Thanks John.

Have you done that before or seen where it's been done and worked longterm? My "concern/question" has been whether that has inherent risk for breaking down over time, eg high pressures, air pockets, water tricking in, and eventually failing....

Also, is concrete the material to use or are there newer products that are better suited for a lasting solution? Someone mentioned bentonite, but perhaps that's no good and perhaps there are other materials...