Grouting For Ground Water Control In Underground Mining-Books Pdf

Grouting for ground water control in underground mining
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may range from the permeation grouting of waterbearing alluvial and. granular rock formations to void filling grouting operations such as. the backwall grouting of shaft and tunnel linings and the formation of. underground plugs and dams, Grouting at the development stage of a mine is usually a pre planned. operation based on detailed borehole investigations and is aimed at. reducing the difficulties for shaft sinking or tunnel driving. Groundwater problems i association with phase i above and to a. lesser degree phase iii are generally the results of Unexpected. inrushes or increasing seepages and here the use of grouting is more a. firefighting application In most instances the mine development and. mining method will be designed to minimise the risk of such water inflows. during production, In the following sections of this paper the various groundwater problems. and methods of grout treatment are addressed in some detail The. different types of grout employed and t e approach to design of typical. grouting programmes are described Finally the various methods are. illustrated by reference to examples of grouting operations carried out. in recent years by the authors company, ASSESSMENT OF GROUNDWATER PROBLEM AND METHODS FOR GROUTING. Purpose of Grouting, Although grouting has a number of quite different applications the main. purpose in the context of this paper is always to eliminate or reduce. the flow of groundwater into an existing or proposed underground. excavation Grouting is only one of several methods of ground treatment. for excluding water which have to be assessed on their respective merits. for each situation However grouting does provide the benefits of a. permanent or at least semi permanent ground treatment and the bonus of. increased stability in some situations as compared with purely temporary. expedients such as dewatering and ground freezing, The acceptable level of groundwater in any mine situation will depend on.
a number of factors including the type of mine particular client. requirements safety aspects pumping economics and environmental. considerations In some instances the nature of the mineral involved. dictates that a dry mine is required In other circumstances where. perhaps pumping and disposal costs are low mining can continue with. little or no ground treatment and relatively high levels of water inflow. The philosophy of British Coal formerly the National Coal Board with. regard to controlling water is one of exclusion rather than pumping Dunn. 1982 In terms of shaft sinking and tunnelling it is considered that. the present mining construction techniques can tolerate a flow of up to. some 3 81 s 50 gpm into the excavation before action must be taken to. reduce the flow In the recent Selby mine developments involving both. shafts and surface drifts the requirement was for a final inflow after. permanent lining of less than one gpm This can be compared with. potential inflows from the major aquifer zones of several thousand. gallons per minute and gives an indication of the overall efficiency. required from the grouting process For instance a 99 per cent effective. grout treatment is required in order to obtain a ten fold reduction in. water make to the excavated shaft This degree of cover is difficult to. achieve and explains the long time that is often required for. satisfactory chemical grout treatments,Origins of Groundwater Problems. Mine water inflows have a variety of origins Access shafts or surface. drifts will often encounter substantially horizontal lying aquifers. before reaching the mining horizon These aquifers may be relatively. shallow water table aquifers or deeper confined aqulfers and their. nature may range from high intrinsic permeability alluvial deposits or. sandstones to intensely fractured and vuggy limestones Underground. tunnels drlvages and galleries may encounter major faults which can. transmit groundwater through relatively impermeable zones from such. aquifers into mine workings Similarly the presenq9 of faults and other. discontinuities in the rock mass may present water problems for. developments under bodies of water such as lakes reservoirs large. rivers and the sea or to developments n close proximity to abandoned. mines unsealed or poorly sealed shafts and boreholes which are likely to. he flooded e g Dunn 1982 Wilson 1985 Garritty 1983 Slatcher 1985. In most respects all of these situations represent water inflow problems. related to naturally occurring primary and secondary permeability in the. ground To these must be added the range of problems connected with. induced or modified permeability due to mine development and mineral. extraction Channels may be created through basically impermeable roof. strata to connect with the sources of groundwater described previously. e g Singh 1982 Massey 1984, It is essential therefore that the nature of the groundwater problem is. thoroughly investigated and fully understood to enable the optimum method. of ground treatment to be selected and in the case of grouting for the. appropriate form of grouting to be introduced Daw 1986. Grouting Methods and Materials, The various grouting methods can be described by reference to the. mechanisms by which the groundwater flows are eliminated or reduced. Permeation Grouting, In permeation grouting the grout material penetrates the interconnected. porous structure of the soll or rock which may comprise both the. intergranular voids and the fissure network Whilst in most instances. the fissure permeability represents the major contribution to the total. permeability of the ground and hence the main agent for transmitting. groundwater flow to the excavation there are instances where. intergranular permeability is equally important This will he most. evident in the shallower coarse sand and gravel aquifers but can also. occur at depth in medium to coarse grained sandstones e g in U K the. Bunter Sandstone Basal Permian Sands and Coal Measure Sandstones. In such aquifers it will be necessary to inject chemical grouts in order. to achieve the required penetration of the intergranular network and also. the finer fissures The choice of a particular chemical grout will. depend on a number of factors including permeability and pore size of the. aquifer rock cost strength and permanence requirements of the grout. and environmental considerations In order to penetrate the finest. rock structures that still pose a groundwater problem it may be. necessary to go to the lengths of clarifying the chemical grout. component solutions by filtration and or centrifugation. For wider fissures and a width of about 2 x lO 4m is usually considered. to be the lower limit standard cement based grouts will be used Since. these wider passages will generally be carrying the major water flows it. is the usual practice to inject the cheaper cement grout as the first. phase In some instances this procedure will reduce the inflow to a. tolerable level whereas in other cases a secondary treatment using. chemical grout will be necessary,Hydrofracture Grouting.
Although both intergranular and fissure grouting is described here under. the heading of permeation grouting the actual penetration process is. obviously quite different in a fissure than in a pore However in both. instances the injection is carried out at pressures insufficient to. disturb the ground structure The grout advances steadily displacing air. and water outwards with the predominant direction of flow being that. offering the least resistance i e the path of highest permeability In. some circumstances usually in relatively shallow alluvials it is. permissible to use hydrofracture grouting where deliberate overpressurlng. is used to either widen existing fissures or create new fissures This. procedure has the advantage of rapidly creating direct access through low. permeability ground to a more permeable and treatable zone from the. widely spaced array of injection holes In addition by creating new. passages a greater injection surface is available for the grout to. impregnate the ground From relatively shallow surface boreholes the. tube a manchette injection technique would most likely be used and. both chemical and cement grouts could be employed depending on the. particular application,Squeeze Grouting, Problems sometimes occur where unconsolidated but relatively impervious. deposits under high groundwater pressure and within an otherwise. competent rock structure need to be consolidated These may comprise. finely fractured or pulverised rocks silts soils millonites mud runs. etc none of which can be permeated with grouts The squeeze grouting. technique is used by which grouts are used to apply high pressures to the. ground to squeeze out excess pore water and consolidate the unstable. material by increasing its density and shear strength Consolidation is. achieved by either forming a grout bulb which does not penetrate the. soil or preferably by deliberate hydrofracture using a grout of llmited. capability to penetrate far The use of hydrofracture enables a larger. zone of ground to be stressed from a single grout pipe Viscous fluid. grouts are required but a number of combinations and variations of. approach are possible ranging from neat cement grout to clay cement. grouts and thickened chemical grouts Greenwood 1982. Vold Filling Grouting, In void filling grouting the requirements and methods will usually be. quite different from those described above In most instances. cementltious grouts will be employed often with fillers or cement. replacement materials in order to reduce costs where large volumes are. involved Frequently chemical admixtures will be incorporated in order. to impart the optimum combination of fluidity workability and setting. characteristics, In back wall grouting the first stage of the operation will be to fill. the bulk of the cavity between the shaft or tunnel lining and the. excavated profile The choice of grout will be dictated b y t h e. anticipated volume of the void as the void volume increases the fraction. of cement replacement material used in the grout will generally be. increased Despite the use of admixtures to control the settlement of. particles that occur in a cementitious grout prior to setting some. bleed often occurs This can result in bleed channels through which. water can still migrate around the lining The second stage of backwall. grouting serves the dual purpose of sealing these bleed channels and also. of locking the lining into the surrounding rock and is effected by the. injection of thin neat cement grout mixes, After the construction of a shaft plug or tunne dam thin cement grout. mixes are used to lock the structure and on occasion to form a cut off. curtain by injecting an array of holes drilled out from the plug or dam. into the surrounding strata,Combined Techniques, In some circumstances grouting has been used in combination with other.
methods of groundwater control during shaft sinking and tunnelling In. particular the incorporation of groundwater pressure relief wells. together with grout injection was used at two of the Selby shaft sites. to facilitate excavation through deep sandstone aquifers Fotheringham. 1983 The two specific applications were i where weak rock near the. face of the excavation would have been subject to collapse under the. action of the ground water pressure and ii where the shaft lining. would have been subjected to excessive hydrostatic loading during. backwall grouting Such a combination of grouting and depressurising can. be expected to be very effective even if the grout cover is only 80 per. cent complete, Depending on the circumstances the relief wells may be sunk from surface. and the water pumped by submersible pump Juvkam Wold 1982 or may be. sunk as a steep conical array from a temporary shaft sump above the. aquifer Fotheringham 1983 Scott 1983, Although ground freezing and grouting are not generally considered as. suitable combination techniques they were used for successive sections. of waterbearlng strata at the Gascoigne Wood Drift site Selby Mine and. the somewhat novel techniques used at the overlap zone are of interest. GROUTING AT DEVELOPMENT STAGE,Introduction, In most mining situations where groundwater presents a problem and. grouting is employed it is at the development stag that the major grout. treatments take place the purpose being to enable the shafts or drifts. to be constructed both safely and efficiently Four particular. applications of grouting can be considered pre grouting from surface. boreholes cover grouting from the shaft sump or tunnel face backwall. grouting and in certain circumstances the placement of shaft plugs and. drift or roadway dams,Pre grouting from Surface, The use of the pre grouting method from surface horeholes has in the. U K and North America at least been restricted in general to relatively. shallow depths of up to about 200 to 300 metres Jones 1979 In other. parts of the world e g South Africa U S S R where particular. hydrogeological conditions exist the method has been used at. considerably greater depths Dietz 1982 Kipko 1984, Probably the main application has been when a relatively thin alluvial.
or fractured rock aquifer zone occurs at shallow depth and other. processes such as dewatering ground freezing or cover grouting are. considered impractical or uneconomical The main advantage is that the. grouting is carried out prior to excavation and hence does not interrupt. the sinking process In addition it is not subject to the space and. environmental problems encountered when grouting in a shaft sump. However the two methods are not usually competitive being dependent on. specific ground conditions and it is possible for both approaches to be. used in the same shaft or drift, In the case of a shaft a ring of vertical grout holes typically six to. twelve in number are drilled around and somewhat outside the periphery. of the shaft Fig l The actual number of holes will depend on the. diameter of the shaft and the results of the ground investigations. Often and particularly for large diameter shafts a central hole is also. drilled which may be used initially as a test hole. In the case of surface drifts rows of grout holes will be drilled along. the llne of the drift with the number and spacing of holes chosen to. give adequate coverage of that area of the drift which will intersect the. aquifer zone Fig 2, In rock grouting the drill holes will most often be drilled to full. depth with the grout injections carried out in ascending stages using. borehole packers Other techniques such as the tube a manchette. method may be employed for shallower alluvial aquifers A sequence of. primary and secondary treatment will normally be adopted. Cementitious grouts are most commonly associated with pregrouting. although where conditions have dictated a special requirement chemical. grouts have also been used In such cases a larger number of holes may. well be required in order to obtain the required grout penetration for. closure of the curtain,Cover Grouting, Probably a more common approach and certainly that used in most U K. coal mine developments is to grout from within the excavation and ahead. of the advancing shaft sump or tunnel face Keeble 1981 Pocock 1982. Black 1982 This has the advantage of being a closer controlled. grouting process than pre grouting from surface boreholes as the shaft. sump can be taken quite close to the aquifer zone In addition the hole. drilling requirements are not so stringent as the hole lengths are. I I 6rouf Injection Hole,Drilled from Sur e,i II iiii ii iiii Ii ii Water. illl I Excavation Line,Surface Plan of Boreholes, Figure I T y p i c e l Arrangemenf of Pregroufing from.
SurfcIce Shaffs, generally much shorter However there are a number of disadvantages as. mentioned previously, On the basis of a detailed pre sinklng ground assessment and regular. probe hole investigations ahead of the face the excavation will be taken. to within some 5 10m of the aquifer zone requiring the grout treatment. A cone of injection holes are then drilled through valved standpipes. sealed into the shaft sump and fanned out from the proposed excavation. llne of the shaft A typlcal grout cover may be a out 30m in length and. if an extensive aquifer is intersected two or more over applng covers. may be required in order to achieve a full grout treatment Fig 3. Within each cover the holes will be drilled and injected in depth stages. of perhaps 3m or in some instances extended until a certain level of. watermake is encountered before grout is injected In rock where the. main waterflow channels may be vertical and sub vertical fissures it can. be beneficial to spin the drill holes Fig 4 in order to give a better. chance of intersecting all the fissures, A central test hole will often be used to monitor the effectiveness of. the grout treatment in restricting potential water inflows to the shaft. All grouting plant including pumps and mixing tanks are located within. the shaft and usually in the actual sump although sometimes use is made. of the shaft sinking stage Both cement and chemical grouting can be. carried out in this manner In circumstances where very highly. penetrating chemical grout is required a special grout clarification. plant will be set up on surface close to the shaft and the refined. components are then taken into the shaft in special hatching tanks. A similar approach is used for grouting from the face of a drift Fig 5. although various modifications have to be made to the grout mixing. set up depending on the differing geometry of the drift and the space. available at a particular face,Backwall Grouting, Backwall grouting is the process of filling of the void between the. excavated rock face and the installed shaft or tunnel lining Efficient. backwall injection and drying off any residual water seepages behind the. lining can be as important as any ground injections to the successful. completion of the shaft or tunnel, A typical procedure for back wall grouting of a section of shaft is shown.
in Fig 6 Grouting generally connnences after the concrete in a. particular length of lining has been cured adequately with injections. proceeding upwards from the lowest ring through injection pipes cast into. the lining Injections are carried out in different phases using. progressively increasing grout pressures until all leakages are. eliminated over the particular length of lining, In general three rings of holes are drilied out and grout valves fitted. to each injection pipe Grouting commences with thin cement grout W C. i0 I until connections between the holes are established then the grout. consistency is thickened and the holes closed off Injection continues. with a thin mix and the grout is allowed to rise and establish. connections with the next ring of holes The grout is then thickened and. the holes closed off as before The injection point is moved to this. iiiii iiiiiii i iiiii iiii Wats raB aring,Plan of injection Sump. Thick Aquifer Requires Multiple,rout Covers Solid Cones. Plan of Injection Sump,Thin Aquifer Requires Single. Grout Cover Hollow Cone, Figure3 Typical Arrangements for Cover Grouting Shafts.
Figure IUustrafion of Spinning of Drill Holes for,Cover Groufing. I Construction Joint, Figure 6 Typical ArrQngemenf for BQckwall Injection in a Shoff. second ring of holes and the procedure repeated Any small leaks are. dealt with by caulking with small wooden wedges or lead wool and. temporarily increasing the grout consistency, The grout is allowed to set then the injection holes are drilled out and. a second pass made with a thin cement grout mix The injection is. terminated at a pre determined finishing pressure, Cement grouts for backwall injection are best mixed in a high shear mixer. and usually contain a plastlcising admixture to give better dispersion of. the cement particles The thicker mixes used to fill voids will contain. antl bleed admixtures which by increasing the viscosity of the aqueous. phase reduce the rate at which settlement of cement particles occurs. Shaft Plugs, Despite detailed pre lnvestigatlon work and forward probing there are.
rare occasions where it is necessary to cast temporary plugs of concrete. in the sump of the shaft prior to further progress with the excavation. Whilst plugs and dams will be discussed in more detail in later sections. there are two applications that are relevant to this development phase of. For example in an extensive aquifer zone where more than one grout cover. is necessary it may not be possible to find a suitable section of. competent rock to establish as a sump for the second or subsequent grout. covers In these circumstances it can be beneficial to cast a. consolidation plug in the sump designed to provide the resistance. required for satisfactory setting of standpipes and high pressure. grouting Fig 7 Once the grouting is complete and the water zones. are satisfactorily sealed the plug is removed and shaft excavation. The other application can be regarded as an emergency plug required if. the shaft is subject to a major unexpected inrush of water In the worst. situation this may cause a temporary abandonment of the shaft A. concrete plug may then have to be cast underwater before the necessary. remedial action can be taken the water pumped out and the shaft. A detailed account of the design of the various types of underground. plugs is given by Auld 1983,GROUTING AT PRODUCTION PHASE. In most underground mine developments the need for grout treatment will. not be anticipated Either drlvages and ore extractions will be in dry. conditions due to the absence of aquifers or by deliberate design to. avoid groundwater problems or the level of inflow can be handled. conveniently and economically by pumping, However on occasion unexpected groundwater inrushes are encountered for. which some form of grout t r e a t m e n t proves beneficial and indeed. essential before further mining progress can be made The possible. causes of the inflow are numerous e g the intersection of unknown fault. zones or uncharted former shafts or workings which have become flooded. In the act of mining itself a strata zone of modified permeability is. Skin Grouf,Aggregote Bose Woven,Concrete Plug Polypropy ene Sheet. Figure7 Arrengemenf for Consolidefion Plug in Shoft Sump.

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