INSTALLATION OF DEWATERING AND GROUNDWATER CONTROL SYSTEMS
5-1. General. The successful performance of any dewatering system requires that it be properly installed. Principal installation features of various types of dewatering or groundwater control systems are presented in the following paragraphs.
5- 2. Deep-well systems.
a. Deep wells may be installed by the reverse-rotary drilling method, by driving and jetting a casing into the ground and cleaning it with a bailer or jet, or with a bucket auger.
b. In the reverse-rotary method, the hole for the well is made by rotary drilling, using a bit of a size required by the screen diameter and thickness of filter. Soil from the drilling is removed from the hole by the flow of water circulating from the ground surface down the hole and back up the (hollow) drill stem from the bit. The drill water is circulated by a centrifugal or jet-eductor pump that pumps the flow from the drill stem into a sump pit. As the hole is advanced, the soil particles settle out in the sump pit, and the muddy water flows back into the drill hole through a ditch cut from the sump to the hole. The sides of the drill hole are stabilized by seepage forces acting against a thin film of finegrained soil that forms on the wall of the hole. A sufficient seepage force to stabilize the hole is produced by maintaining the water level in the hole at least 7 feet above the natural water table. No bentonite drilling mud should be used because of gelling in the filter and aquifer adjacent to the well. If the hole is drilled in clean sands, some silt soil may need to be added to the drilling water to attain the desired degree of muddiness (approximately 3000 parts per million). (Organic drilling material, e. g., Johnson’s Revert or equivalent, may also be added to the drilling water to reduce water loss.) The sump pit should be large enough to allow the sand to settle out but small enough so that the silt is kept in suspension.
c. Holes for deep wells should be vertical so that the screen and riser may be installed straight and plumb; appropriate guides should be used to center and keep the screen plumb and straight in the hole. The hole should be some deeper than the well screen and riser. (The additional depth of the hole is to provide space for wasting fdter material first put in the tremie pipe if used.) After the screen is in place, the fdter is tre-
mied in. The tremie pipe should be 4 to 5 inches in diameter, be perforated with slots У1в to %2 inch wide and about 6 inches long, and have flush screw joints. The slots will allow the filter material to become saturated, thereby breaking the surface tension and “bulking” of the filter in the tremie. One or two slots per linear foot of tremie is generally sufficient. After the tremie pipe has been lowered to the bottom of the hole, it should be filled with filter material, and then slowly raised, keeping it full of filter material at all times, until the filter material is 5 to 10 feet above the top of the screen. The fdter material initially poured in the tremie should be wasted in the bottom of the hole. The level of drilling fluid or water in a reverse-rotary drilled hole must be maintained at least 7 feet above the natural groundwater level until all the filter material is placed. If a casing is used, it should be pulled as the filter material is placed, keeping the bottom of the casing 2 to 10 feet below the top of the filter material as the fdter is placed. A properly designed, uniform (D90/D10 ^ 3 to 4) fdter sand may be placed without tremieing if it is poured in around the screen in a heavy continuous stream to minimize segregation.
d. After the filter is placed, the well should be developed to obtain the maximum yield and efficiency of the well. The purpose of the development is to remove any film of silt from the walls of the drilled hole and to develop the filter immediately adjacent to the screen to permit an easy flow of water into the well. Development of a well should be accomplished as soon after the hole has been drilled as practicable. Delay in doing this may prevent a well being developed to the efficiency assumed in design. A well may be developed by surge pumping or surging it with a loosely fitting surge block that is raised and lowered through the well screen at a speed of about 2 feet per second. The surge block should be slightly flexible and have a diameter 1 to 2 inches smaller than the inside diameter of the well screen. The amount of material deposited in the bottom of the well should be determined after each cycle (about 15 trips per cycle). Surging should continue until the accumulation of material pulled through the well screen in any one cycle becomes less than about 0.2 foot deep. The well screen should be bailed clean if the accumulation of material in the bottom of the screen becomes more than 1 to 2 feet at any time dur-
ing surging, and recleaned after surging is completed. Material bailed from a well should be inspected to see if any foundation sand is being removed. It is possible to oversurge a well, which may breach the filter with resulting infiltration of foundation sand when the well is pumped.
e. After a well has been developed, it should be pumped to clear it of muddy water and sand and to check it for yield and infiltration. The well should be pumped at approximately the design discharge from 30 minutes to several hours, with periodic measurement of the well flow, drawdown in the well, depth of sand in the bottom of the well, and amount of sand in the discharge. Measurements of well discharge and drawdown may be used to determine the efficiency and degree of development of the well. The performance of the well filter may be evaluated by measuring the accumulation of sand in the bottom of the well and in the discharge. A well should be developed and pumped until the amount of sand infiltration is less than 5 to 10 parts per million.
f. Deep wells, in which a vacuum is to be maintained, require an airtight seal around the well riser pipe from the ground surface down for a distance oflO to 50 feet. The seal may be made with compacted clay, nonshrinking grout or concrete, bentonitic mud, or a short length of surface casing capped at the top. Improper or careless placement of this seal will make it impossible to attain a sufficient vacuum in the system to cause the dewatering system to operate as designed. The top of the well must also be sealed airtight.
g. After the wells are developed and satisfactorily tested by pumping, the pumps, power units, and discharge piping may be installed.
h. Where drawdown or vacuum requirements in deep wells demand that the water level be lowered and maintained near the bottom of the wells the pumps will have to handle a mixture of water and air. If such a requirement exists, the pump bowls should be designed to allow increasing amounts of air to enter the bowl, which will reduce the efficiency of the pump until the pump capacity just equals the inflow of water, without cavitation of the impellers. The impellers of
deep-well turbine pumps should be set according to the
manufacturer’s recommendations. Improper impeller settings can significantly reduce the performance of a deep-well pump.