The Competent Person responsible is for inspecting excavation safety. This includes analyzing the stability of the soil as well as inspecting the surroundings for other associated hazards. An inspection must always be performed daily and before anyone enters a trench and at best is done continuously throughout the life of the project.
This paper for the ASSE proceedings offers inspection insights gained from the school of hard knocks and the Federal OSHA Standard. It is designed to promote awareness into identifying, then controlling or abating, some of the hazards in excavation work and to help keep people safe.
First, excavation inspections include both visual and manual tests performed by the Competent Person to gauge a soil's stability and resistance to sheer (referred to as unconfined compressive strength in the OSHA Standard).
Factors that limit a soil's ability to resist shearing forces include water, vibration, previously disturbed soil, freeze/thaw cycles, and prior consolidation. They can all impact the stability of soil.
Numerous manual tests to gauge soil stability are available. They range from using an instrument that costs between $50 and $100 (pocket penetrometer or torvane) to lower-cost methods, such as timing the rate at which soil constituents settle in a jar of water, inserting your thumb into the soil, or creating a ribbon or thread of dirt.
All of these tests, and others can indicate how stable a soil is and can reveal its OSHA federal soil classification (A, B, C, or rock). These classifications help determine the proper slope or structural member needed to prevent failure of an excavation less than 20 feet deep.
Second, there is much more to excavation safety than just classifying the soil and selecting the proper slope, shore, or shield.
What is the big deal if a bit of dirt falls on you or a co-worker? Why does it matter? How great is the risk? Well, a single cubic yard of dirt weighs over 3000 pounds and could reach 4000 pounds for some types of soil and moisture content. Therefore, one cubic yard of soil weighs more than a small pickup truck! In fact, a 2×2 foot triangular wedge of soil just 5 feet long could weigh up to 1000 pounds.
Figure 1 A single cubic yard of dirt weighs between 3000 and 4000 pounds -more than a small pickup truck (available in full paper).
Most people who die in an excavation collapsed so from suffocation, although many injuries result from being crushed. Some excavation fatalities are from drowning. Excavations generally fail in waves. These multiple failures called secondary failures, often cause the most serious injuries compared with the initial or primary failure. Once a supporting mass is removed, there is little to hold up the remaining portion of soil above.
