Working for Eclipse Engineering Inc. over the past four years has allowed me the opportunity to work on a wide variety of projects within the Structural specialty of Civil Engineering. The latest such project involved evaluating the Certification-listed maximum pressures for Excavation Protection Equipment, trench boxes, purchased by a road construction company, Johnny Cat, Inc.
OSHA has a set of rules to govern most, if not all, activities that employees of any company might be required to perform. OSHA requires that construction companies either bench/tier the walls of excavations; or use trench boxes to support the walls of narrow excavations for any excavation deeper than 4’-0” below the existing, adjacent grade. OSHA also requires that any trench box being used on a job site have a written certification stating the physical dimensions of the box and the maximum allowable lateral soil pressure that the box is able to support. These certifications are to be stamped and signed by a licensed Professional Engineer.
OSHA has another requirement beyond those listed above, for excavations deeper than 20’-0”. The construction company shall hire a Professional Engineer to either design the bench system for the excavation, or evaluate the maximum pressures allowed by the trench box certifications by comparing them to the actual soil pressures produced by the soils present on the job site. Not being a Geotechnical Engineer myself, I was thankful to be part of the latter option only.
OSHA has three main soil types that they use for evaluating Excavation Benches or Protection Equipment. Soil Type ‘A’ is a very stable soil with a high percentage of clay in its make-up. Soil Type ‘B’ is less stable, but often contains a combination of crushed rock, silt and sand. Soil Type ‘C’ is the least stable type of soil according to OSHA since it is primarily dry sand and gravel. The soil at the project we are involved contains bedrock that was blasted out to expose the desired depth, then loose, dry gravel and sand were backfilled against the trench boxes, leaving us with an OSHA Soil Type ‘C’ situation. In conclusion, the trench boxes that we reviewed are all of a sufficient capacity for their intended depths of use and construction is well under way.
Nick Burnam, PE