Collapse


Problem:

Workers that excavate sites may face hazards from collapse.

Risk Description:

Workers who dig or excavate trenches are at risk of death if they enter an unprotected trench and the walls collapse.  There is no reliable warning when a trench fails.  Even though small amounts of dirt may not seem treacherous, a single cubic yard of dirt can weigh more than 3,000 pounds, which can fatally crush or suffocate workers (Deatherage et al. 2004).  There are four common types of trench collapse hazards: (1) the spoil pile slide, (2) the shear wall collapse, (3) the belly slough, and (4) the slip slide.

The Spoil Pile Slide

Spoil Pile slides (figure 1) are very common and occur when dirt is piled too close to the edge of the trench.  Usually this type of hazard does not cause serious injury initially, but care must be taken during the rescue to avoid causing a more serious collapse of the trench wall.


Figure 1. Spoil Pile Slide

Shear Wall Collapse

Shear wall collapses (figure 2) occur most frequently in clay or layered soil.  They are deadly, happen very quickly, and usually cause death or very serious injury to the victim.   An average collapse is about 2-3 yards of soil and weights approximately 5,400 - 8,100 pounds.

Figure 2. Shear Wall Collapse

Lip Slide

The lip slides (figure 3) are similar to the shear wall collapse, but is smaller and less severe.  These occur when placing the spoil pile too close to the trench.  Weight from the spoil pile, rain, or vibration from equipment and vehicles traveling nearby can cause the lip to fracture and allow the spoil pile to slide into the trench, trapping the victim.

Figure 3. Lip Slide

Belly Slough

The belly slough (figure 4) occur in areas near underground utilities or where running water is present in the trench.  Often, a fracture line will appear as an indicator near the bottom wall of the trench signaling an impending collapse.  This type of collapse is rapid and usually buries the victim deeply, and likely to suffocate the victim.


Figure 4. Belly slough


Level of Risk:

From 2000-2009, 350 workers died in trenching or excavation cave-ins - an average of 35 fatalities per year [BLS 2010].

An analysis of OSHA data from 1997-2001 showed that 64% of fatalities in trenches occured at depths of less than 10 feet [Arbodela and Abraham 2004].

Arbodela CA, Abraham DM [2004].  Fatalities in trenching operations analysis using models of accident causation.  Journal of COnstruction Engeineering Management.  130(2): 273-280


Assessment Info:

OSHA breaks down soil types into three groups: A, B, and C. However, the mineral composition of the soil is not the only factor. Absorbed water in the soil helps determine whether it is a cohesive soil, that is, holding together and being pliable or plastic. The presence of clay in a soil also contributes to the cohesiveness. So, if you put a soil into group A, for example, it may change as it dries out or gets wetter and may be re-classified as type B or C.  Therefore, when a competent person classifies a soil, you are really classifying both the soil and the environment it's in at the time it is tested.

Type A: cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater.

Type B: cohesive soils with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf.

Type C: cohesive soils with an unconfined compressive strength of 0.5 tsf or less.


Regulations & Standards:

The OSHA standard for excavation and trenching, known as 29 CFR 1926 Subpart P, describes teh precautions needed for safe excavation work.

Regulations adopted by a state must be at least as protective as the corresponding federal standard. Work may also be subject to rules of other federal, state and local agencies. Even where there is no hazard specific standard, OSHA provides a general duty for the employer to provide a work site free from recognized hazards.

Federal OSHA Standards are enforced by the U.S. Department of Labor in 26 states. There are currently 22 states and jurisdictions operating complete State plans (covering both the private sector and state and local government employees) and 5 - Connecticut, Illinois, New Jersey, New York and the Virgin Islands - which cover public employees only. If you are working in one of those states or jurisdictions you should ensure that you are complying with their requirements.

Federal OSHA Standards are enforced by the U.S. Department of Labor in 26 states. There are currently 22 states and jurisdictions operating complete State plans (covering both the private sector and state and local government employees) and 5 - Connecticut, Illinois, New Jersey, New York and the Virgin Islands - which cover public employees only. If you are working in one of those states or jurisdictions you should ensure that you are complying with their requirements.