There have been many questions raised in many forums regarding borewell performance, monitoring and maintenance/rehabilitation.
This post shall be dedicated to discussions on these aspects. Here I have discussed the performance aspect of borewells.
Processes Affecting Performance
1 Physical Process
drilling fluid invasion damage at the time of construction
inter-mixing of aquifer horizons as a result of wash-out and caving during drilling and development
inter-mixing of aquifer and gravel pack material due to poor installation procedure or overaggressive development
migration of fines from the formation into the gravel pack material due to high flows during operation
migration of aquifer material and infilling of the borehole and pumping system
high velocity particle laden water
2 Chemical Process
coupling of dissimilar metals
impurities in the composition of metals
degree of surface protection
exposure to different conditions
differrence in metal treatment
3 Microbial Process
Microbially induced corrosion
sulphate reducing bacteria
4 Operational Factors
Inappropriate operating schedules
high pumping rate
Aquifer over abstraction
5 Structural and Mechanical Factors
Poor design and construction
poor installation of screen and casing
inappropriate use and placement of grouting materials
poor selection and installation of gravel pack
poor drilling technique
Inappropriate selection of borehole component materials
use of mild steel materials
sand pumping due to enlarged screen
pump failure due to corrosion
clogging by increased iron concentration and hence iron fouling
Hydraulic design is very important for sustainable borewell performance. It decides the screen enterance velocity and the vertical velocity inside the borewell. Commonly recommended value for enterance velocity is 0.03 m/s.
The velocity can be calculated using the following formula.
Ve - Enterance velocity
Q - Discharge in m/s
D - Diameter of screen in m.
L - Net length of screen in m.
p - Proportion of open area in screen
b - blocking factor (take a value of 0.5)
It is recommended to take a design value of 0.02 m/s for borewells with problems of incrustation and biofouling. In fissured aquifers the velocity need to be calculated emperically. (compare with existing borewells in same formation; carry out a trial and error to reduce sand ingress)
Reducing enterance velocity / design discharge have many benefits.
Sand ingress is reduced
Erosion of screen slots are reduced
Supply of nutrients are reduced
Loss of kinetic energy is reduced
Hello, I'd like to seek your guidance on th geological aspects on the design of Borewell on an artificial island. For de watering an area for construction, I'd like to use borewell pumps adjacent to the said site which has a water column height of 9.5 m from the bottom and short of 2. 5 meters from above (the ground level). The area is 4 m in length and 6 meter in breadth. The total volume of water at any point of time is presumably 228 m3 (4x6x9.5). I was thinking of putting 4 borewell pumps of 6 "casing size with 29 m3/hr and suitable head, around the periphery of the site. I'd appreciate your guidance on any precaution to be taken for the job.