As I appreciate what we’ve learned so far at Metabunk, the underslab drains - vitrified clay pipe (vcp), perforated, 6” dia. - run crossways to the spillway, with a slight downhill bias. Herringbone pattern was the way it was described.
They rest in a gravel bed to allow seepage to percolate into them. Kind of like a french drain. They were covered with a polyethylene (Visqueen) sheet to prevent concrete infiltration when the slab was placed.
From the center, they extend both ways from the center to just beneath the wall footings and there connect to a 12” vcp collection drain. We think the were on 20’ centers, with about 10 each per collection drain.
The collection drain runs longitudinally along the outside of the wall footing, following the natural slope of the spillway.
At some point just past the last underslab drain, the collection pipe elbows to a horizontal run (probably slightly pitched to facilitate drainage). Then the collection pipe elbows into the spillway wall and discharges.
There are multiple systems on each side.
Thanks abb. Did I get that wrong about certain drainage pipes somehow pointing uphill?