Casing is the tubular structure that is placed in the drilled well to maintain the well opening. Along with grout, the casing keeps possibly contaminated surficial water from reaching the aquifer zone underground and prevents contaminants from mixing with the water. The casing also holds back unstable earth materials so that they do not collapse into the well. Some states or local governing agencies have laws that require minimum lengths for casing.
The most common materials for well casing are carbon steel, plastic (most commonly, but not exclusively, PVC), and stainless steel. Different geologic formations and groundwater quality dictate what type of casing can be used. For example, parts of the country where hard rock lies underground are known strictly as “steel states.”
Residents in some areas have a choice between steel and PVC, both of which have advantages. PVC is lightweight, resistant to corrosion, and relatively easy for contractors to install. However, it is not as strong and not as resistant to heat as steel. Steel, though, is susceptible to corrosion, can have scale build-up, and can cost more than PVC.
Some contractors also use concrete, fiberglass, and asbestos cement casing.
On the top of the casing should be an approved well cap. It should fit snugly so debris, insects, or small animals can’t find their way into the well system.
Well caps are usually aluminum or a thermoplastic, and include a vented screen so that the pressure difference between the inside and outside of the well casing may be equalized when water is pumped from the well.
The casing and cap should extend at least 12 inches above the ground. If the well is near a river or stream, it should extend at least past the flood level to prevent overflows from contaminating the groundwater.
For information on well caps, click here.
Well screens are filtering devices used to prevent excess sediment from entering the well. They attach to the bottom of the casing or in intercepted water-bearing zones, allowing water to move through the well, while keeping out most gravel and sand. The most popular screens are continuous slot, slotted pipe, and perforated pipe.
Perforated pipe is a length of casing that has holes or slots drilled into the pipe, often after the casing is installed, perforated in place. It is not efficient for aquifers that feature a lot of sand and gravel because it has wide openings.
Continuous slot screens are made of wire or plastic wrapped around a series of vertical rods. This configuration provides consistent, regular slot openings that can be engineered to the particle sizes found in the screened zone. Slotted pipe screens, which have the least amount of open area, feature machine-cut slots into steel or plastic casing at set distances. Both of these are typically installed with a filter pack (gravel pack) that assists in screening and stability of the well.
Pitless adapters provide wells with a sanitary — and frost-proof — seal between the well casing and the water line running to the well system owner’s house.
After a frost line is determined for the area where the well is being installed, the adapter is connected to the well casing below the frost line. Water from the well is then diverted horizontally at the adapter to prevent it from freezing.
For more information on materials used in a water well system, contact a professional contractor in your area.