Soil Classification and Testing in Construction Excavation
In the United States, more than 800 construction workers die while working on the project site every year. Of this number, 40 workers die in accidents related to trenching. Trenching accidents are extremely dangerous. Workers can suffer death or serious injury in mere minutes if caught in a trenching cave in.
As a contractor, it’s your responsibility to facilitate a safe project site for your workers and supply them with the necessary personal protective equipment (PPE) to protect themselves from potential hazards. Failing to comply with Occupational Safety and Health Administration (OSHA) regulations pertaining to trenching and excavating (29 CFR 1926.650, Subpart P) could result in severe penalties.
In this article, a Florida OSHA lawyer will discuss some related topics that are often overlooked, including soil classification and testing in construction excavation. Remember, for assistance with OSHA defense or citation prevention, consult a Florida OSHA defense lawyer with years of experience representing the construction industry.
Types of Soil
Soil can either be cohesive or granular. Cohesive soil contains fine particles and enough clay to stick to itself. More cohesive soil contains more clay, and has a reduced chance of cave in as compared to granular soil. Granular soil is more akin to sand or gravel, and has coarse particles that are significantly less cohesive. When excavating in granular soil, additional safety precautions must be utilized to prevent a cave in.
OSHA uses a measurement referred to as “unconfined compressive strength” to classify soil into one of three categories: A, B, or C. Measuring unconfined compressive strength, which is reported in units of tons per square foot, informs us of the amount of pressure that will cause soil to collapse.
Type A Soil: The most stable and cohesive soil with an unconfined compressive strength of 1.5 tons per square foot or greater. Type A soils generally include clay, silty clay, sandy clay, and clay loam. Type A soil never includes soil that is fissured, previously disturbed, has water seeping through it, or is subject to vibrations from heavy machinery or local traffic.
Type B Soil: Less stable but still cohesive, Type B soil doesn’t stick to itself quite like Type A soil and possesses an unconfined compressive strength of .5 to 1.5 tons per square foot. Examples of Type B soil include angular gravel, silt, silt loam, and soils that are fissured or near sources of vibration that would otherwise be classified as Type A.
Type C Soil: The least stable type of soil, Type C soil is granular and has an unconfined compressive strength of .5 tons per square foot or less. Examples of Type C soil include gravel and sand. Due to a lack of stability, any soil with water seeping through it is automatically classified as Type C regardless of its other characteristics.
It’s important that a contractor or other competent person performs a visual test upon breaking ground to help determine factors that could compromise the strength of the soil on the project site. You should be seeking answers to questions such as:
- Is the soil clumpy or granular?
- Are there sources of heavy vibrations near the excavation site?
- Are there signs of previously disturbed soil, such as from utility lines?
- Are there signs of water seeping through the soil?
- Is the soil fissured or otherwise showing crack-like openings or chunks of soil crumbling off the side of a vertical excavation wall?
Choosing a Sample
When choosing a soil sample, be sure to collect a sample that is indicative of all the soil in the excavation area. As you dig deeper and deeper, continue to take new samples. Always remember that a trench can be cut through multiple types of soil, so it’s imperative that you understand the soil profile of your entire excavation. Failure to account for the different compressive strengths of these layers could result in a trench collapse. If this happens, consult an OSHA lawyer.
OSHA recommends taking a large clump from a freshly excavated pile that hasn’t been compacted as opposed to taking a sample from the excavation wall. Once you have your sample, test it immediately to avoid an error. When soil dries up, test results can change. There are three types of soil tests: the plasticity test, thumb penetration test, and pocket penetrometer test.
The plasticity test, or pencil test, is used to determine if the soil on your excavation site is cohesive. This test is performed by rolling a moist soil sample into a small piece one-eighth of an inch thick and two inches long. Then, hold the piece on one end and see if it breaks. If you can hold the rolled sample on one end without it breaking, it is cohesive. If it breaks, it is not cohesive and is, therefore, classified as Type C soil.
Thumb Penetration Test
This test is used to quickly estimate the compressive strength of a cohesive soil sample. To perform this test, you simply push your thumb into a fresh sample of soil. If…
- Your thumb only makes an indentation with great effort, it is Type A soil.
- Your thumb sinks into the soil up to the end of your thumbnail, it is Type B soil.
- Your thumb goes all the way in, it is Type C soil.
If you are unsure whether a sample qualifies as Type A, B, or C, collect a new sample, utilize a different testing method, or consult another competent construction worker on the excavation site to obtain a more accurate result.
Pocket Penetrometer Test
The first two testing methods we mentioned do not require any numerical data to verify the type of soil present at the excavation site, nor do they require the use of tools. By contrast, the pocket penetrometer test requires the use of a tool that works like a tire pressure gauge to give a numerical reading that can identify cohesive and non-cohesive soil samples. This numerical data is valuable, as it could be used as evidence if a trench collapses unexpectedly. For more information about protecting yourself in the instance of a trench collapse, consult one of our OSHA lawyers.
The pocket penetrometer has a thin metal piston that pushes into the soil sample and measures its compressive strength. When you start, make sure the scale indicator is inserted into the body of the pocket penetrometer until only the zero mark is visible. Push the device into the sample until it reaches the engraved line, then take the reading. Be sure to run this test on multiple samples to ensure that your results are consistent. Soil that contains rocks or pebbles may not compress, compromising your results.
Avoid an OSHA Citation
Arming yourself with knowledge is one way to adopt better practices and potentially prevent an OSHA citation, but partnering with an experienced OSHA defense attorney can give you access to a full suite of services, including defense in active cases. Keep in mind that workers in trenches and excavations have limited options for protecting themselves in the event of a collapse. One cubic yard of soil can weigh as much as a car, or roughly 3,000 pounds. When your workers are digging a trench, it’s vital that they understand the type of soil they are working with so they can apply the proper techniques to properly slope, bench, or shore the trench.
If you would like to speak with an OSHA defense lawyer, please contact us today.
Disclaimer: The information contained in this article is for general educational information only. This information does not constitute legal advice, is not intended to constitute legal advice, nor should it be relied upon as legal advice for your specific factual pattern or situation.