types of wells.pdf

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TYPES OF WELLS

TYPES OF WATER WELLS

ARBA MINCH UNIVERSITY INSTITUTE SCHOOL OF POST

GRADUATE STUDENT

DEPARTMENT OF WATER RESOURCE AND IRRIGATION

ENGINEERING

COURSE OF GROUND WATER HYDROLOGY

By ELIAS AWOL

ID NO.RMSC/159/05

SUBMITTED TO: GUCHIE GULIE (Ph.D.)

DATE: JUNE/2013


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B Y E L I A S A W O L A R B A M I N C H U N I V E R S I T Y Page 1

Table of Contents Page

1. INTRODUCTION ...........................................................................................................................2

2.

TYPES OF WELLS ................................................................................................................................ 3

2.1.TUBE WELLS .....................................................................................................................................4

2.1.1. Based on entry of water .4

2.1.2. Based on method of construction ...8

2.1.3. Based on depth .11

2.1.4. Based on Types of aquifer ...14

2.2. OPEN WELLS18

2.2.1. Open wells in unconsolidated formations.19

2.2.1.1Unlined wells...19

2.2.1.2.Wells with pervious lining ......20

2.2.1.3.Wells with impervious lining ..20

2.2.1.4.Dug-bore wells ........21

2.2.2.Open wells in hard rock formation ....22

2.2.2.1. Dug wells ...23

2.2.2.2. Dug-Cum-Bore Well..24

3.

References ................26


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1.Introduction

A water wells is a hole or shaft, usually vertical, excavated in the earth for bringing ground water

to the surface .Occasionally wells serve other purposes, such as for subsurface exploration and

observation, artificial recharge, and disposal of waste waters. Water wells have been a source of

water for people, animals and crops since the earliest civilizations in Africa and Asia. In Europe,

the development of many towns and cities in the middle ages and on through the industrial

period was aided considerably by the abstraction of relatively pure water supplies from wells and

springs.

Uses of wells are for irrigation purpose, livestock watering, industrial supplies, geothermal or

ground-source energy, construction, dewatering, brine mining, water injection to oil reservoirs,

aquifer clean up, river support and artificial recharge of aquifers. Wells also used extensively for

monitoring water levels and groundwater quality.

Water wells come in many forms, orientations and sizes. Traditionally most water wells wereexcavated by hand as shallow, large diameter, shafts; nowadays, the majority is constructed from

relatively small diameter boreholes drilled by machine, sometimes to great depths. Water wells

are typically vertical but can be horizontal (infiltration gallery), a combination of vertical and

horizontal well (radial collector well), or occasionally inclined. The water may be abstracted by

hand-operated or motorized pumps, or it may flow to the surface naturally under positive upward

pressure (artesian well) or by gravity drainage.


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2.

Types of Water Wells

Diverse geological formations require different types of wells for tapping ground water for

irrigation and water supply. The choice of type of well for irrigation is influenced by the size of

farm holdings and the relative preference given to private, cooperative and public wells. There

are two broad classes of Wells:

o Tube wells and

o HandDug well/Open wells

Fig.2.1Types of wells


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2.1.

Tube wells

Tube well is: a hole drilled in the ground for the purpose of extracting ground water. Some tend

to limit the term to shallow wells.

Tube wells are classified on the basis of the entry of water into the well, the method of

construction, the depth and the type of aquifer tapped. These are listed below:

2.1.1. Entry of water

a) Screen wells

i. Strainer wells

ii. Slotted pipe gravel pack wells

c) Cavity wells

2. 1.2. Method of construction

a) Drilled wells

b) Driven wells

c) Jetted wells

2.1.3. Depth

a) Shallow wells

b) Deep wells

2.1.4. Type of aquifer

a) Water table wells

b) Artesian wellsc) Semi-artesian wells

d) Bore wells in hard rock areas

e) Skimming wells

2.1.1. Based on Entry of water

Tube wells are classified as screen wells or cavity wells on the basis of the entry of water from

the aquifer into the well

a)

Screen wells

Several types of well screens are used to suit the specific requirements of the aquifer and

economic status of the farmer. Johanson (1966).listed the following as the desirable features for a

properly designed well screen:

Openings in the form of slots which are continues and uninterrupted around the

circumstance of the screen.


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Close spacing of slot openings to provide the maximum percentage of open area.

V-shaped slot openings that widen inward.

Single metal construction to avoid galvanic corrosion.

Adaptability to different ground water and aquifer conditions by the use of various

materials.

Maximum open area consistent with adequate strength.

Ample strength to resist the force to which the screen may be subjected during and after

installation.

Full series of accessories to facilitate screen installation and well completion operations.

There are many screen well types like Strainer, slotted well type, Louver-Type

Screen, Coir-Rope strainer; bamboo strainer, agricultural strainers but here

mentioned the two types of screen well.

i. Strainer wells: It essentially consists of a perforated or a slotted pipe with a wire

mesh wrapped around the pipe with smaller annular space between the two. In thistype of tube well a screen is placed against the water bearing stratum. The wire screen

prevents sand particles from entering the well pipe through the fine mesh (screen) and

the sand particles of size larger than the size of mesh are kept away from entering the

pipe. This reduces the danger of sand removal and hence, larger flow velocities can be

obtained. In the strainer type tube well drilling is continued through different layers

and after ascertaining the water bearing strata, strainers are located opposite these

strata to allow the water to come into the tube well. Plain pipes are to be located

against those layers which are not water bearing. This is done by joining the whole

length of strainers and the plain pipes in the same length and order in which they are to

be lowered in the bore. For the strainer type well drilling is to be started with pipes oflarger diameter (known as casing of pipes) than the strainer and these pipes are to be

extract after the strainers are installed.

ii. Slotted Type Tube Wells: It uses a slotted pipe without being covered by any wire

mesh. If proper depth of water bearing strata is not available even at deep depths of 85

to 100 m, so as to obtain the required discharge from a strainer well. After placing the

assembly of plain and slotted pipes in the bore hole, gravel is poured into the bore hole

between the well pipe assembly and the casing pipe. Gravel packs are highly

perforated for deep wells and tap more than one aquifer.

b) Cavity Type Tube Wells:

A cavity well is the shallow tube well drilled in alluvial formation .A cavity type tube well draws

water from bottom of the well and not from the sides. The difference in flow pattern of a screen

well and cavity well is that whereas in a strainer well, the flow is radial, the flow in a cavity well


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B Y E L I A S A W O L A

is spherical. In a strainer well, t

pipe, while in a cavity well the

cavity formed with a certain dis

The cavity type well does not h

started with the correct size of

developed by drawing out a part

reservoir for pumping. The cavit

top of the water bearing stratum.

ground strata permits its constru

Fig.2.2.Sche

TYPES OF WELLS

R B A M I N C H U N I V E R S I T Y

e area of flow is increased by increasing the

area of flow is increased by enlarging the si

charge enlarges in size if an increased discharg

ve a strainer and draw water from one stratu

ipes which are left to serve as tube for the w

of the sand from this layer so that such a cavi

y well requires a strong and dependable stratu

Cavity wells are very economical and can be a

tion.

atic sketch of a cavity well (Anjaneyulu, 19

Page 6

ength of strainer

e of cavity. The

e is pumped out.

only. Drilling is

ell. The cavity is

y acts as storage

immediately on

dopted where the

72)


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Fig.2.3A.Cav

Fig.2.3B.cavity w

TYPES OF WELLS


ty well (for centrifugal pump)

ll (for turbine submersible pump)

Page 7


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2.1.2.

Based on Method of construction

Under this classification wells are grouped according to their method of construction. They are

drilled wells, driven wells and jetted wells.

a)

Drilledtubewells:

Drilled wells are smaller in diameter, usually ranging from 10-20 cm (4-8 in.), and completed too

much greater depths than bored wells, up to several hundred meters. The producing aquifer is

generally less susceptible to pollution from surface sources because of the depth. Also, the water

supply tends to be more reliable since it is less affected by seasonal weather patterns.

There are two primary methods of drilling:

Rotary

Cable tool.

Rotary (air or hydraulic) drilled wells are constructed using a drill bit on the end of a rotating

drill stem. Drilling fluid or air is circulated down through the drill stem in the hole and back tothe surface to remove cuttings. Rotary drilling rigs operate quickly and can reach depths of over

300 m (1000 ft.), with casing diameters of 10-45 cm (4-18 in.).

Cable tooldrilled wells are constructed by lifting and dropping a heavy drill bit in the bore hole.

The resulting loose material, mixed with water, is removed using a bailer or sand pump. This

method, also called percussion drilling, reaches depths up to 300 m (1000 ft.). Well diameters

can range from 10-45 cm (4-18 in.). The drilling rate is typically much slower than for a rotary

rig, but when aquifers are low yielding, they may be more easily identified using this method.

Fig. 2.4a.KS 1000 Water Well Drilling Fig.2.4b. Rotary drill

Rig KiloloStar.org


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Advantages of cable tool drilli

o One person operation

o Low equipment cost

o

Simple rugged design

o Requires minimal mainte

o Requires low horsepowe

o Virtually indestructible

b) Driven tube wells:

A driven well consists of a series o

below the water table. Driven well

water-bearing sand or gravel.

casing before driving. These w

can tap only shallow water and

they are not sealed with grouti

deep; machine-driven wells can

Fig.2.5A driven well with dri

TYPES OF WELLS


g

nance

connected lengths of pipe driven by repeated impac

s are constructed by driving a small-diameter

sually a screened well point is attached to t

lls are relatively simple and economical to c

are easily contaminated from nearby surface

g material. Hand-driven wells usually are onl

e 50 feet deep or more.

ing mechanism

Page 9

ts in the ground to

ipe into shallow

e bottom of the

nstruct, but they

sources because

y around 30 feet


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Yields from driven wells are small, with discharges of about 100-250 m3/day. Driven wells are

best suited for domestic supplies, for temporary water supplies, and for exploration and

observation.

Driven wells are limited to unconsolidated formations containing no large gravel and rocks that

might damage the driven point. To drive a well the pipe casing and threads should be protected at

the top with a drive cap.

Important advantages of driven wells are that they can be constructed in a short time, a

minimum cost, and even by one person.

c) Jetted tube wells:

Jetted wells are constructed by the cutting action of a downward directed stream of water. The

high- velocity stream washes the earth away, while the casing, which is lowered in to the

deepening hole, conducts the water and cuttings up and out of the well. Small-diameter hole of 3

to 10 cm are formed in this manner (although the method is capable of producing diameters up to

30cm or more) to depths greater than 15m. Jetted wells have only small yields and the portability

of the equipment; jetted wells are useful for exploratory test holes, observation wells, and well-

point system.

Fig2.6.Jetted well with self-jetting well point


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2.1.3.

Based on Depth

Tube wells are classified as shallow or deep tube wells on the basis of their depth.

a) Shallow wells:

Shallow tube wells are of low capacity. The average depth of the well is usually less than 35 m.

Cavity tube wells and strainer tube wells with coir strainers generally fall in this category. The

latter usually tap only the unconfined aquifer.

The most common "dry well" problem has been with dug wells. Most dug wells are shallow and

excavated in poorly permeable material; consequently they are readily affected by drought or by

seasonal declines in the water table. Pumping during the drought would cause the water to

decline to or below the pump intake. Excavating this well deeper to match the well on the right

would solve this problem. Dug wells should be constructed during seasonal or climatically low-water-level periods.

Many dug wells extend only to the bedrock surface and tap the perched water (unconfined

ground water separated from an underlying main body of ground water (aquifer) by an

unsaturated (impermeable) zone) on top of the bedrock. These wells cannot be easily deepened.

In such cases a new drilled well is the only long-term solution.

Some drilled wells that tap shallow bedrock will yield only 1 or 2 gallons of water per minute.

These wells are not deep enough to provide adequate storage of water for short-term pumping

cycles. Such a well may contain only 50 feet of water above the pump intake. As an example,when the water table declines 10 feet because of drought conditions, only 40 feet of water is

available in the well for one pumping cycle, and the well seems to "go dry." In that situation,

deepening the well may solve the problem as long as the deeper water is of good quality. If

usable water is not available at a greater depth, the pumping rate must be reduced so that less

water is pumped during each cycle.

Performance and Protection of Shallow wells

Shallow wells in clay formation are good for stability of their walls, but recharge is usually slow

if many people draw water from it. The result is a common long queue of people awaiting

water recharge in the well. On the other hand, shallow-wells in sandy formations tend tobe hazardous as they collapse easily, but their recharge is usually fast due to much higher

porosity compared to the clay formations. This problem is usually managed by lining the

wells walls with locally made materials such as bricks or baskets. A concrete slab is often put on

top to prevent pollution. Where resources allow, a pulley system or better still, a hand-pump is

installed to lift the water to the surface


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Fig. 2.7Shallow and

Advantages of shallow

o They can acquire a well usin

compared to borehole or pip

o Some Areas needing water s

o Some areas have naturally

while shallow water is of bet

o

When external support iscommunities opt for digg

which would have gone into

TYPES OF WELLS


ery Shallow Shrouded Tube well

wells

g their own resources (low cost and locally ava

d water technologies).

pplies are not accessibility by big drilling mac

polluted deep water (eg too salty for hum

er quality).

relatively low, compared to the magnitudng many shallow wells for the same a

much fewer boreholes.

Page 12

lable technology

ines.

n consumption),

of need, someount of money,


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Limitations of shallow wells

Despite the above advantages of shallow wells, the following are some of their typical

limitations;

Shallow wells dry quickly in protracted dry season since the water table of theannually recharged perched water aquifer in the superficial deposits (eg the sand, clay

formation etc) goes down fast.

Easy water pollution due to poor disposal of human and industrial waste on the

ground.

The large diameter wells are hazardous to people when they collapse.

If unprotected, animals can also cause pollution when they drink from it.

b) Deep wells:

Deep tube wells are wells of high capacity, tapping more than one aquifer. Their depth usuallyranges from 60-300 m. Deep tube wells may be strainer wells or gravel-pack wells, depending

upon the characteristics of the aquifer formation.


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Fig.2.8. Deep well detail

2.1.4. Based Aquifer characteristics

Tube wells under this category are classified as water table wells, semi-artesian wells, artesian

wells and hard rock bore wells. The classification is based on the location of the well and the

characteristics of the aquifer. Wells may be defined as water table or artesian wells, depending

upon whether they tap a water table aquifer or an artesian aquifer. Artesian wells are further

classified as semi-artesian wells and flowing artesian wells. Tube wells bored in hard rock

formations are classified as hard rock bore wells.


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a) Water table wells :

These are installed in unconfine

level is not under pressure. Gene

Fig. 2.9. Water table wells

b)

Semi artesian wells :

Semi-artesian wells are installe

pressure, but not so high as to fl

TYPES OF WELLS


d aquifers which are under water table conditi

rally, shallow tube wells fall under this categor

under semi artesian conditions of aquifer. T

w out of the well.

Page 15

ns, i.e. the water

.

e water is under


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c) Artesian wells :

A flowing well gets its supply from an aquifer where the water is under such high pressure that it

overflows at the top. The well is so named because the initial knowledge about such wells wasderived from Artois in France. The static water level in this case is above the ground and can be

measured within the well casing, if the pipe is extended high enough so that the flow does not

occur. Alternatively, flow can be contained by capping the well casing, after which the shut-in

head can be measured with a pressure gauge.

Fig.2.10 Artesian well

d) Bore wells in hard rock areas :

Tube wells in hard rock areas are called bore wells because the bore hole is able to hold on its

own for most of its depth and the tube is put only against the upper weathered soil zone. Bore

wells have proved their merit as drinking water wells when the discharge requirement is limited.

Bore wells for irrigation purpose also coming into vogue, especially with drip irrigation systems.


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In case of wells with very low

application. In such cases, a sto

rates of flow could be allowed i

desirable. The other disadvanta

head and leading to high suction

be used.

Fig.2.11. Schematic sketch

rock area. Note the soil cove

e) Skimming wells

The Indo-Gangetic plain is most

deposits which at some places c

highly brackish (Zuberi and Mc

accumulated through seepage a

fields. This upper layer of fresh

This layer of water is of good qu

TYPES OF WELLS


ischarge, there is considerable loss of water in

rage tank is constructed to store water, from

to the water conveyance system. A lined conv

es of such wells are the excessive drawdown,

heads. Therefore, generally, submersible and j

illustrate a bore well tapping fissured

overlying the hard rock.

ly underlain by a huge water-bearing aquifer, f

ontain native saline water of the sea. The dee

Whorter, 1973). In the upper portion, however

d deep percolation from rainfall, rivers, cana

water is thick near the source of recharge (riv

ality and can be used for irrigation.

Page 17

conveyance and

hich the desired

eyance system is

resulting in high

et pumps have to

one in a hard

rmed by alluvial

ground water is

, fresh water has

ls and cultivated

ers, canals, etc.).


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ADVANTAGES AND DISADVANTAGES OF TUBE WELLS

S.No. Advantages Disadvantages

1 do not require much space Requires costly & complicated drilling

equipment & machinery.

2 Can be constructed quickly- not time consuming Requires skilled workers & great care todrill & complete the tube well.

3 Fairly sustained yield of water can be obtained

even in years of drought.

Installation of costly turbine or submersible

pumps is required.

4 Economical when deep seated aquifers are

encountered.

Possibility of missing the fractures, fissures

& joints in hard rock areas resulting in

many dry holes.

5 Flowing artesian wells can sometimes be struck.

6 Generally good quality of water is tapped.

2.2 Open wells

Open wells have been the major means of domestic water supply throughout the span of the

recorded history of mankind. They are also used extensively in small-scale irrigation. Compared

to tube wells, open wells are shallow and usually used to tap water table aquifers. Open wells are

used mainly for three purposes:

(1) To extract ground water from fine grained aquifers of shallow depth, where the danger of

entering small particles requires a large area of contact with the aquifer,

(2) To tap ground water in hard rock areas and

(3) To serve as reservoirs for ground water slowly replenishing the well. Storage of water in an

open well permits its periodic extraction at a rate greater than the rate of recuperation ofground water into it.

Open wells are best suited to shallow and low-yielding aquifers. They do not require

sophisticated equipment and skilled personnel for construction. They can be operated by

indigenous water lifts driven by man or animal power, or low-cost mechanically operated

centrifugal pumps. Open wells can be revitalized by deepening or providing bores at the

bottom or sides.

Limitations of open wells

oLarge space is required by the well structure and for dumping excavated material.

o

Construction of well is slow and laborious.

oOpen dug wells are economically unsuitable for tapping deep aquifers, as the cost of

construction becomes excessive as the depth of the well increases (Deeper aquifers could,

however, be tapped by resorting to dug-cum-bore wells).

oThey are susceptible to contamination or pollution from surface sources, unless properly

protected.


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oDue to shallow water table t

the well drying up, especiall

Depending upon the nature of t

as: (i) Open wells in unconsoli

Open wells may be either circulin alluvial and other porous for

in well sinking. However, open

For the same area of cross-secti

area exposed to seepage of wate

rectangular well than in a circula

2.2.1. Open wells in unconsoli

1. Unlined wells

2. Wells with pervious li

3. Wells with impervious4. Dug-cum-bore wells.

2.2.1.1. Unlined wells

Well dug for purely temporary

the cost of construction, which

wells are not protected, it is es

under natural conditions. The w

level. To ensure stability, the de

Fig. 2.12 Open wells

TYPES OF WELLS


ere are large water level fluctuations and ther

y during drought periods.

e ground water formation to be tapped, open

dated formations, and (ii) open wells in hard-

r or rectangular in cross-section. The circular sations because of its greater structural strength

wells in hard-rock formations are usually rect

on, the perimeter of a rectangular well is mo

r into it from fractures and fissures, are substa

r one.

dated formation classifiedas:

ing,

lining,

urposes are not usually protected by lining, si

cannot be justified in a particular situation. A

sential that the sub-soil is compact enough t

ter table should not be lower than about 4 m

ths of unlined wells are limited to about 6.5 m.

unlined and lined with pervious lining

Page 19

is possibility of

ells is classified

rock formations.

hape is preferredand convenience

ngular in shape.

e, and hence the

tially higher in a

ce this increases

the sides of the

stand vertically

elow the ground


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2.2.1.2. Wells with pervious Lining

This type of wells is usually lined with dry bricks or stone masonry. Water flows from the

surrounding aquifer into the wells through the sides of the well. Pervious lining is suitable when

the water-bearing formation consists of gravel or coarse sand deposits. When the formationconsists of layers of fine sand, the sand particles escape along with water into the well, through

the pervious lining. As a result, a hollow space or cavity is formed behind the well lining, thus

endangering the structural stability of the well. The annular hollow space around the well lining

will be self-sealing in loose formations but, in cohesive materials, it must be filled with brick or

stone ballast. The ballast is about 2 cm in size and packed behind the lining. It should extend at

least up to the static water table.

2.2.1.3. wells with impervious lining

Open wells with permanent masonry lining, laid in cement mortar, are commonly used inalluvial formations. Once constructed, they form a permanent structure for tapping water, as

long as ground water conditions remain favorable. Though wells with impervious linings are

usually deeper than the two types described earlier, their depths generally do not exceed 30 m

as, beyond that, the cost becomes excessive and the well tends to be uneconomical. Such linings

are provided with weep holes for the lateral entry of water.

Wells with Reinforced Cement Concrete (RCC) linings are also sometimes used, especially for

higher depths. In some shallow water table regions, RCC collar wells, sometimes referred to as

ring wells are used, though mainly for domestic water supply.


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Fig.2.13. open well with masonry lining and inverted filter at bottom

2.2.1.4. Dug-Cum-Bore wells

Dug wells are sometimes provided with vertical bores at their bottom, to augment their yields.

Such wells are referred to as dug-cum-bore wells. Boring consists essentially of drilling small

diameter holes of sizes ranging from 7.5 to 15 cm in diameter, through the bottom of the well,

and extending them up to or into the water-bearing formation lying underneath the bottom of thedug well. In unconsolidated formations there is usually only one hole, which is bored at or near

the centre of the well bottom. In hard-rock areas, however, the number of holes may range from

1 to 6 depending on the nature of the rock and the size of the well.

The bore in open wells may be of two types, according to whether they tap the water-bearing

medium through a cavity formed below hard impermeable strata or through strainers or screens

provided opposite the water-bearing stratum/strata. In the first type, the bore extends to the top of

the water bearing formation, where a cavity is formed. In the second type, well screens are laid

opposite the water bearing formations and blind pipes provided opposite the non-water bearing

strata. The first type i.e. a cavity bore is cheaper but feasible only for shallow artesian aquiferslying beneath the bottom of a dug well.


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Fig.2.14.Sectional view of an open well with bore at the bottom ending in a cavity.

Dug-cum-bore wells are hydraulically superior to ordinary dug wells. However, their success

will depend on the availability of confined aquifers at reasonable depths below the dug section of

the well. If a dug-cum-bore well is operated by a pump set and the suction pipe of the pump isinstalled on the bored section itself, the well practically becomes a tube well. However, in such

wells, the flow of ground water from the upper unconfined aquifer into the well is restricted. The

well staining then serves the purpose of a pump house only. In such a situation, the construction

of a regular tube well will be more economical than a dug cum-bore well. The only advantage,

however, is that it permits a phased development of the well structure.

2.2.2. Open wells in hard rock formation

Open wells, in hard rock areas, often called hard rock wells; the aquifer is directly dependent on

precipitation for recharge. Hence, the water table is prone to considerable fluctuations in relation

to the incidence of rainfall. The shallow ground water reservoir is constituted by the weathered

mantle covering the unaltered rock, and by the fracture porosity of the unaltered rock itself. They

are characterized by limited permeability. Hence, they are capable of yielding only limited

quantities of ground water. Due to their poor permeability, tube wells are usually unsuitable in

such formations. This is because they have to be pumped at heavy drawdown, for considerable

lengths of time, to derive even meager supplies of water. This factor is detrimental to aquifer


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efficiency, due to the reduction in its saturated thickness. It is, therefore, desirable to have open

wells in such formations.

Open wells are capable of storing fairly large supplies of water during a given recovery period.

Thus, the available supplies of water can be obtained at small drawdown in relatively short

periods, thereby allowing sufficient recuperation between successive periods of pumping. Openwells also expose a greater surface area of the aquifer for seepage into them. In hard rock terrains

where the ground water occurrence is spurious, the most important zone in which ground water

invariably occurs is the weathered zone. The thickness of the weathered zones depends upon

topography, climatic conditions and rock type. Therefore, a study of the weathered zone profile,

mode of weathering, structural features, and correlation of all these features with litho logy are

useful. In the basement rock, one can expect a good yield only if it is located in a shear, fracture

or fault zone. Dykes, which act as barriers to ground water movement, are often encountered

intermittently in granitic terrain.

Dykes are actually dark colored, medium-to-fine-grained igneous rocks occurring as intrusions ina wall-like form. They have a high specific gravity of about 3, low porosity and are very durable.

They are more resistant to weathering. Unless, the rock is highly fractured and weathered, which

is not common, the chances of striking water within a dyke are remote. Thus, a dyke is a good

negative indicator of ground water. Dykes act as subsurface dams and effectively stop and/or

change the lateral movement of ground water. On the upstream side of a dyke, one may get high

yielding wells, while on the downstream side the availability of ground water may be meager.

Open wells in hard rock areas may be dug wells or dug-cum-bore wells

[2.2.2.1. Dug wells

In spite of the rocky sub-stratum, well construction is undertaken in hard rock areas, as they

provide the only convenient local source of irrigation and water supply. These wells are usually

open, excavated pits through the rock, and lined only a couple of meters. Usually, pneumatic

rock blasting equipment, using jack-hammers and explosives, is employed for the excavation of

the well through hard rocks. Ordinary horizontal centrifugal pumps are commonly used for

pumping water from the open wells. The inside view of an open well in a fissured formation

fitted with a motor-driven centrifugal pump is shown below.


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Fig.1.15. Schematic sketch of a dug well in a hard rock formation

2.2.2.2. Dug Cum-Bore Well

Boring of dug wells in hard rock areas, to augment their discharge, has been gaining popularity.

Boring helps to tap embedded water-bearing materials, if existing underneath, as in the case of

the Deccan trap areas. Boring also helps to tap additional fissures and cracks in crystalline hardrock areas, even if embedded layers of water-bearing materials are not available.

It may be seen that several types of operations (deepening by blasting, large diameter central

borings, slim boring in different directions, etc.) are available to augment the discharge of dug

wells in hard rock areas. The comparative efficiency and merit of each of these operations

depend upon local hydro geological situations.


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OPEN WELL- ADVANTAGES & DISADVANTAGES

S.No. Advantages Disadvantages

1 Storage capacity of water is available in

the well itself.

Large space is required for the well and for the

excavated materials.2 Does not require sophisticated equipment

& skilled personnel for construction.Construction is slow and laborious

3 Can be easily operated by installing acentrifugal pump.

Subject to high fluctuations of water table duringdifferent seasons.

4 Can be revitalized by deepening by

blasting or drilling vertical or side bores.

Susceptibility to dry up in years of drought.

5 5. High cost of construction as the depth increases in

hard rock areas.

6. Deep seated aquifers cannot be economicallytapped.

7. Uncertainty of tapping good quality water.8. Susceptibility for contamination unless sealed

from surface water ingress.


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Reference

1.

Water Wells and Boreholes BRUCE MISSTEAR ,Department of Civil, Structural andEnvironmental Engineering, Trinity College, Dublin, Ireland DAVID BANKS Holymoor

Consultancy, Chesterfield, UK LEWIS CLARK (Deceased) formerly of Clark Consult

Ltd, Henley on Thames, UK ,2006.

2. Code of practice for construction and testing of tube wells /bore wells, part 1 cocystru

ction ,Second Revision October 1994.

3. www1.agric.gov.ab.ca/.../waterwells_Module3.Design and construction of water wells

module 3.

4. www.amazon.com/Ground-water-rural-homeowner-Waller/dp/.Ground Water and

the Rural Homeowner by Roger M. Waller.

5.

Ground water Hydrology, Third edition, David Keith Todd (University of California,Berkley and Todd Engineers) and Larry W. Mays (Arizona State University) 2005.

6. Water wells and Pumps, Second Edition, A. M. Michael,S. D. Khepar, and S. K. Sondhi,

2008.

types of wells.pdf

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