In an aquifer is of slits and sands, the bore well is generally provided with a filter pack. However if an aquifer has gravelly sands, gravel grains of size to which 10 percent of the formation material is fine or sandy gravel the gravel pack is not provided with and such wells can be naturally developed to create graded zone at the screen in the intake zones. Bore wells are designed for a rate of discharged of over 25 percent than the required rate of discharge. This is done to ensure that discharge is achieved even under conditions of continuous pumping.
The annular space between the bore well and the casing pipe need to follow the specified regulations. The filter pack material and sealing grout are generally placed around the screen by specially engineered methods. This is achieved by strategically placing a rigid pipe of 1 inch diameter on the maximum grain size of the filtering material.
Slotted pipes can be used as screens in case of fine grained aquifers but generally a continuous wire wound screen is preferred for all types of aquifers. The total surface area of the screen is designed to give entrance velocity less than a few centimeters and the local regulations for all these factors are strictly adhered to.
The slot size is selected based on factors such as bore wells with a filter pack have a slot size that helps in retaining a majority (90%) of the pack material. However, bore wells that are naturally developed without any filter pack have a slot size that will facilitate about 40% of the aquifer material to freely pass through.
Filter packs should consist of well-rounded particles that have a uniform coefficient which is less than 2.5. The gravel and sand that is used in the filter pack should be free from foreign matters, properly washed and disinfected before use. The filter pack should also extend above the screen at least for a distance of one to two meters and should account for setting and any loss during the development to prevent the filter pack which is around the screen from being fouled.
Casing pipe is generally made of mild steel that should meet the government development regulations. However, in areas of high corrosion or place that is prone to drastic contamination. Casing pipes that are made out of PVC can be a good option to explore. The intake zone is also selected in such a way that it covers a continuous aquifer that is of the same type as formation. The practice of providing housing pipes of a much larger diameter as compared to the casing pipe is not practiced anymore. Modern submersible pumps do not have the need of large housing pipes.
The grain size distribution is can also be measures by the field method of classification by placing a sample of the formation mixed with water in a calibrated cylindrical jar of glass. This jar is the shook well and the mixture is allowed to settle down so that the finest particles are at top and the coarsest particles settle down at the bottom.