitous, spray-on liners (TSLs) since that time (e.g. Spearing & Hague 2003). These polymer-based products are applied in layers of typically 6 mm or less in thickness, largely as a replacement for mesh or shotcrete. Stacey & Yu (2004) explore the rock support mechanisms provided by sprayed liners.

The author’s experience at the Neves Corvo Mine, Portugal, is that TSLs are useful in providing immediate support to prevent rock mass deterioration and unravelling in special circumstances (Figure 2), but that they do not yet provide a cost-effective replacement for shotcrete in most mainstream support applications. In some circumstances, they can be applied more quickly than shotcrete and may be used to provide effective immediate support when a fast rate of advance is required. Recently, Archibald & Katsabanis (2004) have reported the effectiveness of TSLs under simulated rockburst conditions.

Overcoming the limitations and costs associated with the cyclic nature of underground metalliferous mining operations has long been one of the dreams of miners. More closely continuous mining can be achieved in civil engineering tunnelling and in longwall coal mining than in underground hard rock mining. Current development of more continuous underground metalliferous mining systems is associated mainly, but not only, with caving and other mass mining methods (Brown 2004, Paraszczak & Planeta 2004).

Several papers to this symposium describe developments that, while not obviating the need for cyclic drill-blast-scale-support-load operations, will improve the ability to scale and provide immediate support and reinforcement to the newly blasted rock. Jenkins et al. (2004) describe mine-wide trials with hydro-scaling and in-cycle shotcreting to replace conventional jumbo scaling, meshing and bolting at Agnew Gold Mining Company’s Waroonga mine, Western Australia. Neindorf (2004) also refers to the possibility of combining hydro-scaling with shotcreting to develop a new approach to continuous ground support in the development cycle at Mount Isa. These developments form part of the continuous improvement evident in support and reinforcement practice in underground mining.

As was noted at the 1999 symposium, although backfill has been used to control displacements around and above underground mining excavations for more than 100 years, the great impetus for the development of fill technology came with the emergence of the “cut-and-fill era” in the 1950s and 60s (Brown 1999a). It was also noted that fill did not figure prominently in the papers presented to that symposium. A few years earlier, paste fill made from mill tailings and cement and/or other binders, had been developed in Canada (Landriault 2001). Since that time, the use and understanding of paste fill have increased dramatically, so much so that Belem et al. (2004b) suggest that it is “becoming standard practice in the mining industry throughut the world”.

Cemented paste fill is now used with a range of mining methods including sublevel open stoping, cut-and-fill and bench-and-fill. In some applications, it is necessary that unsupported vertical paste fill walls of primary stopes remain stable while secondary stoping is completed. In common with Landriault (2001) and Belem et al. (2004a), the author has had success using the design method proposed by Mitchell (1983). A particular requirement in some applications is to include enough cement to prevent liquefaction of the paste after placement (Been et al. 2002).

In two papers to this symposium, Belem et al. (2004a, b) discuss a range of fundamental and applied aspects of the use of cemented paste fill in cut-and-fill mining generally, and in longhole open stoping at La Mine Doyen, Canada. Varden & Henderson (2004) discuss the use of the more traditional cemented rock fill to fill old underground mining voids at the Sons of Gwalia Mine, Western Australia.

Static and Pseudo-Static Support and Reinforcement Systems

It is perhaps remarkable to find that, although rock and cable bolts have been used in underground mining and construction for several decades (if not more than 100 years in the case of rock bolts), bolt elements and bolting systems continue to evolve and improve. The papers presented to this symposium detail advances made in fully encapsulated resin and cement grouted bolts (Mikula 2004, Mould et al. 2004, Neindorf 2004), one pass mechanized bolting (Mikula 2004, Neindorf 2004) and bulbed cables (Yumlu & Bawden 2004), for example.

The developments in ground support practices that have accompanied greater productivity, larger excavations and larger equipment are especially well-illustrated in the paper by Neindorf (2004) describing the evolution of ground support practices at the Mount Isa mine over the past 30 years.

In a detailed and valuable review paper, Windsor (2004) concludes that “the quality and performance of cable bolts used to stabilise temporary, non-entry, production excavations have improved over the last 20 years to the point where they are now an essential part of modern mining practice. Cable bolts have provided the industry with increased production, increased safety and increased flexibility in the extraction process.

However, with the development of wider span haulage and other larger mine openings, cable bolts are now also used to secure longer life, infrastructure excavations.” Windsor (2004) recommends “that greater care and attention to detail be invested during selection and installation of cable bolts for mine infrastructure excavations than that given to mine production excavations”. He identifies, in particular, the importance of the control of the geometry, material quality, installation and testing of the barrel and wedge fittings used as cable grips.

It is also important to recognize that the use and effectiveness of rock and cable bolts in Australia’s underground coal mines have developed considerably in the recent past. Hebblewhite et al. (2004) suggest that the significant trends over the last decade have included:

- use of longer bolts;

- use of partial and predominantly full-encapsulation, polyester resin anchored bolts;

- use of threaded bolt fixing systems;

- adoption of bolt pre-tensioning in an increasing number of applications;

- adoption of different grades of steel to achieve stiffer and stronger bolts; and

- variations to bolt deform patterns and ribbing systems for improved anchorage and load transfer performance.

An issue that has long existed, but has often been over-looked, is the corrosion resistance and longevity of rock and cable bolts. The initial Snowy Mountains installations which are generally regarded as having pioneered the systematic use of rock bolting in Australia (e.g. Brown 1999b) are now more than 50 years old. It was inevitable, therefore, that this issue would assume the increasing importance accorded it by the papers presented to this symposium (e.g. Bertuzzi 2004, Hassell et al. 2004, Hebblewhite et al. 2004, Satola & Aromaa 2004, Windsor 2004). As noted by Hassell et al. (2004) and Potvin & Nedin (2004), the long-term corrosion resistance of the popular friction rock stabilizers, remains an issue. Corrosion protection is one of the advantages offered by fully encapsulated bolts and cables.

However, there are suggestions that cement grouting alone does not provide long-term (e.g. 100 year) corrosion protection (Bertuzzi 2004). For long-term protection, two independent corrosion barriers are usually required. Depending on the atmosphere and the mineralogy and groundwater conditions in the rock mass, corrosion may also affect surface fixtures such as plates and nuts as well as the bolts and cables themselves. Of course, galvanizing provides protection to the steel underneath but not necessarily for long periods of time (Hassell et al. 2004, Windsor 2004).

Interestingly, in a detailed inspection of 50 km of 35–40 year old tunnels in the Snowy Mountains Scheme, Rosin & Sundaram (2003) found the mainly fully cement grouted, hollow core mild steel bolts to be in excellent condition, showing little evidence of corrosion. An approximately 5 mm protective grout or bitumen coating applied to the bolt threads and face plates appeared to have worked very well. Carefully controlled installation and grouting is a necessary pre-condition for the achievement of such performance (Windsor 2004).

With increasing knowledge, experience and the availability of a range of analytical and numerical tools, rock and cable bolt installations are now being designed for increasingly demanding operational conditions in both civil engineering and underground mining. However, the most successful installations are usually those whose performance is monitored by a well-designed instrumentation system as part of a systematic observational approach (e.g. Moosavi et al. 2004, Thibodeau 2004, Thin et al. 2004, Tyler & Werner 2004, Yumlu & Bawden, 2004).

Shotcrete

Over the last decade, increasing use has been made of shotcrete for ground support and control in infrastructure, development and production excavations in underground mines in Australia and elsewhere. Clements (2003) reports that nearly 100,000 m3 of shotcrete is applied annually in some 20 underground mines in Australia. Advances have been made in mix design, testing, spraying technology and admixtures which have combined to improve the effectiveness of shotcrete. Wet-mix fibre-reinforced shotcrete is now the industry standard.

Of course, shotcrete has long been an essential part of support and reinforcement systems in underground civil construction where its use is well-established even for softer ground than that commonly met in underground mining (Kovari 2001). In underground mining, shotcrete is now used to good effect not only for infrastructure excavations, in weak ground (e.g. Yumlu & Bawden, 2004), for rehabilitation, and in heavy static or pseudo-static loading conditions (e.g. Tyler & Werner 2004), but as a component of support and reinforcement systems for dynamic or rockburst conditions (e.g. Li et al. 2003, 2004).

The toughness or energy absorbing capacity of fibre-reinforced shotcrete is particularly important in this application. A new toughness standard, the Round Determinate Panel test, has been developed in Australia and adopted in some other countries (Bernard 2000, 2003). The performance of fibre-reinforced shotcrete measured in these tests can vary significantly with the type (usually steel or polypropylene structural synthetic fibres) and dosage of fibres used.

Mesh and sprayed liners

Another important change in support and reinforcement practice in underground mining in recent years has been the increasing emphasis being placed on mesh and sprayed liners of several types as a primary ground control mechanism. Although, because of the large quantities used and its importance as a support technique, shotcrete has been treated here as a special category of support, it is often included with other techniques in the class of spray-on liners (e.g. Spearing & Hague 2003). The overall subject of mesh and sprayed liners has become so significant that it now has its own series of specialist international meetings.

In some mining districts such as those in Western Australia and Ontario, Canada, mining regulations and codes of practice now require that some form of surface support, usually mesh, 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). These provisions form part of the steps being taken to understand and alleviate the rockfall hazard in Western Australia’s, and Australia’s, underground metalliferous mines (Lang & Stubley 2004, Potvin & Nedin 2004).

Immersion of Metals and Alloys

It is the differential electrical potential between the anode (+) and the cathode (-) which is key to the moist corrosion example described above. This differential is primarily generated by the difference in oxygen availability between the edge and the centre of the water droplet.

Differential potentials can also be generated by the presence (and contact) of dissimilar metals immersed in an oxygenated electrolyte solution (Illston et al., 1979; Bryson, 1987). Corrosion induced by such a coupling can be extremely aggressive and can result from the designed use of dissimilar metals (steel cables with aluminum plates or anchors) or from the presence of cablebolts in a rich sulphide ore. Indeed, rock bolts in sulphide ore bodies have significantly reduced service lives (Hoey and Dingley, 1971; Gunasekera, 1992).

Corrosion cells can also be generated on cablebolt surfaces at the point where abrupt transitions in environment occur. These include differential grout coverage, for example, at the borehole collar, at penetrating cracks in the grout, where the cable crosses a local water table, or within voids in the grout column. Oxygen (atmospheric or dissolved) is the critical component of the cathodic reaction discussed so far.

The concentration of oxygen is therefore a critical factor governing the rate of corrosion. In aqueous environments with high levels of acidity or low pH, however, the hydrogen (H ) ions in the acid solution react +cathodically with the free electrons in the steel to form hydrogen gas (H ). This 2 reaction is countered as before by the release of iron ions from the steel and does not require the presence of oxygen. While oxygen concentration normally controls corrosion rate (loss of iron ions), the acid (H ) reaction dominates below a pH of +4 and can become extremely aggressive.

Although it is not as common as oxygen related corrosion, acid corrosion can pose a serious hazard to mine support (Gunasekera, 1992) due to its accelerated rate. Sampling of groundwater and/or mine water for pH is relatively simple so the risk can be easily determined. In Canada, mine water with a pH of 2.8 has been recorded in underground mines, and measurements of 3-4 are not uncommon (Minick and Olson, 1987). Acidic mine water can often be linked to the oxidation of sulphide ores (primarily pyrite and marcasite) resulting in the generation of sulphuric acid and pH levels as low as 1.5-2 (Gunasekera, 1992).

In addition, there are many species of bacteria which flourish in the underground environment and which greatly accelerate the breakdown of sulphides to form sulphuric acid. Different species are active with and without the presence of oxygen. Such bacteria can accelerate the production of acid in mine waters by a factor of four with a related increase in corrosion rate.

Accelerated Corrosion

Of primary consideration in cablebolting is the acceleration of any of these corrosion processes at points of excessive strain in the cablebolt. As steel is strained in tension or in shear across a joint in the rock by rockmass movement, or bent by improper plate installation, the susceptibility to all forms of corrosion increases. Any protective surface rust is cracked by such strain exposing fresh surfaces. Microscopic cracks formed in areas of high strain create corrosion conduits beyond the steel surface. In addition, the strained ionic bonding in the metal increases the potential for iron-electrolyte interaction and hydrogen embrittlement (Littlejohn and Bruce, 1975).

This so-called stress corrosion cracking is important because cables will tend to corrode much more rapidly in aggressive environments exactly when and where their mechanical integrity is most tested and is most critical. In the case of grouted cablebolts, load concentrations along the cable length are usually related to full cracking and separation across the grout column. This allows direct and focussed attack on the stressed steel by corrosive agents. Stress corrosion is often the final mechanism in cablebolt failure in corrosive environments.

Cablebolt Geometry Effects

In general, the high carbon steels used in the manufacture of cablebolt strand are more corrosion resistant than the steels used in conventional rock bolts. Nevertheless, certain features of the grouted cablebolt which increase its potential for detrimental corrosion include the presence of flutes (v-grooves), internal channels between the outer wires and the king wires, as well as the formation of concentrated corrosion sites at separation planes in the rock and grout. Voids and bubbles in the grout column also create potential corrosion cells.

Summary Recommendations for Corrosive Environments

Corrosion is rarely a problem in open stope cable support, simply due to the short service life involved. Cut and fill stopes can be open for up to a year or more and overhead cables should, therefore, not be allowed to corrode to unacceptable levels during this time. Fractured, sulphide ore bodies require special attention in this regard. Corrosion of cablebolts (and other steel support) in permanent mine openings can cause serious problems in terms of safety and rehabilitation. In addition to normal capacity reduction, corroded cables tend to become brittle and can suffer reduced effectiveness in dynamic loading situations. The factors which contribute to corrosion are often complex, are compounded in an underground environment, and are very difficult to combat in areas of high severity. Nevertheless, the following is a brief list of remedial measures for use when corrosion has been identified as a problem (Littlejohn, 1990; Gunasekera, 1992).

Cablebolt storage

- Store cablebolts in a dry location, preferably moving them underground to the working site only when required. Long-term storage outside, under the sun or exposed to the elements should also be avoided.

- Do not allow water to collect on the cablebolts. Corrosion will quickly fill the flutes reducing bond strength and potentially pitting the steel.

Installed cablebolts

- High humidity accelerates corrosion. Good ventilation at all times can help to reduce this factor.

- Use caution when installing cables in areas with flowing water.

- Avoid any use of cements, mixing water or admixtures containing chlorides, sulphides or sulphites.

- Grout voids and bubbles increase corrosion potential.

- Request that plates, barrels and wedges, and other fixtures are electro-chemically compatible with the high strength carbon steel used in strand.

- Long rust stalactites growing rapidly from the ends of uphole cables indicates potentially severe strand corrosion up the hole.

- Sulphate resistant grouts are alkaline and can counteract acidic mine waters. The use of this cement does not permit the use of such waters for grout mixing.

Severe corrosion

- Epoxy-encapsulated cables are available for use in corrosive environments (Windsor, 1992). Note that such coatings may not be resistant to all forms of corrosion and that the coating must penetrate the strand, encapsulating the king-wire to prevent focussed corrosion down the centre of the strand.

- Galvanized cable would be of use against non-acidic corrosion.

- Grease can protect ungrouted lengths of cable (at the collar, for example).

Other more costly measures such as cathodic protection are discussed in Littlejohn and Bruce (1975) and Littlejohn (1990; 1993).

Corrosion of Steel Strand

Corrosion of high carbon steel strand can be a serious problem in long term civil engineering applications. In mining, however, the incidences of cablebolt corrosion causing serious problems are rare. This is due primarily to the short time frame involved in open stope support in underground mining.

Corrosion problems observed by the authors in mining environments were typically in long term support in open pits where the groundwater was acidic or saline and in long term support in underground sulphide deposits. Cut and fill applications in wet conditions where fractured stope backs could remain (supported) for up to a year were notably susceptible to corrosion. Serious failure, due to corrosion and rupture of the strand, can occur in such applications.

The nature of corrosion is extremely complex and a fundamental discussion is beyond the scope of this book. It is the intent here to discuss some of the important factors involved in corrosion so that the engineer may assess the potential for problematic corrosion and take steps to prevent it or make the appropriate design allowances for it.

Most common refined metals are inherently unstable ionic materials composed of arrays of single atoms which possess a full compliment of electrons. Metals such as iron normally tend to give up electrons at room temperature (gold is a notable exception) and become involved in reactions leading to the formation of more stable compounds such as iron oxide or iron hydroxide (rust). The release of electrons is termed an anodic reaction and the acceptance of electrons a cathodic reaction. Both reactions must occur for corrosion to take place. Since metals such as the iron found in steel cable are normally willing to give up their electrons, it is normally the presence of a cathode which determines the corrosion potential.

The cathodic reaction (involving the consumption of electrons released anodically from the iron) can be made possible by the presence of an acid, sulphate, water and/or oxygen.

Corrosion of steel (iron) can be divided into four basic categories (Illston et al., 1979; Pohlman, 1987):

- Dry corrosion

- Wet corrosion

- Corrosion of immersed metals and alloys Induced or accelerated corrosion (includes influence of stress)

The following discussion is confined to corrosion of cablebolts and as such is incomplete as a comprehensive examination of general corrosion.

Dry Corrosion

Dry corrosion is an inevitable consequence of medium- to long-term storage of cablebolts in even the most ideal conditions. It involves the formation of iron oxide (Fe0) as iron atoms combine with atmospheric oxygen. Once the process initiates on a clean surface, it spreads fairly rapidly to involve most of the exposed surface. While Fe0 forms an adherent film on steel surfaces and can actually form an impervious layer, it can be vulnerable to cracking and as such fresh iron is constantly being exposed and the process continues. In the perspective of cablebolting in mining, however, dry oxidation is a relatively slow chemical process and is of only minor consequence. Light surface (dry) corrosion has been shown (Goris, 1990) to improve bond performance of cablebolts by up to 20% in ideal conditions, although deliberate rusting of cablebolts is not advocated by the authors. The process is accelerated by higher surface temperatures (e.g. if the cables are exposed daily, over long periods, to direct and intense sunlight).

Heavy surface rust on newly shipped cables is usually the result of exposure to moisture and subsequent atmospheric corrosion which can be very detrimental to the performance of the cablebolts.

Wet or Atmospheric Corrosion

In a wet or humid environment, the corrosion process is accelerated and can involve a wider variety of cathodic reactions. Water and oxygen become jointly involved in the cathodic reaction and result in other compounds such as 2Fe(OH) ,3Fe O (magnetite), or Fe O (hema 3 4 2 3 tite). These compounds are much less adhesive then FeO and less likely to form a self-arresting film.

Corrosion products formed on cablebolts by wet corrosion are more likely to have a greasy feel as compared to the dry, rough texture of FeO film and are more likely to be associated with other film substances such as oils and additional moisture. These products are likely to have a detrimental effect on bond capacity of cablebolts. Clearly, unchecked corrosion reduces the cross-sectional area of steel in the cable and ultimately reduces the tensile capacity of the steel to unacceptable levels. Ductility and displacement capacity is also reduced (embrittlement).

The presence of water on the surface of the cablebolt also increases the potential for galvanic corrosion. The same wet corrosion cathodic reactions occur, accelerated by the presence of an electrolyte such as chloride, sulphate or hydroxide. Without electrolytes in a static solution, the corrosion process is self-limiting. Iron ions (e.g. Fe ) move into solution adjacent to the steel surface 2+ leaving behind free electrons (2e ) in the steel solid. The concentration of iron ions -in solution and free electrons in the steel creates an electrical potential difference which resists further dissolution of iron ions.

The effects of electrolytes in the surface water is best illustrated in the above example. A drop of water on the surface of the steel contains a dissolved electrolyte such as sodium chloride (which forms a solution of free sodium, Na ,+ and chloride, Cl , ions). The presence of electrolytes permits the transport of iron - ions as FeCl away from the corrosion (anode) site at the centre of the drop. At the same time, water and oxygen combine at the perimeter of the drop with the free electrons from the steel to form hydroxide ions (OH ) balanced by Na in solution. - + These move in the opposite direction to the FeCl generating a current (electron flow) in the steel supplying electrons to the drop perimeter as more iron ions go into the solution at the drop centre. Between the active centre (anode) and the drop perimeter (cathode) the iron ions combine with the hydroxide to form ferrous hydroxide.

This in turn becomes a relatively stable and complex hydrated oxide known as rust. The sodium and chloride transport ions are freed to carry on the process. The cyclic nature of the process combined with the fact that the corrosion product (rust) is not deposited at the anode (as it is with dry corrosion) means that this form of galvanic corrosion is not self-limiting and can be very aggressive. This is particularly true in mining environments given the high concentration of chloride and sulphate ions in mine waters (Minick and Olson, 1987).

Moist corrosion is particularly enhanced by crevices such as those formed by the flutes of a cable. Crevices are particularly good at retaining moisture and the conditions are perfect for differential aeration with low oxygen supply at the tip of the crevice compared with the rest of the cable. If a weak electrolyte is present, an aggressive corrosion cell is thus generated. This corrosion is particularly detrimental as the corrosion product (rust) readily fills the flutes of the cable preventing the penetration of grout and seriously reducing the cable/grout interlock essential for cable bond strength.

Cara membuat wallet Monero Indonesia yang benar di Android

Mungkin kamu akan bertanya, Apa itu Monero? Monero adalah sebuah kripto yang memiliki fitur sangat anonim dan private (rahasia), Hal ini yang memungkinkan pengguna untuk melakukan transaksi / pembayaran bertaraf internasional. Layaknya Bitcoin, Monero juga dapat di tambang melalui mining pool menggunakan perangkat keras berupa GPU. 



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