SURFSIDE, Fla.—An engineer hired to help figure out why a South Florida condominium collapsed last month is warning officials that the site may still not be safe. Structural engineer Allyn Kilsheimer told Surfside and Miami-Dade officials in a letter Thursday that Collins Avenue could crumble, the Miami Herald and WPLG reported. All that remains of the Champlain building are the walls of the underground parking garage, around a hollowed-out foundation, and Kilsheimer says that without more support for those walls, traffic could make them collapse, with parts of the street falling into the void. “If the wall were to collapse or rotate substantially, the retained soil under the street and sidewalk could move with it,” wrote Kilsheimer, of KCE Structural Engineers. He recommends building an earthen berm to support the walls near the street and sidewalk. Otherwise, the movement “could cause portions of the street to collapse and could seriously …