That is weird. You must have a real good topping slab that doesn't have the cracks in the structural slab transmit through it. I've walked my share of in place clarifiers and the topping slab is usually cracked. As a thin unreinforced grout pour, it's expected. But I've never had a gas pressure upward on a clarifier either.
A permeable concrete might work or there might be some secondary effect that causes other issues. It's worth a try.

JedClampett....yep, it pushed the topping slab up into "bubbles" I've attached a video where we poked a hole in one.

• https://files.engineering.com/getfile.aspx?folder=e9e10d61-c5c7-4c7c-a5f2-e

Is that ever neat... first time I've heard of that... have to get rid of the source of the gas, first... I would think.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Dik. Best that I've been able to figure is there is an old contaminated site uphill from this site (supposedly being remediated) and a combination of nasty chemicals appear to be entering the ground water and moving toward this site. The bottom of the clarifier is almost a dome (thickened edges around the perimeter with piles and then thickened parts of the slab with piles. The spaces between all the piles and the perimeter traps the gas and it's under about 14' of head from the surrounding groundwater. So any gas that comes out of solution below there gets trapped and pressurized. I was expecting water to squirt out when they drilled the hole and was shocked when this gas started jetting out. I don't think there's anyway to prevent the gas from accumulating and the gas is able to migrate through the structural slab cracks so I have to figure out a way to let it vent thru the topping slab or it will all delaminate and bubble up again.

I've seen vents installed into the soil for methane sites... maybe something similar...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

Pervious concrete would likely clog rather quickly with sludge. Novel idea, but probably limited. I would opt for dik's solution....vent the source.

Venting the source would be good but the source appears to be groundwater that has dissolved gases in it that come out of solution, slowly, very slowly over time in the irregular volume below the clarifier slab. There are probably multiple little pockets of gas, that aren't connected to each other, below the clarifier. I'm thinking that the sludge would be pushed out of the pervious concrete voids by the pressure from below. Might have to experiment with one of the six clarifiers to see if it works. Thanks for the suggestions!