Article Courtesy : Ground Water India
It is common practice to observe that only DTH rigs are operating in Hard rock areas, Direct Rotary drilling machines in unconsolidated or alluvial formations and Percussion drilling machines in the areas where boulders, gravel and karst topography(limestone solution cavities) are prevalent. Percussion drilling also can be taken up in consolidated and unconsolidated formations but the drilling rate is slow and financially not viable compared to DTH and Direct rotary drilling methods. But the problem arises when alluvial formations or unconsolidated formations encountered in predominantly dominated Hard rock areas.Similarly,several problems occur when the local boring methods are being used instead of Direct Rotary drilling method in unconsolidated formations,particularly when the depth of Tubewells increases from 25 feet to more than 100 feet due to increase in utility of groundwater.
Common problems in Hard Rock Areas
1.Problems due to occurance of unconsolidated formations:
The alluvial formations occur in hard areas in patches due to buried river channels,along stream courses,foodplains, palaeo-channels etc or lake beds which are enchroached for building construction.As only DTH rigs are available in hard rock areas,(as Direct Rotary drilling or local hand boringmethods are not available )drilling for borewells in this area taken up by DTH rigs have to be abruptly stopped due to collapse of unconsolidated formations and as such drilling could not be proceed beyond 12 to 15 metres. And the owners of the buildings who have no other option except to satisfy with these shallow borewells having very limited quantity of water.But ater few years, the residents of the building face water scarcity as the shallow borewell goes dry. Direct Rotary drilling rigs or combination Rigs(combination of DTH and Rotary drilling) are not available in these areas to take up drilling under such conditions.
2. Problems due to occurance of Limestone Karst topography:
In Singanamala mandal of Anantpur district, drilling of borewell came to an abrupt end at
100 feet as the DTH rig could not proceed further as continuation of granite rock is missing. This area is transition zone for Archaean Granite Gneisses and Kadapa rock formations.Infact Limestone formation having solution cavities(Karst limestone) struck at 100 feet.In Karst topography, Percussion drilling rigs could be able to drill borewells but such rigs are not available in this area.
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