The type of well drilling method chosen depends on many factors, including: cost, ease of construction, geologic conditions, well diameter, depth, sanitary protection, and intended use of the well itself. The well-drilling methods commonly used in drilling wells for private water systems are:
Percussion Cable Tool
The cable tool method involves up and down movement of a
tool string on a cable to pulverize the geologic formation.
Water is added to loosen the material as well as to facilitate the
removal of the resultant cuttings by a bailer. In stable formations,
drilling progresses for 5 to 10 feet, then casing is driven to the
bottom of the borehole, and the cuttings are then removed. In unstable
formations, however, the casing is driven down first and drilling is
completed to the bottom of the casing before bailing.
Wells drilled by this method may be as deep as 1000 feet and have diameters ranging from 4 to 18 inches. The upper portion of the borehole is made 4 inches wider than the rest of the hole and is grouted to prevent any possibly contaminated water from entering the aquifer by flowing between the casing and the borehole wall. The depth of the grouted section is dependent upon the geologic formation.
In addition, cable tool drilling cost 1/2 to 1/3 as much
as rotary drilling methods of the same capacity. The drill
rigs are relatively compact requiring less accessory equipment and are easier to move making this method ideal for
drilling in remote or rugged areas. The motors also require
less fuel than motors on rotary drill rigs, and much less water
is required. Lastly, data on the water bearing formations and
water samples for testing can be collected while the casing is
being driven and bailed.
Rotary Cable Tool
Rotary drilling combines the use of a rotating bit
for forming the borehole and a continuously circulating drilling
fluid for removal of the cuttings.
Drilling fluid is pumped down the drill stem and out through nozzles in the bit. The pressure from the pump forces the fluid, mixed with cuttings from the borehole bottom, to flow upward in the annular space around the drill pipe to the surface. The fluid is then channeled to a settling pit and then to a storage pit before being picked up again by the pump for reuse.
Rotary drilling may be used for boreholes reaching as deep as 1000 feet and having a diameter from 3 to 24 inches wide. The upper portion of the hole is made 2 inches wider than the rest and grouted. As with the cable tool method, the depth of grout required depends on the geologic formation.
Jetted
This method of well drilling involves the use of a high velocity stream or jet of fluid to cut a hole in the ground and
transport the loosened material up and out of the hole. The equipment used may be the same equipment that is used for rotary drilling
minus the bit.
Actual jetting of the hole may take only minutes, therefore well pipe assembly needs to be on hand and ready to install before jetting begins.
The water under pressure should be at least 50 ppm chloride to prevent contamination of the aquifer by jetted water.
The jetted wells may be up to 50 feet deep and have diameters ranging from 2 to 12 inches. Protective casing should be installed to at least 25 feet and the well should be grouted to a minimum depth of 10 feet to protect the well against contamination from the surface.
Jetted wells can only be installed in unconsolidated formations and are best suited for bore holes 4 inches in diameter.
Bored
In this method and earth auger is 'screwed' into the earth
by rotating it either by hand or automatically. Two different
methods of boring are used; one uses a rotary bucket and the
involves the use of a continuous flight stem auger.
Neither method can bore through consolidated materials, or dense rock. When boulders or cobbles are encountered, the stem must be pulled and the rock must be removed by stone tongs, ram''s horn tool, or an orange-peel grab. In addition, augers cannot excavate saturated sand or unconsolidated material, therefore these methods only work well for formations having enough clay in them to support the borehole walls until the hole can be cased. Some difficulties in boring in saturated sand may by overcome by keeping the borehole full of water.
Bored wells can have diameters ranging from 2 to 30 inches, and hand bored wells are limited to depths of 15 to 20 ft. while power driven augers can bore holes up to 125 ft. deep.
Driven
To install a driven well a special driving point with
a well screen on a series of short pipe sections is driven into the
ground, either by hand or power tool. It helps to keep the pipe
vertical if the well point is started at the bottom of an augured
hole slightly larger than the well.
Concerns during the driving process include: 1) Pounding too hard on the pipe may damage the screen, 2) Using a maul for driving may cause pipe to break from glancing blows, 3) Pipe should be turned with a wrench periodically to keep couplings tight.
Well depths may reach 30 feet if driven by hand and 50 feet if driven by power tool, and well diameters range from 1.25 to 12 inches.
Well yields are usually relatively small due to small pipe diameters and comparatively small screened sections. If yields are too small, there are several options to increase the yield:
Dug Wells
The hole is dug the desired diameter and depth by hand or power.
The hole may be shored; often casing is installed as digging progresses and
is allowed to sink by its weight as the hole is excavated under the casing.
The walls are often brick, stone, concrete or precast concrete pipe. Be sure
to seal the liner tightly to help prevent contamination. When digging deep holes,
ventilate the hole.
Many dug wells fail during droughts, because they can only be dug a few feet under the water table. They are also easily contaminated. They also cannot be dug into dense rock.
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