Where a water table exists at a shallow depth in an unconsolidated aquifer, bored wells can furnish small quantities of water at minimum cost. Bored wells are constructed with band-operated or power-driven earth augers. Hand augers are available in several shapes and sizes, all operating with cutting blades at the bottom that bore into the ground with a rotary motion.
When the blades are full of loose earth, the auger is removed from the hole and emptied; the operation is repeated until the desired hole depth is reached. Hand-bored wells seldom exceed 20cm in diameter and 15m in depth. Power-driven augers will boreholes up to 1m in diameter and, under favorable conditions, to depths exceeding 30m. The auger consists of a cylindrical steel bucket with a cutting-edge projecting from a slot in the bottom.
The bucket is filled by rotating it in the hole by a drive shaft of adjustable length. When full, the auger is hoisted to the surface and the excavated material is removed through hinged openings on the side or bottom of the bucket. Reamers, attached to the top of the bucket, can enlarge holes to diameters exceeding the auger size.
A continuous-flight power auger has a spiral extending from the bottom of the hole to the surface. Cuttings are carried to the surface as on a screw conveyor, while sections of the auger may be added as depth increases. The simple equipment, usually truck-mounted, can be operated rapid by one person and functions to depth exceeding 50m in unconsolidated formations that do contain large boulders.
Augers work best in formations that do not cave. Where loose sand and gravel are encountered in a large-diameter hole, or the boring reaches the water table, it may be necessary to lower a concrete or metal casing to the bottom of the hole and continue boring inside. After the desired depth is reached, the permanent well casing and screen are entered in the hole, the outer casing is removed, and the peripherical space is backfilled with gravel.
Augers sometimes supplement other well- drilling methods where sticky clay formations as encountered; here augers are most effective than any other penetrating device.
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