I would NOT suggest adding pure bentonite! There are a few grades of bentonite also...some swell little, while other swell a LOT. You'll need a swelling bentonite, BUT, what swell with moisture, also shrinks, and that creates cracks and voids! If you go through a dry spell, and the bentonite has dried, shrunk, and levy behind voids, and then you get a large rain event that causes water to flow through voids, you could have a bigger problem, even a sudden breech.
It's best to mix the bentonite into moisture holding soils/substrates at proper percentages.
Consider contacting Mike Otto at
Otto's Dirt Service and ask Mike's advice on how to best compact around drain pipes...also consider buying his book, "Just Add Water".
