However, the standards are applicable only to trenches 5 feet or more in depth. These requirements are intended to approximate, for each soil type, the safe angle of repose. The standards also define minimum requirements for shoring of trenches for various classifications of soils. The standards require additional shoring and bracing procedures when excavations or trenches are located adjacent to previously backfilled excavations or where excavations are subjected to vibrations from railroad or highway traffic, the operation of machinery, or other sources. A major issue surrounding existing OSHA standards for excavations, trenching, and shoring is the classification of soils.
Soil classification provides an empirical estimation of the soil's stability.
If the potential for cave-ins or other dangerous movements of ground is thought to exist, appropriate decisions to provide adequate protection must be reached. It also presents a simplified soil classification system for use in determining appropriate side-sloping requirements and for calculating the lateral soil pressures imposed on shoring systems. Table 1 suggests that compliance with existing regulations or adherence to recommended work practices might have prevented important causative circumstances in each of the four fatal incidents. Stable rock is exempt. One hydraulic shore was in use on an intersecting trench.