Trenching is ranked as one of the most dangerous types of construction work in the United States, according to the Occupational Safety and Health Administration (OSHA), with an average death rate of 40 construction workers per year. Within mere minutes of being caught in a trench cave-in, workers are at risk of suffering serious injury or peril. Given the significant soil excavation hazards, it’s necessary for construction teams to implement safe practices, including conducting tests to classify soil types prior to digging.
OSHA requires every employer engaged in this work to have a competent, trained individual assess and define soil types for the purpose of predicting its potential to move or collapse while employees are on site. The types are determined based on specific characteristics related to consistency, visual appearance and other factors.
This article breaks down the various soil types as classified by OSHA, offering valuable insights into the precariousness of soil excavation and the need for related safety measures. It also discusses the role air knife methods play in preventing cave-ins and other soil excavation hazards.
Overview of Soil Conditions
Before jumping into the specific classifications of soil for excavation purposes, let’s take a closer look at what soil conditions exist and how they are characterized. Soils are often described as either cemented or cohesive:
- Cemented soil is that in which the particles are held together by some chemical agent (like cement). This soil is unable to be broken by hand or finger pressure, but it is not quite solid rock.
- Cohesive soil is moist and formable. It does not crumble or break into pieces, and it tends to come out in clumps when excavated.
The kinds of soil being excavated can range in consistency and generally fall into one of the following categories:
- Sand or gravel, which is found in various sizes and shapes. This category of soil breaks easily and doesn’t clump.
- Silt contains a mixture of particles (like sand, clay and loam), and falls on the sizing scale between sand and clay. Silt is usually found suspended in water at the edge or bottom of a creek or pond.
- Clay is a moist and formable soil. Its smallest particles are cohesive.
- Loam is light and fluffy, consisting of equal parts sand, silt and clay plus additional organic material. Loam is similar to mature compost and better than top soil.
The larger the grain size of soil, the less stable that soil is. So for soil excavation purposes, it’s necessary to compare cohesiveness versus granularity. If it’s cohesive, that means it contains fine particles and enough clay so that the soil sticks to itself. The more cohesive the soil, the more clay it has, making it less prone to caving in.
Granular soil, on the other hand, is made of coarse particles such as sand or gravel and does not stick to itself. The more granular (or less cohesive) the soil, the greater the measures needed to prevent a cave-in. As soil is composed of different elements, including organic material, air, water and mineral matter, the particular amount of each substance found in the soil plays a part in its overall cohesiveness.
Breakdown of Soil Classifications
It’s certainly true that hundreds of different types of soil exist in the world, but for the purposes of classifying them for safe soil excavation, OSHA uses four designations. Following are the four soil types listed in decreasing order of “unconfined compressive strength,” or a measurement (usually reported in units of tons per square foot) of how much pressure will cause the soil to collapse:
- Stable Rock
- Type A
- Type B
- Type C
Type A soil is the most stable in which to excavate, while Type C is the least stable. Of course, soil properties can vary widely within a single trench, which means the type of soil changes as you move from top to bottom or across its length. Where soils are configured in layers, their classifications must be based on the weakest soil layer. OSHA requires that classifications of the soil be based on at least one visual and one manual test (such as the Plasticity Test, the Thumb Penetration test or the Pocket Penetrometer test).
Essentially, all of these classification efforts help safeguard workers against dangerous soil excavation hazards like cave-ins. Knowing the type of soil makes it possible to determine the right protective system to keep workers safe when working at an excavation site.
Air Knife Method for Safe Soil Excavation
When it comes to keeping workers involved in soil excavation protected from harm, there are a number of factors to consider (and compliance standards to maintain). That’s why it’s essential to employ the smartest, safest excavation methods.
The air knife is an increasingly leveraged tool at various types of construction sites because of its ability to excavate in an effective yet non-destructive way. Using a high-pressure airflow, the air knife works to safely penetrate and loosen ground soil, which is then vacuumed into a debris tank for later backfilling or disposal. Air excavation also eliminates mud disposal issues.
Soil excavation by way of air knife enables workers to remain at a safe distance from trench hazards and help prevent cave-ins. Given its softer approach to digging, it also mitigates accidental damage to dangerous underground lines and buried infrastructure. Using an air knife, crews can accurately locate the depth of existing utilities.
Air is an essentially unlimited resource, given that air compressors provide for an infinite supply of the pressure needed to dig. And as an excavation method, it’s often the optimal choice when working around live electrical wires, as air is nonconductive. Because it’s an immensely more advanced and less cumbersome option than traditional methods, air knife excavation supports greater safety and reduced risk for soil excavation projects.
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