Water & Wastewater Treatment

In the know Technically speaking: boreholes Drilling a borehole. The most common used method today is rotary drilling. 36 | AUGUST 2015 | WWT | www.wwtonline.co.uk to be encountered. Drilling techniques Drilling methods are generally categorised as either 'percussion' or 'rotary', depending on the principal drilling technique employed. Some methods use a combination of both techniques. The percussion technique is regarded as the oldest form of drilling but nowadays is best suited to the construction of shallow boreholes in unconsolidated sediments. 'Down- the-hole hammer' drilling combines percussion drilling with a rotary action and is suited to hard rock formations. Today, the most common method of constructing water wells is rotary drilling. The technique was developed in the late 19th and 20th century and uses a circulating fluid (usually a drilling 'mud') to support the borehole wall during construction and carry rock fragments back to the surface. Rotary drilling allows boreholes to be advanced to much greater depths than can be otherwise achieved with percussive methods. Borehole construction Drilling is generally carried out in two stages, the first to install the permanent casing and then the final drilling to completion depth. In cases where there is some uncertainty about the geology, a field geologist should be on hand to determine where casings should be set and the final completion depth. Once drilling is completed, techniques such as acidising or targeted air li…ing (surging) may be used to enhance the yield and efficiency of the borehole. This part of the construction phase is referred to as borehole 'development'. Where required, a well screen and casing will be installed and the annular space between the borehole wall filled with a sand or gravel pack. Specialist, artificial or engineered media are also available for use. The gravel pack is commonly referred to as a 'formation stabiliser' when the primary purpose is to provide stability when the target formation is unstable. If the primary purpose is to prevent sand and other fine material entering the borehole and damaging the pump, it is referred to as a 'filter pack'. There are a number of options for screen type and materials range from plastic to coated, or stainless steel. The choice of screen will have a significant impact on project budget Installing a plastic well screen.

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