Abstract: At approximately 9:00 a.m. on February 1, 2010, Employee #1 was assisting in the construction of a micropile system a large municipal office building. At the time of the accident, a large, split-level excavation had had been dug for the building foundation. The employer was in the process of drilling a series of 68-foot deep holes into the floor of the excavation for micropiles. The employer had positioned two micropile rigs into the deeper portion of the split-level excavation: one yellow rig and one red rig. The employer had also placed a 900 cfm/300 psi air compressor on the lip of the upper portion of the excavation. Both rigs were to be hooked up to the air compressor so that the compressor could supply the rigs with compressed air to blow excavated sand and soil out of the micropile holes during the drilling process. At the time of the accident, a fifty-foot, two-inch-diameter hose was connected to the compressor and extended down the slope of the excavation to the lower level, near to where the rigs were located. At the end of the fifty-foot hose, there was a T-shaped manifold connected with dual outlets - one outlet at the end of the manifold, in line with the supply hose (the "in-line valve"), and the other branching out perpendicularly to the right (the "secondary valve"). Each of the two outlets was equipped with a quarter-turn ball valve, and each valve had a lever attached to the top of the manifold to allow a user to open or close the valve. At the time of the accident, the yellow micropile rig was already connected to the air compressor via a hose attached to the open in-line valve on the manifold. The air compressor was pumping air into the hose, and the yellow rig was operating. To place the red micropile rig into operation, the employer tasked Coworker #1 Coworker #2 to connect the red micropile rig to the air compressor through a hose attached to the secondary valve. Coworkers #1 and #2 determined that the hose they planned to use to connect the red micropile rig to the manifold would not reach from the rig to the manifold, so they decided to reposition the manifold closer to the red rig. Coworker #2 grasped the live air hose on the supply side of the manifold to drag the manifold across the ground and into position. Almost immediately after Coworker #2 began to pull the supply hose, the manifold turned over, either onto its side or upside down, so that the valve levers on top of the manifold were in contact with the ground. The lever for the secondary valve, which was in the closed position when Coworker #2 began to drag the manifold, caught against the ground or another obstruction and rotated into an open, or at least semi-open, position. As the compressed air began to rush out of the opened secondary valve, the manifold and supply line lifted off the ground and began to whip about. Employee #1, who was not involved in hooking up the red micropile rig, was struck in the leg and knocked to the ground. The manifold, still energized with compressed air, continued to flail and struck Coworker #1 in the chest. Coworker #1 grabbed the manifold and held it against his chest, was lofted momentarily into the air, and then knocked to the ground. Coworker #1 was on the ground, still clutching the manifold to his chest, when Coworker #2 threw himself onto Coworker #1 to help arrest the whipping of the manifold. Shortly thereafter, another coworker was able to shut the compressor off, deenergizing the manifold and air hose. Employee #1 was transported to San Francisco General Hospital, where he was admitted and remained over 24 hours for treatment of a fractured leg. The San Francisco Fire Department reported the incident at 11:00 a.m. the same day. The employer, Dees-Hennessey, Inc. reported the accident the next day at 9:00 a.m. The employer is a foundation/site- preparation contractor, and had been engaged by the general contractor of the new office building.