Deep-well systems

a. Deep wells can be used to dewater pervious sand or rock formations or to relieve artesian pressure be-

Atmospheric

Header, pressure

Deep-well systems

Note: Vacuum in header = 25 ft; vacuum in filter ond soil in vicinity of well point = approximately )0 ft.

(From “Foundation Engineering,” G. A. Leonards, ed.,

1962, McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.)

Figure 2-5. Vacuum wellpoin t system,

neath an excavation. They are particularly suited for dewatering large excavations requiring high rates of pumping, and for dewatering deep excavations for dams, tunnels, locks, powerhouses, and shafts. Excava­tions and shafts as deep as 300 feet can be dewatered by pumping from deep wells with turbine or submersi­ble pumps. The principal advantages of deep wells are that they can be installed around the periphery of an excavation and thus leave the construction area unem – cumbered by dewatering equipment, as shown in fig­ure 2-7, and the excavation can be predrained for its full depth.

b. Deep wells for dewatering are similar in type and construction to commercial water wells. They com­monly have a screen with a diameter of 6 to 24 inches with lengths up to 300 feet and are generally installed with a filter around the screen to prevent the infiltra­tion of foundation materials into the well and to im­prove the yield of the well,

c. Deep wells may be used in conjunction with a vac­uum system to dewater small, deep excavations for tunnels, shafts, or caissons sunk in relatively fine­grained or stratified pervious soils or rock below the groundwater table. The addition of a vacuum to the well screen and filter will increase the hydraulic grad­ient to the well and will create a vacuum within the surrounding soil that will prevent or minimize seepage from perched water into the excavation. Installations of this type, as shown in figure 2-8, require dequate vacuum capacity to ensure efficient operations of the system.

2-6. Vertical sand drains. Where a stratified semipervious stratum with a low vertical permeability overlies a pervious stratum and the groundwater table has to be lowered in both strata, the water table in the upper stratum can be lowered by means of sand drains as shown in figures 2-9. If properly designed and in­stalled, sand drains will intercept seepage in the upper stratum and conduct it into the lower, more permeable stratum being dewatered with wells or wellpoints. Sand drains consist of a column of pervious sand placed in a cased hole, either driven or drilled through the soil, with the casing subsequently removed. The ca­pacity of sand drains can be significantly increased by installation of a slotted 1% or 2-inch pipe inside the sand drain to conduct the water down to the more per­vious stratum.

2-7. Electro-osmosis. Some soils, such as silts, clayey silts, and clayey silty sands, at times cannot be dewatered by pumping from wellpoints or wells. How­ever, such soils can be drained by wells or wellpoints combined with a flow of direct electric current through the soil toward the wells. Creation of a hy­draulic gradient by pumping from the wells or well – points with the passage of direct electrical current through the soil causes the water contained in the soil voids to migrate from the positive electrode (anode) to the negative electrode (cathode). By making the cath­ode a wellpoint, the water that migrates to the cathode can be removed by either vacuum or eductor pumping (fig. 2-10).

2-8. Cutoffs. Cutoff curtains can be used to stop or minimize seepage into an excavation where the cutoff can be installed down to an impervious formation. Such cutoffs can be constructed by driving steel sheet piling, grouting existing soil with cement or chemical grout, excavating by means of a slurry trench and backfilling with a plastic mix of bentonite and soil, in­stalling a concrete wall, possibly consisting of overlap­ping shafts, or freezing, However, groundwater within the area enclosed by a cutoff curtain, or leai ^ge through or under such a curtain, will have to be pumped out with a well or wellpoint system as shown in figue 2-11.

a. Cement and chemical grout curtains. A cutoff around an excavation in coarse sand and gravel or por­ous rock can be created by injecting cement or chem­ical grout into the voids of the soil. For grouting to be effective, the voids in the rock or soil must be large enough to accept the grout, and the holes must be close enough together so that a continuous grout curtain is obtained. The type of grout that can be used depends upon the size of voids in the sand and gravel or rock to

Deep-well systems

a. PLAN

PRESSURE HEADER

 

PRESSURE PUMP,

 

RETURN HEADER

 

RE ‘URN PRESSURE 15 PSI

 

PRESSURE PI PE 100 PSI

 

PRESSURE LINE RETURN LINE 100-150 PSI 10-15 PSI

 

– 1-1/2" RISER PIPE

 

– 1 " RISER PIPE

 

-1-1 /2" WELLPOINT

 

VACUUM UP TO 25"

 

c. TYPICAL EDUCTOR WELLPOINT

 

Deep-well systemsDeep-well systemsDeep-well systemsDeep-well systems

Deep-well systems

U. S Army Corps of Engineers

<£ EXCAVATION

Deep-well systems

U. S. Army Corps of Engineers

 

Figure 2-7. Deep-well system for dewateringan excavation in sand.

be grouted. Grouts commonly used for this purpose are Portland cement and water; cement, bentonite, an ad­mixture to reduce surface tension, and water; silica gels; or a commercial product. Generally, grouting of fine or medium sand is not very effective for blocking seepage. Single lines of grout holes are also generally ineffective as seepage cutoffs; three or more lines are generally required Detailed information on chemical grouting and grouting methods is contained in TM