The blow-out of a cap on a large pipe under Centre Street is to blame for the spectacular flooding at the corner of Boylston and Centre. The flood appears to have started sometime after 11 p.m. Friday near the intersection with South Huntington Avenue where work crews had been active well into the evening. As of noon Saturday, 20 residents remained without water, according to Mark Van Dam, spokesperson for the Boston Water & Sewer Commission. Crews expect to have water service restored to those persons by 2 p.m. Saturday, he said. Here's what caused the flooding: Workers from Feeney Brothers removed a "thrust block," essentially a giant piece of granite from the end of an older water main.