OPTIMAL HOLE DIAMETER FOR WATER WELL
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OPTIMAL HOLE DIAMETER FOR WATER WELL

S.S. Chauhan

Member ( ED & MM )

C.G.W.B., Faridabad

Various conventional methods of water well drilling quite often dictates the criteria for well design i.e.  selection of proper hole size and casing diameter. The assumption that an increase in hole diameter will produce a greater well yield is ambiguous and vague. The subject of optimum  hole  diameter is difficult one and loaded with variable  line, cost of drilling/ reaming, cost of  casing and screens, cost of gravel pack etc. These variables shorts up rapidly with an increase in diameter of  a hole.

Cost is usually and certainly should be a factor while designing the well assembly in any  formation.

In determining a hole size a decision must reach regarding diameter of casing which will accommodate airlines, water level measuring device, bowl assembly of pump etc. If a gravel pack is desirable the thickness of gravel envelope may increase the requirement of hole size.

As the hole size increase the development process of well becomes vary difficult. Without proper cleaning or development a huge loss in yield can be expected, the thickness of gravel pack becomes more difficult to develop the well to its  natural formation. During the field visit of author in USA with California State water department a well pump test was attended for 3 No of 50 mts depth wells drilled in identical situation in a formation consists of clay , sand and gravel. The three wells drilled were lowered with a casing diameter of  12, 14 and 18 tapping the same aquifer thickness.

 The typical testing of these wells at constant draw down reveals a relative increase of % in the yield of these wells, as the diameter of casing increases  along with radius of influences as indicated in table below :-

It is evident from table above that when the radius of influence is 200 ft. a well lowered with 24 dia. Casing yields only 9% more water than 12 diameter well.

Experience of the author in water well construction in various hydro geological formation and conditions in India recommends strongly that keeping in view cost benefit ratio that well casing size for water wells no larger diameter than 12 is justified. An exception to the 12  maximum diameter may be encounter while dealing with a high permeability aquifer located near the source of recharge in Yamuna flood plain   or around Delhi region , gengatic plain of U.P., Bihar and West Bengal. Thus due to relatively minor significance of large diameter wells which affects well yields under favorable conditions. But at the same time relatively major significance of large diameter wells is noted which affects the cost and development of well in particular under normal conditions.

Therefore an approach and practices followed towards holes as small as practical rather than as large as possible should govern the system design of water well construction.