Replies

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.

>>HerringBone Drain Pipe - Fracture pattern in Main Spillway...

Hi Jim 0216, Perhaps this image may clear this up (re: question discussion). The fractures reveal an "upward" pattern where the drain pipe is still under the slab, but angled slightly upslope to facilitate flow. Key: Pipe is angled "upslope" with respect to the resulting "downslope" flow of drainwater. If the drain pipe were constructed strictly horizontal there were design considerations discussed of "self-cleaning" and "better flow".

These series of fractures follow design information on the placement of the drains under the main spillway slabs. These fractures likely occurred after the original blowout failure. The downstream hydraulic turbulence would have pounded these slabs. As Freeper abb has provided prints on the gravel & drain pipe placement in the dimensions of the slabs, this becomes a narrowing area of the thickness of the slabs. Thus the cracks below the blowout area offer insight into many factors (stagnation pressures, thin section in slab concrete, strength of hydraulic pressures from turbulence, etc).

Of course, these insights are from an after-failure. The spillway was not designed for the after-failure - BUT - this after blowout failure cracking gives an indication of what the actual design may perform to in extreme destructive conditions (i.e. useful as a form of a destructive test to provide data in original safety factor margins & design vunerabilities).