Installation of wells

(a) Locution of wells. The dewatering wells shall be installed at the designated locations. Soil conditions in the areas where the wells are to be installed are de­picted in a general way by the boring logs made around the excavation. Subsurface conditions and stratifica­tion are erratic at this site, and clay, silt, and sand strata (with or without gravel and cobbles) may be en­countered in drilling the holes for the specified wells. Logs and wood debris have also been encountered in making borings and drilling wells at some locations at the site,

(b) Depth of wells. The wells shall be installed to

a depth so that the top of the specified length of screen (100 feet) is set approximately 5 feet below the top of fine (or coarser) sand as determined at the time of drilling by the C. O.R. The elevation at which the top of the screen shall be set is shown on certain logs of bor­ings along the line of the dewatering wells. The top of fine sand (D10 ± 0.12 mm) below which the screen is

to be set will probably vary from what is shown on the boring logs. The hole drilled for the wells will be logged by the C. O.R. If buried logs or boulders are en­countered which, in the opinion of the C. O.R., render it impractical to advance the drill hole to the design depth, the C. O.R. may adjust the depth in order to uti­lize the well in the system at the depth actually ob­tained, or he or she may request the Contractor to abandon the well, plug the hole by backfilling, and construct another well at an adjacent location.

(c) Drilling. The dewatering wells shall be drilled by the reverse rotary method to a depth which will permit setting the top of the screen in fine (or coarser) sand as determined at the time of drilling by the C. O.R. The water level in the sump pit and the well shall be maintained a minimum of 8 feet above the groundwater table at all times until the well screen, riser pipe, filter, and backfill have been placed. Drill­ing of the well shall be carried out so as to prevent any appreciable displacement of materials adjacent to the hole or cause any reduction in the yield of the well. (A temporary surface casing with a minimum length of 20 feet shall be set prior to the start of drilling.) The diameter of the hole drilled for the well shall be 28 to 30 inches. The hole shall be advanced using a reverse­rotary drill rig with a (minimum) 5-inch I. D. drill stem. While drilling and installing the well, the drill hole shall be kept full of (natural) drilling fluid up to the ground surface with turbidity of about 3000 parts per million. No bentonite drilling mud shall be used while drilling or installing the well. Silt may be added to the drilling water to attain the desired degree of muddi­ness (approximately 3000 parts per million) if nec – esary. If natural turbid water, or with silt added, proves insufficient to keep the hole stable, an ap­proved organic drilling compound such as Johnson’s Revert, or equivalent, may be added to the drilling fluid. The Contractor shall dig a sump pit large enough to allow the sand to settle out but small enough so that some silt is kept in suspension. The drilled hole shall be 3 feet deeper than the well screen and riser to be in­stalled in the hole. All drilling fluid shall be removed

from the well filter and the natural pervious informa­tion after the well is installed.