Wells & Borings
Drilled wells with pumps are currently used throughout the world to provide water for agriculture, industry and domestic purposes. Typically wells are used in rural or sparsely populated areas, but many urban areas are supplied totally or partially by wells.
Drilled wells are typically created using either top-head rotary style, table rotary, or cable tool drilling machines, all of which use drilling stems that are turned to create a cutting action in the formation, hence the term 'drilling'. Most shallow well drilling machines are mounted on large trucks, trailers, or tracked vehicle carriages. Water wells typically range from 20 to 600 feet, but in some areas can go deeper than 3,000 feet.
Rotary drilling machines use a segmented steel drilling string, typically made up of 20 foot sections of steel tubing that is threaded together, with a bit or other drilling device at the bottom end. Some rotary drilling machines are designed to install (by driving or drilling) a steel casing into the well in conjunction with the drilling of the actual bore hole. Air and/or water is used as a circulation fluid to displace cuttings & cool bits during the drilling. Another form of rotary style drilling, termed 'mud rotary', makes use of a specially made mud, or drilling fluid, which is constantly being altered during the drill so that it can consistently create enough hydraulic pressure to hold the side walls of the bore hole open, regardless of the presence of a casing in the well. Typically, boreholes drilled into solid rock are not cased until after the drilling process is completed, regardless of the machinery used.
The oldest form of drilling machinery is the Cable Tool, still used today. The 'spudding' of the drill causes the bit to be raised & dropped onto the bottom of the hole, and the design of the cable causes the bit to twist at approximately 1/4 revolution per drop, thereby creating a drilling action. Unlike rotary drilling, cable tool drilling requires the drilling action to be stopped so that the bore hole can be bailed or emptied of drilled cuttings.
Drilled wells are typically cased with a factory made pipe, typically steel or plastic / PVC. The casing is constructed by welding, either chemically or thermodynamically, segments of casing together. If the casing is installed during the drilling, most drills will drive the casing into the ground as the bore hole advances, while some newer machines will actually allow for the casing to be rotated & drilled into the formation in a similar manner as the bit advancing just below. PVC or plastic is typically welded & then lowered the drilled well, vertically stacked with their ends nested & either glued or splined together. The sections of casing are usually 20' in length, and 6" - 12" in diameter, depending on the intended use of the well and local ground water conditions.
Surface contamination of wells is typically controlled by the use of a 'surface seal'. A large hole is drilled to a predetermined depth or to a confining formation (clay or bedrock, for example), and then a smaller hole for the well is completed from that point forward. The well is typically cased from the surface down into the smaller hole with a casing that is the same diameter as that hole. The annular space between the large bore hole & the smaller casing is filled with bentonite clay, concrete, or other sealant material. This creates an impermeable seal from the surface to the next confining layer that keeps contaminants from traveling down the outer sidewalls of the casing or borehole & into the aquifer. In addition, wells are typically capped with either an engineered well cap or seal that vents air through a screen into the well, but keeps insects, small animals, and unauthorized individuals from accessing the well.
At the bottom of wells, based on formation, a screening device, filter pack, slotted casing, or open bore hole is left to allow the flow of water into the well. Constructed screens are typically used in unconsolidated formations (sands, gravels, etc.), allowing water & a percentage of the formation to pass through the screen. Allowing some material to pass through creates a large area filter out of the rest of the formation, as the amount of material present to pass into the well slowly decreases & is removed from the well. Rock wells are typically cased with a PVC liner/casing & screen or slotted casing at the bottom, this is mostly present just to keep rocks from entering the pump assembly. Some wells utilize a 'filter pack' method, where an undersized screen or slotted casing is placed inside the well & a filter media is packed around the screen, between the screen & the borehole or casing. This allows the water to be filtered of unwanted materials before entering the well & pumping zone.
Driven wells may be created in unconsolidated material
with a "well point", which consists of a hardened drive point
and a screen. The point is simply driven into the ground, usually with
a tripod and "driver", with pipe sections added as needed. A
driver is a weighted pipe that slides over the pipe being driven and is
repeatedly dropped on it. When groundwater is encountered, the well is
washed of sediment and a pump installed. This is the cheapest and simplest
type of water well known today, however it is only useful at relatively
shallow depths and for small capacity wells.
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