Water within the drain pipes won't become "pressurized" in any way that I can imagine, other than that which results from the combination of the force of gravity and resistance to flow (leading to the head pressure which causes the water to spurt from the outlets). The collection pipes simply provide an easy route for gravitational flow to the main pipe just outside the wall, and the flow in that pipe also operates by gravity. If water beneath the slab has a pressure somewhat greater than atmospheric, and that is possible since the head pressure within the stream of water above is the most basic reason the water comes through cracks from above in the first place, that would only encourage under-floor water to find an easy path of escape to an area of lower pressure. That easy path of escape would be into the collection pipes. The fact that water can run out of the collection pipes as easily as in makes no difference as long as the net gravitational flow ends up in the non-perforated pipe just outside the wall. Any water lost from one collection pipe, if that water runs far enough downslope beneath the slab that it can't return, will simply end up encountering another collection pipe during its downhill journey soon enough. If the water runs downhill (and it will), and if other void spaces beneath the floor are not excessive (which is a whole other topic), it will end up being taken out from beneath the floor by the drain system as fast as it enters (assuming the rate of entry is not excessive, which again is a whole other topic).
If you have an idea how the water within the collection pipes could somehow have a pressure that's greater than that of their surroundings, maybe you could elaborate on that.
In general I agree.
Over the past few days I have been regretting my prior statements about those high outflows must be pressure driven. It is possible (likely?) those high exit velocities are merely due to high velocities -- free stream, not full pipe section. (Also my earlier comments re pressurization were prior to me seeing description of the drain system - herringbone w perforated pipe, etc.) They were based merely on seeing some plumes that appeared similar to fire hoses without nozzles.
That said, if there were no flow out from under the slab, the pressure at the bottom of the approx. 550 ft spillway drop would be roughly 250-300 psi (assuming 1/2 psi/foot). Obviously there is leakage between the slab and rocks and through the drain system which greatly reduces that pressure. The pressure builds up to where the driving pressure is sufficient to vent the incoming water. It is a very complex and distributed network. It depends on the flowrates and the drain rates (both natural and manmade) everywhere under the slab. We know the pressure is zero at the sidewall openings and zero at the top of the sidewall backfill. Everywhere else it depends on the pressure required to drive the flow through the system. The backfill looks like that should be a very low pressure, so the pressure at the sidewall footing should be close to zero.
Upstream of that sidewall (ie as you move under the slab toward spillway centerline), the natural and man-made flow channels to the sidewalls may have low capacity relative to the inflows.
The system is unstable because erosion creates preferential flow channels which results in increased local flow and local erosion. Lather, rinse, repeat.
If all the perforations were taking away all their share of the local water, underslab erosion would be minimal, but if preferential flows developed, look out. Eventually any flow at all will remove the fines and reduce the resistance to flow, developing preferential flow channels. French drains typically are wrapped in Geotechnical fabric which acts as a filter to keep the fines from being carried away.
Question on different issue: Would upflow through the slab seams wash away the interslab seals and thereby open the seams to later leakage of spillway water to beneath the slab? (Why/where would there be underslab pressure in one scenario and later top side pressure at same location in different scenario?)
