An environmentally friendly transit center in Scott County, Minn., has turned out to be a $50,000 white elephant, officials say. Porous paving units designed to minimize toxic runoff crumbled under the weight of the buses, and a rain garden quickly turned into a clogged, festering mess. “We failed,” said the county’s public-works chief. “I learned from a professor at the U[niversity], if you are afraid to show the public your mistakes, skip civil engineering.”