The 10-story, 3,000-plus-car garage is being built in three phases by a joint venture of Zachry Construction, San Antonio; Vaughn Construction, Houston; and Layton of Phoenix, as part of a $900-million system-wide expansion project by the public health system owned by Bexar County.

Just before the collapse, workers reported hearing a rumbling sound, and construction supervisors evacuated the site, according to Melissa Sparks with the San Antonio Fire Dept. “Thank God they paid attention,” she says. Some 120 employees were working on the site. Some had left the structure for a lunch break, she says.

The accident occurred at approximately 11:40 a.m. according to Sparks. Two workers were injured. One is stable and the other remains in critical condition at University Hospital, according to Julie Wiley, a spokesperson for the University Hospital. The fire department completed two searches, found no one trapped, and the construction firms were able to account for all of their workers.

Concrete columns fell atop vehicles, construction equipment and storage units leaving behind approximately 1,500 tons of fallen debris, according to Sparks. The area is adjacent to UHS’ existing hospital as well as the University of Texas at San Antonio’s Health Science Center.

The contractor broke ground on the garage in March 2010 and was scheduled for completion this summer, with a portion of it open to vehicles. Hospital officials say a new section of the garage that opened last month was inspected, and aside from a small section that will require additional shoring, was found to be “structurally sound.” The fire department assessed the connecting parking deck to make sure the vehicles parked in it could be safely removed and then cordoned off the area.

Sparks says all work has been suspended, pending an investigation of the cause.

The Zachry-Vaughn-Layton team broke ground last month on the nearly 1-million-sq-ft hospital tower directly in front of the existing University Hospital.