[KC Burke]

Do you think some misguided person thought some bentonite clay would stay in place ignoring drainage?

[EarthResearcher333]

Do you think some misguided person thought some bentonite clay would stay in place ignoring drainage?

I have no idea (answer to your query). The only link I can think of, by a person or person(s), is how Bentonite is used as a low permeable barrier in lining landfills. But to be amiss in knowledge of the pressurized drain waterflow dynamics is very revealing.

I can't imagine an experienced contracting company specializing in dams and concrete spillway repairs would ever do this (unless there is a formulation & technique to a void repair technology that may have been interpreted as a "clay" by error). If you exclude the experienced contracting concrete specialist companies, this leaves DWR engineers directing custom repairs.

[EarthResearcher333]

Re: Bentonite: Perhaps they were thinking of the old method of sealing cracks from water leaks in basement walls.....A basement wall is a far cry from a "washed" underside of a pressure gradient spillway slab.

Bentonite Stops Leaks in Basement Walls

[EarthResearcher333]

"Do you think some misguided person thought some bentonite clay would stay in place ignoring drainage?"

I believe you nailed the thinking. The constant repairs of leaking cracks in the concrete would drive to this solution space (thinking). Without understanding the cause of the constant re-cracking (repairs) from slab flexure (top layer rebar only, drain design thinning of the slab, high levels of subslab waterflow, hydraulic jacking..) a flexible type of Sodium Bentonite water seal fits.

The irony is that pressurized waterflow & other underslab waterflow would defeat this approach. It will be telling to find out what more information comes out from the independent consulting experts (FERC recommended).