UNITED STATES
DEPARTMENT OF LABOR

On December 10, 2008, Employees #1 though #5, with Hydro-Arch, were repairing the floor of the south bypass conduit water tunnel located under Lake Las Vegas in Henderson, Nevada. The tunnel was two miles long and seven feet in diameter. They had cleaned the floor of the tunnel and were preparing to pour concrete on the tunnel floor. Two coworkers initially entered the tunnel with a golf cart at approximately 7:45 a.m. to drop off equipment after activating the ventilation fan from outside. Approximately 8:00 a.m., they exited the tunnel to wait for the concrete truck to arrive at the site. Approximately 9:30 a.m., Employees #1 through #5 entered the tunnel with the concrete load. Concrete was carried in eight buggy carts pulled by bobcat. Behind them, the mechanic and Project Superintendent followed on a propane-operated golf cart. Around 10:00 a.m., these employees arrived at the work location and found that the mechanical ventilation fan had been shut down. Several unsuccessful attempts were made to start the fan. The Project Superintendent and foreman decided that work would continue, after measuring the natural air flow using a handheld Mini Thermo-Anemometer by Extech Instrument. At 11:15 a.m., the foreman radioed from inside tunnel that employees were experiencing difficulty breathing, chest pains, and fatigue. The project superintendent sent golf carts and removed all employees from the tunnel about 12:16 p.m. Employees #1 through #5 were treated for asphyxia and carbon monoxide poisoning at University Medical Center (UMC) in Las Vegas and St. Rose De Lima in Henderson. All five employees were treated at UMC with hyper-oxygen therapy. Reports from the employer confirmed that employees' carboxy-hemoglobin concentrations ranged between 12 and 32.