SDR excavation worries in Tamilnadu : Why JCB or blasting techniques fail in SDR excavation?

I work as an Assistant Executive Engineer in Rural Development And Panchayat Raj Department of Govt of Tamilnadu. Under a state scheme called AGAMT(Anaithu Garma Anna Marumarachi Thittam) it is decided to deepen and renovate at least one village pond by deepening, construction of inlet/outlet, bathing ghat and retaining walls. Also the supply channels leading to the inlet is de-silted.
The guideline says all ponds should be deepened to a depth of 2 m from the existing bed level of the pond. Further a baby pond should be excavated to a depth of 1 m covering 50% area of the mother pond which is 2.00m depth. I am in a district where I have varied surface soil namely hard gravel, block cotton soil, sandy soil near river bed and also loose sand near sea shore. The rate for excavation using machinery remains the same.

My problem is, in some area after excavating a depth of about 1.5m to 2.00 m the soft disintegrated rock (kankar) is reached. On reporting to our administrative officials, we get a reply, do blasting and go 2m plus 1m. My explanations are viewed as being intractable or non-coperative.  I wish to inform my district project officer and other officials why each site has to be considered as unique.

Please provide me with technical details like why a JCB or Exacavator cannot excavate SDR. Also reasons why blasting fails in SDR. Information on how I can remove, tackle SDR will also be helpful. Please advice.