Trenching and Excavation Safety (Part 5)

Trench Protective Systems

There are four basic ways to protect workers in a trench. Trenches that are more than 20 feet deep must be protected by a protective system that was designed by a registered professional engineer.

  1. Sloping
    The type of soil will determine the ratio of the slope:
    1. Type A soil should have a ¾ to 1 ratio
    2. Type B Soil should have a 1 to 1 ratio
    3. Type C soil should have a 1 ½ to 1 ratio

    If different layers of soil are encountered as you go deeper, the slope must be adjusted in order to have the right ratio for each.

  2. Benching
    Benching follows the same ratios as for sloping with the last 4 feet being straight down (without a slope) and the top of this 4 foot deep trench within a trench being at least 4 feet from the slope of the rest of the trench
  3. Shoring
    Shoring is essentially plating that is held in place with expandable braces. There are different types of shoring systems but the basic system is similar. The plates are lowered in and the brace is expanded until it holds them in place and keeps the soil on either side of the trench from collapsing.
  4. Shielding
    Shields are similar in nature to shoring except that the plates and the braces are permanently fixed together. The braces are not expandable.

Trenching and Excavation Safety (Part 3)

Testing the Soil

There are three ways to test the soil:

 

  1. 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)

  2. 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)
  3. 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.

 


Trenching and Excavation Safety (Part 2)

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:

  1. Clay, caliche or hardpan
  2. Loam, silt, sandy loam or silty loam
  3. 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:

  1. Soils with an unconfined comprehensive strength of 1.5 tsf (tons per square foot) or greater.
  2. Soils with an unconfined comprehensive strength between 0.5 and 1.5 tsf.
  3. Soils with an unconfined comprehensive strength of 0.5 tsf (tons per square foot) or less.

 

 


Trenching and Excavation Safety (Part 1)

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

  1. 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.
  2. 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.
  3. The Belly Slough – A pocket of dirt between the top and the bottom of the trench breaks off and slides into the trench
  4. 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).