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r��r��ax be used immediately above and below the well screen and approximately every 30 to 50 feet along the length of the casing. Centralizers need not be placed on well assemblies installed within augers or drill casings because the auger or drill casing will adequately center the well casing and screen in the borehole. For borings drilled by the mud rotary method, potable water may be added to the drill mud and circulated in the borehole after completion of the boring. Circulation will continue until the suspended sediment in the return fluid has been decreased. If borehole conditions are relatively stable, the mud will be thinned before the casing assembly is lowered to the specified depth. This is preferred because it reduces the potential for clogging the well screen with thick mud. Conversely, if borehole conditions are relatively unstable, the mud will be thinned after the casing is placed at the specified depth but prior to installation. of annular fill materials. After installation of the well assembly, a slurry of filter sand and potable water will then be tremied into the annular space. For borings drilled using the hollow stem auger method, the filter sand will be placed after the well assembly has been lowered to the specific depth through the augers. The augers will be incrementally raised as the filter sand is placed by free fall through the augers. The depth to the top of the filter pack will be measured after each increment to detect possible bridging. If bridging occurs, it will be broken by washing the filter materials into proper place with potable water or by repeatedly raising and lowering the augers slightly. The amount of water, if any, added to the borehole should be noted on the BORING LOG or DAILY FIELD RECORD. For monitoring wells, the filter sand will be placed in a calculated quantity sufficient to fill the annular space to a level of about 1 to 2 feet above the top of the well screen. For extraction wells, the level of filter sand above the well screen will be based on site conditions. The depth to the top of the filter pack will be verified by measuring, using a tremie pipe or a weighted tape. Groundwater extraction wells or monitoring wells may be surged before placement of the transition seal to promote filter material settlement, as specified by the project manager. INSTALLATION AND DESTRUCTION OF VvMELLS Page 3 of 8 MS0PR0T0-[W.TYT REVISION DATE:MARCH 1996 -3