Construction of Open wells & Types of Tube wells

Ground water:

It is the part of subsurface water which occur within the saturated zone of earth's crust, where all openings are filled with water. It is that part of the subsurface water which can be lifted or which flows naturally to the earth surface.

Occurence of ground water depends upon two factors:

1. Porosity of rocks: it is the percentage of void present in a given volume of soil

2. Permeability: it is the ability of a rock or unconsolidated sediment to transmit or pass water through it.

Transmissibility: it represents the same physical meanings of permeability.

Water table: it can be said as the upper limit of saturated zone.

Aquifer:

A saturated geological formation capable of yielding water economically insufficient quantity is known as aquifer or water bearing strata or G. W reservoir.

1. Unconfined aquifer: when the water table is under atmospheric pressure and is free to rise or fall. Depending upon the availibility of water.
2. Confined aquifer: artisan or pressure aquifer: it is separated from unsaturated zone by impermeable foundation.
   

The ground water is under pressure and water level in the well indicates the piezometric pressure at that point.

Availibility, movement and quality of ground water depends upon characteristics of soil:

1. Porosity
2. specific yield
3. coefficient of permeability
4. transmissibility

Specific Yield:

It is the ratio of volume of water drained by gravity to its own volume

It indicates the yielding capacity of unconfined aquifer.

Specific retention:

It is the amount of water (volume) retained against the force of gravity to its own volume.

Coefficient of permeability:

K of a medium is measured in terms of hydraulic conductivity which is equal to the volume of water which flows in unit time through unit cross sectional area of medium under unit hydraulic gradient at the prevailing temperature units m/d or m/hr

Transmissibility (K)

b = aquifer thickness

T = Kb

T is the amount of water which flows through unit width of saturated aquifer under unit h.g

Darcy's law

Q = KIA or V = KI

i = H G

Wells

A water well is a hole usually vertical excavated in the earth for bringing Gw to surface

1. Open wells
2. Tube wells

Types of tube wells

1. Strainer type
2. Cavity type
3. Slotted type

Design of strainer or well screen

In design, find its length, slot size, opening area, diameter and material requirements

1. Corrossion resistant
2. Strong enough to prevent collapse
3. Prevent excessive movement of sand into well
4. Minimum resistance to flow of water into the well

Materials

1. Zinc free brass
2. Stainless steel
3. Low carbon steel
4. High copper alloy