Shale Bedrock

It looks intact at the outside when it was brought up 21m below ground level which was also probably the actual in-situ state, lying deep beneath the Earth. 

But during the drilling process which used water plus the vibration of the drilling machine, the Shale deteriorated fast. The core broke into pieces which might not reflect the actual condition beneath. 

The longer you drill, the worst the core will be. That is why it is always a good practice to drill short pieces and bring up fast. The use of standard 1.5m length is not good for this type of rock. But Contractor will find this way too time consuming and costly. They prefer the maximum 1.5m. He may not be able to bring up anything (lost core) but it does not mean there is nothing down there, just change into mud slurry with the drilling water.