Plastic sump tank (18-in.-dia. by 24 ... Once all the concrete is out of the hole, you should see soil. ... Pour water into your sump if water hasn't collected there naturally already.

Once concrete is hard enough to stand on after it’s finished, water is its friend. If you can keep fresh concrete wet for days after you pour it, it helps the curing process.

The biggest issue with pouring concrete under water is movement. ... This small hole or sump would be about 6 inches deep below the bottom of where the slab is supposed to be.