Trench Rescue: Soil Types, Trench Wall Failures and More


CarolinaFireJournal - Bob Twomey
Bob Twomey
10/18/2019 -

This article is designed to provide the rescuer with an understanding of the forces associated with soil “failures” and factors directly related to soil destabilization and collapse. It is not all-inclusive of how to perform trench rescues; this can ONLY come from taking certified trench rescue coursework, such as in the Chapter 8 Trench Rescue Standard for Technical Rescue. Likewise, my intent is not to make soil scientists out of you, but rather to give you the basic components of geologic features you will deal with when performing trench rescues.

Generally, if you encounter a soil type that has rock particles in it, the soil will usually be less stable. Soils containing a significant amount of gravel will almost always be unstable, meaning they will be more likely to be unable to support themselves when a trench is dug through them.

I cover these aspects of trench rescue as they relate to soils, water, hydraulic and atmospheric loading, and related forces/weights that are important factors in understanding how to evaluate — and then perform — a trench rescue. This is similar to the information format used in the article I wrote regarding earthen dams and how/why they fail (CFRJ Summer 2017). I believe that to understand the principle characteristics — of the elements that cause a rescue to be needed — are critical as the FIRST step in understanding how a rescue is performed. As I have said many times in the past: “you don’t know where you are going until you know where you have been.” You cannot understand and perform any rescue without knowing the forces of nature that substantially contribute to the cause, in this case, to trench wall failures. Now that the groundwork (no pun intended) has been established, let’s proceed.

Soils

With the exception of organic soils, which are formed from decomposed vegetative materials like leaves, stems, roots (plants and plant components) and woody vegetation that have collected over hundreds or many thousands of years, mineral soils are formed from decomposed rock “weathered” and broken down over geologic time. Rock breaks down, ever so slowly, over millions of years, to become small particles called “soil.” Over time, geo-technical forces, freezing, thawing, heat, erosion, water, wind, plant acids and so forth break rock down into smaller and smaller particles. This is what we call soil. Soil gets moved (eroded) from one place and deposited in another. Examples? The depth of the Grand Canyon or the build-up of the Mississippi delta. The soil in the delta had to come from somewhere, right?

The basic mineral soils are sand, silt and clay, going from the larger particle size to the smallest particle size. In the general landscape where trench rescues will occur, common soil types described above are mixed. Exceptions would be deep sands like are found in the coastal plain areas or clay deposits or layers found in all parts of the state. But most of the time you will encounter soil mixes, like sandy clay loam, silty clay loam, loamy sand, loam, silty clay and various other mixes. The terminology relates to the amounts of sand, silt and clay in the soil mixtures. This is important, because it relates to the relative stability of a soil when trenched.

Cohesiveness of Soils

Fine-soil particles, like pure clays and soils containing a significant proportion of clay, tend to be more “cohesive,” that is, the particles tend to stick together. As such, when a trench is dug through soil types containing more cohesive particles, a ditch wall tends to be more stable. More stable here means that the trench wall is, under most circumstances, able to stand and self-support itself. Other factors that can affect this natural stability will be discussed later in this article. However, soil types that have a greater proportion of sand, or larger less cohesive soil particles, tend to be much less stable when exposed on vertical trench wall excavation. This means that the soil profile exposed in an excavation will have less stable trench walls. Look at it this way: can you “wad up” a ball of sandier soil in your hand as well as you can a more clay soil type?

Then you can encounter soils with a higher silt content, the soil size particle bigger than clay particles but smaller than sand particles. This can tend to make a soil less cohesive than clay but more cohesive than sandier soil; thus, the rescuer wonders just how stable this soil type will be in an exposed trench wall. Generally, if you encounter a soil type that has rock particles in it, the soil will usually be less stable. Soils containing a significant amount of gravel will almost always be unstable, meaning they will be more likely to be unable to support themselves when a trench is dug through them. Therefore, you can see now that soil types are most important as it relates to trench sidewall stability.

Soil Categories

In the Chapter 8 Trench Rescue curriculum, the program divides soils into three type categories: Type A soils as the cohesive soils, for example clay dominate soil types, like clay, clay loam and sandy clay loam. Type B soils of a less cohesive nature, mainly dominated by silt, like silty clay, silty clay loam and silty loam. Type C soils are dominated by more sand content, like sand, loam and sandy loam. As you can see, trench rescue is a specialty within itself.

Other Factors Affecting Trench Wall Stability

Soil weighs 90 - 120 pounds per cubic foot. A five-gallon bucket contains 0.67 cubic feet of volume. Fill up a five-gallon bucket and see how heavy it is to pick up. It will generally weigh 60 to 100 pounds and that’s if the soil is dry. Soil contains water, commonly referred to as the field moisture content. Soil at “normal field moisture content” is damp to the touch. A gallon of water weighs 8.34 pounds depending upon how saturated the soils profile is. So, the soil weight now also contains water weight.

An item commonly not thought about in rescue evolutions is the weight of the atmosphere bearing down upon all things, including the earth’s surface. A contractor digs a trench six feet deep into the surface of the soil, now exposing the vertical cut banks to no support, that is, soil bearing upon soil for its lateral support. It would be similar to building a block wall on a footer, then you remove most of the footer, leaving the block wall to support itself. This is the cohesion of the mortar holding the blocks together as they are now suspended in the air, like the soil particles are having to do now, minus their support.

In almost all circumstances, wetter soils are less stable than drier soils. Saturated soils are very unstable. If a trench is dug seven feet deep and the contractor encounters very wet soil conditions, the ditch will almost always “slough-in” from the bottom, meaning the lower trench walls literally “wash into” the trench. Why? Hydraulic loading. Water in soil profiles usually either just sits there within the soil profile until it gradually drains deeper under the effects of gravity, or it “flows” or moves laterally through the soil profile. Either way, this water is present, and if a trench is dug that exposes this water content, it will tend to flow into the trench bringing soil with it. Consider it like this: If you saturate a sponge, then take a razor and cut the saturated sponge in half, the water runs out of the exposed sponge cuts. Likewise, in a trench excavation. Even though the water is contained within the soil profile, it is still subject to lateral and gravitational forces, only in a soil profile the water brings the soil with it.

Ground vibrations tend to make any ditch or trench cut unstable. As you can see from all that’s been discussed above, open cuts, or faces, of a trench are already destabilized. Now add ground vibrations from construction equipment, trains running close by, vehicular traffic, or even your department’s rescue equipment, and you greatly enhance the probability of a trench cave-in or collapse. Anything that causes vibration of the trench walls will make them susceptible to collapse.

Scientific studies have concluded that the speed at which a cave-in occurs is approximately 55 to 66 feet per second, or roughly 45 miles per hour. A worker caught under the top of the trench cave-in covered with only two feet of material over his body has 700 to 1000 pounds per square foot of pressure exerted upon his body, and it happened so quickly he was powerless to move out of the way.

Being trapped under this soil is very similar to being trapped in the snow of an avalanche. The material sets up tight around the body, and the victim cannot move. Over a very short period of time, the material compacts itself tighter around the victim until breathing is not possible You understand the rest of the scenario if rescue cannot occur in time.

Post Log

Trench rescue is a highly specialized type of rescue. MANY factors influence how trench rescues are performed. Here, I have tries to present the most basic components involved in trench wall failures or trench collapses. Chapter eight of the Technical Rescuer Standards is the authoritative course for trench rescue training, along with federal and state OSHA standards as they relate to trenches and rescue. I would encourage any of you interested in physics and engineering concepts of soils and trench rescue to “go for it.” It is one of the most technically rewarding rescue skill blocks you can learn and use to save someone’s life.

Until next time, go make a difference in this world one rescue at a time.

Bob Twomey has been in the volunteer rescue service for 46 years, having served on five Rescue Squads from the coast to the mountains. He is currently a member of Transylvania County Rescue Squad, past Chief and Training Chief, an EMT for 45 years, and is an IFSAC and OSFM certified Rescue Instructor. Bob has been active in SAR, Mountain Rescue, and teaches high-level rescue. He is the chief pilot of Wolf Tree Aviation, and flies helicopter searches and rescue support locally. He is a Crew Chief for the NC Forest Service. He can be reached at 828-884-7174 or at [email protected].
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