In this article we will discuss about the design of open wells with the help of suitable diagrams.
‘Draw Wells’ or Open Wells are shallow wells which are usually confined to soft ground, sand and gravel. The diameter of these wells may be between 1 m to 4 m and depth may be up to 20 metres depending on the requirement and geological structure of the earth.
These wells are suitable for small discharge of about 20 cu.m/hour. The walls of these wells may be constructed with precast R.C.C. blocks, bricks or stone masonry. The thickness of the staining of the well mainly depends on the depth of the well and varies from 50 cm to 75 cm.
For the construction of such wells first of all a well curb of steel, wood or R.C.C. is constructed at the site over which masonry work is done up to about 1.5 metre height. Now the earth from the inside of the curb is excavated by means of pick-axes and shovel. Due to self-weight the masonry wall will sink gradually.
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More and more masonry work will be done as the excavation proceeds till the well is sinked upto required depth. After complete sinking parapet wall will be constructed over the well, and a platform with outward slope will also be constructed around the well. Sometimes roofs on pillars are also constructed over the well to project it from contamination.
Dug wells are very cheap in construction, therefore these are very popular in rural areas and small towns due to their large diameter, these wells act as a small storage reservoir also and can meet all types of small demands. These wells should be disinfected frequently to avoid the risk of contamination, because these wells generally are in poor sanitary conditions.
As the open wells are constructed up to limited depth, their yield is also limited, if the ground water storage is also limited. The quantity of water which can be withdrawn from these wells also depends on the critical velocity for the soil. Water cannot be withdrawn at higher rate, as it will lead to the disturbance of the soil particles which may further lead to the sinking of the well staining.
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The yield of the open wells can be increased by providing a 8-10 cm diameter bore-hole in the centre of the well, which will tap more quantity of water from the aquifer below the hard stratum as shown in Fig. 4.9 (b). If the well curb of the existing open-well is resting on soft stratum, and the mota stratum is not at greater distance, it will be better to sink the well further as shown in Fig. 4.9(a).
Mota stratum of layer is the name given to the layer of clay, kankar, cemented layer and other hard materials layer which is commonly available few metres below the first water table in the sub-soil. The mota layer can safely take the load of the moderate depth well staining. But it cannot support the staining of deep wells. These layers are generally available at different depths in different thicknesses in the ground.
The main difference between the shallow well and deep that shallow well draws its water only from the top most water bearing stratum, whereas deep well draws its water from more than one water bearing stratum. Their names do not depend on the depth of the wells, as shallow wells may be deeper than the ‘deeper wells’ which depends on the geological formation of the ground sub soil.