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There are many kinds of well construction, most of which require a pump or some bailing device to lift the water to the ground surface. Occasionally, ground water will flow naturally from a well to the ground surface in sufficient volumes to be used as is.
Dug and Drilled Wells
Two basic kinds of wells, dug and drilled, are illustrated in Figure 24. A dug well is a large opening made into unconsolidated sediments and occasionally bedrock by hand, backhoe, or auger. It must be deep enough to go a few feet below the water table. The hole is kept from caving in by installing a lining that may be stone, tile, cement blocks, or some other material. A drilled well in bedrock is of much smaller diameter and made with a drilling rig. It is deep enough to encounter one or more water-bearing fractures. The hole is lined with casing, usually steel but occasionally plastic, which penetrates through the overburden and terminates at some depth in the bedrock depending on local conditions (typically 3 to 5 feet). The remainder of the bedrock hole is left open in order to allow ground water to flow into the borehole through fractures in the bedrock.
Often wells are drilled into gravel deposits when substantial yields are required for municipal, industrial, or commercial use. Such wells are generally between 6 and 16 inches in diameter. They are cased from top to bottom except for a cylindrical well screen at the lower end. Screen length and size opening are selected to fit the water needs, textural characteristics, and saturated thickness of the particular gravel aquifer. A 20-ft screen might be required for a well yielding 500 gallons per minute, while a 3-ft screen might suffice for a 20 to 50 gpm yield. Occasionally, to increase the yield, and to decrease the energy required to pump a given yield, a gravel well is drilled to a much larger diameter than the the final casing of 6 to 16 inches in diameter. The oversized space around the well screen is filled with selected gravel that increases the permeability of the aquifer in the immediate vicinity of the well, and therby greatly increases the pumping efficiency of the well. Such gravel-packed wells are the norm in the case of municipal service (Figure 25).
Driven and Jetted Wells (Point Wells)
Some gravel deposits, where the water table is within about 20 feet of the ground surface, can be tapped by driving a 2 to 3 inch diameter pipe down to the appropriate depth. This pipe acts as a casing and water conductor, and is equipped with a well screen (or "point") at its lower end (Figure 26). Similar to this driven well is the jetted well, in which the water conductor/casing pipe is fitted with a special type of screen, and is washed down into a water-bearing layer by pumping water down the pipe as it is lowered into the ground. Sometimes the enlarged hole that is created by jetting is filled with a very permeable sand of a particular size that allows free movement of water into the well, but does not pass through the openings in the well screen (Figure 27).
Last updated on March 25, 2009
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