A water well is defined as a hole or shaft that excavated in the earth to bring the groundwater on the surface. Occasionally wells serve other purposes, such as for subsurface exploration and observation, artificial recharge, and disposal of wastewaters.
Test holes and Well Logs
Before drilling a well in a new area, it is common practice to drill a test hole. The purpose of a test hole is to determine depths to groundwater, quality of water, and physical character and thickness of aquifers without the expense of a regular well, which might prove to be unsuccessful.
Test holes may be put down by any method for well construction; however, cable tool, rotary, and jetting methods are commonly used. If the test hole appears suitable as a site for a finished well, it can be reamed with hydraulic rotary equipment to convert it to a larger permanent well.
Dating from Biblical times, dug wells have furnished countless water supplies throughout the world. Depths range up to 20m or more, depending on the position of the water table, while diameters are usually 1 to 10m.
Dug wells can yield relatively large quantities of water from shallow sources and are most extensively employed for individual water supplies in areas containing unconsolidated glacial and alluvial deposits. Their large diameters permit storage of considerable quantities of water if the wells extend some distance below the water table.
In the past, all dug wells were excavated by hand, and even today the same method is widely employed. A typical dug well in under-developed portions of the world is often no more than an irregular hole in the ground that intersects the water table. A pick and shovel are the basic implements.
Loose material is hauled to the surface in a container by means of suitable pulleys and lines. Large dug wells can be constructed rapidly with portable excavating equipment such as clamshell and orange-peel buckets. For safety and to prevent caving, lining of wood or sheet piling should be placed in the hole to brace the walls.
A modern dug well is permanently lined with a casing of wood staves, brick, rock, concrete, or metal. Curbs should be perforated or contain openings for entry of water and must be firmly seated at the bottom. Dug wells should be deep enough to extend a few meters below the water table. Gravel should be backfilled around the curb and at the bottom of the well to control sand entry and possible caving.
A properly Constructed dug well penetrating a permeable aquifer can yield 2500 to 7500 m3/day although most domestic dug wells yield less than 500m3/day.
A serious limitation of large open dug wells involves the ease of their pollution by surface water, airborne material, and objects falling or finding entrance into the wells.
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