resource of a particular area. Theinformation available normally will giveyou an idea as to the geologicalconditions, such as terrain and typematerial, approximate depth of anaquiferor series ofaquifers, quantities of waterthat may be expected during differentseasons of the year, average depth of thewater table, and drilling procedures andproblems that may be encountered.Access to maps and publications isusually through hydrographic officeseither on a state or national level,offices of the United States Coast andGeodetic Survey, or its equivalent inother countries, geological or universityarchives, battalion engineering offices,or native drillers.When very little is known about thewater resources in a particular area, themost valuable clues may come from aninspection of the outcropping of rockformat ions. You may need to verify yourconclusions byexploratory drilling, which isthe surest way to establish the existenceof water-bearing formations.Exploratory drilling is usuallyinitiated as part of the groundwater studyof an area before construction of waterwells at a particular site.WELLSWells are classified into five methodsof construction. The methods are as follows:dug, bored, jetted, driven, and drilled wells, Each type ofwell has certain advantages and limitations;and the type of well to be developed dependsupon the ease of construction, storage,capacity, limitations as to formations it canpenetrate, and ease of safeguarding itagainst pollution.Adug wellis one in which theexcavation is Made by the use of picks,shovels, spades, or digging equipment,such as sand buckets or clamshell buckets.Abored well is one in which the excavationis made by the use of hand or power augers.A jetted well is one in which theexcavation is made by the use of a high-velocity jet of water.A driven well is one which isconstructed by driving a pointed screen,referred to as a drive point, into theground. Casings or lengths of pipes areattached to the drive point as it is beingdriven into the ground.Dug, bored, jetted, and driven wells arerelatively shallow, Generally, they areless than 100 feet deep and may beconstructed with hand tools. Drilled wellsin the NCF are normally drilled to thedepth of 1,500 feet and are constructedwith portable well drilling machines.ROTARY DRILLINGWhen using a hand drill to bore ahole, you press the cutting tool or bitinto the material to be bored. Thematerial is cut as you turn the bit bymeans of the drill handle. During thedrilling process, chips of the cutmaterial are carried to the top of thehole by the flutes of the bit. A rotarydrilling rig operates on the same principle,except for the method of raising the cutmaterial, This material is washed to thesurface by a fluid substance instead ofbeing carried by the bit itself. The bitof arotary drilling rig is attached to thelower end of the drill pipe.The methods of drilling for waterare referred to asrotary mud or rotary airdrilling. Rotary mud drilling is currently themost common method used to drill wells andis used where ground formations are looseand unconsolidated. Mud drilling is alsoused in consolidated formations but is notvery efficient. Therotary air drilling methodis preferred when ground formations areconsolidated. A limitation of theairdrillingmethod is the cfm output of the aircompressor.When rotary mud or rotary air drillingoperations are being conducted and aformation that is hard to penetrate isencountered, thedown-hole-drilling hammerattachment can be used. Thedown-hole-drilling hammer attaches to the lower endof the drill pipe and rotates as well ashammers (short rapid blows) against thehard formation (fig. 9-4). To use theFigure 9-4.-Down-hole-drilling (DHD).9-4
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