Water Stewardship

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

Part 2 — Technical Information Appendices

APPENDIX 10: PITLESS ADAPTER UNITS

1. General Information

A well pit can be a source of well contamination from surface or near surface drainage. Pitless well construction sharply reduces the possibility of contaminated water entering the well and system. Pitless adapter units allow termination of the well casing above ground and connection of the discharge pipe underground. Pitless adapters and pitless units are water-tight in design and construction to maintain a sanitary water supply. Water from the well coming up through the drop pipe is connected to the discharge connection by one or more removable seal joints. This permits the drop pipe and pumping equipment in the well to be easily removed from above ground for repair or servicing without disturbing the underground piping. The tank, pump controls, etc., can be placed in a heated location such as a basement offset from the well. Maximum sanitary protection of the offset piping from a pitless adapter requires that the piping be kept under pressure by use of a check valve, etc.

APPENDIX 11: THERMOPLASTIC WATER WELL CASING GENERAL

In preparing the following considerable use was made of material given in two publications:

1. Standard Specifications for Thermoplastic Water Well Casing Pipe and Couplings made in Standard Dimension Ratios (SDR) — ASTM F480-76.

2. Manual on the Selection and Installation of Thermoplastic Water Well Casing 1981, produced by National Water Well Association and the Plastic Pipe Institute.

The reader should refer to these two publications for further details. Thermoplastic water well casing is made primarily from three thermoplastics:

PVC or polyvinylchloride

ABS or acrylonitrile-butadiene-styrene

SR or styrene rubber

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 products complying with the ASTM F480 specifications have markings identifying the manufacturer, nominal size, type of material, dimension ratio and manufacturers code.

Which 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. Technique of installation including backfilling technique and method of grouting.

6. Temperature.

7. Technique of well development.

Casing Thickness and Depth of Use

In the tables attached to this Appendix are given 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.

Standard Dimension Ratio

Standard Dimension Ratios (SDRs) as given in the appendix tables are defined as the ratio of average outside pipe diameter to minimum wall thickness. The advantage of establishing four standard dimension ratios with four groups of minimum wall thickness means that for any given SDR group the collapse pressure is independent of pipe size. For example in the first table in the appendix under SDR 26, PVC 12454, the well casing has the same constant hydraulic collapse strength of 59 psi or 136 ft., (head) of water, over the entire range of pipe diameters.

The SDR system adopted in the appendix tables has definite advantages over the schedule systems i.e., schedule 40, schedule 80, where the SDR ratios of outside diameter to wall thickness may vary from one size to the next and because of this the hydraulic collapse pressures are not constant for all sizes within a schedule and tend to decrease in value as the casing diameter increases.

Hydraulic Collapse Pressure

The hydraulic collapse pressures as given in the appendix tables are a most significant property of thermoplastic water well casing. It has been determined that a thermoplastic water well casing is most vulnerable to collapse during installation when it has not yet been confined or restrained by the placement of fill material, gravel packing or grout.

Some reasons for differential pressures occurring in the thermoplastic water well casing are:

1. Rapid injection of air during well development, which results in excessive external pressure brought about by the reduction of internal fluid level and density.
   
   
2. Rapid removal of the bailer full of drilling mud from the bottom of the well which reduces internal pressure through the suction effect.
   
   
3. Extreme drawdown caused by overpumping during some development procedures.

The 1981 NWWA Manual on Thermoplastic Water Well Casing (given above) recommends that a casing be selected so that its resistance to hydrostatic collapse is significantly larger than that required to resist external hydrostatic pressures alone and in the appendix tables attached, a recommended safety factor of approximately two is used in computing the maximum recommended depth of use for three types of thermoplastic water well casing. If we take the same example we used before in the first table in the appendix under SDR 26, PVC 12454, the hydraulic collapse strength is 59 psi or 136 ft., (head) of water. Using a safety factor of approximately 2, the maximum depth of use is 70 feet, i.e., just over half of the collapse strength. In the final analysis 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.

Temperature

ABS thermoplastic water well casing displays the best impact resistance over a wide range of temperatures. Air temperatures lower than about 1.60C (350F) will, however, significantly impair the impact resistance of SR thermoplastic. PVC and ABS are impaired at about -260C (-150F) and -400C (-400F) respectively.

Long-term exposure to direct sunlight will cause a gradual loss of impact strength (toughness) that may be significant depending on the plastic, extent of exposure and susceptibility of the casing to mechanical damage. It is therefore recommended that installed well casings projecting above ground be suitably protected.

See also Appendix 3, Well Grouting, 2.7

Cement Grouting and Thermoplastic Water Well Casing, for effects of higher temperature from heat of hydration.

Joints

Thermoplastic water well casing pipe couplings should meet the designated dimensional requirements given in ASTM F480 and the procedures for assembly should conform to that standard.

All thermoplastic water well casing 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. Use of proper techniques for joining casing sections with solvent cement is critical in providing adequate handling strength and in maintaining the integrity of the completed well.

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 material should be employed.

Thermoplastic casing may also be attached to metallic screens through plastic coupling adapters. Extreme care should be taken while threading the metallic screen into the plastic adapter.

Installation Problems

The most common factors which cause problems in the installation of thermoplastic water well casing are:

1. Selection of the incorrect casing for a given installation i.e., the selection of a thin walled casing for a well that must withstand the stress of frequent redevelopment.

2. Joint separations or joint leaks.

3. Placement of gravel pack and/or grout in such a way that substantial, sudden and sometimes unsymmetrical. loads are applied to the casing.

4. A well development procedure which produces large pressure differentials on the casing.

5. The rapid shifting of formation material creating point and impact loading on the casing.

Table 9: U.S. - Imperial - Metric (SI) Factors

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