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Pipeline Trench Shoring

Bracing the walls of a pipeline trench to prevent collapse is what trench shoring is all about. Shoring trenches is a large part of pipeline construction. As such, shoring safely is one of the most important components of excavation. Why? According to OSHA, trenching is one of the most dangerous types of construction work, causing over 40 deaths per year and seriously injuring many more, in what could otherwise be preventable situations.

So what can be done to avoid becoming part of this unfortunate construction statistic from lack of, or poor trench shoring techniques? First and foremost, one must be competent to know and recognize the different types of soil (also known as dirt, loam, earth, dust or mud) prevalent throughout new construction sites, especially in Southern California.

Did you know that as little as one cubic yard of soil can weigh as much as a 3,000 pound car? What this means is that if you are in a pipeline trench surrounded by dirt, and it caves in, it can lethally crush you. This is why it is so important to know the type of soil you have to work with so you can take the proper steps to protect yourself from a trench cave in.

Basically, there are four main groups of soil that OSHA has classified: Type A, Type B, Type C soil and Stable Rock.

Type A is known to have a high unconfined compressive strength of 1.5 tons per square foot or greater. It is cohesive and sticks together. The more cohesive the soil, the more clay it has and is less likely to cause a cave-in. It is clay-like, and includes clay, silty clay, sandy clay and clay loam in its composite structure. Note that soil is not considered Type A if it is fissured or subject to vibration of any kind, or has been previously disturbed or is part of a sloped, layered system where the layers dip into the excavation on a slope of 4 horizontal to 1 vertical, or greater 4H:1V, or has seeping water.

Type-A-Soil.jpg

Type B soil has a medium unconfined compressive strength between 0.5 and 1.5 tons per square foot, and includes soil that is near sources of vibration, fissured, split up or separated in parts, angular gravel, silty and silty loam. It's often disturbed or cracked, and doesn't stick together as well as Type A soil.

Type-B-Soil.jpg

Type C soil is granular and the least stable. Examples of Type C soil are gravel and sand. This soil doesn't stick together and is generally not cohesive. In fact, any soil with water seeping through it, and submerged rock that is not stable is automatically classified as Type C soils, regardless of other characteristics. This soil is equal to or less than 0.5 tons per square foot.

Type-C-Soil.jpg

Have you ever heard the phrase, "Go pound sand?" It is because Type C soil is does not easily compress and in fact, would constitute a waste of time even trying to compact the soil.

Stable Rock is composed of solid, natural mineral deposit matter, such as granite or sandstone that can be excavated with vertical sides that remain intact while exposed. It may not be as easy to determine this type of stable rock as one thinks, unless one can identify whether there are cracks and whether these cracks run into, or away from the excavation.

Stable-Rock.jpg

Geography plays an interesting role in pipeline construction. Oftentimes, in the course of pipeline excavation work near an ocean or other waterway, the soil tends to be sandy or saturated with water. As areas of construction are closer to mountainous areas, soil can be more rigid, and clay-like, and have rocks or granite in it, making it difficult to excavate using standard equipment like an excavator or backhoe, and require a pricier, and more complex form of hard rock excavation measures.

Typically before starting wet utility pipeline excavations, civil plans have been studied, the specifications have been read and the soils report has been reviewed which gives a good indication of the project and the soil that it sits in. However, an idea of the soil type is not good enough, and must be field verified and tested. Confirming the right type of soil makes it possible to determine the correct protective system to use in order to keep workers safe while working in trenches.

Now that we have covered the types of soil there are, stay tuned to our next Blog on pipeline excavation which will dive deeper into the various testing measures to confirm the different types of soil, discussed above, and the typical shoring methods used for trenches under 20 feet deep.

For more information, go to the United States Department of Labor, and visit OSHA's Technical Manual (OTM) on Excavations: Hazard Recognition in Trenching and Shoring.

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Published on March 31, 2014 | Comments: 0

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