A borehole is the common term for each narrow shaft drilled in the ground, either vertically or horizontally. A borehole may be made for many different purposes , containing the extraction of water or certain liquid or gases (such as natural gas), as part of a geotechnical investigation or environmental site assessment, for mineral exploration , or as a pilot hole for setting up piers or underground utilities. Boreholes used as water wells are described in more depth in that article. Boreholes may be drilled with the help of a drilling rig, or by a hand-operated rig. The machinery and technique to upgrade a borehole differs considerably depending upon the producer , geological conditions, and the intended purpose .

Water wells

A water borehole can be dug, driven or drilled. To dig a water well, the ground must be quite soft to dig and the water table shallow enough to avoid collapse of the soft walls. Soil can be hauled up by bucket supported with a tripod. Metals rings and braces are placed against the walls to stop collapse throughout excavation.

Driven wells are made by pounding the top of a pipe with a weight. The structure has three pieces . A rotary bit, or drive point, attached to the bottom of a short pipe gyrates as it is pounded from above. The short pipe may have a screen over perforations, so that water can flow in when the aquifer level is reached . The short pipe’s diameter at the top is tapered, therefore the well pipe can fit onto it. A rod of narrower diameter, named a guide rod, adjusts inside the two pipes. It is weighted from the top , and drops down onto the rotary bit.

Another way is to jet water through a narrow pipe into the bottom of the gap , where a wide drill bit has loosened soil to be transported back up through the wider channel on the outside of the pipe. This is called a jetted well.

An alternative of the jetted well is to use a percussion bit to loosen earth , while pouring in water. When the resulting paste is too thick to allow the bit to solidly hit soil , the percussion bit is put up and a bailer is lowered in, to move the liquid.

Drilling requires a drilling rig, which are usually mounted on trucks. Drill bits can be alternated between rotatory and percussion type . A rotatory auger bit may be needed close to the surface to lift out soft earth .

The price of a borehole can be significantly affected by the applied design as well as the difficulty to make a borehole in a specific geological formation. Many times people want to save on costs and therefore they will budget for the drilling of a borehole, but not for the casing of a complete hole .

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