Selection of dewatering system

a. General. The method most suitable for dewater­ing an excavation depends upon the location, type, size, and depth of the excavation; thickness, stratifica­tion, and permeability of the foundation soils below the water table into which the excavation extends or is

GROUNDWATER TABLE DUE TO SAND DRAINS

Подпись:Подпись:Selection of dewatering systemAVERAGE HEAD AT LINE OF SAND DRAINS DUE TO PUMP WELLS OR WELLPOINTS

underlain; potential damage resulting from failure of the dewatering system; and the cost of installation and operation of the system. The cost of a dewatering method or system will depend upon:

(1) Type, size, and pumping requirements of proj­ect.

(2) Type and availability of power.

(3) Labor requirements.

(4) Duration of required pumping.

The rapid development of slurry cutoff walls has made this method of groundwater control, combined with a certain amount of pumping, a practical and econom­ical alternative for some projects, especially those where pumping costs would otherwise be great.

WELLPOI NT PUMP

 

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PLAN

 

WELLPOINT HEADER

 

SECTION B-B

 

Selection of dewatering systemSelection of dewatering systemSelection of dewatering system

U. S. Army Corps of Engineers

Selection of dewatering system

U. S. Army Corps of Engineers

Figure 2-11, Grout curtain or cutoff trench around an excavation.

 

b. Factors controlling selection. Where foundations must be constructed on soils below the groundwater level, it will generally be necessary to dewater the ex­cavation by means of a deep-well or wellpoint system rather than trenching and sump pumping, Dewatering is usually essential to prevent damage to foundation soils caused by equipment operations and sloughing or sliding in of the side slopes. Conventional deep-well and wellpoint systems designed and installed by com­panies specializing in this work are generally satisfac­tory, and detailed designs need not be prepared by the engineer. However, where unusual pressure relief or dewatering requirements must be achieved, the engi­neer should make detailed analyses and specify the de­watering system or detailed results to be achieved in the contract documents. Where unusual equipment and procedures are required to achieve desired results, they should be described in detail in the contract docu­ments. The user of this manual is referred to paragraphs 6b, 14b, and 2f of Appendix III, TM 5-818-4/AFM 88-5, Chapter 5, for additional discus­sions of dewatering requirements and contract speci­fications. Major factors affecting selection of dewater­ing and groundwater control systems are discussed in the following paragraphs.

(1) Type of exca va tion. Small open excavations, or excavations where the depth of water table lowering is small, can generally be dewatering most economically and safely by means of a conventional wellpoint sys­tem. If the excavation requires that the water table or artesian pressure be lowered more than 20 or 30 feet, a system of jet-eductor type wellpoints or deep wells may be more suitable. Either wellpoints, deep wells, or a combination thereof can be used to dewater an exca­
vation surrounded by a cofferdam. Excavations for deep shafts, caissons, or tunnels that penetrate strati­fied pervious soil or rock can generally best be dewa­tered with either a deep-well system (with or without an auxiliary vacuum) or a jet-eductor wellpoint system depending on the soil formation and required rate of pumping, but slurry cutoff walls and freezing should be evaluated as alternative procedures. Other factors relating to selection of a dewatering system are inter­ference of the system with construction operations, space available for the system, sequence of construc­tion operations, durations of dewatering, and cost of the installation and its operation. Where groundwater lowering is expensive and where cofferdams are re­quired, caisson construction may be more economical. Caissons are being used more frequently, even for small structures.

(2) Geologic and soil conditions. The geologic and soil formations at a site may dictate the type of dewa­tering or drainage system. If the soil below the water table is a deep, more or less homogeneous, free-drain­ing sand, it can be effectively dewatered with either a conventional well or wellpoint system. If, on the other hand, the formation is highly stratified, or the saturat­ed soil to be dewatered is underlain by an impervious stratum of clay, shale, or rock, wellpoints or wells on relatively close centers may be required. Where soil and groundwater conditions require only the relief of artesian pressure beneath an excavation, this pressure relief can be accomplished by means of relatively few deep wells or jet-eductor wellpoints installed around and at the top of the excavation.

(a) If an aquifer is thick so that the penetration of a system of wellpoints is small, the small ratio of

Method

Applicability

Remarks

Sumps and ditches

Collect water entering an excava­tion or structure. ’

Generally water level can be lowered only a few feet. Used to collect water within cofferdams and excavations. Sumping is usually only successful in relatively stable gravel or well-graded sandy gravel, partially cemented materials, or porous rock formations.

Conventionalwellpoint

system

Dewater soils that can be drained by gravity flow.

Most commonly used dewatering method. Drawdown limited to about 15 ft per stage; however, several stages may be used. Can be installed quickly.

Vacuum wellpoint system

Dewater or stabilize soils with low permeability. (Some silts,

sandy silts).

Vacuum increases the hydraulic gradient causing flow. Little vacuum effect can be obtained if lift is more than 15 ft.

Jet-eductor wellpoint

Dewater soils that can be drained by gravity flow. Usually for deep excavations where small flows are required.

Can lower water table as much as 100 ft from top of excavation. Jet-eductors are particularly suitable for dewatering shafts and tunnels. Two header pipes and two riser pipes, or a pipe with­in a pipe, are required.

Deep-well systems

Dewater soils that can be drained by gravity flow. Usually for large, deep excavations where large flows are required.

Can be installed around periphery of excavation, thus removing dewatering equipment from within the excavation. Deep wells are particularly suitable for dewatering shafts and tunnels.

Vertical sand drains

Usually used to conduct water from an upper stratum to a lower more pervious stratum.

Not effective in highly pervious soils.

Electroosmosis

Dewater soils that cannot be drained by gravity. (Some silts, clayey silts, clayey silty sands),

Direct electrical current increases hydraulic gradient causing flow.

Cutoffs

Stop or minimize seepage into an excavation when installed down to an impervious stratum.

See paragraph 2-8 for materials used.

 

Подпись: TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418

U. S. Army Corps of Engineers

 

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screen length to aquifer thickness may result in rela­tively little drawdown within the excavation, even though the water table is lowered 15 to 20 feet at the line of wellpoints. For deep aquifers, a deep-well sys­tem will generally be more applicable, or the length of the wellpoints should be increased and the wellpoints set deep and surrounded with a high-capacity filter. On the other hand, if the aquifer is relatively thin or stratified wellpoints may be best suited to the situa­tion.

(ib) The perviousness and drainability of a soil or rock may dictate the general type of a dewatering sys­tem to be used for a project. A guide for the selection of a dewatering system related to the grain size of soils is presented in figure 2-12. Some gravels and rock for­mations may be so permeable that a barrier to flow, such as a slurry trench, grout curtain, sheet pile cutoff, or freezing, may be necessary to reduce the quantity of flow to the dewatering system to reasonable propor­tions. Clean, free-draining sands can be effectively de­watered by wells or wellpoints. Drainage of sandy silts and silts will usually require the application of addi­tional vacuum to well or wellpoint dewatering sys­tems, or possibly the use of the electroosmotic method of dewatering where soils are silty or clayey. However, where thin sand layers are present, special require­ments may be unnecessary. Electroosmosis should nev­er be used until a test of a conventional system of well – points, wells with vacuum, or jet-eductor wellpoints has been attempted.

(3) Depth of groundwater lowering. The magni­tude of the drawdown required is an important con­sideration in selecting a dewatering system. If the drawdown required is large, deep wells or jet-eductor wellpoints may be the best because of their ability to achieve large drawdowns from the top of an excava­tion, whereas many stages of wellpoints would be re­quired to accomplish the same drawdown. Deep wells can be used for a wide range of flows by selecting pumps of appropriate size, but jet-eductor wellpoints are not as flexible. Since jet-eductor pumps are rela­tively inefficient, they are most applicable where well flows are small as in silty to fine sand formations.

(4) Reliability requirements. The reliability of groundwater control required for a project will have a significant bearing on the design of the dewatering pumps, power supply, and standby power and equip­ment. If the dewatering problem is one involving the relief of artesian pressure to prevent a “blowup” of the bottom of an excavation, the rate of water table re­bound, in event of failure of the system, may be ex­tremely rapid. Such a situation may influence the type of pressure relief system selected and require inclusion of standby equipment with automatic power transfer and starting equipment.

(5) Required rate of pumping. The rate of pump­ing required to dewater an excavation may vary from 5 to 50,000 gallons per minute or more. Thus, flow to a drainage system will have an important effect on the design and selection of the wells, pumps, and piping system. Turbine or submersible pumps for pumping deep wells are available in sizes from 3 to 14 inches with capacities ranging from 5 to 5000 gallons per minute at heads up to 500 feet. Wellpoint pumps are available in sizes from 6 to 12 inches with capacities ranging from 500 to 5000 gallons per minute depend­ing upon vacuum and discharge heads. Jet-eductor pumps are available that will pump from 3 to 20 gal­lons per minute for lifts up to 100 feet. Where soil con­ditions dictate the use of vacuum or electroosmotic wellpoint systems, the rate of pumpage will be very small. The rate of pumpage will depend largely on the distance to the effective source of seepage, amount of drawdown or pressure relief required, and thickness and perviousness of the aquifer through which the flow is occurring.

(6) Intermittent pumping. Pumping labor costs can occasionally be materially reduced by pumping a dewa­tering system only one or two shifts per day. While this operation is not generally possible, nor advan­tageous, it can be economical where the dewatered area is large; subsoils below subgrade elevation are deep, pervious, and homogeneous; and the pumping plant is oversize. Where these conditions exist, the pumping system can be operated to produce an abnor­mally large drawdown during one or two shifts. The recovery during nonpumping shifts raises the ground­water level, but not sufficiently to approach subgrade elevation. This type of pumping plant operation should be permitted only where adequate piezometers have been installed and are read frequently.

(7) Effect of ground wa ter lowering on adjacent structures and wells. Lowering the groundwater table increases the load on foundation soils below the ori­ginal groundwater table. As most soils consolidate upon application of additional load, structures located within the radius of influence of a dewatering system may settle. The possibility of such settlement should be investigated before a dewatering system is de­signed. Establishing reference hubs on adjacent struc­tures prior to the start of dewatering operations will permit measuring any settlement that occurs during dewatering, and provides a warning of possible dis­tress or failure of a structure that might be affected. Recharge of the groundwater, as illustrated in figure 2-13, may be necessary to reduce or eliminate distress to adjacent structures, or it may be necessary to use positive cutoffs to avoid lowering the groundwater level outside of an excavation. Positive cutoffs include soil freezing and slurry cutoff techniques. Observa­tions should be made of the water level in nearby wells before and during dewatering to determine any effect

Selection of dewatering system

Подпись: TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
Selection of dewatering system

(Courtesy of Moretrench American Corp.)

 

Figure 2-12. Dewatering systems applicable to different soils.

 

Selection of dewatering system

Selection of dewatering system

U. S. Army Corps of Engineers

Figure 2-13. Recharge ofgroundwater to prevent settlement of a buildingas a result of dewatering operations.

of dewatering. This information will provide a basis for evaluating any claims that may be made.

(8) Dewatering versus cutoffs and other proce­dures. While dewatering is generally the most ex­peditious and economical procedure for controlling water, it is sometimes possible to excavate more eco­nomically in the wet inside of a cofferdam or caisson and then seal the bottom of the excavation with a tremie seal, or use a combination of slurry wall or other type of cutoff and dewatering. Where subsurface construction extends to a considerable depth or where high uplift pressures or large flows are anticipated, it may occasionally be advantageous to: substitute a caisson for a conventional foundation and sink it to the
design elevation without lowering the groundwater level; use a combination of concrete cutoff walls con­structed in slurry-supported trenches, and a tremied concrete foundation slab, in which case the cutoff walls may serve also as part of the completed struc­ture; use large rotary drilling machines for excavating purposes, without lowering the groundwater level; or use freezing techniques. Cofferdams, caissons, and cut­off walls may have difficulty penetrating formations containing numerous boulders. Foundation designs re­quiring compressed air will rarely be needed, although compressed air may be economical or necessary for some tunnel construction work.