Tubewell
The discharge rate from an open well is typically limited to 3 to 6 litres per second. Mechanical pumping of small discharges from open wells is not cost effective. Tubewells are those types of wells which are used to achieve high discharge rates. A tubewell is a long pipe or tube bored or drilled deep into the ground to intercept one or more water-bearing strata. Larger discharges in tubewells can be obtained by increasing the velocity as well as the cross sectional area of the water bearing stratum. Although tubewells are commonly used for irrigation, they provide an excellent method of supplying water. The tubewells can be classified according to the type of soil on which they are constructed. The tubewells for the alluvial soils and for the hard rocky solis are discussed below:-
Tubewells for Alluvial Soil
The majority of our land, particularly the area from the Himalayas to the Vindhya mountains (such as the Indo-Gangetic plain), coastal areas, the Narmada valley, and so on, is made up of deep alluvial soils. Except in desert areas, subsoil water slowly penetrates and is stored in porous sand and gravel beds that are widespread in India. Tubewells, which are very useful for irrigation, can be easily installed in such soils. In this context, tubewells are becoming increasingly important for tapping our ground water resources, particularly in alluviums. The tubewells for the alluvial soils can be categorised as shallow tube wells and deep tubewells.
Shallow tube well
These types of wells have a depth of 20 to 70 metres and taps only one aquifer. These wells are typically built by private individual cultivators. If properly placed, such wells can produce as much as 15-20 litres per second. Each well irrigates approximately 8 hectares.
Deep tube well
These types of wells can reach depths of up to 300 metres and tap into more than one aquifer. In our country, they are typically built by state governments and are known as State Tubewells. Such wells can produce up to 200 to 220 litres per second. The average yield of standard tubewells is in the range of 40 to 45 litres/second.
It should be noted that the name given to shallow wells and deep wells is not related to their depths and is purely technical.
Fig.3: Shallow Tubewell and Deep Tubewell
Tubewells For Hard Rocky Soils
A tubewell irrigation system is extremely difficult to build in rocky areas. As a result, in rocky areas, tubewells are used only when no other water sources are available. Thus, only isolated holes of 10 to 15 cm diameter are drilled in rocky areas using down the hole hammer rigs (DTH rigs). They are typically 100m deep, but tubewells up to 300m deep have been successfully bored. Such tubewells are called bored wells because the bore hole is able to stand on its own in the bottom portion, and a tube is pushed only in the upper weathered zone. These wells typically get their water from joints and fissures in the rocks. Even with a heavy drawdown of 20 to 30 m, such wells typically cannot produce more than 5-10 litres/sec, unless they tap into an embedded aquifer. These tubewells have mostly been built in our country's southern states.
Types of Tubewells
Depending upon the entry of the water through a cavity or a screen, the tubewells can be broadly classified into the following categories:-
- Cavity-type tubewell.
- Screen-type tubewell.
- Slotted-type tubewell.
Cavity-type Tubewell
These types of wells draw water from the bottom of the well rather than the sides, as screen wells do. Flow in a cavity well is spherical rather than radial. Furthermore, gravity wells draw water from the bottom, so they can only access one water-bearing stratum. However, such a tubewell is very cost-effective because it only requires a plain well pipe, which is lowered into the bore made through the ground strata up to the required depth in order to tap the requisite aquifer. The working principle of these types of wells is essentially the same as that of a deep open well, with the exception that a deep open well must tap the first aquifer just below the 'mota' layer.
In contrast, a cavity-type tubewell does not have to do so and may even tap the lower or even lower stratum. A cavity-type tubewell is made up of a pipe that is bored through the soil and rests on the bottom of a strong clay layer. At the bottom, a cavity forms, and water from the aquifer enters the well pipe through this cavity. During the initial stage of pumping, fine sand is discharged with the water, forming a hollow or cavity. As the spherical area of the cavity expands outwards, the radial critical velocity for the same discharge decreases, lowering the flow velocity and, as a result, stopping sand entry. As a result, the flow is sandy at first but becomes clean as time passes. The cavity of such a tubewell should be developed carefully using a centrifugal pump rather than a compressor or a turbine pump.
Fig.4: Cavity Type Tubewell
Screen-type Tubewells
These types of wells are the most widely used and extensively built in our country, particularly for irrigation. Screen-type tube wells can easily tap a number of aquifers and thus do not rely solely on one aquifer, as a cavity well does. These types of wells can further be classified into two categories:-
Strainer-type tube well- In these types of wells, strainer lengths are lowered into the bore hole and placed opposite the water-bearing formations, while plain pipe lengths are placed opposite to the non-water-bearing formations. At the bottom, there is a bail plug. Water enters the well from the sides through these strainers, and the flow is radial. A strainer is basically a perforated or slotted pipe with a wire mesh wrapped around it and a small annular space between the two. Sand particles are kept out of the well by the wire screen. As a result, water enters the pipe through a fine mesh, and particles larger than the mesh's size are prevented from entering the well. This reduces the risk of sand removal, allowing for higher flow velocities. These types of wells are generally unsuitable for fine sandy strata because the size of the mesh openings must be significantly reduced in that case, which may result in strainer choking. If the mesh openings are larger, the well will begin discharging sand. Some of the types of strainers which can be preferred for these types of wells are-
Cook strainer- It is made from a solid drawn brass tube with wedge-shaped horizontal slots. The slots are cut from the inside of the tube using a slot-cutting machine. The slots are larger on the inside and narrower on the outside. The gauge of the slots varies from 0.15 to 0.4 mm depending on the coarseness of the surrounding soil. This relatively expensive strainer is widely used in the United States and other Western countries.
Tej Strainers- It is similar to the Cook strainer, but it is made of a brass tube bent round to form the tube, with its vertical joint brazed. Before the sheet is bent, the slots are cut. This strainer is typically manufactured in sizes ranging from 75 mm to 2.5 m in length and can be joined together using brass screwed collars. Tej strainers are made in India and are widely used in the country.
Phoenix Strainer- It is made of a mild steel tube with slots cut from the inside by a slot cutting machine. The tube is cadmium plated to prevent choking from corrosion which can be caused by various chemical actions.
Ashford Strainer- It is made up of a perforated tube with a wire wound around it and a wire mesh soldered over it. The mesh is kept away from the tube by the wire. A wire net surrounds the wire mesh, protecting and strengthening it. It is a delicate strainer that must be handled with care.
Leggett Strainer- This strainer includes a cleaning device in the form of cutters. The cutters can be controlled from the ground. When the cutters are turned in, the slits are cleaned.
Layne and Bowler Strainer- A wedge-shaped steel wire is wound to a suitable pitch around a slotted or perforated steel or wrought iron pipe. Screwed collars hold the strainer pipes together. This strainer is very strong and is commonly used in oil wells.
Brownlie Strainer- It is made up of a polygonal convoluted steel plate tube with perforations and a copper wire mesh wrapped around it. The mesh is made up of heavy parallel copper wires that are woven with copper ribbons. This is the best strainer because it keeps the wire mesh (or straining material) away from the perforated tube. The convolutions are straight-sided, and the tube is shaped like a star.
Fig.5: Strainer Type Tubewell
Slotted-type Tubewell
These types of wells are also called slotted pipe gravel pack tubewell, which use slotted pipes covered by any wire mesh. As with strainers in a strainer tubewell, such slotted pipe lengths are located opposite the water-bearing formations. These slotted pipes are also known as screen pipes, slotted pipe strainers, or simply pipe strainers. Following the installation of the plain and slotted pipes in the bore hole, a mixture of gravel and bajri is poured into the bore hole between the well pipe assembly and the casing pipe, so that the well pipes are surrounded by a designed optimum thickness of gravel pack. Gravel packing is required around the screen pipes. But because the material is poured from the ground level into the bore hole, the pack must be installed throughout the depth of the well below the shallowest screen's top level. To achieve a stable and efficient tubewell, the gravel pack is sometimes provided even above the top level of the shallowest screen and up to the ground level.
Fig.6: Slotted Type Tubewell
