a. Many complex seepage problems, including such categories as steady confined, steady unconfined, and transient unconfined can be solved using the finite element method. Various computer codes are available at the WES and the NAVFAC program library to handle a variety of two- and three-dimensional seepage problems. The codes can handle most cases of nonhomogeneous and anisotropic media.
b. A general computer code for analyzing partially penetrating random well arrays has been developed based on results of three-dimensional electrical analogy model tests at the WES. The computer code provides a means for rapidly analyzing trial well systems in which the number of wells and their geometric configuration can be varied to determine quantities of seepage and head distributions. Wells of different radii and penetrations can be considered in the analysis.
4-6. Wellpoints, wells, and filters. Wells and wellpoints should be of a type that will prevent infiltration of filter material or foundation sand, offer little resistance to the inflow of water, and resist corrosion by water and soil. Wellpoints must also have sufficient penetration of the principal water-carrying strata to intercept seepage without excessive residual head between the wells or within the dewatered area.
a. Wellpoints. Where large flows are anticipated, a high-capacity type of wellpoint should be selected. The inner suction pipe of self-jetting wellpoints should permit inflow of water with a minimum hydraulic head loss. Self-jetting wellpoints should be designed so that most of the jet water will go out the tip of the point, with some backflow to keep the screen flushed clean while jetting the wellpoint in place.
(1) Wellpoint screens. Generally, wellpoints are covered with 30- to 60-mesh screen or have an equivalent slot opening (0.010 to 0.025 inch). The mesh should meet filter criteria given in c below. Where the soil to be drained is silty or fine sand, the yield of the wellpoint and its efficiency can be greatly improved by placing a relatively uniform, medium sand filter around the wellpoint. The filter should be designed in accordance with criteria subsequently set forth in c below. A filter will permit the use of screens or slots with larger openings and provide a more pervious material around the wellpoint, thereby increasing its effective radius id below).
(2) Wellpoint hydraulics. The hydraulic head losses in a wellpoint system must be considered in designing a dewatering system. These losses can be estimated from figure 4-25.
b. Wells. Wells for temporary dewatering and permanent drainage systems may have diameters ranging
from 4 to 18 inches with a screen 20 to 75 feet long depending on the flow and pump size requirements.
(1) Well screens. Screens generally used for dewatering wells are slotted (or perforated) steel pipe, perforated steel pipe wrapped with galvanized wire, galvanized wire wrapped and welded to longitudinal rods, and slotted polyvinyl chloride (PVC) pipe, Riser pipes for most dewatering wells consist of steel or PVC pipe. Screens and riser for permanent wells are usually made of stainless steel or PVC. Good practice dictates the use of a filter around dewatering wells, which permits the use of fairly large slots or perforations, usually 0.025 to 0.100 inch in size. The slots in well screens should be as wide as possible but should meet criteria given in c below.
(2) Open screen area. The open area of a well screen should be sufficient to keep the entrance velocity for the design flow low to reduce head losses and to minimize incrustation of the well screen in certain types of water. For temporary dewatering wells installed in nonincrusting groundwater, the entrance velocity should not exceed about 0.15 to 0.20 foot per second; for incrusting groundwater, the entrance velocity should not exceed 0.10 to 0.20 foot per second. For permanent drainage wells, the entrance velocity should not exceed about 0.10 foot per second. As the flow to and length of a well screen is usually dictated by the characteristics of the aquifer and drawdown requirements, the required open screen area can be obtained by using a screen of appropriate diameter with a maximum amount of open screen area.
(3) Well hydraulics. Head losses within the well system discussed in paragraph 4-2a(5) can be estimated from figure 4-24.
c. Filters. Filters are usually 3 to 5 inches thick for wellpoints and 6 to 8 inches thick for large-diameter wells (fig. 4-30). To prevent infiltration of the aquifer materials into the filter and of filter materials into the well or wellpoint, without excessive head losses, filters should meet the following criteria:
Slot or screen openings minimum filter D50
Max filter D50 Min aquifer D50
Min filter D15 ^ 0 , _
——————— 2 to 5
Max aquifer D15
If the filter is to be tremied in around the screen for a well or wellpoint, it may be either uniformly or rather widely graded; however, if the filter is not tremied into place, it should be quite uniformly graded (D90/D10 ^ 3 to 4) and poured in around the well in a heavy, continuous stream to minimize segregation.
d. Effective well radius. The “effective” radius rw of a well is that well radius which would have no hydraulic entrance loss H*. If well entrance losses are considered separately in the design of a well or system of wells, rw for a well or wellpoint without a filter may be considered to be one-half the outside diameter of the well screens; where a filter has been placed around a wellpoint or well screen, rw may generally be considered to be one-half the outside diameter or the radius of the filter.
e. Well penetration. In a stratified aquifer, the effective well penetration usually differs from that computed from the ratio of the length of well screen to total thickness of the aquifer. A method for determining the required length_of well screen W to achieve an effective penetration W in a stratified aquifer is given in appendix E.
f Screen length, penetmtion, and diameter. The length and penetration of the screen depends on the thickness and stratification of the strata to be dewatered (para 4-2a(6)). The length and diameter of the screen and the area of perforations should be sufficient to permit the inflow of water without exceeding the entrance velocity given in 6(2) above. The “wetted screen length hvg” (or h* for each stratum to be dewatered) is equal to or greater than Qw/q,, (para 4-2a(4) and (6)). The diameter of the well screen should be at least 3 to 4 inches larger than the pump bowl or motor.
4-7. Pumps, headers, and discharge pipes. The capacity of pumps and piping should allow for possible reduction in efficiency because of in
crustation or mechanical wear caused by prolonged operation. This equipment should also be designed with appropriate valves, crossovers, and standby units so that the system can operate continuously, regardless of interruption for routine maintenance or breakdown.
a. Centrifugal and wellpoin tpumps.
(1) Centrifugal pumps can be used as sump pumps, jet pumps, or in combination with an auxiliary vacuum pump as a wellpoint pump. The selection of a pump and power unit depends on the discharge, suction lift, hydraulic head losses, including velocity head and discharge head, air-handling requirement, power available, fuel economy, and durability of unit. A well – point pump, consisting of a self-priming centrifugal pump with an attached auxiliary vacuum pump, should have adequate air-handling capacity and be capable of producing a vacuum of at least 22 to 25 feet of water in the headers. The suction lift of a wellpoint pump is dependent on the vacuum available at the pump bowl, and the required vacuum must be considered in determining the pumping capacity of the pump. Characteristics of a typical 8-inch wellpoint pump are shown in figure 4-31. Characteristics of a typical wellpoint pump vacuum unit are shown in figure 4-32. Sump pumps of the centrifugal type should be self-priming and capable of developing at least 20 feet of vacuum. Jet pumps are high head pumps; typical characteristics of a typical 6-inch jet pump are shown in figure 4-33.
(2) Each wellpoint pump should be provided with one connected standby pump so as to ensure continuity
U. S. Army Corps of Engineers
U. S. Army Corps of Engineers
of operation in event of pump or engine failure, or for repair or maintenance. By overdesigning the header pipe system and proper placement of valves, it may be possible to install only one standby pump for every two operational pumps. If electric motors are used for powering the normally operating pumps, the standby pumps should be powered with diesel, natural or LP gas, or gasoline engines. The type of power selected will depend on the power facilities at the site and the economics of installation, operation, and maintenance, It is also advisable to have spare power units on site in addition to the standby pumping units. Automatic switches, starters, and valves may be required if failure of the system is critical.