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Bores must be lined with an adequate lengthof appropriate casing to prevent the collapse

of the strata penetrated. The casing also acts as asafe housing for any pump installed in the hole.

9.1 GENERAL CONSIDERATIONSThe casing must be of sufficient strength andcomposition to withstand the pressure exerted bythe surrounding strata and other forces imposedduring installation, bore development and anycementing operations, and to resist rapidcorrosion by the soil and water environments. Itshould provide a secure and leakproof conduitfrom the water source to the surface throughunstable formations and through zones of actualor potential contamination. It must be joined andinstalled so that it is reasonably straight and freeof kinks or twists.

The selected diameter of the bore casing shouldfirstly comply with the minimum requirementsof the respective licensing authority and beadequate to accommodate the size of pumpselected to meet design or supply requirements.It should take into account:� the efficiency of the pumping unit� the expected pump life� the extra clearance required in the event that

the casing is not perfectly straight� the possibility of weld metal projecting inside

at the joints of steel casing.

The wall thickness or class of the bore casingshould be selected in accordance with gooddesign practice and experience as applied toconditions found at the bore site. Where it isnecessary to drive casing only steel should beused. Casing that is placed in an oversized drillhole may be of other types of materials as notedin these guidelines. Irrespective of what materialis used all joints should be watertight and havethe same structural integrity as the casing itself.

9.2 TYPES OF CASINGCasing selection depends on several majorfactors: strength requirements, corrosionresistance, ease of handling, cost considerations,type of formation, method of drilling, theparticular bore design, construction techniquesand licence or permit requirements. Casing musthave the column, collapse and tensile strengthsrequired for a specific borehole.

The improved drilling methods and techniquesnow available allow the use of a range ofmaterials for bore casing:� steel� PVC� thermoplastic� fibreglass

Each of these has advantages over the others andmay be more suited for a particular application.

9.2.1 SteelSteel is a commonly used casing material becauseof its greater strength.

Today much of the steel used as casing is in theform of piping or tubing and each length isjoined by butt welding. Some screwed andsocketed casing is used but the former is lessexpensive.

If using steel, choose casing or piping that isclean, new and of approved quality. Do not usesteel tubing that is defective or reject pipe.

Steel has these advantages over other types ofmaterials:� The diameter of drill hole can be smaller in

soft formations.� It is stronger than other materials.� It can be pressure-cemented to greater depths

due to its higher collapse strength.� It can withstand rougher treatment.

A disadvantage with steel is that its life can bereduced in a corrosive environment. This can bethrough corrosive soils, water or by galvanicaction arising from the use of dissimilar materialsin the bore. Stainless steel has generally fewerproblems but cost may be a prohibiting factor.

It is also best not to use steel in situations whereiron bacteria are present. These bacteria cancause corrosion and use steel as an additionalsource of energy.

Some potable waters can be very corrosive tosteel because of the dissolved gases they contain,carbon dioxide being the most common.Indications of high CO2 water qualities that canaccelerate the corrosion of steel are listed inTable�9.1.

chapter

9.0 CASING

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The short-term strength of plastic casing is muchhigher than its strength over time. In general,short-term test results for strength are not a goodindication of long-term strengths.

Other factors to consider when using plasticmaterials include impact resistance, toughnessand pipe stiffness. When casing protrudes aboveground level, for example, it must be protectedbecause it can be severely damaged by movingvehicles or contact with drilling tools. The casingshould also be shielded from the sun�s ultravioletrays if exposed above ground for long periods,because the impact strength of the material maybe reduced significantly over time. Care must beused during cold weather to prevent shatteringof the plastic during handling. Because theweight of plastic casing is only one-fifth to one-seventh that of steel, tensile strength is usuallyless important. Occasionally, it will float in abore during installation, thus creating specialhandling problems.

Plastic pipe may present a hazard to drinkingwater quality in areas where groundwatercontamination has occurred. If volatile organicchemicals exist in groundwater near a bore, butabove the intake section, it is possible for some ofthese chemicals to move into the discharge bypassing through the wall of the casing. Althoughthis process is not fully understood, it appearsthat plastic casing can be permeable in thepresence of certain chemicals.

9.2.3 PVC (unplasticised polyvinyl chloride)PVC pipe is made as piping for a wide range ofdrainage and general water distribution uses. Itis made in a variety of wall thicknesses andinternal diameters.

The only PVC piping suitable for use as borecasing is pressure rated pipe manufactured toAS�1477 standards. The piping is swell-jointedand solvent-welded. The only solvent and primerto be used is Type 'P' conforming to AS 3879.

If screws are `used to support the joint while thesolvent cures, only stainless steel screws are to beused. Care must be taken to ensure the screwsdo not protrude internally.

All other PVC piping is unsuitable for use in boreconstruction. PVC sewer or drainage pipe shall

Reactive Water Reactionagent quality

pH less than 5.5 corrosive

O2 more than 4 mg/L corrosive

CO2 more than 100 mg/L corrosive

CO2 50 to 100 mg/L marginal/corrosive

CO2 less than 50 mg/L acceptable

Table 9.1 Reactivity of steel casing tocorrosive waters

The reactivity shown in the table can varydepending on the chemistry of the particularwater.

Nonferrous or plastic materials are commonlyused as casing materials where corrosive waterspreclude the use of steel.

9.2.2 PlasticPlastic materials have much lower strength andweight than steel and require care in handling,storage and installation to prevent breakage ordistortion of their shape and must be derated inaccordance with the manufacturer�sspecifications when used with waters which areabove certain temperatures.

Plastic casing is low in compressive strengthrelative to steel casing. High temperatures deratethe pressure rating of the casing, so care must beused when grouting to minimise the effects of theheat of the curing grout around the casing.

Plastic material is much more flexible than steel.Therefore, plastic casing must be centred in theborehole before backfilling or filter packing iscompleted. Any voids in the backfill or filterpack material may lead to sudden collapse offormation materials against the casing, causing itto break.

The collapse strength of unreinforced plasticcasing is much less than for steel casing. Theactual strength for any situation will depend onthe wall thickness uniformity, roundness of thecasing, rate of loading, and the temperature ofthe casing when the loading is applied. Whereunreinforced plastic casing is fully supported byevenly compacted non-clayey fill or by grout, thecollapse strength and hence possible settingdepth is significantly increased.

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not be used. Class 6 pressure pipe has beenfound to have insufficient strength to be used asbore casing. Class 9 can be used with care forshallow bores. Class 12 piping is therecommended casing for most bore constructionapplications. Under NO circumstances is mPVCto be used as bore casing.

9.2.4 ABS (acrylonitrile butadiene styrene)In recent years thermoplastic ABS casings havebeen developed specifically for use in water bores.

ABS is generally a low-density material with ahigher tensile strength and lower temperaturederating than PVC. It is light and easy to handleand inert in aggressive groundwater conditions.The only acceptable ABS piping to be used shallcomply to AS 3518.1.

There is a variety of joining methods available.ABS can be supplied in a flush internal andexternal screw joint as well as a tapered solvent-weld flush internal and external joint. Thesolvent used must conform with the pipemanufacturer�s recommendations andcompliances.

9.2.5 FRP (fibreglass-reinforced plastic)Casing for water wells may also be constructedfrom various types of fibre-reinforced plasticmaterials. This type of casing is usually referredto as fibreglass casing as the reinforcing fibre isnormally glass strand. Fibreglass casing isresistant to most forms of corrosion, is notconductive, and has a higher strength to weightratio than steel. It is used for deep water supplybores in some areas. Fibreglass casing has morecollapse resistance than other plastic pipes. Ahigher initial cost may be offset by longer bore life.

9.3 CHOOSING CASING TYPEWhere resistance to collapse is the most criticalstrength requirement for boreholes with unstablestrata, steel or FRP casing is usually selected tomeet the strength characteristics required. Forcorrosive water, PVC, ABS, FRP or stainless steelcasing provides the longest life possible. Becauseof the many and sometimes conflicting factorsinvolved in selecting the most suitable casingmaterial, the driller or contractor should consultwith the water authority, owner and local drillingcontractors before selecting the type of casing.

9.3.1 Collapse resistance of pipes or casingsPlastic pipe or casing which is subjected to a highenough pressure externally (or vacuuminternally) can be potentially unstable. For anygiven diameter:wall thickness ratio, there is acritical collapse pressure at which the pipe wallwill commence to buckle inwards. The failuremode is unstable, (that is the more it buckles theless resistance to buckling) and so total collapseoccurs rapidly.

Situations where pipe buckling can arise arecommon in bore drilling and bore developmentand this can be the controlling factor in theselection of a pipe class.

The maximum possible differential pressureagainst collapse resistance should be used tocalculate the theoretical maximum depth for eachcasing class.

Plastic casings have been and can be installed ina water-filled hole to greater depths than thetheoretical maximum provided the pressuredifferential does not exceed the external collapsepressure rating for that particular casing typeand size.

Maximum differential pressures that should beallowed for have been calculated to range from12 kPa per metre head for an evenly compactednon-clayey filled bore annulus to 23 kPa permetre head for bores in swelling clays.

Plastic casing should not be set to a depth inunconsolidated formations where the maximumpotential pressure differential could exceed thecollapse rating of the pipe with consideration alsogiven to any loss of strength if the pipe is slotted.

Drillers need to be aware that problems caneventuate when the bore is developed or air-tested especially if a long column of mud orwater is rapidly discharged. Pumping of the borecan also cause a pressure difference.

A comparison of the similar strengths of casingmaterials is shown in Table 9.2.

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MINIMUM REQUIREMENTS FOR CASING OF BORES

Table 9.3 Bore casing manufacturing standards

Casing material Standard

Steel AS 1396–2000 steel water bore casingAS 1579–2001 arc welded steel pipes and fittings for water and wastewaterAPI–5L–350 MPa linepipe

ABS AS 3518.1 Acrylonitrile Butadiene Styrene (ABS) pipes andfittings for pressure applications – pipes

PVC AS/NZS 1477–1999 PVC pipes and fittings for pressure applications

Table 9.2 Comparison of strengths of bore casing materials

Material Specific Tensile Tensile Impact Uppergravity strength modulus strength temp.

103 kPa 105 kPa (relative to PVC) limits, 0C

ABS 1.04 31 20.0 x6 50

PVC 1.40 55 28.0 x1 40

Fibreglass (FRP) 1.89 115 158.0 x20 80(1)

Steel 7.85 350 (yield) 2068.0 very high(2) 800–1000Stainless steel 8.0 517 (yield) 2000.0 very high(2) 800–1000(1) FRP higher temperature with special resins.(2) The impact strength of steel and stainless steel is so high relative to PVC and to the demands of water well work, that it is generally not a design consideration.

R9.1 Material: The casing material types listed in Table 9.3 shall comply with manufacturer'sstandards shown. (Drillers may be required to supply copies of these compliances.)Second grade or reject steel pipe or tube shall not be used as bore casing.

Joints in steel casing may be welded or screwed. Care should be taken with welded joints toensure a round, straight, secure and leakproof joint is obtained.

Holes should not be cut into casing for the purpose of raising or lowering a string of casing in a bore.

Casing of nonferrous or plastic type inert material shall be used in all areas where a corrosiveproblem from bore water or strata is known to exist.

R9.2 Casing diameter for bores: The minimum casing size shall be 100 mm for shallow low-yield bores and 125 mm for deeper bores unless otherwise formally approved by the licensingauthority. As a general guide the diameter of the bore casing for high-yield bores shall beapproximately 50 mm larger than the pump size specified for the bore. It is recognised thatsmall diameter casing may be used for monitoring bores.

R9.3 Casing wall thickness: Bore casing wall thickness shall be sufficient to withstand theanticipated formation and hydrostatic pressures imposed on the casing during its installation,bore development and use. The minimum wall thickness shown in the following tablesrepresents the minimum thickness required in the absence of unusual stresses placed on thecasing in the course of installation, bore development, or use.

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MINIMUM REQUIREMENTS FOR CASING OF BORES (continued)

Class Unconsolidated Consolidatedformation/clays formation (ii)(metres head) (metres head)

9 26 60

12 56 110

15 100 200

18 200 300Note:(i) uPVC should be derated in pressure rating when temperature exceeds 20°C.(ii) The maximum difference in metres between external and internal water level.

Includes fully grouted and gravel pack.

The minimum acceptable wall thickness for steel casing or steel tube is listed in Table 9.4.

Outside Minimum Outside Minimumdiameter wall thickness diameter wall thickness(mm) (mm) (mm) (mm)

101 3.86 250 7.79

127 4.76 273 7.79

152 4.76 323 9.53

168 4.80 355 9.53

203 6.35 457 12.70

219 6.35 508 12.70

Table 9.4 Minimum acceptable wall thicknesses for steel casing or steel tubes

Table 9.5 Maximum potential pressure differential (i)for PVC bore casing (head difference)

R9.4 uPVC bore casing: The class of plastic casing used shall conform to the requirementslisted in Table 9.5. Class 9 uPVC pressure pipe shall be the minimum class allowable for use asbore casing. Class 6 uPVC pipe, uPVC sewer or drainage pipe shall not be used as bore casing.Type P solvent cement (AS/NZS 3879) for pressure pipe applications must be used for joininguPVC piping/casing. Applications are to be applied evenly to both spigot and socket ends,applied to the socket end first.

It should be noted that PVC is approximately1.25 times stronger than ABS for collapsepressure and approximately two thirds as rigidas GRP. Thus, as an example, the equivalents to150 mm Class 12 PVC (which has a wallthickness of 8.8 mm), would be 8.8 x 1.25 = 11mm for ABS and 8.8 x 2/3 = 5.6 mm for GRP.

This example is provided as a guide only withinthe normal limits of sizes and depths used bymost drillers. It should not be regarded as asubstitute for proper bore casing design.

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