No matter how well engineered a structure is, if it’s not built on a firm foundation, the life of that structure will be significantly shortened.
The United States Department of Agriculture (USDA) classifies soils into three major categories: sand, silt, and clay.
1. Sand is the most popular soil type. It’s the largest in size of all the soil types and it contains little to no organic material. Sand is basically small rock fragments and has hardly any chemical properties, so it retains little water.
2. Silt is smaller than sand but larger than clay. It’s spherical in shape, so it contains a large amount of water and feels slippery or soapy, which makes it difficult to compact. It has more chemical properties than sand, but not as many as clay, and contains little to no organic content.
3. Clay is 2000/millimeter in diameter or smaller, yet it possesses the most chemical properties and greatest range in characteristics, such as color. Clay retains large amounts of water, which makes it stick together more easily than other soil types.
The word “loam” is used to describe soils that are a mixture of sand, silt and/or clay; these soils are generally classified as sandy loam or clay loam.
Soil testing and analysis
Soil composition identification is usually completed using boreholes (and sometimes test holes) prior to the bidding process so contractors and the owner can get a snapshot of the soil conditions the construction company will be dealing with during the construction process.
“The bored samples are sent to a lab; they analyze it and describe it according to the different parameters of the material, such as whether it’s silt, sand, or clay, as well as other qualities,” says Lou DeVuonno, Project Manager, MGI Construction Corp. “With this information, contractors will be more aware of whether the soil is a reusable material.”
Wet, silty material is not structurally sound and can’t be engineered, so oftentimes it needs to be taken off-site. If so, you may then need to backfill the excavated material with a more solid soil to bring it to grade. Contractors need to know this information ahead of the bidding process, as to include these costs in the bid.
Engineering soils
There are seven common methods for stabilizing soil; some are permanent and some are temporary:
1. Chlorides are one of the oldest and most popular methods of soil stabilization for unpaved roads. It comes available as either a solid or a liquid and both forms are applied using a water truck. It works by attracting moisture, which dries the surrounding soil, and it’s suitable for most soils but it needs to be reapplied every 6-12 months.
2. Polymers glue particles together; they are mineral-based and have many of the same properties as Portland cement. Most popular applications include dirt roads, paths, and pond linings.
“We primarily use polymers,” says DeVuonno. “When you’re trying to bridge two layers of soil, polymers provide structural integrity that allows you to support the granular material on top of the less suitable material underneath. It creates a kind of bridge between two different materials—or layers of soil—that then complement each other to create a stabilized environment.”
3. Mechanical includes the use of either vibration compaction or dynamic compaction. With vibration compaction, the soil is vibrated repeatedly until the soil particles become interlocked and physically changed. With dynamic compaction, a heavyweight is repeatedly dropped in place, which creates a stress wave throughout the soil and the weight creates a compacted surface.
4. Lime/cement is also one of the most popular in the US, especially for paved road construction. Both lime and cement are binding agents; lime is natural and more costly, whereas cement is not natural but is less costly.
5. Fly ash is a coal-based bonding agent used to increase soil strength, similar to lime/cement, and is often used in the sub-base of paved roads. There are two classes of fly ash; Class C, which gets hard when wet, and Class F, which requires an agitator, such as gypsum before it can harden the soil.
6. Geotextiles look like blankets and get laid between two layers of different kinds of soil so each soil type retains its integrity and function while still allowing water to pass through.
7. Enzymes are one of the newest methods in soil stabilization. The enzyme formula is made up of living organisms, which act as a catalyst in creating a specific biochemical reaction. They permanently bond soil particles together in a concrete-like way and continue to harden for years after application. Because it interacts with organic material, the soil should have at least 20-percent clay content.
While there are a few types of soil and a variety of methods to stabilize soil, it is crucial to complete the soil composition identification process in order to determine the correct material and method to be used specific to the project and job site.
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