Water Stewardship

Guidelines for Minimum Standards in Water Well Construction, Province of British Columbia — June 1982

Part 1

3. Guidelines for Well Casing Selection and Installation

3.1 General Considerations

All well casing should be of new material or in like new condition. Casing must be of sufficient strength and composition to resist the pressure exerted by the surrounding materials and other forces imposed during installation, and to resist rapid corrosion by soil and water environments. It should be of the proper length to accomplish the purpose of providing a channel from the source to the surface through unstable formations and through zones of actual or potential contamination.

The thickness of well casing should be selected in accordance with good design practice and experience as applied to conditions found at the well site, All casing that is to be driven should be of steel. Casing that is placed in an oversized drill hole may be of other types of materials as noted in these guide-line. All joints in all casing should be water-tight. Joints in steel casing should be welded or screwed coupled joints. Casing joints should have the same structural integrity as the casing itself. Welded joints should be at least as thick as the wall thickness of the well casing and be fully penetrating. The standards of the American Welding Society should apply if metallic casing is welded. For threaded and coupled joints the couplings should be API, or equivalent and when made up-tight all threads should be buried in the top of the coupling.

3.2 Steel Casing

Recommended wall thicknesses for standard steel line pipe for various diameters for use in water wells are given in Table 1. These thicknesses are taken from EPA's Manual of Water Well Construction Practices EPA-57019-75-001, and are based on both the need for strength in various kinds of construction and resistance to corrosion. The AWWA provide more detailed tables and standards for wall thicknesses in the publication AWWA Standard for Deep Wells.

Due to the number of water well drillers in the Province who use thinner wall casing on domestic wells drilled, especially by the rotary method and from discussions with BCWWDA members, it would appear that a wall thickness of 0.188 inches may be successfully used for wells up to and including 6.625 inches O.D. It is recommended this wall thickness only be used provided the well is drilled by the rotary method and provided the casing is not driven. Table 1 has therefore been amended to include a footnote to accommodate this revision.

Based on the combined, reported experience of its members, the British Columbia Water Well Drilling Association consider that, where ideal site conditions prevail and in the interests of initial economy, the minimum thicknesses for steel casing given in Table 2 can be accepted. The site-specific decision as to whether to use Table 1 or Table 2 thicknesses for steel casing must be predicated upon accepted design practice applied to known site conditions.

3.3 Thermoplastic Water Well Casing

All materials used for wells should meet or exceed the standards set for that material and purpose by the Canadian Society for Testing Materials (CSTM) or the American Society for Testing Materials (ASTM). Thermoplastic water well casing should only be installed in an oversized drill hole and it must never be driven.

The type and diameter of thermoplastic water well, casing to use for a particular job will include consideration of the following:

1. Required internal diameter to accommodate the pump.

2. Nature of the formations.

3. Total well depth.

4. Anticipated drawdown.

5. Techniques of installation including backfilling techniques and method of grouting.

6. Temperature.

7. Techniques of well development.

Tables 3 and 4 give guide-line for the minimum wall thickness and maximum recommended depths of use for three types of thermoplastic water well casing (PVC 12454, ABS 434 and SR 4434), for four standard dimension ratios (SDR 26, 21, 17 and 13.5) listed in the ASTM F480 specifications.

The 1981 NWWA Manual on Thermoplastic Water Well Casing (see Appendix 11) recommends that a casing be selected so that its resistance to hydrostatic collapse is significantly larger than that required to resist external hydrostatic pressure alone, and in Tables 3 and 4 below, a recommended safety factor of 2 is used in computing the maximum recommended depth of use for three types of thermoplastic water well casing.

In the final analyses the depths to which thermoplastic water well casing can be used is a design judgement. A well casing with adequate packing to support the casing and to prevent casing deformation will allow placement to greater depths as long as the drawdown or head differential before packing is in place will always be within design limits.

All thermoplastic water well casing pipe couplings should meet the designated dimensional requirements given in ASTMF480 and the procedures for assembly should conform to that standard. All joints must be water-tight and where bell type ends or moulded couplings are used, all joints should be made utilizing solvent cement in accordance with the manufacturers directions. Threaded thermoplastic water well casing pipe couplings should be of the moulded or formed threads type only. The thread lubricant sealant specifically recommended for use with the designated thermoplastic It is common practice to drive a single material should be employed. steel casing into bedrock without first Further information on thermoplastic enlarging the hole and attempting to obtain water well casing is given in Appendix 11.

3.4 Casing Diameter

The minimum water well casing inside diameter recommended for line shaft turbine pumps is 6 inches, where a submersible pump is used the minimum inside casing diameter recommended is 4 inches. The well casing should be straight and plumb enough (see Appendix 1) to allow for the setting and correct operation of a pump capable of delivering the required water supply.

The selected diameter of the well should be adequate to accommodate the size of pump necessary to supply the designed quantity of water, keeping in mind the efficiency of the pumping unit, the expected pump life, the extra clearance required in the event the casing is not exactly straight and the possibility of weld metal projecting inside at the joints of steel casing. As a general guide the diameter of the well casing should be two nominal sizes larger than the bowl size of line shaft turbine pumps and in no case should it be less than one nominal size larger. Tables 5 and 6 have been prepared to show well casing sizes recommended for line shaft turbine pumps for wells ranging from under 100 U.S. gpm to over 4,500 U.S. gpm.

3.5 Casing Seating

It is a common practice to drive a single steel casing into bedrock without first enlarging the hole and attempting to obtain a good grout seal. In cases where there is a possibility for contamination, the use of a single casing is not recommended for wells drilled into bedrock or for wells penetrating clay, hardpan or other relatively stable unconsolidated material. Under these conditions it is very hard to get a tight seal between the single casing and the formation with the result that potential contaminants may enter the well. An adequate grout seal can be obtained by first drilling and installing an oversized surface casing which can later be withdrawn as the grout is placed around the inner casing. For further information see Appendix 3 - 3.2 Methods of Drilled Well Construction Provide an Adequate Grout Seal Around the Well Casing and 3.3 Methods of Grouting the Annular Space Surrounding a Well Casing.

*0.188 inch well casing thickness is acceptable for wells up to and including 6-5/8 inches O.D., provided the well drilled by the rotary method, and provided the casing is not driven.

Table 1. Wall Thicknesses for Standard Steel Line Pipe Recommended for Use in Water Wells*

Table 2. Minimum Thicknesses for Steel Casing Recommended by the B.C.W.W.D.A.

Table 3. Minimum Wall Thicknesses & Maximum Recommended Depth of Use for Three Types of Thermoplastic Water well Casing (PVC 12454, ABS 434 & SR 4434) for 2 Standard Dimension Ratios (SDR 26, SDR 21) Listed in ASTM F480 Specifications

² From values calculated in NWWA Manual on Thermoplastic Water Well Casing.

³ These sizes are under consideration by ASTM for addition to ASTM F480.

⁴ From values given in NWWA Manual of Thermoplastic Water Well Casing. These minimum inside diameters assume an average wall thickness of six percent above the specified ASTM minimum.

⁵ As given in ASTM F480 and NWWA Manual on Thermoplastic Water Well Casing.


Table 4. Minimum Wall Thicknesses & Maximum Recommended Depth of Use for Three Types of Thermoplastic Water well Casing (PVC 12454, ABS 434 & SR 4434) for 2 Standard Dimension Ratios (SDR 17, SDR 13.5) Listed in ASTM F480 Specifications

¹ NWWA Manual on Thermoplastic Water Well Casing, recommends that a sufficient margin be provided against possible collapse by all normally anticipated external loadings. A casing should be selected so that its resistance to hydrostatic collapse is significantly larger than that required to resist external hydrostatic pressure alone. The above table uses the recommended factor of safety of approximately x 2. Larger factors should be used when it is apparent that greater weight should be given to the other external loadings.

² From values calculated in NWWA Manual on Thermoplastic Water Well Casing.

³ These sizes are under consideration by ASTM for addition to ASTM F480.

⁴ From values given in NWWA Manual of Thermoplastic Water Well Casing. These minimum inside diameters assume an average wall thickness of six percent above the specified ASTM minimum.

⁵ As given in ASTM F480 and NWWA Manual on Thermoplastic Water Well Casing.

Table 5. Casing Sizes Recommended for Line Shaft Turbines (1800 RPM)

Table 6. Casing Sizes Recommended for Line Shaft Turbines (1200 RPM)

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