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be used immed r dy above and below the well screen and approximately every 30 to 50 feet along the lengti L the casing. Centralizers need not be placed on well assemblies installed within augers c trill casings because the auger or drill casing will adequately center the well casing and scre v in the borehole. For borings dr- I :d by the mud rotary method, potable water may be added to the drill mud and circulated -- the borehole after completion of the boring. Circulation will continue until the suspended a iiment in the return fluid has been decreased. If borehole conditions are relatively stab- ; the mud will be thinned before the casing assembly is lowered to the specified dept . This is preferred because it reduces the potential for clogging the well screen with thick mu , Conversely, if borehole conditions are relatively unstable, the mud will be thinned after _ : casing is placed at the specified depth but prior to installation of annular fill materials. A : r installation of the well assembly, a slurry of filter sand and potable water will then be trem_ '. into the annular space. For borings l illed using the hollow stem auger method, the filter sand will be placed after the well assemb has been lowered to the specific depth through the augers. The augers will be increments' raised as the filter sand is placed by free fall through the augers. The depth to the top of t t filter pack will be measured after each increment to detect possible bridging. If bridging or ;irs, it will be broken by washing the filter materials into proper place with potable wa or by repeatedly raising and lowering the augers slightly. The amount of water, if any, adc i to the borehole should be noted on the BORING LOG or DAILY FIELD RECORD. For monit. rng wells, the filter sand will be placed in a calculated quantity sufficient to fill the annular sF c -- to a level of about 1 to 2 feet above the top of the well screen. For extraction wells, the iel of filter sand above the well screen will be based on site conditions. The depth to the top _ the filter pack will be verified by measuring, using a tremie pipe or a weighted tape. Gr( dwater extraction wells or monitoring wells may be surged before placement of the transi _ seal to promote filter material settlement, as specified by the project manager. INSTALLATION AND DESTRUCTION OF WELLS Page 3 of 3 MSC\PROTO- :T REVISION DATE:MARCH 1996 IW-3