Construction Methods and Materials (Ben)

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Water that is drinkable is called potable water, but all water must not be potable in order to be of use. Non-potable water can be used for watering livestock, irrigation, and other uses that do not involve human ingestion of water. The way to get water to the surface for personal use is to construct a well system. A well should be designed to maximize the withdrawal of ground water while protecting the quality of ground water in the aquifer. After a well has been constructed, the only evidence of the system should be a piece of casing that protrudes out from the ground. Excluding the pump system and the distribution lines that connect the well to the surface, there are three basic parts to every well: 1. Casing: The part that connects the aquifer with the surface. The casing allows access to the aquifer and provides protection against contamination from lower quality water migrating between permeable rock layers. Casing is structurally important because it supports the sides of the borehole, which is the hole that penetrates through the earth to reach the aquifer. Casing cannot be easily replaced and doing so will most likely result in significant damage to the well. Therefore, care in the selection and installation of the casing is important. 2. Seal or Grout: This is the material used to seal the original borehole outside the casing and serves two functions. The grout should seal the outside of the casing and borehole to control the movement of water vertically along the sides of the casing and borehole. This is to stop poor-quality water in the soil from flowing down the space between the outmost casing and the borehole wall and contaminating the borehole. The following materials are often used for the seal: cement consisting of a mixture of ordinary cement and clean water in a ratio of 22 liters of water to 50 kg of cement, bentonite clay pellets or a premixed bentonite slurry, sand-cement grout consisting of ordinary cement, clean sand and water in a weight proportion of no more than 2 parts sand to 1 part cement. The material selected must be able to withstand the chemical degradation of the unwanted water as well as maintaining a seal. A combination of materials can be used for the grout. Some grout materials are used for their ability to add strength and protect the casing from the soil and water corrosion and other materials are selected to resist water movement. 3. Screen or Intake Section: The screen lets ground water move freely from the aquifer into the well while stabilizing the aquifer material. The screen or intake section allows the well to penetrate vertically through the aquifer, taking advantage of the pressure situations that are found in the aquifer. (Jay Lehr et al., Design and Construction of Water Wells: A Guide for Engineers, New York, Van Nostrand Reinhold, 1988, 73). Construction Methods and Materials The initial step in constructing a well is proper testing to ensure that the proposed well area is capable of providing water to a designated number of people. It is important to drill a test hole to obtain information about the geology and aquifer characteristics of the proposed well site. This pre-design information allows the designer to anticipate problems with the drilling and construction of the well before they occur. The test drilling program is divided into two phases. First is the exploratory phase of drilling in which you are searching for conditions where ground water is accessible in safe and usable quantities. The second phase focuses on obtaining as much information as possible about the aquifer characteristics and water quality. Samples of the aquifer material are obtained from the test borehole. Water quality information may also be available from other wells in the immediate area. Information about the yield of the aquifer may also be obtained from nearby wells, which can help give an estimate of what can be expected from the proposed well. If available, well logs from nearby wells can provide pertinent data about local drilling conditions of the soils in the area. (Jay Lehr, et al., 74) Since geological conditions are diverse, the choice of a drilling rig to drill a borehole should be controlled by the data collected prior to drilling. Some factors are the types of rock to be drilled, the maximum depth of the borehole, the types of casing and screed to be used, the depth of the water table, and the accessibility of the borehole drilling sites. Most drilling methods require a motorized drilling rig, which is a crane with a motor and a mast equipped with the necessary cables to allow for the lift, fall, and rotation of drill rods. Manual drilling is the cheapest drilling method, but is not possible in most areas due to the geology of a given area. Often manual methods have developed gradually in an area and make use of local materials so expensive equipment is not necessary. If other well projects have been undertaken in the proposed area, the sharing of data and methods may be very helpful in determining which drilling method is best. (Alan MacDonald, et al., Developing Groundwater, 154) If a private drilling company is used to carry out the drilling, then a contract governs the specific actions to be undertaken by the driller. Borehole contracts are much the same as other forms of civil engineering contracts. However, there are three main differences. Each borehole will be different, much of the borehole construction is not visible and cannot be inspected, and the borehole owner is unlikely to know the specialist skills required to drill and construct a successful borehole. Examples of borehole contracts often exist within a country and an excellent guide is provided by the US Environmental Protection Agency’s Office of Water Supply. Although this is dated 1976, it is still seen as a benchmark for contracts. There are several published general conditions of contracts for civil engineering work. Listed below are several of these general conditions that need to be modified for drilling contracts to fit the scope of work: 1. The site is not always clearly defined at the beginning and flexibility must be built into the contract. 2. The engineer may need to make urgent minor alterations without having received written permission from the employer. 3. The site and aquifer conditions are not always known at the beginning of the task. 4. The accuracy of the estimated quantities may be less since the thickness of aquifers is not known prior to work. 5. The period that a contractor must come back to fix problems must be short enough so the problems can be fixed while the equipment is still in the area. 6. The contractor is responsible for health and safety at the drilling site. 7. Because of the scope of work, the contractor should be allowed to work during the night without written permission.

The second part of the contract is the Design Specifications, which specify the work to be done and the specifications. Listed below are several points to address in this part of the contract: 1. General: description, purpose and location of work, expectant drilling conditions, health and safety, unit of measurement used. 2. Borehole drilling: often the contracts leave the drilling method to the contractor, but special conditions must be stated. 3. Borehole construction: design, schedule of construction, grouting, finishing at the top, the project engineer or representative’s role in finalizing design. 4. Materials: casing, screen, reducers, cement, etc. 5. Borehole development: how long the boreholes should be developed for. 6. Tests: pumping tests, quality of the water. 7. Sampling: analyzing rock samples. 8. Recording: drillers log, construction, material used in each borehole, depth and diameter. 9. Site restoration: how the site is to be left, filling in abandoned boreholes. (Alan MacDonald, et al., 167-168) It is important to ensure that all individuals involved in well construction are using the same measurement system in order to cut down on mistakes. Dug Well Construction A dug well is a shallow hole dug into the ground down into the water table. Open unprotected wells can become very contaminated and can spread diseases such as guinea work, typhoid, cholera, hepatitis A and many diarrheoal diseases. Wells that are lined with concrete, covered and fitted with a water lifting device can provide safe drinking water. (Water Supply and Sanitaion, prepared by Catholic Relief Services, [CD-ROM], Ausust, 2005). When sighting a well, the spot should have as much clearance around it as possible and the vegetation in the area should be cleared. Time should be taken to clear the site so that objects don’t fall down the well. Any overhead tree limbs should be trimmed as a clear work area will help make the project safe. Start by digging a hole in the desired location that is 1 or 2 feet deep and fill it with water. When the water disappears, fill it again and repeat several times. This will soften the soil that you are about to dig. At this time you can install a collar at the hole if you have one. This can be a 5 gallon bucket with the bottom removed. The collar will help keep loose soil and rocks from going back into the hole as you work. (Bob Mellin, Waterhole: A Guide to Digging Your Own Well, San Anselmo, CA, Balboa Publishing, 1991, 25) An auger can be used to dig the hole and it should be turned in a slow, steady motion so its blades will draw it into the ground. When the auger bucket fills up it can be pulled up and emptied. To remove soil that is stuck to the bucket, a 2 foot high 2x4 stuck in the ground works well for banging the auger against to remove soil. A digging bar about 5 to 7 feet long and about 40 lbs. will be helpful if you encounter rocks. The bar can be dropped in the hole and rocks will usually break loose and can be removed with the auger. Once you’ve dug beyond the digging bar’s length, you’ll have to attach a rope to the end of the bar after dropping it into the hole so that you can retrieve the bar. If you hit a big rock that cannot be broken, it is necessary to move over 4 or 5 feet and start again. It is important to seal an abandoned hole as it may contaminate your well once it is dug. As you dig deeper, the auger will eventually be too short to use. At this time, the first extension to the auger should be added. It should be a 2 foot length extension and this can be repeated as needed. By the time your well is done, you may have added 4 or 5 additional lengths and the auger may be more than 30 feet long. Emptying water out of the bucket at the end of a 30 foot auger can be difficult and you don’t want any overhead objects to get in your way. (Bob Mellin, 27) When you strike water you will notice that the material you withdraw has changed to more of a sand and gravel consistency. You should continue digging. After a while you may see the material you are removing change to a clay consistency. This means you have dug through the water bearing layer of sand and gravel and are now in a non-water bearing layer. At this time you may want to quit. If you keep digging you may or may not strike an even better water bearing layer. At this point it is very helpful to have some knowledge of other wells in the area. For example, how deep did they go and does the well go dry at times? Once you stop digging and install the casing, it is very difficult to make the well deeper if you change your mind. The water level in your area will go up and down with the seasons and the amount of rainfall you receive. The deeper the well, the more likely it will be to produce when the water table drops during dry periods. (Bob Mellin, 29) Whenever you are not working on the well, it is important to cover the well so no objects drop down it. A good cover includes a board and something heavy enough that a child cannot remove it. When you’ve reached the point that you cannot or do not want to go any deeper, it is time to install the casing. The casing is the lining of the well that prevents the well from collapsing. Schedule 40 PVC white plastic pipe that is 4 inches in diameter is ideal for a hand-augered well. The casing needs to be at least 12 inches above the anticipated flood level for your location. The depth of the well can be measured with a weighted string. The casing should stick out of the ground and it can be trimmed to the proper height after everything else is done. When you measure the depth of the well you must also determine the standing water level in the well. If the water fills the well up to a height of 10 feet below ground level then the high water mark is 10 feet. The slits in the casing should begin at this point and continue to the bottom of the well. (Bob Mellin, 36) Slits that are about 2 inches long and 1 inch apart cut across the pipe allow water to enter the casing. These cuts should be started 6 inches from the bottom end of the casing and should be staggered around and up the length of the casing to the high water level. Coarse sand paper should be used to remove any rough spots that are made when cutting slits in the casing.

After cleaning the casing inside and out, a cap should be glued to the bottom end of the casing. All PVC one inch in diameter or larger should receive a coat of primer before gluing. The primer should be allowed to dry before applying the glue. The bottom cap seals the end of the casing and keeps silt from entering the casing. It also helps with retrieval of any foreign objects that are dropped in the well as the end cap keeps them from falling any further. A cap should be placed, not glued, on the top end of the casing to keep materials from getting into the well as the work continues. At this point the casing can be dropped into the well. First a couple of shovels full of pea gravel should be dropped into the well to form a base for the casing. The casing can now be lowered into the well, being careful not to rub the sides of the well. Use a level to make sure the casing is plumb while centering it in the hole. Pour pea gravel around the casing and stop back-filling when the pea gravel gets to a level 10 feet below the surface. Now it is time to pour the concrete seal in the top 10 feet of the space. (Bob Mellin, 37) To seal the well against contamination, the top 10 feet of the space around the outside of the casing is filled with concrete. The concrete can also form a base for mounting the well cap and pump. The space should be at least 2 inches on all sides of the casing and the surface pad should slope away from the casing in all directions. It should rise at least 4 inches above the ground and extend at least 10 feet below the surface. The concrete should be mixed and poured at the same time to eliminate any potential voids. A mixture of Portland Cement (ASTM C150), sand, coarse aggregate and water in the proportion of at least 5 bags of cement per cubic yard of concrete to not more than 7 gallons of clean water per bag of cement (one cubic foot or 94 pounds) should be used. When pouring the seal the concrete should be poured around the casing evenly and the casing should be kept centered in the well. Avoid disturbing the sides of the well and refrain from mixing any soil into the concrete during the pour. A form for the pad can be made out of scrap lumber and the top of the pad should slope away from the center of the well. Remember that the casing should extend at least 12 inches above the surface. Once the concrete has set, the casing can be cut with a hand saw at least 1.5” above the concrete seal. (Bob Mellin, 39-41) The top of the well must be capped to prevent foreign material from entering. A suitable cap can be made from a rubber gasket between two plates that are metal or PVC and cut to fit the casing. Where the pump is installed directly over the casing, a watertight seal should be placed between the pump head and the pump base. If a concrete slab is constructed around the casing, it should be free from cracks or other defects that will detract from water tightness. The minimum thickness of the concrete slab should be 4 inches. (Bob Mellin, 43-45) The pump selected should match the well and the use. One option is a piston pump, which is a suction device where water is drawn into a cylinder by means of a moving piston, which is fitted with leather seals. This type of pump can be hand powered. The pump has a rod, which is connected to the piston and extends up through the well casing to the power source. The water in the piston is held in place by a valve while another stroke pulls more water into the cylinder and up the riser pipe. With each stroke, more water is raised until it reaches the top of the pipe and spills out at the surface. Piston pumps are fairly inefficient and slow, but are simple and reliable.

Before the water from the well is used, the well must be disinfected. A well that has a depth of about 20 feet should have 10 ounces of regular chlorine bleach (5.25%) added to the well. After pouring this into the well, potable water should be added in an amount at least equal to the amount that had been standing in the well. This will force the disinfectant out through the casing into the annular space. The well should sit for at least 12 hours and at this time can be pumped to clear the well of the disinfectant. A hand pump should be used frequently to keep moving parts working and grease or oil should be added to moving parts where friction occurs. The well site should be kept clear of vegetation and debris for a radius of at least 4 feet. The concrete slab should be maintained by filling any cracks with concrete patching compound to keep it watertight. In the case that a well is abandoned because it is no longer producing, there are procedures that should be followed. The top 20 feet of the well should be filled with concrete and the space below may be filled with clay, silt, sand, gravel, crushed stone, or native soils. The basic concept behind this is to return the site to the conditions that existed prior to digging the well. An improperly abandoned well may allow contaminated water to pollute the aquifer. (Bob Mellin, 57, 62)

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