How to Design and Construct Tubewells? | Ground Water | Geography

Everything you need to learn about designing and constructing tubewells.

Design of Wells:

The design of a tubewell includes decision on:

1. Location of the well,

2. Type of well,

3. Method of construction,

4. Selection of the strata to be screened, length, diameter and material of the screen in case of strainer wells,

5. Diameter, size of slots and size of gravel for shrouding in case of gravel packed wells, and

6. Type of pumping unit.

Location of Wells:

An open well or tubewell is to be located taking into consideration the availability of water at the site within a reasonable depth, water quality and nearness to the area where the water is to be used.

One of the oldest methods which claims to detect the underground water is the water divining method also known as dowsing. In this method a twig from a tree like neem, peepul, willow etc. is held in the hands and the area is traversed by walking. It is believed that the twig will whirl round when it comes over subsoil water.

Some people who claim to be water diviners say that they feel very great strain in their hands catching the twig as they come across the watertable. Though this method is relied upon in some areas with varying degrees of success, this has no scientific background and cannot stand any technical analysis.

A geological survey of the area will give a general indication of the subsoil water supplies. But this does not given any indication of the nature and extent of water at a particular spot. Hydrogeological maps prepared using geophysical methods are useful in general for locating tubewells.

Tubewells present in the nearby area give a good indication about the availability of water. For greater certainty it is necessary to drill experimental bores of small diameter (also known as trial bores) which alone can furnish all the desired information.

The tubewell should be located inside the area which it is supposed to irrigate. It is advantageous to locate the tubewell at or near the highest point of the area so that maximum facilities are provided while planning the distribution system of the tubewell.

While locating all wells and especially shallow wells, care should be taken to keep the selected site away from the source of pollution. Another aspect which has to be taken into consideration in case of tubewells is the interference of wells. If the tube wells are too close, the operation of one well affects the discharge of the other.

The term drilling (also known as boring) refers to the process used in making opening in the ground for well construction purposes.

Construction of Wells:

Construction of Strainer Type Wells:

After the drilling operations are complete, if it is decided to develop the well as a strainer type well, strainers are to be located against the water bearing strata proposed to be tapped and plain pipes against other strata. The strainers and the plain pipes are assembled at the ground level in the same order as they are to be lowered into the bore and then they are serially numbered.

At the bottom end of the tubewell it is advisable to keep a small length of plain pipe (about 1.5 m long) with cap to close at the end. This cap is known as a bail plug and it has an ‘eye’ provided in it. This is useful for extracting the strainers if necessary by lowering a-hook to contact the eye in the bail plug and pulling up the whole strainer assembly.

The plain pipe provides the space for settlement of heavy particles during development of the well so that the bottom end of the strainer may not be choked up cutting out a useful portion of its length.

The strainers and the plain pipes are to be lowered with the help of the tripod and wire rope into the bore in the same order as fixed. The strainers and the plain pipes have threads on both their ends such that these can be screwed together.

At the top of each length of the pipe or strainer, a clamp is fixed and the other end is screwed to the length already lowered. Then the whole assembly is lowered till the clamp rests on the casing pipe.

The procedure is continued till the whole assembly is lowered inside the bore. The whole length of the assembled strainers and plain pipes are kept suspended and the casing pipes are jacked up and removed one by one. As the casing pipes are extracted the soil around them gets loosened and grips the strainer assembly.

Types of Strainers:

There are several types of strainers available for use in tubewells. Each type has its own advantages from view point of efficiency and cost.

Some of the common types of strainers are described below:

1. Esbee Strainers or Coir Strainers:

This is one of the common strainers used in tubewells particularly when the size of the tubewell is small (10 cm dia) and the watertable is very near the ground level. This strainer is made of coir, wound round a longitudinal steel frame.

The steel frame is made of mild steel rods (sometimes strips are also used) which are rivetted or welded to screwed pipe ends, so that each length of the strainer can be joined to the next. There are no separate openings and the whole surface of the coir acts as a screen allowing the water to flow but preventing the sand from entering the tubewell.

This strainer is the cheapest of all and effective but its life is short (10 to 15 years in general). The life of these strainers can be improved by painting the m.s. rods with alluminium paint.

2. Agricultural Strainers or Ashford Strainers:

The agricultural strainers are made of a perforated mild steel pipe over which a copper mesh is soldered. The perforations on the mild steel pipe are round and are made with a drilling machine.

The type of the copper mesh used depends upon the type of the sand strata. Mild steel hoops at regular intervals over the perforated pipe are welded taking care that these do not cover the perforations. The strainer sheet is then wrapped and soldered to these hoops. These types of strainers are reasonably priced and they have long life.

3. Cook-Tej Brass Slotted Strainers:

Among the strainers used in tubewells this type is the costliest because of the brass used for their construction. They, however, have the maximum life as this metal is not corroded by the water and as such these types of strainers are usually adopted for water supply projects. The slots on this strainer are about 2.5 cm in length and about 0.5 cm apart.

The width of the slots are to be determined depending upon the type of sand strata. The pattern of slots may be horizontal or vertical (Fig. 8.7) and may be in line or staggered. In all strainers the slots are tapering with the wider end inside with a view to prevent the sand from chocking the strainer.

4. Ashford Strainers:

In this type of the strainer a copper wire of trapezoidal section is wound round a perforated pipe. While winding the wider edge of the trapezoidal wire is kept on the outside and the narrow edge inside and the whole unit acts as the strainer preventing the sand particles entering the tube. No special advantages are claimed for this strainer.

5. Phoenix Strainers:

These are made of mild steel tubes with slots of desired width. Chromium plating is done to prevent the chemical action of salts. These are similar to Cook and Tej Strainers but are cheaper.

6. PVC Strainers:

Strainers made with PVC material are useful in case of aquifers with poor quality of water. Slot size in case of these strainers should carefully be selected.

7. Diameters of Strainers:

As regards the diameter of the strainer, the head losses reduce with increase in diameter. But larger diameters will involve additional cost. Thus the diameter of the strainer should be chosen in relation to the well discharge and the aquifer characteristics so that the overall cost of the well is a minimum.

Assuming appropriate values of cost of the strainers of various diameters, cost of drilling and running expenditure, and optimum diameter of the strainers have been obtained for various values of discharge and permeability of the aquifer and are given in Table 8.2. These could be used as a guide.

8. Selection of Size of Opening of Strainers:

The size of the slot openings in the strainers in general are selected to retain from 30 to 50 per cent of the aquifer material depending upon the aquifer conditions. The higher percentages are selected (i.e. small size slot openings) in case of fine uniform sands containing corrosive waters as corrosive waters tend to enlarge the slot openings with time. In coarse sand and gravel formations, the enlargement of the openings by a small fraction is not a problem and as such larger slot openings can be selected.

Construction of Cavity Type Tubewells:

The cavity type tubewell does not have a strainer and draws water from one stratum only. For development of the cavity, sand should be drawn out from the water-bearing stratum at the bottom of the clay roof. If it is known that cavity wells are successful in the particular areas, drilling can be started with the correct size of the pipes and they can be left in place to serve as the tube for the completed cavity well.

For constructing the cavity well, drilling is to be carried out in the same way as strainer type wells, but the bailer (or the sand pump) has to be operated slightly ahead of the cutting shoe of the casing pipe. Immediately after the bailer penetrates the sand layer in which the cavity is proposed to be formed, the load on the casing pipes should be removed so that no further sinking of the pipes takes place. The cutting shoe end of the casing pipe should be left at about 1 m above the bottom of the clay layer.

Now before the use of any mechanical equipment for the formation of the cavity, the sand pump should be repeatedly operated and as much sand as possible should be removed. More sand has to be removed to make the formation of the cavity complete and this can be done by mechanical equipment like a centrifugal pump or an air compressor.

Among both these equipment the centrifugal pump is preferable as the discharge can be easily controlled and the equipment is easily available. For the development of the cavity, the centrifugal pump is connected to the bore and in the beginning the discharge is kept low.

At this stage, sand will be drawn out for some time. When the water is clear, the rate of discharge should be increased. Consequently more sand will be drawn out.

The process is to be continued till the desired discharge is obtained free of sand. The pump may be stopped for a few hours and run again. Some more sand will come out with water. The cavity is thus developed gradually till a clear discharge of water is obtained even after stopping the pump for days together.

If an air compressor is used for developing the cavity, it should be used cautiously. The pressure developed by the compressor may be kept low so that the discharge is low in the beginning and the same is gradually increased. Care should be taken to see that the airline of the compressor does not disturb the subsoil excessively.

When the cavity is being developed, if it is observed that large amounts of the clay material are coming out with the water, it can be taken that the cavity has failed. In such a case the bore is to be abandoned or drilling can be continued and strainers installed. If strainers are proposed to be installed, in such circumstances the diameter of the tubewell becomes smaller than the originally planned one.

Construction of Gravel Packed (Shrouded) Wells:

At locations where the water bearing material is predominantly fine grained sand, the strainer to be put should have very fine openings to separate the fine sand from the water. The large number of fine Openings reduces the percentage area of the openings and consequently the yield of water. In such cases gravel packed wells are recommended for obtaining higher water yields.

The gravel packed well is an improvement over the strainer type of well. In this well, gravel packing is done around the strainer and thus gets a larger percolating cylinder of coarse material and thus gets a greater yield of water. Where natural gravel is available, this could be developed as gravel well. In most of the cases adding gravel from outside for shrouding has to be done. In these wells instead of using the strainers, it is advantageous to use the slotted pipe.

The slotted pipe is made out of wrought iron pipes, threaded at both the ends and necessary slots made on it. The slots can either be horizontal or vertical having dimensions of 2.5 cm x 5 cm with spacing of 1 cm to 1.5 cm (Fig. 8.7).

In making the slotted tube the following points are important:

1. Construction of rectangular slots is preferable over square or round slots as there are less chances of clogging with the rectangular slots.

2. The slots constructed should have a sharp outer side and an abruptly widened inner opening. This will facilitate easy movement of the sand grains and prevents the wedging and clogging of the opening.

3. The number of slots should be as many as possible consistent with the strength of the pipe.

The slotted pipes are lowered in the bore just like strainers. The bottom of the slotted pipe is fitted with plain pipe of about 1 m length with a bail plug. Before lowering the slotted pipe, the casing may be pulled up to the proposed bottom level of the slotted pipe and the bore hole is filled with sand or gravel up to this height.

Gravel is now poured from the top in the annular space between the slotted pipe and the casing till at least 3 m above the top of the slotted pipe is filled up. The casing tube is now lifted about 10 cm from the bottom and water is pumped out from the well till clear water is obtained.

The process of lifting the casing pipe, pumping the water till clear water comes out and back blowing is done till the casing tube is extracted. During the whole operation gravel is fed continuously into the annular space to keep up the required level.

It is important to keep the slotted pipe at the centre of the casing all throughout and for this purpose a wooden ring is attached at the bottom of the tube. The whole casing is extracted out without any jerks and taking care that the slotted pipe is not dragged out.

Gravel Pack Design:

The gravel used for shrouding should be selected depending upon the nature of the strata. The sieve analysis curve of the aquifer material is used for the design of the gravel pack.

The material of the aquifer is considered to be uniform if the uniformity coefficient Cu is equal or less than 2 and graded if Cu is greater than 2. Based upon laboratory and field studies several recommendations have been made in literature for the pack-aquifer ratio.

The Central Board of Irrigation and Power (1961) recommended the following criteria to ensure least sand movement and loss of head through the gravel pack:

The gradation of the gravel pack is obtained from the sieve analysis curve of the aquifer material. Using the upper and lower limits of the pack aquifer ratio, the size distribution curve for the gravel pack is drawn keeping a uniformity coefficient of about 2.

Example 1 illustrates this procedure-

Example 1:

Design a gravel pack for the aquifer material whose sieve analysis information is given in Fig 8.8.

Solution:

The sieve analysis curve is plotted on a semilog paper. Since the uniformity coefficient of the aquifer material is more than 2, it can be considered as graded. The d₅₀ is multiplied by 12 (selected P.A. ratio) and the point to the right is marked on the graph. Through this point, the size distribution curve of the gravel pack is drawn by trial and error such that the uniformity coefficient is around 2. The selected gravel should satisfy these criteria.

If 2,000 gms of gravel is analysed, it should have size distribution as indicated below:

Choice of Well Screen for Different Types of Strata:

For coarse and medium sand supplying reasonable quantities of water a strainer without any shrouding can be adopted. For very fine and uniform sand slotted pipes with shrouding may be used. If the strata consist of a mixture of course, fine sand and gravel the well may be developed as natural gravel packed well.

If the thickness of the water bearing stratum is small (say 5 to 6 m) but supplying good amount of water and at the same time there is no chance for developing a cavity, shrouded well will give more discharge than a simple strainer type well.

Construction of Open Wells:

Construction of open wells can be done either as dug wells or as sunk wells. The selection of the particular method for construction depends upon the nature of the subsoil formation.

Dug wells are adopted when the subsoil formations are hard and stable while excavation. The well is excavated, keeping the excavated diameter about 1 m more than the proposed diameter of the well and taking it to sufficient depth below the water table.

Where rock formations are encountered the rock is blasted either with explosives or are broken using rock breakers. After digging the well to the required depth, lining of the well may be done either with brick masonry or stone masonry.

The lining is raised about 1 m above the ground level to provide the well with a parapet. The space between the lining and the dug portion can be filled either with brickbats in mortar or compacted clayey soil.

Sunk wells are constructed in soft formations where the sides are likely to collapse if dug wells are tried. In the construction of the sunk wells, certain height of the lining is first constructed above the ground level and then it is sunk in the subsoil formation by putting load on the lining. The procedure is continued till the required depth of the well is attained.

For constructing the well, excavation is done about 1 m larger than the required diameter of the well and about 1 m deep. At this depth the well curb is laid. The well curb is a circular structure made of either wood, reinforced cement concrete or mild steel plates. The well curb is useful in sinking of the well and ultimately it remains at the bottom of the lining and serves as its foundation.

The inner diameter of the well curb should be the same as the inner diameter of the finished well. After laying the well curb, masonry wall is constructed on the same till the wall comes about 10 to 20 cm above the ground surface.

A temporary platform is now constructed on top of the lining and it is loaded either with sand filled gunny bags or other locally available material. On this platform sufficient opening is kept and through this opening the soil is removed. As the removal of the soil progresses, due to the weight placed, the lining sinks.

When the lining sinks up to the point the loaded platform comes down to the ground level, the load is removed and additional height of the platform is constructed. Again it is loaded and sinking is done. The process is repeated till the required depth of the well is reached.

The space between the lining and the soil is filled with compacted clayey soil. In order to keep the sinking vertical, it is necessary to suspend four plumb bobs from four sides of the well. Unequal sinking in the well can be corrected by suitably shifting the load on the loading platform.

Development of Tubewells:

After the drilling operations are complete and the pipes are installed, development of the tubewells has to be done in order to get the maximum discharge possible from the well. The process of development consists in removing the finer particles around the strainer or the gravel pack as the case may be so that the permeability of the formation through which water moves to the well is increased.

The purpose of the well development is also to agitate water in the formation so that sand bridging is prevented. Bridging refers to the tendency of fine sand grains to wedge against each other and form arches across the opening of voids between coarser grains.

Once bridging takes place so long as the water moves in one direction these particles do not move and cause reduction in the yield of the well. In order to dislodge the bridged particles, it is necessary to backwash or agitate the strata around the strainer.

The procedure for developing the well depends upon the type of the subsoil formation.

Except in case of rock formation, in ordinary sand or alluvial formations the following methods could be used:

1. Development by Surging:

In this method a surge block (a wooden block slightly less than the diameter of the well) is fixed at the end of a mild steel rod or pipe and is lowered into the well. A reciprocating motion is given to the block and consequently the water moves alternatively into the soil with force and comes out.

The finer fraction surrounding the strainer is drawn when the water is coming into the well and is subsequently pumped out. The surging method is very simple in operation and is effective in sand and gravel formations.

2. Development by Pumping:

In this method a pump of large capacity is used to withdraw water and sand from the well. The speed of the pump should be kept low initially. Pumping is started till all the sand particles are withdrawn at this speed and clear water comes out continuously.

The speed of the pump is now increased and more sand is withdrawn. The process is continued till the maximum discharge is reached and no further sand particles are withdrawn.

Development of the well by this method is not fully effective because the flow of water remains in one direction and thus encourages bridging of sand particles. In order to agitate the formation, sometimes the pump is started and stopped intermittently to produce relatively rapid changes in the head of the well.

3. Development with Air Compressor:

Development of the tubewell with an air compressor is convenient and effective and is suited for small diameter wells. There are two general methods viz. the surging method and the backwashing method. Sometimes a combination of these two methods is also used.

In the surging method, a large volume of air released in the water to produce a strong surge and consequently dislodge the finer particles. Air compressor mounted on trolleys and run by diesel engines are normally used for this purpose. A 5 cm diameter air pipe is lowered inside a 15 cm to 20 cm discharge pipe (also known as education pipe or drop pipe). The lower portion of the airline has perforations for about 2 m length and the end is sealed.

In the beginning the airline extends below the discharge line and is used to release a large volume of air. This causes a surge of water into the strata around the strainer and dislodges the sand, silt and clay particles.

The airline is raised into the discharge pipe and consequently an air jet pumping action is created causing the water to flow out. When the water coming out is free from sand, the end of the discharge pipe is raised and the above process is repeated till the entire length of the strainer is subjected to surging and pumping action.

The top of the well is fitted with an air tight cover. Airline and discharge pipes are installed in the same manner as in the previous method. In addition a short air pipe and a three way value are added. To start with, the water is pumped out using the compressed air.

When the water is free from sand, the pumping is stopped and water is allowed to return to normal level. Compressed air is now introduced through the short air pipe till the air starts escaping from the discharge pipe. At this stage air supply through the short pipe is stopped and it is directed through the long airline to pump the well. This process is repeated till the well is fully developed.

4. Removal of Drilling Mud:

If the well is a gravel packed well and is constructed by the direct circulation hydraulic rotary method, a thin layer of the drilling mud is deposited on the walls of the well and it lies between the natural formation and the artificially placed gravel.

To help the removal of the drilling mud suitable dispersing agents are used to prevent the tendency of the mud to stick to sand grains and disperse the clay particles, thus making their removal easier.

Sodium hexa-metaphosphate is one common chemical used for the purpose. Approximately, half a pound of chemical is added to every 100 gallons of water in the well and mixture is allowed to stand about an hour before starting the development.

Testing Yields of Tubewells:

After the well is fully developed, the yield of the well should be tested so that the water supply available from the well is known and the correct specifications of the pump to be used can be worked out. The well test consists of observing the static water level in the well, the yield and the depth of water when the well is pumped at different rates.

The static spring water level is the level of the water in the tubewell when no pumping is done. This level can be measured by means of a measuring tape or a suitable tubewell gauge.

For the purpose of the test, the well is first pumped for the maximum possible drawdown. The pumping rate is then reduced and the discharge is measured for lower depths of drawdown. With this data a graph is drawn showing the relation between the drawdown and discharge.

The graph will be useful in selecting the pump having suitable characteristic to suit the well. The air compressor that is used for well development can be conveniently used for testing of the well also.

Vertically of Tubewells:

If vertical turbine pumps are proposed to be installed in tubewells, it should be ensured that the tubewell is vertical by means of a verticality test. Crookedness of the well will result in misalignment of the line shaft assembly which in turn will result in excessive wear and tear of the shaft bearings (Fig. 8.11). It results in higher power consumption and also requires higher starting currents.

The equipment to check the verticality of a tubewell consists of:

(1) A tripod stand having at the top a metallic disc with a small rotating, pulley at the centre,

(2) A plumb of about 40 to 45 cm long with diameter 0.5 cm smaller than the inside diameter, and

(3) A reel of flexible steel wire.

To test the verticality of the bore, the centre of the pipe at the top is established. The tripod is adjusted over the pipe such that the wire rope from the pulley passes through the centre of the bore. The plumb which is tied to the end of the wire of gradually lowered and deflections from the centre are noted at regular intervals for their value and direction.

The correct deviation at the depth CL is obtained by adding to LE the difference of the diameter of the plumb and the bore. In case the well is very crooked, the wire will swing over until it touches the casing. If this happens, the verticality test should be repeated with a plumb of a suitable smaller diameter so that the wire may not touch the casing.

Usually such a well may not be suitable for a vertical turbine pump. A submersible pump will be suitable for bores which are not within acceptable limits of verticality for turbine pumps.

Incrustation and Corrosion in Tubewells:

After the well has been in use, its discharge might get reduced due to:

(1) Incrustation,

(2) Corrosion,

(3) Mechanical wear or failure

(4) Mutual, interference of wells, or

(5) Declining groundwater levels.

In some cases the wells might completely stop giving any water. Incrustation and corrosion are two important problems in tubewells. Incrustation can fill slot openings and results in reduced transmitting capacity. Even a small amount of corrosion can result in excessive sand production.

Incrustation on the strainers is caused due to the precipitation of dissolved minerals in groundwater. Where groundwater has such dissolved minerals incrustations could occur decreasing the well capacity. Incrustation includes calcium and depositions of clay and silt. Incrustations, particularly scale can be dissolved by treating with an inhibited acid.

Hydrochloric acid is commonly used. An inhibitor is used with the acid to prevent acid attack on the well structure. Bacterial growth and associated slime can be effectively treated by chlorination.

Chloride kills bacteria, and oxidizes slime so that it can be removed from the well. Polyphosphates are chemicals used to disperse the iron and manganese compounds and also silt particles.

In each case the nature of the problem is to be studied to decide about the treatment. After the treatment the water in the well should be vigorously agitated. The dispersed material should be pumped or bailed from the well.

Corrosion usually consists of a chemical reaction in which metal is attacked by constituents in the water, forming a chemical product that dissolves into solution and is carried off. Corrosion may result in general rusting, localized pitting, enlargement of screen slots or perforations, bimetallic corrosion at the junction of dissimilar metals (between a screen and casing) and accelerated wear or failure of pump parts.

Corrosion in wells is increased by the amount of dissolved gases in water, such as carbon dioxide, oxygen and hydrogen sulfide. High total dissolved solids can also increase corrosion because of higher electrical conductivity in the water and corresponding capability to carry metal ions into solution.

Careful examination of the groundwater and experience with wells in the area indicate whether problems of incrustation and corrosion are likely to occur. A corrosion resistant alloy is advisable for screen material in areas with incrusting or corrosive water.

Installation of such screens with the control of corrosion of slot openings and in the event of incrustation acid treatment for the well rehabilitation can be undertaken.

Commonly used materials include stainless steel, copper bearing steel, low alloy steel and brass. Painting of the strainer with a corrosion resistant paint is helpful in controlling corrosion.

In case of coir strainer, the life of the strainer is increased when the metal rods are painted with a suitable paint. P.V.C. casings and screens are very useful for corrosive water, When P.V.C. casings and screens are used necessary precautions about their structural strength are to be taken.