On August 26 we posted an anti-seep collar going in on Basin 1. Here is the same detail on Basin 2, with a wrinkle.

This basin has two outlet barrels leaving it, not one. Both pass through a single collar.

The collar itself does the same job either way. Water that reaches the outside of a buried pipe follows the pipe, works along the wall, and carries fine soil with it until the path opens up. The collar makes the trip long enough that it does not get there. How much longer, and how many collars it takes to get there, comes off the length of barrel sitting in the saturated zone. It is a calculation, not a judgment call.

What changes with two barrels is everything around it.

The two downstream outlet barrels leaving the anti-seep collar, with the unreachable wedge of ground under and between them
The downstream side. The wedge of ground under and between the two barrels is the part that has to be compacted by hand. Click to enlarge

There is a wedge of ground underneath and between the pipes that an excavator bucket cannot reach and a plate compactor cannot sit flat on. It gets done with a jumping jack, in thin lifts, by someone willing to spend the time. A collar sitting in loose backfill accomplishes nothing at all. The concrete is not the part that fails. The soil against it is.

That is the whole reason we stand there while the trench is open. Once it is backfilled, nobody can inspect it again, and the only evidence it was built right is a photograph.

Earlier critical stage stops on this job: the collars on Basin 1 on August 25, then the underdrain and the soil media a week later.

Seeding and matting are what is left.