Wells and Boreholes
Wells and Boreholes in History
Water wells have been dug by hand for thousands of years to supply drinking water to communities across the world, the oldest known well dating from 8,000BC. The deepest hand dug well anywhere in the world is in Woodingdean in Sussex; it was completed in 1862 and measures a staggering 392m.
Hand dug water wells were built with diameters large enough to accommodate one or more men with shovels digging down to below the water table. Today, water wells with much smaller diameters, typically 150mm to 400mm, are drilled using diesel driven drilling rigs. Drilled water wells are more commonly known as boreholes and usually measure between 25m and 100m in depth but can go as deep as 1000m. There are two principal drilling methods available, percussion and rotary drilling.
Percussion drilling involves repeatedly lifting and dropping heavy drilling tools into the borehole which break up consolidated rock into small fragments which can then be lifted from the borehole by a bailer which forms an integral part of the drilling tool. Percussion drilling is more economical than rotary drilling in many situations, but less effective at penetrating hard rock.
Rotary drilling uses a sharp rotating drill bit to dig down into the prevailing geology, the spinning drill bit enabling penetration into the hardest rock formations. As drilling progresses fluid will be circulated through the well hole to cool and lubricate the rotating drill bit, to bring cuttings up to the surface, and when drilling in unconsolidated formations such as sand to provide hydraulic pressure and prevent the walls of the borehole from caving in during the drilling process.
Protecting Water Quality
Modern boreholes are designed and built to protect water quality – plastic or steel casing, combined with bentonite grouting and a sealed well cap ensure that source water remains protected and free from contamination.