Tests will show that the contractor was trying to pad his margins by using substandard concrete. He needed to do this so he could pay the graft to the politicians who made sure he won the contract and still make a Tbill type return.
Low bid. Always gets the gig. Think about that the next time you go over an overpass or a tall, tall bridge.
Sure what it sounds like, though the diversion tunnels -- were they trying to let water out too fast? did they have a caviatation problem like we had at Lake Powell back in the big Colorado River Floods of 1983? They found a way to fix the cavitation problem that is supposed to be in use in dams around the world today.