Test: Tube Well- 2 - Civil Engineering (CE) MCQ

A tube well is a type of water well in which a long, 100–200 millimetres (3.9–7.9 in)-wide, stainless steel tube or pipe is bored into an underground aquifer. The lower end is fitted with a strainer, and a pump lifts water for irrigation.
Advantages of Tube well irrigation

• Tube well can be constructed quickly
• The quality of water is good
• Land acquisition is less as compared to canal
• Tube well can be privately owned but canal is not privately owned
• Tube well water is much costller to canal water
• Tube well require less space
• Tube well give fairly sustained yield even in dry years.

Classification of tube wells:
(a) Strainer type tube well:
The strainer type tube well is generally unsuitable for very fine sandy strata, because in that case the size of mesh opening will have to be reduced considerably which may result in choking of the strainer, and if the screen openings are kept bigger, the well will start discharging sandy water.
(b) Cavity type tube well
If a suitable strong clay stratum is not available then cavity type tube well cannot be provided because it does not have strainers and hence, it draws its supplies from the bottom and not from the sides.
(c) Slotted type tube well
In this type of tube well, the gravel and coarse sand shrouding prevents the fine particles from entering the well pipe.  If sufficient depth of water bearing stratum is not available, then it is not possible to provide strainer or cavity type tube wells, hence, slotted type tube wells provided in that case. 

Important Points
Although the maximum discharge of a deep tube well exceeds 200 liters/sec but can never reach 500 liters/second.
Discharge through tube well is much more than 5 liters/second and thus the most appropriate answer is 50 liters/second.
Note: The discharge of an open well is limited to 3 to 6 liters/second. 

Strainer type tubewell is a bored hole in which a metal pipe with suitable perforations is inserted. A strainer type tubewell is suitable when water-bearing stratum is available at less depth and there is a possibility of tapping water through sides. It is generally unsuitable for fine sandy strata.

A strainer type tubewell is suitable when water-bearing stratum is available at less depth and there is a possibility of tapping water through sides. The slotted type tubewell is intermediate to the strainer and cavity type tubewell and is used where both the above cannot be used.

Effective diameter is an index of the measure of the fineness of an aquifer. It is used for designing various intake components of a tubewell. For permeability D10 (i.e. 90% grains retained on the sieve) is taken as the effective size.

The Central Board of Irrigation and Power suggested the following criteria for PA ratio.
i. For uniform aquifers (having C_u < or = 2.0), the PA ratio should be in between 9 – 15.5
ii. For graded aquifers (having C_u > 2.0), the PA ratio should be in between 12 – 15.5.

Jet pumps have low efficiency of the order of 35% as compared to 65-85% for other centrifugal pumps. It is not used for irrigation tube-wells.

In rocky areas, down the hole hammer rigs (DTH rigs) are used to drill isolated holes of 10 to 15 cm in diameter. They are usually in the depth range of 100 m and mostly been constructed in southern states of India. This type of wells usually depends on joints and fissures in the rocks for their water supply.

Percussion method is suitable for soft and fissured rock formations. The rotary boring method can be successfully used for rock as well as unconsolidated formations. The reverse rotary method is used for soft unconsolidated formations made of sand, silt or soft clay.

Tube well irrigation is very suitable where the subsoil formation is suitable for storing water. The general average yield from the standard deep tube-wells is of the order 40 to 50 liters/sec. Shallow tube-wells may yield as high as 15-20 liters/sec if located at proper places.

In cavity-type tubewell, water contribution to the bored hole takes place through the bottom layer only as is done by screen well. The flow in a cavity well is essentially spherical. In strainer type tubewell, water enters into the well from the sides and the flow is radial.

The gravel pack is usually designed on the basis of Pack Aquifer (PA) ratio. It is usually defined as the ratio of D₅₀ size of the gravel pack material to the D₅₀ size of the aquifer material. It should be designed before designing the size of the slots to be made in the well-screens.

The uniformity coefficient was proposed by Hazen. The value of the coefficient of the gravel material is preferably kept to be equal to or less than 2.5. A higher value may cause segregation of the gravel during pouring which will result in poor efficiency of the well.

In India most of the land especially the Indo-Gangetic plains, Narmada valley consists of deep alluvial soils. The sub-soil water slowly penetrates and is stored in porous sand and gravel beds which are extensively found in India. Tube wells can be easily installed in such lands and are useful for irrigation.

Percussion method of drilling the well hole is also known as the cable method of drilling. It is ineffective in loose material because of slumping and caving of the material around the drilling bit. This method is useful for cutting consolidated rocks from soft clay to hardest rocks.

Hydraulic rotary method can handle hard and soft foundations with ease and the danger of accidents is lesser. The rate of drilling by this method for consolidated rock formation may vary from 10 to 15 m per day and in unconsolidated formations 100 to 150 m per day.