Water and Wastewater Engineering

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WELLS 4-3

quality standards, the strata that serve as aquifers may differ both in the constituents and the

concentration of the constituents.

Adverse Resource Impact

A s noted in Chapter 3, one method of withdrawing water from a su rface water body is by the

installation of a gallery or a Ranney well system. Individual wells may also cause water from a

surface water body to flow out of the water body into the aquifer. If the pumping rate is too high,

this may lower the water level in the surface water body enough to have a negative impact on its

aqu

atic life.

4-3 WELL PROTECTION

The elements employed to protect the well include:

• I solation from sources of contamination.

• Sanitary considerations in well construction.

• Provision of structural protection.

Although the provisions of the Michigan Safe Drinking Water Act (MSDWA, 1976) that

are discussed in the following paragraphs s erve as guidelines for good

practice, individual state

requirements vary, and these should be consulted as a matter of normal engineering practice.

Isolation

For the purpose of this discussion, the Michigan Safe Drinking Water Act definitions of different

types of water supplies discussed in Chapter 2 are classified as follows:

• T ype I: all community supplies.

• Type II: all noncommunity supplies.

• T ype IIa: average daily water production for the maximum month eq

ual to or greater

than 75 m

/ d.

• Type IIb: average daily water production for the maximum month less than 75 m

/ d.

• Type III: all public supplies that are not Type I or Type II.

This classification is used to set isolation distances based on the probable radius of influence

of the well and the risk to the population exposed. The recommended isolation distances are

shown in Table 4-1 on page 4-5.

Well Construction

Geologic conditions dictate two general types of well construction. A well that taps an aquifer

of water-bearing sand is cased through the overburden and screened in the water-bearing sand as

shown in Figure 4-1 a (Johnson, 1975). A well that taps an aquifer of consolidated rock consists

4-4 WATER AND WASTEWATER ENGINEERING

Gravel fill pipe

with cap

Air-line

gauge

Grout seal

Inner casing

Static water level

Pump column and

shaft assembly

Outer casing

(note: a third

outer casing may

also be used)

Vertical turbine

pump bowl assembly

and intake

Gravel pack

material

(a)

Unconsolidated-depos

its

aquifer

(e.g., sand and

gravel)

Continuous slotted

screen intake

Drive shoe

Low-permeability

nonaquifer

material

(e.g., clay)

Soil

Discharge pipe

Well head

(includes pump motor, discharge

head, and base plate)

FIGURE 4-1

( a ) Typical construction of a screened, gravel-packed well in an unconsolidated-deposits aquifer.

WELLS 4-5

TABLE 4-1

Typical isolation distances

Contaminant sourceType I Type IIa Type IIb Type III

Barnyards 60 60 20 20

Chemical or waste chemical storage

or disposal facilities

600 600 250250

Land application of wastewater or

sludges

600 600 250250

Municipal solid waste landfill 600 600 250250

Pipelines 60 60 20 20

Septic tanks, drain fields, d

ry wells,

cesspools, leaching beds

60 60 20 20

Sewers—storm or sanitary 60 60 20 20

Surface water 60 60 20 20

Distances are in meters.

Adapted from MSDWA, 1976.

of a cased portion that passes through the loose overburden material and an open borehole in the

rock ( Figure 4-1 b).

Well Drilling. There are numerous methods for drilling the well. A few of these are high-

lighted here.

• Cable-tool percussion: The drilling operation is carried out by lifting and dropping a heavy

string of drilling tools. The reciprocating action of the drilling tools mixes the crushed

loosened particles with water to form a slurry that is removed by a sand pump or bailer.

• Jet drilling: The drill tools for the jet-percussion method consist of a chisel-shaped bit

attached to the lower end of a string of pipe. Water is pumped through the drill bit and

flows upward in the annular space around the d

rill bit carrying the cuttings to the surface.

• Hollow-rod or hydraulic percussion: This method is similar to jet drilling except the water is

pumped down through the annular space, and the cuttings are carried up through the pipe by

the reciprocating motion and a set of valves that keeps the water from flowing downward

• Rotary drilling: The borehole is cut by a rotating bit. The cuttings are removed by drilling

fluid that passes down the drill pipe system and out through the nozzles of the bit. When the

fluid reaches the surface, it is pumped to a pit where the bulk of the cuttings settle out. The

drilling fluid is then reused.

• Reverse c

ircu lation rotary drilling (RCR): The flow of drilling fluid is reversed from

that used in conventional drilling. A schematic of this drilling method is shown in

Figure 4-2 on page 4-7. This is a common method for drilling community wells. This

method is favored when completion of the well is to be by artificial gravel packing.

The suitability of thes e methods for different geological conditions is summarized in

Table 4-2 on page 4-8.

4-6 WATER AND WASTEWATER ENGINEERING

Open borehole

construction

Vertical turbine

pump bowl assembly

and intake

Drive shoe

Fractured

bedrock

Column

and

shaft assembly

Casing (note: an

outer casing may

also be used)

Overburden

Discharge pipe

Well head

(includes pump motor, discharge

head, and base plate)

Grout seal

Overburden

Air-line

gauge

(b)

Static water level

FIGURE 4-1 (Continued)

( b ) Typical construction components of an open-borehole well in a bedrock aquifer.

Casing. The purpose of the casing is two-fold. It provides a means of maintaining the bore

opening by keeping loose and unstable soil from falling in. In addition, it is a key feature in pro-

tecting the well from contamination.

Steel pipe is commonly used for the casing. There are several types of pipe available: drive

pipe, line pipe, reamed and drifted pipe, standard pipe, and water well

casing. Electric-resistance–

welded and seamless “line pipe” is preferred (Johnson, 1975). The most commonly accepted

specifications for water well casing are those prepared by:

WELLS 4-7

• A merican Society for Testing Materials (ASTM).

• American Petroleum Institute (API).

• American Iron and Steel Institute (AISI).

• American Water Works Association (AWWA).

• U.S. federal government.

Typically one of the following is specified: AWWA A-100, ASTM A-120 and A-53B grade,

“stenciled” or API 5-L. Minimum weights and thicknesses may

be found in Recommended

Standards for Water Works (GLUMRB, 2003).

Large diameter suction hose

High capacity, low head pump or jet

Pump discharge

Settled sand and cuttings in pit

Reverse circulation drill bit

Rotary table

Kelly with

large bore

Swivel

Large diameter drill pipe

FIGURE 4-2

Basic principles of reverse-circulation, rotary drilling are shown in this schematic diagram. Cuttings are lifted by upflow inside

the drill pipe. The Kelly is a part of the Kelly drive. It is square, hexagonal-or octagonal-shaped tubing which connects to the

drill pipe for drilling wells.

4-8 WATER AND WASTEWATER ENGINEERING

Grouting and Sealing the Well Casing. The annular space between the casing and the drill

hole is one of the principal avenues for c ontamination to enter the well. To block this route of

entry into the well, the space is filled with neat cement grout.

Neat cement grout is a mixture of cement and water in the ratio of between 1.9 to 2.25 kg of

cement to 1 L of clean water with the higher ratio of cement preferred. Hydrated lime, up to 10

percent of the volume of cement,

may be added to make the grout mix more fluid. Up to 5 percent

by weight of bentonite clay may be added to reduce shrinkage (U.S. EPA, 1973).

Careful attention should be given to see that:

• The grout mixture is properly prepared and mixed well.

• The grout material is placed in one continuous

mass.

• The grout material is placed upward from the bottom of the space to be grouted.

One method of introducing the grout is shown in Figure 4-3 . A packer connection with a

one-way valve is provided at the bottom of the casing to prevent the grout from flowing into the

casing while grouting is in progress. The valve is called a cementing shoe or a float shoe. Grout

is pumped through the grout pipe and forced upward around the casing.

The procedure to be followed with this arrangement is as follows:

The casing, with float shoe attached and grout pipe properly installed, is suspended a short distance above

the bottom (3 cm is recommended to prevent clogging). The casing is filled with water to provide sufficient

mass to prevent flotation of the pipe. Grout is pumped into the annular space until it appears at the surface

TABLE 4-2

General suitability of well construction methods

Characteristics Hollow-rod Jetted Cable-tool

Rotary

drilling

Reverse

circulation

rotary drilling

Practical range of depth 30 m 60 m 600 m 1,000 m 1,000 m

Diameter 0.1 m 0.3 m 0.6 m 1.5 m 1.5 m

Geologic formation

Clayyesyesyesyesyes

Silt yesyesyesyesyes

Sand yesye

syesyesyes

Gravel < 0.5 cm < 0.5 cm yesyesyes

Cemented gravel no no yesyesyes

Boulders no no no yesyes

Sandstone no no no yes no

Limestone no no no yes no

Igneous rock nononoyes no

The ranges of values in this table are based on general conditions. They may be exceeded for specific areas or conditions or

advances in technology.

Sources: Campbell and Lehr, 1973; Johnson, 1975; Stollhans, 2007: U.S. EPA, 1973.

WELLS 4-9

and all other fluid has been displaced. The grout pipe is disconnected from the float shoe, and it is washed

in place with water. It is then removed. After the grout has set, the bottom plug is drilled out and drilling is

continued below the grouted section (Johnson, 1975).

The bore hole for this method of placing the grout should be 10 cm larger in diameter than

the nominal casing size (U.S. PHS, 1965). The entire casing length is to be grouted. Screened

wells are to be grouted from a point not more than 3 m above the screen to the ground surface

(MSDWA, 1976). For wells ending with an open borehole in a rock formation, the casing is

grouted from the top of the borehole to the grou

nd surface. For artesian aquifers, the casing is

FIGURE 4-3

Grout pipe inside casing connects to drillable cementing plug or float shoe at bottom of casing pipe (a). After the grout has set,

the bottom plug is drilled out and drilling continues. Detail of cementing shoe or float shoe with ball check value (b).

Pump

Grout

Mixer

Top soil

Grout pipe

Non-caving

formation

(clay)

Fractured

bedrock

Pipe off

the bottom

(a)

Grout

Casing filled

with water or

mud fluid

Well casing

Drilled hole

diameter at

least 5 cm

larger than

well casing

Grout overflow

Steel casing

Rubber seat

Plastic ball valve

Plastic cage

Concrete

(b)

4-10 WATER AND WASTEWATER ENGINEERING

sealed to the upper confining layer from within 1.5 m of the top of the aquifer to the ground sur-

face (U.S. PHS, 1965).

Grout made from Type I, Ia, or III cement should cure a minimum of 24 hours before drill-

ing operations resume. When a cement/bentonite mixture is used, a waiting period of 48 hours is

recommend

ed (MSDWA, 1976). Some rules require a curing time of three to seven days.

Well Screen. A well screen is installed when the water-bearing formation is unconsolidated

material such as sand. The well screen prevents fine material from entering the well and then

causing undue wear on the pump and, potentially, filling the well.

Gravel-Packed Well. When the water-bearing formation is unconsolidate

d, the gravel-packed

well ( Figure 4-1 a) is us ed. As shown in Figure 4-4 , there are numerous configurations for the

gravel pack. The gravel wall retains the water-bearing sand while the screen keeps the gravel

Gravel

Underreamed—sin

le casin

Underreamed—dual casin

Strai

ht hole—dual casin

Casing Casing Casing

Gravel

Grout Seal

FIGURE 4-4

Some alternative schemes for packing gravel in a gravel-packed well.

WELLS 4-11

from entering the well casing. The gravel-packed well is one of the most commonly used well

designs in the United States. This system improves the efficiency of the well by increasing the

inlet volume, specific capacity, retention of sand, and size of the screen openings, and by reduc-

ing the entrance friction, entrance velo

city, drawdown, operating cost, and maintenance. All of

these factors contribute to increasing the life of the well.

The reverse circulation rotary method of drilling is well suited to the installation of a gravel-

packed well system. As shown in Figure 4-5 , the bore-hole is begun with a temporary surface

casing. The annular space between the d

rill and the bore-hole wall is about 10 to 15 cm. When

the well is completed to its final depth, the casing and screen are lowered into the hole. Water in

the hole maintains static pressure on the assembly and the walls of the hole. The annular space is

filled with gravel to the desired level. The assembly must be supported from the ground surface

ntil the gravel and grout are in place as the column is too long to be self-supporting.

Well Development. The aim of well development is to remove finer material from the aquifer

to enlarge passages in the aquifer formation, so water can enter the well more freely.

Development has the following benefits:

• It corrects any damage or clogging that may occur during drilling.

• It increases the porosity and permeability of the natural formation in the vicinity of the well.

• It stabilizes un

consolidated material around a screened well.

The fundamental intent of the development operation is to cause reversals of flow through

the well screen to rearrange the particles in the formation to break down bridging of small groups

of particles. There are a number of procedures used to create the reversal. Some of these are

mechanical surging with air and

backwashing. Alternatively, high-velocity jetting with water

may be employed. Details of these techniques are described in Hanna (2007).

Drill

Water level in bore hole

Temporary surface casing

Reverse circulation

rotary bit

Bore hole

(a)

Minimum of

4 m of water at start

of drilling below

temporary casing

Casing

(b)

Neat cement grou

Screen

Gravel pack

Casing

welded

screen

Approx 8 m

FIGURE 4-5

Installation of gravel-packed well. ( a ) Start of drilling. ( b ) Finished bore hole with casing and screen lowered into place, gravel

in place, and casing grouted.

4-12 WATER AND WASTEWATER ENGINEERING

Both gravel-packed wells and those in consolidated formations benefit from well development

because the placement of the rock and the drilling operations tend to smear and seal the surface

of the aquifer material around the well. The development operations will break up this layer. This

effort is worthwhile to obtain the maximum capacity of the well.

Other Sanitary Considerations.

Figure 4-6 summarizes several sanitary considerations in

well construction. The sanitary well seal is a gasket to prevent contamination from entering the

top of the casing. It is to be water tight with an air vent to relieve the negative pressure caused

by lowering of the water in the well. Because it keeps the air above the water at atmospheric

Discharge

Plug

Air, vent

Ground surface sloped

to drain away from well

Sanitary well seal

Connection to

source of power

Top soil

Artesian pressure surface

or piezometer surface

Cement grout

formation seal

Submersible pump

Dynamic (pumping)

water level

Clay

Concrete pad

0.6 m

6 m

Highest

flood

level

Drive shoe

Water-bearing sand

Packer

Screen

Power cable

Drop pipe connection

Check valve

Pump casing

Inlet screen

Diffusers & impellers

Inlet body

Power leads

Motor shaft

Motor section

Lubricant seal

Submersible pump details

FIGURE 4-6

Drilled well with submersible pump.

( Source: U.S. EPA, 1973.)