PVC pipe specifications for borewell use in coastal Orissa - Need standard recommendation

We are residing in the coastal belt of Orissa, India. We are using Sch.80 PVC Pipe of ASTM-D. Sometimes we find that the Tubewell collapsed while washing or after washing. What could be the problem? The Tubewell was constructed in Kendrapara, Orissa, India. It is in the coastal part of Orissa. The sea is 10kms from the borewell site. The dia of bore is 18'' (450mm). The pipe lowered 8'' (200mm) Sch.80 PVC Pipe. The depth of borewell is 318mtrs. Date of installation is 21.01.2009.

After 2 days of installation, during compressor washing, huge quantity of water with mud was coming out. It has seen that after 250mtrs from below ground level, the PVC pipe had collapsed. This has lead to the following query.
 
Further, the standard for borewell refers to IS : 12818 / 92. But due to faulty design (so far as my knowledge since last 20yrs) the pipes belonging to this specification might not be recommended. So I suggest PVC pipes belonging to ASTM 1785 / 89. But I cannot find any specification that recommends the pipe type in relation to the depth of the borewell. Kindly shed some light on this issue.

Purna Chandra Sahoo
sahoopvc@gmail.com

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