Two workers were injured on Wednesday by a collapse in the concrete on the Caldecott Fourth Bore in California. Only one worker had injuries that required a hospital visit that day, and both were back at work on Thursday.
Ivan Ramirez, the senior engineer in charge of the 1.03km tunnel’s NATM excavation, said the concrete that fell was part of the shotcrete lining.
“We had finished the ground excavation and we were in the process of installing the rock dowels. And some of the initial shotcrete fell,” he explained.
The initial layer of shotcrete, 50mm thick, had been about 12 hours old. The second layer, 150mm thick, was less than one hour old. It had passed the quality control set in place for the project, Ramirez said. “It was one of the situations where the shotcrete didn’t adhere well to the previous shotcrete and it fell right at the joint.”
Panel testing is being used for the Fourth Bore, and early strength tests are done with a Meyco needle at 10 minutes and one hour. There is a one-day comprehensive stress test on cylinders cored out of the panel, a seven day strength test and a 20 day strength test. An ASTM flexural toughness test is done after seven days, as well.
The 10 minute test had been performed and the shotcrete had passed. But the area that fell, did so before the one-hour mark.
“It’s hard to tell right now why it happened,” Ramirez said. “But a very real possibility is maybe we’re getting there too quickly. One of the things we’re going to be doing is waiting before we get in there and install the dowels. We’re going to allow the shotcrete to have more time to cure.”
Beside the incident on Wednesday, Ramirez reported everything is going well in the excavations, which are being done by roadheader. Boring started in August on the east end has progressed 90m to date.Work on the west end will start next week to install the pile canopy ahead of excavations, starting next year.
The Caldecott Tunnel currently has three bores, and the USD 420M Fourth Bore project will add a two-lane tunnel north of the three existing tunnels. Tutor-Saliba is the contractor for the project.
