Four construction workers were injured, three seriously, after a barrier encasing wet concrete collapsed Wednesday, sending the sludge spilling onto the men as they worked on a housing complex, authorities said.
The concrete was being poured on the roof of the six-story building when the structure being used to support the concrete while it dried gave way around 8 a.m. Wednesday.
The collapse sent the workers plunging as many as three stories down a stairwell, Fire Department spokeswoman Mindy Talmadge said.
"They were covered in concrete," Talmadge said. "Their co-workers uncovered them and pulled them out."
All four men were taken to San Francisco General Hospital, where three of them were listed in serious condition. A fourth was upgraded to good condition and was set to be released, hospital spokeswoman Rachael Kagan said.
The workers range in age from early 20s to late 30s, Kagan said. Their names have not been released.
The injured men worked for Nibbi Brothers General Contractors of San Francisco. Rick Fedick, Nibbi's chief financial officer, said he could not speculate on what caused the structure, known as a form, to fail.
State workplace safety investigators and city building inspectors were at the apartment complex in the city's Dogpatch neighborhood to probe the accident. The San Francisco District Attorney's office was also investigating. Work resumed later Wednesday on other parts of the $40 million project, which will include 196 apartments.
The investigation will focus on what caused the form to collapse, said Dean Fryer, spokesman for the California Division of Occupational Safety and Health.
"We need to dig into their procedures and the materials they're using. We're going to want to talk to employees on the site and take a close look at the site itself," Fryer said.
Investigators have inspected the rest of the forms surrounding the spot where the collapse occurred and found no other safety issues, he said.
Inspections of the job site in January and July found no safety violations, according to federal workplace safety records.