Thumbs up for NeilN.You have put it in a nutshell.I imagine...

Thumbs up for NeilN.You have put it in a nutshell.I imagine removing the concrete 'filler'would be possible using a 'bit' similar to a hole saw cutter {surely they must exist]**To add a few more cents worth; as someone said heavier casing must be used not to speak of reinforced ccncrete in the upper region; shale common earth and sand are plastic [as opposed to granite being elastic] in nature and offer little or no resistance to internal pressures and containment value is low, which possiblly lead to cracked concrete and metal fatigue of the casing.************************************just a thought just a laymam.