PRIVATE WATER SYSTEMS - Well Drilling Methods

Well Drilling Methods

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:

  1. Percussion Cable Tool
  2. Rotary Cable Tool
  3. Jetted
  4. Bored
  5. Driven
  6. Dug

Percussion Cable Tool

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.

-Disadvantages-

This method is slower than rotary drilling, and casing must be done coincident to drilling.

-Advantages-

Cable tool drilling can be done in all types of formations, and usually will provide an ample supply of pollution free water (That is if the water was pollution free to begin with).

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.

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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.

-Disadvantages-

It is difficult to drill in rock formations (basalt, limestone, etc.), and the presence of the drilling fluid often makes it difficult to identify cuttings from the water bearing formation.

-Advantages-

Drilling with this method usually penetrates unconsolidated formations faster than any other method. Since the well is not cased until drilling has stoped, abandoning the well does not entail pulling the casing or leaving it behind. Also, it is easier to construct artificial gravel packs when holes are drilled using this method.

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Jetted

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.

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Bored

Bore Well 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.

-Advantages-

In the proper formations, a power auger can bore up to 40 feet per hour, and continuous soil samples are readily available so the water bearing layer is easily known.

-Disadvantages-

Only formations having enough clay to support the borehole walls can be bored. Bored wells are also easily contaminated, so their use as a domestic water supply is ill advised.

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Driven

Driven Well 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:

  1. Drive more wells and combine the amounts of water;
  2. Drive the screen deeper in hopes that more permeable material will be reached;
  3. Pull the pipe out and try elsewhere;
  4. Pull the pipe and install a coarser well screen.

-Disadvantages-

It is impossible to drive well points through rocks, boulders, cemented sandstone, limestone or formations with too much clay. Driven wells also cannot be grouted to prevent contamination from the surface or from perched water tables holding contaminated water.

-Advantages-

Given the right geologic conditions, a driven well is the easiest, fastest, cheapest, and simplest type of well to install.

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Dug Wells

Dug Well 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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