diameter in monitoring well installations. An inconsistent inner diameter causes problems

when tight fitting downhole equipment (development tools, sampling or purging devices, etc.)

is used; an uneven outer diameter creates problems with filter pack and annular seal

placement. The latter problem tends to promote water migration at the casing/seal interface

to a greater degree than is experienced with uniform outer diameter casing (Morrison, 1984).

Because all joints in a monitoring well casing must be watertight, the extent to which

the joints are tightened should comply with recommendations of the manufacturer.

Overtightening casing joints can lead to structural failure of the joint (National Water Well

Association and Plastic Pipe Institute, 1981). To maximize the watertightness of the joint

where threaded joints are used, fluoropolymer tape may be wrapped around the threads prior

to joining male and female sections; also, an O ring may be added for extra security.

Solvent cementing of thermoplastic pipe should not be used in the construction of

ground water monitoring wells. In solvent cementing, a solvent primer is generally used to

clean the two pieces of casing to be joined and a solvent cement is then spread over the

cleaned surface areas. The two sections are assembled while the cement is wet. This allows

the active solvent agent(s) to penetrate and soften the two casing surfaces that are joined. As

the cement cures, the two pieces of casing are fused together; a residue of chemicals from the

solvent cement remains at the joint. The cements used in solvent welding, which are organic

chemicals, have been show to adversely effect the integrity of ground water samples. (See

Aller et al., 1989 for a summary of relevant research.)

6.2.4

Well Casing Diameter

While casing outside diameters are standardized, variations in wall thickness can cause

casing inside diameters to vary. In "scheduled" casing, wall thickness increases as the

scheduling number increases for any given diameter of casing. Nominal 2 inch casing is a

standard 2.375 inches outside diameter; wall thicknesses vary from 0.065 inch for schedule 5

to 0.218 inch for schedule 80. This means that inside diameters for nominal 2 inch casings

vary from 2.245 inches for schedule 5 thin walled casings (typically of stainless steel) to only

1.939 inches for schedule 80 thick walled casings (typically of PVC). Wall thickness also

changes with pipe diameter in scheduling. Because schedule 80 PVC is thicker than schedule

40 PVC, schedule 80 PVC wells will extend the life of the monitoring system compared to

schedule 40 PVC. The cost differential between these two schedules is fairly insignificant.

Another method of evaluating casing strength is by standard dimension ratios (SDR).

A SDR is the ratio of the wall thickness to the casing diameter. The ratio is referenced to an

internal pounds per square inch (psi) pressure rating such that all casings with a similar SDR

will have a similar psi rating. Where strength of casing is important, scheduling and SDR

numbers provide a means for choosing casing.

Although the diameter of the casing for a monitoring well depends on the purpose of

the well, the casing size is generally selected to accommodate downhole equipment.

November 1992

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