Understanding the Importance of Soil Types and Grades


Knowledge of specific soil types and grades are an essential part of any construction project. Diverse soil types have different properties and based on the project type, certain soil grades are better suited for construction work than others. Builders use a variety of tools and techniques to assess the soil on a construction site and determine its properties, such as its bearing capacity, permeability, and compressibility. This information is then used to design the foundation of the building, which is the most critical part of any structure. The foundation must be strong enough to support the weight of the building and resist any forces that may act upon it, such as wind, water, snow, or seismic activity.

In previous posts, we’ve examined how erosion and sediment control measures go hand-in-hand with state and federal regulations on construction projects. Today, we will discuss the local soil types, why they matter, and how to properly identify these types with your next construction project in mind.

Oregon Soil Types

There are thousands of soil varieties all over the world. In Oregon, we have a specific soil type perfectly suited for wine growing, aptly named the Jory soil. The Jory soil is a reddish-colored volcanic soil that is rich in clay, iron, and other essential nutrients that provide excellent growing conditions for pinot noir grapes. In addition to the Jory soil, the state of Oregon has over 2,000 soil types that make it prime for agricultural growth.

Agriculture and winery uses are not the only industries affected by soil types. For example, in construction work, soil types and grades are essential in determining load-bearing capacity for building designations, excavation safety measures, and more.

Soil Categories and Types

There are various types of soil that include:

Most soils are a combination or mixture of clay, silt, and sand, and although its composition cannot be fully identified in the field, it still can be evaluated in a few different ways. In addition to types of soil, there are two characteristics of soil:

Cohesive soil is made up of fine particles and contains enough clay to stick to itself. The more clay in the soil, the more cohesive it is and the less likely it is to cave in.

Granular soil, on the other hand, is made up of coarse particles like sand or gravel and will not stick to itself. The less cohesive the soil, the more measures are needed to prevent a cave-in.

Soil Classifications

There are four types of soil classifications:

  • Solid Rock
  • Type A
  • Type B
  • Type C

Type A soil is the most stable for excavation, while Type C is the least stable. It’s important to note that a single utility trench, for example, may cut through more than one type of soil.

Type A

Type A soil is identified as cohesive and has a high, unconfined compressive strength, with a minimum of 1.5 tons per square foot. Examples of this type of soil include clay, silty clay, sandy clay, and clay loam. However, soil cannot be classified as type A if it has been previously disturbed or is currently fissured, has water seeping through it, or is subject to vibration from heavy traffic or pile drivers.

Type B

Type B soil is cohesive but not as well-bound as Type A soil. It is frequently cracked or disturbed and may have pieces that do not stick together well. The unconfined compressive strength of Type B soil is medium, ranging from 0.5 to 1.5 tons per square foot. Some examples of Type B soil are angular gravel, silt, and silt loam, as well as soils that are fissured or near sources of vibration but could otherwise be classified as Type A.

Type C

Type C soil is the least stable type of soil, consisting of granular soils with non-sticky particles and cohesive soils with an unconfined compressive strength of 0.5 tons per square foot or less. Examples of Type C soil include gravel and sand. Soil with water seeping through it is also classified as Type C soil, regardless of its other characteristics.

Determining Soil Types

When determining which soil type is found at the construction site, OSHA uses a measurement called “unconfined compressive strength” to classify each type of soil. This measures the amount of pressure that would cause the soil to collapse and is usually reported in tons per square foot. A competent person should complete the following tests to determine which type of soil before construction begins:

The Pencil Test

Also called the plasticity test, this is an easy way to determine how cohesive the soil is. The test is done by rolling a moist sample of soil into a thread that is about 1/8 of an inch thick and 2 inches long–to resemble a slim pencil. If the sample does not break, it is cohesive.

The Thumb Penetration Test

This can be used quickly to estimate the compressive strength of the soil sample. To test soil type, press your thumb into a fresh clump. Type A takes great effort to indent, Type B sinks to the thumbnail, and Type C sinks all the way. The results of this test may vary.

Pocket Penetrometer Test

This small piston device offers a numerical measurement, although results may vary based on soil samples. To carry out the test, insert the piston into the soil until it reaches the marked line. After that, obtain the reading from the scale indicator. Please note that if the soil has rocks or pebbles, which cannot compress, the penetrometer might produce inaccurate results.

As you can see, there are numerous tests that can be performed to determine soil types; however, results are not always 100% accurate. To ensure worker safety, OSHA suggests conducting at least two tests to determine the appropriate methods for sloping, benching, or shoring to prevent cave-ins.

Final Thoughts

In conclusion, soil types and soil grades have played a significant role in the history of construction work, and they continue to be important today. By understanding the properties of the soil on a construction site, builders and contractors can ensure that their structures are safe, stable, and built to last for generations to come.

If you need an expert opinion or consultation on your new construction project, please contact us, and our dedicated team and project managers can help you build it right.