Tube Well- 1 - Free MCQ Practice Test with solutions, GATE Civil Engineering

There two sources form which water can be extracted:

• Surface water: It involves water from rivers, runoff, rainfall etc. This can be collected by making structures such as dam, reservoirs, canals etc.
• Sub-surface water: The water available below the ground is called sub-surface water. it can be extracted by constructing either tube wells or open wells.

Runoff occurs when there is more water than land can absorb. The excess water flows across the surface of the land and into nearby streams, rivers or ponds are called runoff.
The water supplied by local municipalities at our homes for domestic needs is called potable water.

Abyssinian tube well is the most common type of tubewell used for household purposes. It is also a strainer type of the tube well and the length of the strainer used here is about 2 meters.

Coir rope strainers are low-cost strainers and are generally used for shallow irrigation tube wells. The main disadvantage of such a strainer is its short life of 3 to 5 years. The rusting of the iron bars of the supporting frame and the loosening of the coir rope which expands on wetting are the causes of its failure.

Development of well by high velocity jetting is one of the most effective methods. In this method, the fine particles are washed out of the aquifer due to the shooting of high-velocity water stream through the screen openings. These fines get back into the well due to the turbulence created by the jet. The entire surface of the screen can be covered by gradual raising, lowering and slowly rotating the jetting tool.

The deposition of alkali salts on the inside walls of the pipe is called incrustation. It reduces the effective diameter of the well pipe and hence reduces the discharge of the tubewell. It cannot be completely eliminated.

A centrifugal pump works effectively under the maximum suction head of 6 to 8 m. Such pumps can be used at places where the fluctuations in water table plus the depression head are limited to a maximum of 8 m. The borehole type pump is used for larger values.

Resistivity surveys are based on the fact that water increases the conductivity of the rocks and thereby reducing their resistivity. Resistivity logs indicate the values of electrical resistivity of the rocks at different depths.

Airlift pumps are useful in removing objectionable gas, highly corrosive or abrasive water that damages parts of the pump submerged in the well. It is also used for testing or development of wells but is rarely used due to low efficiency.

In well irrigation water is lifted intermittently. The duty of a well is the area of land in hectares the well can irrigate annually and fully to bring a crop to maturity. It cannot be expressed in hectares/m³/sec for a base period of a crop.

Rewhiding involves starting and stopping of pumping intermittently. It provides a relatively rapid change in the head of the well and can be done with any type of pump. However, the turbine pump installed with a foot valve is the most effective.

A normal tubewell lasts for about 15 to 20 years in Northern India. It may fail due to incrustation and corrosion.

Due to the action of acidic water on the pipe, it is gradually destroyed by corrosion. The aquifer sand surrounding the well pipe comes out through the worn-out pipe walls bringing sand along with water. It results in the excessive withdrawal of sand along with the well water.

Delta in well irrigation is the total depth of water required to mature a crop in meters or centimetres. The delta of well water is about 66% of canal water because conveyance losses are minimum and well water is used at the right time in the right quantity.

The basic principle in well development is to cause reversals of flow that will rearrange the aquifer particles through the screen openings. This breaks down the bridging of the groups of particles. When the flow is in one direction small particles bridge between large particles across the screen openings.

Resistivity logs indicate the values of the electrical resistivity of rocks at different depths. Sonic logs indicate the value of the velocity of compressional waves at different depths. Radiometric logs indicate the variations of radioactivity with depth.