Driven Point Wells used for Drinking Water
Driven Point Small-Diameter Water Well Installation, Diagnosis & Repair
DRIVEN POINT WELLS - CONTENTS: Driven Point Wells for Drinking Water - Problems, limits of water quantity, sanitation issues, & Repair Advice. Types of jetted wells, wash wells, or driven point wells & water supply systems & what to watch out for with each.
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Driven point well construction, capacities, troubleshooting & repairs: here we define driven point wells & describe how a driven point water well is constructed. We discuss why water flow and pressure are ultimately lost from a driven point well and we explain how to restore water flow (sometimes) without having to construct a new well.
This article series explains installing, diagnosing, and repairing small diameter water wells including driven point wells, wash wells, and jetted wells, three types of water sources that may be used where water is close to the ground surface and a well pipe or point can be driven into the soil mechanically or by using hydrojetting. We include an excellent UN FAO small diameter well document reference that will be helpful to those needing to construct a water well in areas where water is close to the ground surface and money or other resources are limited.
Small Diameter Driven Point Wells, Components, Performance, Maintenance
This article series describes various types of drinking water sources like wells, cisterns, dug wells, drilled wells, artesian wells and well and water pump equipment. We provide advice about what to do when things go wrong.
Readers of this document should also see Water Tank Types and before assuming that a water problem is due to the
well itself, see WATER PUMP REPAIR GUIDE an specific case which offers an example of diagnosis of loss of water pressure, loss of water, and analyzes the actual repair cost.
A variety of methods are used to install small-diameter water wells in areas where an adequate water supply is sufficiently close to the surface, where cost must be minimized, where well installation speed is important, and where there may be less concern for drinking water contamination from surface runoff. This article describes the two most common small diameter well methods, driven point wells and jetted wells.
Other small diameter well types include bored or augured wells, hydraulic percussion wells, cable tool percussion wells, bail down wells, and hydraulic rotary-drilled wells. All of these small diameter well methods have the advantage of comparatively low cost, rapid installation, and simplicity, and the risk of limited water availability and surface runoff contamination of the aquifer - considerations we explain in more detail below.
Driven point wells consist of a thick-walled metal pipe whose end is shaped into a point and drilled to permit water to enter the pipe. The sketch at left shows two types of driven point well tips or screens - from a small well article provided by the United Nations, FAO.
The driven point is then hammered into the soil, usually to a very shallow depth, perhaps 6' to 8' in depth, and usually in sandy soil or in other soil where a lot of water flows easily to enter the end of the pipe.
The maximum depth for driven point wells is about 80 feet. If local soils are hard or rocky a driven well cannot be used.
The well point may be purchased from a manufacturer (a forged well point) or hand made by cutting and hammering the end of the first pipe section. Holes to permit water entry into the well pipe may be drilled or sawn.
The pointed end of the perforated pipe aids soil penetration and is further protected from soil entry into the pipe itself by a well screen. Additional perforations in the lower length of pipe increase the water intake of the system.
As the UN FAO document explains:
Well screens for driving must have sufficient strength to withstand the forces caused by the driver and the abrasion of the material through which they pass. One common type (Figure 14a above) consists of a perforated drive pipe fitted with a point.
The perforated section of the pipe is wrapped with a layer of brass screen of the desired fineness and the screen is protected from damage by wrapping it with a layer of perforated brass sheet. Both layers are soldered to the pipe.
Another type of well screen (Figure 14b) is manufactured by wrapping trapezoidal rod in a spiral around a set of round longitudinal rods placed in a circular pattern with all intersections welded. This type of screen has the advantage of having a high percentage of open area and a slot shape which cannot become wedged full of fine sand particles.
How are driven point wells installed?
The driven point is then hammered into the soil, usually to a very shallow depth, perhaps 6' to 8' in depth, and usually in sandy soil or in other soil where a lot of water flows easily to enter the end of the pipe.
While special heavy-walled pipe and couplings are needed for the pipe to withstand the forces of being hammered into the ground (usually by a weight that is repeatedly lifted and dropped onto the upper pipe end), driven point wells are often home made or fabricated by a local well driller
The well pipe with attached point and screen is usually driven by a slide hammer much like those used to drive steel fence posts. A larger diameter pipe that will slide over the upper end of the well pipe is raised and dropped repeatedly to drive the well point into the soil. A screw-on pipe cap protects the upper end of the driven well pipe from damage. See the UN FAO sketch of a well driving apparatus (below left).
Alternatively the driving pipe may slide inside the well pipe as shown in our second UN FAO drawing (below-right). Two additional well point driving schemes are included in the illustrations available in the full UN document "Small Diameter Wells", Natural Resources Management & Environment Department, Food and Agriculture Organization of the United Nations, FAO Corporate Document Repository - Self-Help Wells. That excellent resource includes other well boring and drilling methods as well as techniques for dealing with problems such as retrieving broken well pipes or tools dropped into a well. (Local copy provided).
The upper end of the driven point pipe is usually connected
to a one line jet pump which "sucks" water out of the ground. In areas of very wet sandy soil a driven point well may be able
to produce a barely functional water supply quantity (by modern standards)
Well Water Quantity - Yield of Driven Point Wells
Driven point well water quantity - well yield capacity: Compared with a modern 6" steel casing drilled well, a driven point well installed to the same depth in an aquifer of the same yield capability (the cone of depression is the same), a larger diameter well will yield 1.6 times that of a small diameter well according to United Nations documents on water wells.
Driven point wells often have rapid reduction in water flow rate, depending on the type of soil into which the driven point was inserted. For this reason multiple driven point wells may be connected together to obtain an adequate water supply.
If the water quality from a driven point small diameter well is adequate but the water quantity is not, it may be possible to improve well yield by driving the point deeper into the soil. In general, well yield increases more by increasing well depth than by increasing the well diameter.
Driven point well sanitation and water quality: Even in areas of sandy soils where these shallow wells are frequently used, water quality is questionable as surface contaminants easily
enter the water supply, and water quantity is unreliable in areas where the water passage holes in the driven point become easily clogged
Our clients who have bought homes serviced by a driven point well have ultimately converted to a modern drilled well for better water
quantity and for a more sanitary water supply.
Driven point wells are also used in soil de-watering applications during construction and in some locations driven point wells are used to test for water in soils before drilling or digging a higher capacity well.
Restoring the Yield of a Driven Point Well - how can I get my water back?
What is a wash well? Jetted wells are similar to driven point wells in that a pipe is forced into the soil and connected (most often) to a single line jet pump. In some communities the term "wash well" is used for this water source, as suggested by New Hampshire reader Jack Allen.
How are jetted wells or wash wells installed? In either case, the pipe that is to be used to obtain water is forced into the soil using water at high pressure (40 psi for sandy soils, up to 150 psi for clay or gravel) from an existing water source.
Unlike a driven point well, however, the pipe used in combination with water to force an opening into the ground (the jetting tube) may be a temporary one (the jetting casing is removed from the ground after the jetting process is complete, followed by the insertion of a new casing and casing end screen) or it may be permanent (left in the ground at the end of the jetting process, jacked up just enough to accommodate a well screen lowered inside the casing to its bottom end).
An alternative jetted well process permits soil material to actually be removed from the well opening during the jetting process (soil flows up from the bottom of the jetting casing around its outside surface.
Water flowing from the tip of the jet dislocates soil sufficiently to permit the well pipe to be pushed into the ground. Using this process a jetted well (or wash well) may be driven deeper into the soil than a driven point well, and a jetted well may be driven through soils harder than those penetrated by a driven point well.
Check valves or foot valves in jetted wells: To avoid losing prime in a jetted well a check valve may be used at the lower end of the casing, above the well screen. This detail is important for a homeowner to know, because if a jetted well or wash well stops working the problem could be a failed check valve (or foot valve) rather than a loss of water in the aquifer.
Jetted well water capacity or well yield: In soils that contain large amounts of water, particularly in areas of gravel or sand, a jetted well or wash well may deliver good water flow or quantity.
Jetted well water sanitation: However the water quality questions that apply to a driven point well might need to be considered for a jetted well too: a well of this design has little protection from unsanitary groundwater compared with a steel-casing drilled-well that is cut into water bearing rock and that is sealed against surface water entry. We suspect that a jetted well installed using the alternative process that actually removes material from the well opening by flowing soil to the surface along the outside of the well casing may be more prone to surface water leaking into the well and its aquifer.
Mr. Allen points out that when well repair or service is required for a jetted well, the homeowner will need to contact a company who is familiar with this particular well type.
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Thanks to Jack Allen, Brookline, NH for pointing out our omission of wash wells from the discussion of water source types and who also provided the small diameter well reference below for well drilling information from the United Nations. 31 July 2009
"Small Diameter Wells", Natural Resources Management & Environment Department, Food and Agriculture Organization of the United Nations, FAO Corporate Document Repository - Self-Help Wells - see http://www.fao.org/docrep/X5567E/x5567e05.htm This article has a nice section on well pipe or dropped tool fishing tools (WELL RETRIEVAL TOOLS) at its end. If a tool is dropped into a well or if a section of well piping becomes disconnected and drops into a well, special devices can be lowered into the well to attempt to grasp or hook the lost object (or pipe) to pull it back out of the well. See http://www.fao.org/docrep/X5567E/x5567e05.htm#4.11%20problem%20solving
This document describes the following types of small-diameter (hand-built) water wells: [Quoting] 4.1 Bored or augered wells - This method of excavation consists of shaving or cutting material from the bottom of the hole by the rotation of a cylindrical tool with one or more cutting lips. The process is quite analogous to boring a hole in wood or metal with an auger or drill. The excavated earth normally feeds upward and is contained in the body of the auger where it remains until the auger is emptied. The auger is both rotated and raised and lowered by means of a vertical shaft which extends upward from the auger to a convenient point above ground level from which it can be rotated. Rotation is frequently accomplished by human power applied to a handle attached to the vertical shaft. However, the auger may be driven by other power sources such as animal or engine power. In this case, the power source drives a horizontal ring gear. Two projections extending upward from the ring gear drive a bar known as a "kelly" which lies across a diameter of the ring. A square section of the auger shaft fits through a square hole at the centre of the kelly bar which causes the auger shaft to rotate with the kelly bar while allowing it vertical freedom.
4.2 Driven wells - The driven well consists of a pointed perforated pipe or a pipe with a pointed well screen attached which has been driven into an aquifer. The pipe with pointed well screen is driven into place in much the same way a nail is driven into wood. Normally special pipe with thick walls and specially designed couplings are used to resist the driving forces. Under suitable conditions this method can yield a finished well in a very short time. While the well diameter is normally small and the yield relatively low, a number of driven wells may be coupled together and pumped with a single pump. Because driven wells are quick to construct, they may be used as a temporary source of water and then be pulled up when no longer needed. Driven well points may be installed and used for dewatering an excavation during construction. Unlike other well construction methods, material is merely forced aside and not excavated by the driving process. This means that little is learned about the material through which the well pipe passes. This kind of well can, however, be used for exploratory purposes to determine static water level and rate of inflow versus drawdown. Hard formations cannot be penetrated by this process. Barring impermeable strata the depth to which such a well can be driven depends on the build-up of friction between the well pipe and the material penetrated and the transmission of the force of the driver down the length of pipe. Twenty-five to thirty metres (80-100 feet) would probably be a maximum. A driven well point might be employed to finish a hole which had been excavated down to the water table by some other method such as an auger.
4.3 Jetted wells - This method makes use of a high velocity stream of water to excavate the hole and to carry the excavated material out of the hole. It therefore requires some type of pump, either motor or hand-powered, of reasonable capacity, as well as a supply of water. It is possible to separate the water and the excavated material in a settling pool or tank and to reuse the water, thus minimizing the quantity required. Since this method depends on the erosive action of water, it is obvious that extremely hard materials cannot be penetrated. However, semi-hard materials may be penetrated by a combination of hydraulic and percussion effects. This is accomplished by raising and dropping a chisel-edged jetting bit. Coarse materials such as gravel require a greater water velocity to move them vertically out of the hole than do finer materials. However, very fine, hard packed materials such as clays require a high water velocity to dislodge them. Water pressure of 3 kg/cm2 (40 psi) for sand and 7-11 kg/cm2 (11-150 psi) for clay or gravel have been recommended. Under good conditions, drilling progress is very fast.
4.4 Hydraulic percussion (also hollow rod method) driven wells - In this method the hole is kept full of water and a combination of mechanical and hydraulic action do the excavating (Figure 17). A chisel-edged cutting bit is attached to the bottom of a string of drill pipe. The hollow bit has inlet ports a small distance above its cutting edge. During drilling the drill pipe is alternately raised and dropped. Pressure due to the impact of the cutting bit in the bottom of the hole and the inertia of the water cause a mixture of water and cuttings to enter the inlet ports of the cutting bit. This causes the already full drill pipe to overflow. A check valve in the cutting bit prevents the mixture of water and cuttings from flowing out of the ports when the drill stem is raised. The cuttings may be settled out from the water in a pool or barrel after the mixture overflows from the drill pipe and the water can then be recycled. Hydraulic percussion is limited to drilling through relatively fine materials, since coarse materials will not rise to the surface through the drill pipe. This method has been used to depths of more than 900 metres (3 000 feet) in alluvial areas where neither hard formations nor coarse materials were encountered.
4.5 Percussion (also cable tool method) driven wells - This method consists of repeatedly raising and dropping a chisel-edged bit to break loose and pulverize material from the bottom of the hole. A small amount of water is kept in the hole, so that the excavated material will be mixed with it to form a slurry. Periodically the percussion bit is removed, and a bailer is lowered to remove the slurry containing the excavated material. The bailer or bailing bucket consists of a tube with a check valve at the bottom and a bail for attaching a cable or rope to the top. When it has been raised and dropped a number of times to fill it with the slurry it is brought to the surface for emptying. Bailing is repeated until the hole has been adequately cleaned, at which time drilling is resumed; drilling and bailing are then alternated. If the hole is unstable, casing is lowered and the driving of casing is alternated with the other two processes. In loose granular material, such as sand, bailing alone may be sufficient to remove the material from the bottom of the hole and allow the casing to be sunk. A heavy bailer with a cutting edge at its lower end, known as a "mud scow" is used for this purpose.
4.6 Hydraulic rotary drilled wells - This method employs a drilling bit at the bottom of a stem of rotating drill pipe. Cuttings are removed by pumping water or a mixture of water and various clays down through the drill stem. This "mud" entrains the cuttings and carries them up through the annular space between the drill pipe and the wall of the hole. When they reach ground level, the cuttings can be settled out in a small pond and the "mud" re circulated. If the reverse flow path is used ("mud" pumped to the surface through the hollow drill pipe) the system is called reverse rotary. The reverse rotary system allows larger particles of cuttings to be brought to the surface, because the upward flow velocity inside the pipe is greater than that through the annular space, due to the smaller flow cross section inside the pipe.
Thanks to reader Lloyd McVey at AKR for discussing using compressed air to restore water flow in a driven point well, July 2010
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Water Fact Sheet #3, Using Low-Yielding Wells [ copy on file as /water/Low_Yield_Wells_Penn_State.pdf ] - , Penn State College of Agricultural Sciences, Cooperative Extension, School of Forest Resources, web search 07/24/2010, original source: http://pubs.cas.psu.edu/FreePubs/pdfs/XH0002.pdf
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WATER PUMP REPAIR GUIDE an specific case offers an example of diagnosis of loss of water pressure, loss of water, and analyzes the actual repair cost
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