I don't have any idea about that sort of stuff, but if I'm understanding your situation correctly, might the water not just find its way around the cement and put you back where you are now, with the exception of an even bigger pain the butt to remove the next time you have to work on it?

Todd La Neve

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1.5 & .5 ac ponds - LMB, BG, RES, YP, GC, HSB