Borewell Drilling - Soft Rock Conditions - Bangalore
Submitted by kiran on December 24, 2009 - 13:12
We have implemented a 6.5 inch borewell in Vidyaranyapura, Bangalore a few days back. The area is known for loose soil because of proximity to the lake bed. Loose soil was encountered till 90 feet and soft rock (white color ) from 90 to 200 feet. MS casing was laid toll 90 feet. Boulders were found from 90 to 300 feet. Water at 100 feet, around 1.5 inches was also struck. Drilling was halted at 300 feet and 5" PVC pipe was inserted, but only till 115 feet as the boulders and water prevented further insertion.
The driller suggests the placement of a submersible pump at 105 feet. On further questioning the driller currently plans to re-evaluate the depth and use high pressure air/water to clear the blockage. But he cautioned that this might in some cases damage the casing. And that depth reduction could be due to the soft rock/silt collapse.
My questions are as follows:
1. Is it worth investing further funds on pumps etc in this well?
2. Is slotted PVC a good solution in the current situation?
3. Should the MS casing be removed and re-insertion attempted till 300 feet?
4. Is it necessary to drill till we reach hard rock?
Thanks!
Kiran
Comments
1. Soft Rock Conditions -
Dear Kiran,
From what you have described there is loose soil down till 90 and then beyond it there was soft rock.
It is quite common that due to the soft nature of rock that get displaced the insertion of pipe is hindered. May be the annular space has been reduced or total filled up.
This could be checked by checking the depth of the bore well with a piece of nylon rope and a weight tied to it, if it goes down to 300 ft it is only the reduction of annular space locally, if not then it is the bore well has collapsed below the point .
1. You could install a test pump and test it s performance before buying one
2. Slotted casing are good enough, but there could always be a possibility of some damage to it due to the sharp edges of the rocks.
3. I doubt that it will be possible to remove the MS cashing
4. Is it not necessary to drill till we reach hard rock- many well in the soft rock terrain are only in soft material, but they usually have an end plug at the base of the casing to prevent sand and clay from rising into the tube well
Hope this answers your queries to a certain extent.
Regards
Anil Lalwani
www.wellwaterworks.com
contactus@wellwaterworks.com
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