There are general 2 main types of bore wells that are distinguished by the diameter of the bore hole. They are commonly known as bore well and bore hole. Bore wells are constructed when low yielding ground water sources are found close to the surface, that is, usually lesser than 30 meters. They are also constructed by perforating the casing or even by using a large sand screen with continuous sot openings. One of the biggest advantage of using bore wells is that they have a large diameter of the casing which helps in providing water during the peak seasons and reduce the chances of water crisis in that area.
A major disadvantage of using a shallow ground water aquifer and storage may occur after long and dry periods during summer. Thus these kinds of wells are more prone to surface level contamination.
Drilled wells refer to wells that are of smaller diameter and are constructed at much deeper levels as compared to bore wells. They often go as deep as several hundred meters. The aquifers of drilled wells are not as susceptible to pollution as compared to bore wells as they are very deep and surface contamination cannot reach it. The amount of water supply also tends to be more reliable as it is not based on the seasonal changes.
During the stage of test hole drilling it is important to prepare a lithological or formation log. The soil and rock samples from various depths are taken and properly recorded for further analysis. This helps in identifying all the zones with the best water potential, where the well is ended to the bottom of an aquifer. Thus the highest possible water output can be expected from the well and the aquifer is completely utilized.
The casing of the well should be large enough to house a pump and allow for sufficient clearance during the installation and operation process. It is sufficient for the casing to be of nominal size larger that the external diameter of the pump. Casing is prone to corrosion and hence the development of plastic casings is becoming very common. However, the plastic casing that is used should be made of virgin resin and not any recycled materials. Whichever casing is used should be new and not contaminated from previous use.
Water generally moves from the aquifer to the well either through a screen o r a slotted perforated casing. The screens are manufactured with properly shaped and sized openings and are designed in such a way that they are strong and can withstand corrosive water. They also allow for the maximum amount of water with the entry of minimum amount of sediments. The can also be manufactured with various slot sizes and shapes that allows for specific well designs and aquifer requirements.
Cuttings from the bore well should be analyzed in depth to understand if a screen should be used or a perforated casing. A good screen should allow smooth flow of water and hold back sediment formation.