What is Borehole Drilling?
Drilling has over the years developed to a highly specialized and technical activity. Drilling with jumper rigs as in the olden days rarely takes place and most of the boreholes drilled are drilled with the air percussion technique. This technique is fast and efficient. However, when drilling a borehole for groundwater abstraction purposes, drilling becomes much more than just making a hole in the ground.
For many people drilling is only the activity above the ground that they can see and observe. A water borehole is however a specially engineered hole in the ground, making provision for water to flow into this hole and allowing for a pump to be installed inside the hole to allow abstraction of water. Part of this engineered design is to prevent the surrounding geological formations to collapse and closing the borehole. This could mean cost and energy to re-drill or clean the hole to be able to use it again. The task of the Driller is to drill and construct a borehole, not to get water. This task should be left to the scientist dealing with the nature and study of the geological formations. None can however operate in isolation and it important to see the actions of the scientist and driller as a team effort.
There are very few people that take time to consider how the drillers manage to keep a vertical borehole, which went through sand and hard rock, open until the casing is installed. How does one manage to remove the drilled pieces of rock from deep below the surface? Skill is needed to guide and control a water well drill as it penetrates sand, gravel, clay and solid rock formations underground. The drill rods are quite heavy and can weigh several tons; if the drill pushes too hard or turns too fast, the drill bit will wear out too fast; if it does not push hard enough, it won’t penetrate. Quite often several rock layers are encountered in a single borehole, each requiring different drilling pressures and speeds. When water is encountered the drilling becomes even more complicated and the driller needs to understand exactly what forces need to deal with to further penetrate.
There are a number of aspects that needs to be taken into consideration when testing a borehole. According to Prof van Tonder at the Institute for Groundwater Studies (Free State University) there are two important rules that needs to be kept in mind when determining the sustainable yield I.e.: The total abstraction from a borehole should be less than the natural groundwater recharge, and secondly, a borehole should be pumped in such a manner that the water level never reaches the position of the main water strike (normally associated with a fracture). Should this happen the yield will inevitably be affected and the borehole would eventually dry up.
Specific information is required to properly test a borehole, these can be listed as follows: what is the rest water level before the start of the test, how does the water level change over time once pumping has started, how long does it take for the water level to recover after the pump has been stopped to recover to the original level – or how far does the water level recover after the same amount of time allowed as for pumping – leaving a residual drawdown.
Test pumping of boreholes is normally carried out to meet two main objectives:
To establish borehole potential. To estimate the sustainable yield and hydraulic erformance of individual boreholes for water supplies.
To establish aquifer potential. To assess the hydraulic characteristics of the aquifer.