You wouldn’t think that you’d need to shore up a drainage trench that is only 3’9″ deep but think again. Watch this Youtube video to see what happened to one young man when the drainage ditch he was working in collapsed on him…
General Trenching and Excavation Rules
- Any and all encumbrances on the surface, outside the trench must be either removed or supported in order to make sure that they do not become a hazard
- Heavy equipment (including dump trucks and other vehicles) must be kept well clear of the edge of the trench (2 feet at the very least)
- You must determine where utilities (sewer, telephone, electricity, water pipes, septic systems, etc…) are located before you start digging
- Access and egress must be provided for any trench that is over 4 feet deep. The access and egress should be located so that the worker should have to go no more than 25 feet to get to it.
- Inspect trenches at the start of each shift and following a rainstorm
- If the trench is such that employees might be exposed to vehicular traffic, employees must use safety vests that are rated for it.
- Employees are never allowed under raised loads
- Monitors must be used to test for harmful gases as well as for the oxygen levels both before any workers enter the trench as well as while they are working in the trench
- Measures must be taken to remove water from the trench.
Testing the Soil
There are three ways to test the soil:
In order to determine the unconfined comprehensive strength of the soil, a penetrometer is used. To use the penetrometer simply collect an uncompressed handful of the soil and push the penetrometer into it up to the indicator ring. The reading on the penetrometer is in tons per square foot (tsf). The reading, when compared to the classification above will let you know which type of soil you have.
(image from http://www.strength.com.au/products/details/prod_pentpkt1.aspx. You can purchase the pocket Penetrometer from this website)
- The Torvane Soil Test (aka “shear vane”) measures shear rather than unconfined comprehensive strength. To use it, simply push the vane end of the Torvane into the soil sample and turn the dial until it breaks free. Read the number on the dial and multiply the number by 2 in order to get the unconfined comprehensive strength.
(image from http://www.durhamgeo.com/testing/soils/field-testing-torvane.html. You can purchase the Torvane from this website)
- The final method utilizes only your thumb to test the soil sample. To do this, simply push your thumb into the sample and measure the depth of penetration. A penetration of less than the length of your thumb nail (¼”or less) is probably a type A soil, a penetration that is the depth of the thumb nail (between ¼” and ¾”) is probably a type B soil and a penetration the is greater than the depth of your thumb nail (greater than ¾”) is probably a type C soil.
Understanding the different types of soil
All soil is not created equal. Different soil acts differently and you therefore need to know the type of soil you are dealing with in order to know which type of protective shoring to use. Ultimately, analysis of and proper identification of soil is something that should be handled by a competent person who is properly trained.
With regards to this, OSHA states “OSHA standards require that trenches be inspected daily and as conditions change by a competent person prior to worker entry to ensure elimination of excavation hazards. A competent person is an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to employees and who is authorized to take prompt corrective measures to eliminate or control these hazards and conditions.”
There are three classifications of soil, classified as A, B or C:
- Clay, caliche or hardpan
- Loam, silt, sandy loam or silty loam
- Sand or other types of sand-like soil
The classification of soil, however, is not just in reference to the nature of the soil. The conditions, including humidity, water content, etc… is also to be taken into account. As temperature and humidity content change, so may the classification of the soil. What this classification is ultimately trying to determine is cohesiveness (how well it sticks together as opposed to crumbling and breaking apart). The above type classifications then are only valid for each of the soils when there is average water content. Even A type soils can became C type soils when there is little to no water content. A more technical classification, based on cohesiveness looks like this:
- Soils with an unconfined comprehensive strength of 1.5 tsf (tons per square foot) or greater.
- Soils with an unconfined comprehensive strength between 0.5 and 1.5 tsf.
- Soils with an unconfined comprehensive strength of 0.5 tsf (tons per square foot) or less.
An excavation is defined as a “man-made cut, cavity, trench, or depression in the earth’s surface formed by earth removal”
A trench is defined as “a narrow underground excavation that is deeper than it is wide, and is no wide than 15 feet”
(Definitions are from OSHA)
Trench and excavation work kills an average of 54 workers each year (these number includes only the reported numbers. Many others may be listed as having other causes of death and not officially be counted as trench related fatalities); injuries number in the hundreds. The costs associated with each trench collapse can reach into the hundreds of thousands of dollars for each incident.
Being buried alive is not a nice way to go, especially considering that most if not all of these injuries and deaths could easily have been avoided by following a few simple safety measures. According to the CDC, up to 95% of injuries and fatalities occurred in trenches where no preventative measures were taken.
Types of accidents
- The Spoil Pile Slide – This type of slide is where the dirt that is excavated is piled too close to the trench. When the dirt is piled too high, too close or at a critical angle it can slide back into the trench.
- The Shear Wall Collapse – This type of slide occurs when the shear wall breaks off, dumping a large amount of dirt and debris in the trench. This type of collapse usually occurs in clay or layered soil and is most often catastrophic.
- The Belly Slough – A pocket of dirt between the top and the bottom of the trench breaks off and slides into the trench
- The Lip Slide – Similar to the spoil pile slide, this collapse occurs when the excavated dirt is too close to the top lip of the trench causing pressure. Vibrations from excavating equipment and other machinery can cause a small chunk of dirt to collapse. Because the spoil dirt is resting on this lip, however, the amount of dirt can be much larger than just the amount of dirt from the break.
(Too see a flash animation of each of these types of slides, visit the CDC website at http://www.cdc.gov/niosh/docs/2006-133D/flash/index.html).
Most people aren’t overjoyed to see the OSHA inspector roaming around. One particular employee from the Northern Chicago area however, is extremely glad that this particular inspector did stop by when he did.
For the OSHA inspector it was a routine inspection but for the worker who was working at the time in the unprotected trench it was anything but. The inspector, noticing the worker in the trench identified himself and asked the worker to get out of the trench. The worker climbed up the ladder and agreed not to re-enter the trench until something had been done to shore up the wall to keep it from caving in. As the OSHA inspector began to show the worker why he thought the trench might be dangerous the wall of the trench collapsed. Had the OSHA inspector not stopped by when he did the employee might have been trapped and/or seriously injured.
This incident, along with several others, is posted, along with photos and sometimes video clips (as in this particular instance) on the OSHA website. Check out why OSHA is necessary and how important it is that they do what they do. Read the articles, view the photos and watch the videos here.
So next time the OSHA inspector stops by, instead of acting like he’s pestering you, stop and thank him.