Casing, Screen, and Seals.

The sanitary seal is the top covering of a well. Its function is to keep insects, small animals, dirt and debris from entering the well and contaminating the aquifer . This cap usually has an access port that allows periodic water-level measurements, and may have a screened breathing tube to allow air pressure to equalize as water levels rise and fall in the well.

The casing has several important functions. It is a conduit used to hold the wellhole open in soft or unconsolidated geologic formations. It commonly serves as the framework needed to support any pumping mechanism. It is an integral part of the casing seal, which is constructed to prevent surface water and shallow groundwater from entering the well and degrading deeper waters. To prevent surface runoff from flowing into the well, the

Wells and Well Drilling
casing usually extends 30 centimeters (1 foot) or more above the land surface. In formations prone to collapsing, such as in sands and gravel, the casing may extend the entire depth of the well. Where the well is constructed in hard formations such as shale or basalt, only the very top portion of the well may be cased. Even in ideal circumstances, a 5.5-meter-long (18-footlong) long casing generally is considered the minimum.

 

The casing seal is an impervious barrier, usually of cement grout or clay, placed between the well casing and the native material. It prevents surface water and undesirable groundwater from other aquifers from entering the wellbore or from flowing down the outside of the casing and degrading deeper water. Where wells penetrate confining layers at greater depths, seals are commonly constructed at least 1.5 meters (5 feet) into a confining layer. Occasionally, seals are also constructed at greater depths between different aquifers. These seals prevent one aquifer from draining into another, potentially resulting in resource loss or degradation.


Screens in the casing are used to filter out sand and gravel while allowing maximum water flow into the well. Their construction varies from highly engineered screens over 15 meters (50 feet) long, to a few holes punched through the casing. The formations encountered in drilling and the intended use of the well indicate the type of screen, if any, that is needed.

Pumps.

While some wells may flow at land surface under artesian pressure, most require some type of pumping mechanism to lift the water. Almost all of the common well pumps are centrifugal in design, using spiral impellers to push or pull water up and out of the well. The most common of these is the above-ground centrifugal, where the motor and pump sit at or near the well. These pumps are capable of moving large volumes of water, but they cannot typically lift from depths greater that 8 meters (25 feet). Wells with deeper static water levels and relatively low discharge requirements, such as domestic wells, often use submersible pumps. These pumps usually are attached directly above the motor and the entire unit is suspended in the well. Wells with deep water levels used for large quantities of water often use turbine pumps. Turbine pumps have the motors sitting on top of the well with a long shaft turning the impellers deep below the surface.

Constructing a Well

Siting.

One of the first things to decide is where to place the well. Assuming that an aquifer has been found at depth, the important considerations include maximizing distances to potential contaminants, access for machinery and potential future uses of the property. Septic systems, waste disposal lines, animal feedlots and buried fuel tanks all have the potential to degrade groundwater quality. Wells should always be at least 1.5 meters (5 feet) from any building site, in an area where future access can be preserved when repair, alteration or abandonment become necessary.

Construction.

There are many ways to construct a satisfactory well. The intended use and local geology usually dictate which methods will work in a specific area. Some of the common construction methods include digging, driving, and drilling.

A dug well is built by excavating a hole into a water-bearing zone, lowering a casing into the aquifer, and constructing the seal. Dug wells usually

A worker reacts as water surges from a valve as a water well is activated in 1999 in Tekoa, West Bank. The well was the first to be owned and operated by the Palestinian Water Authority on the West Bank, and was financed by the U.S. Agency for International Development. The water was earmarked for the Bethlehem and Hebron areas.
A worker reacts as water surges from a valve as a water well is activated in 1999 in Tekoa, West Bank. The well was the first to be owned and operated by the Palestinian Water Authority on the West Bank, and was financed by the U.S. Agency for International Development. The water was earmarked for the Bethlehem and Hebron areas.
are inexpensive, but are restricted to the near-surface, which may be insufficient to provide ample water quantity of suitable quality. A driven well typically consists of steel point attached to a section of perforated pipe or stiff screen, and sections of solid pipe. After digging or auguring an oversized hole to later accommodate the seal, the well constructor drives the assembly into the earth until the screen or perforations penetrate an aquifer; then he pours the seal. Most wells are constructed with well-drilling equipment. The two main types of drilling machines are the cable tool and the rotary drill.


The cable tool uses a heavy string of tools (bits), dropped repeatedly over the same point, to loosen, fracture and pulverize the earth so that it may then be lifted out of the hole with a bailing bucket. The loosened material and pulverized materials are referred to as drill cuttings. In dry formations, water may be added to the hole to hold the cuttings in suspension so they may be more easily lifted. The bailing bucket is a long cylinder, fitted with a one-way valve at the bottom. The bucket is lowered into the hole, then lifted and drained as the hole is advanced.

Rotary drills use hardened drill bits to cut through the earth. Compressed air, water, or special water-based fluid called drilling mud is used to suspend the cuttings and lift them from the hole. This mud is usually forced down the inside of the drillstem, past the drillbit, where it picks up the cuttings, then up the outside of this drillstem to the land surface. The drilling mud also helps hold the hole open and forms a cake or barrier inside the hole so the fluid and cuttings are forced up the hole rather than into the formation.

Sealing.

Regardless of which method is used, care must always be exercised in constructing the casing seal. Improperly sealed wells can contaminate water over a wide area. To construct the casing seal, the well constructor creates an oversized hole down to the intended sealing depth. The casing then is inserted, and the annulus (the space between the casing and earth) is filled with the sealing material.

Where the formation is prone to caving, another temporary casing may be necessary to hold the hole open while the seal is constructed. If the sealing depth is shallow (less than 18 meters [50 feet]) and there is little standing water in the sealing area, a special type of clay, known as Bentonite ® , may be used as the sealing material. Dry Bentonite ® chips or pellets are slowly poured into the annular space between the casing and the natural formation. The other common sealing material is cement grout, which is similar to concrete without the gravel component. This fine-grained cement is pumped into the bottom of the annulus until it reaches the land surface.

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