A highway rehabilitation project included the removal and replacement of concrete pavement, drainage modifications and new concrete barrier walls in the center median.
Video inspection of the pipeline indicated that the pipe was either damaged during construction or mis-installed. The contractor that had installed the buried drainage pipe decided to open a trench to repair or replace the pipe, and had its employees hand-dig the trench along the concrete barrier wall. The trench was approximately three and one-half feet deep, approximately two feet, 10 inches wide and more than 130 feet long. The concrete barrier wall involved in the incident was approximately 121 feet long.
Two employees of the contractor were inside the trench working on the pipe when the entire concrete barrier wall collapsed into the trench, killing the two employees.
What went wrong?
Based on the observations and measurements obtained from an OSHA post-accident field visit, the concrete barrier wall sat directly on the utility trench, which was back-filled with flowable fill. Other than the subgrade soil underneath, the concrete barrier wall was not supported by or secured to any other structures, such as an inlet or pavement shoulder.
The load imposed on the subgrade soil from the concrete barrier wall was not limited to the immediate area of the wall, but also extended some distance away from the wall, estimated as being equal to the depth of the excavation. Thus, a critical plane is formed sloping up from the bottom of the excavation toward the wall at an angle of 45 degrees.
Since the footing of the concrete barrier wall remained completely above the critical plane, the stability of the concrete barrier wall was endangered by the excavation operations. Support systems should have been provided to ensure stability of the concrete barrier wall to protect employees working the trench.
