Soil Stabilization and Your Highways
If your road is too soft in the spring, too dusty in the summer, and too rough year round, the problem may be the base, and stabilization might be the solution. Let’s take a closer look at what stabilization is and how it can work for you.
What is soil stabilization?
The Industrial Resources Council (IRC) defines soil stabilization as the “process of changing soil properties to improve strength and durability.” Methods include compaction, dewatering, and adding new clean aggregate to the base. Stabilizers can be used to treat the upper few inches of soil, as in dust suppression, or to treat the base to help the existing in-place material support traffic.
The plastic limit is the moisture content at which a snake of soil 1/8” in diameter begins to break apart. It is a measure of how plastic the fine particles are.
The liquid limit is the moisture content where the fine material begins to behave more like a semi-liquid than a plastic or solid.
The difference between the liquid limit and the plastic limit is the plastic index. The greater this difference the more problems will occur in a roadbed when there is excess moisture.
Gravel: Testing. Cornell Local Roads Program.
The US Forest Service has classified soil treatment processes into several categories with two being commonly used in New York State:
- Chemical or Bituminous
Improves the compaction of soils, acts as a binder and/or water repellant, and makes soil stronger. Chemical stabilization is often used in dust control, and erosion control.
Involves the mixing of soils and other materials to improve compaction and drainage by changing the engineering properties of the existing soil.
In this article, we will focus on the chemical / bituminous options.
Chemical stabilizers are identified as traditional and non-traditional (see table below for examples). In order to get the best results, several factors should be considered including, soil type, the goal of the stabilization, the necessary strength and durability required, cost, and environmental conditions.
Traditional chemical stabilizers
Non-traditional chemical stabilizers
Additionally, it is important to determine soil classification for the correct application. This can be determined by conducting grain size distribution (sieve analysis), and Atterberg limits tests (plasticity). The grain size distribution separates the material by particle size to measure the amount of fines in a given soil sample. The Atterberg limits test measures the water content and plasticity of fines component of a soil. For more information on the necessary testing and the stabilizer selection process, refer to “Testing the soil” in the Forest Service publication Stabilization Selection Guide for Aggregate and Native-Surfaced Low Volume Roads, Appendix D and B respectively.
Traditional stabilizers have been around and used for many years. They have been studied and tested to determine the effectiveness of each process. Many have been used for over 50 years and have proven track records as stabilizers.
Cement is commonly used to stabilize a subgrade or base. The addition of cement to the soil increases soil strength, decreases compressibility, reduces the potential for swell, and makes it more durable. Cement can be used in most situations, except in soils with high organic contents. It is readily used with fine grained and/or sandy soils. Adding the proper amount of cement is critical and dependent upon soil characteristics; excess cement can cause the subgrade/base material to become brittle, resulting in cracking and other distresses. In general, cement stabilization creates a hard, bound, impermeable layer on which a road can be built. Existing bases treated with cement can be crushed/pulverized and reused as fill material.
Lime, like cement, is typically used as a base stabilizer. Lime works best on clayey soils, but not very well with silts and granular materials. Since most subgrade and base soils in New York State are silts or granular in nature, lime is not commonly used here. In general, the addition of lime decreases the soil density, decreases plasticity, and improves workability. An improvement in the soil strength will depend on the soil type. Lime can also reduce the shrink/swell properties of clay soils.
Lime has been used in New York State to help dry out wet soils during construction. Removing water reduces the plasticity of the material making it more workable. If lime is used as a temporary surface treatment, it should be further treated with a surface wearing course since it does not have the durability to resist traffic abrasion.
Fly ash is mainly a base/subgrade stabilizer. Surface use is limited to reducing the water content of the soil to make it more workable. Fly ash, a product of coal combustion from power plants, can be used with most materials including silts, clays, sand, and gravel, to reduce moisture, improve workability, lower plasticity, and increase the strength and stiffness of the soil. The combination of too much fly ash and excessive moisture, from drainage or rainfall, can make the subgrade soft and weakened.
Fly ash content is dependent on the source of the coal burned, however most fly ash contains heavy metals which can leach out and cause water quality issues impacting aquatic life. Fly ash application around sensitive waters should be controlled to avoid any potential leaching of the material into the water body.
The use of bituminous materials for stabilization is a different approach than using lime or cement. Bituminous material acts to waterproof cohesive (sticky) soils such as clay. It both waterproofs and binds non-cohesive soils (sand and gravel). Waterproofing increases soil strength by preventing or reducing the intrusion of moisture into the base. Fines contents should be between 10 and 15 percent in most cases to be economical.
Asphalt emulsion is typically used for stabilization. The type of emulsion will depend upon the soil type and the amount of fine material present. Medium- and slow-setting asphalt emulsions are most commonly used, but you should test the soil to determine the correct type of bituminous material for your project.
Non-traditional stabilizers are being increasingly used due to their effectiveness and because they can be used on the surface for dust control. Many are proprietary and have not been thoroughly tested independently, so confirming their effectiveness can be a challenge.
Chlorides, or salts, such as calcium chloride (CaCl), magnesium chloride (MgCl), and sodium chloride (NaCl) have been used for years in New York State and many local agencies would consider them traditional. They are most commonly used for dust suppression. Chlorides work by using moisture from the air to bind the surface fines together. Calcium and magnesium tend to have a longer lifespan as they pull moisture from the air while sodium chloride (rock salt) does not. They also help the road surface to retain moisture and resist evaporation. The life expectancy and durability of chloride application is dependent on the amount of traffic and rainfall. As the speed and volume of traffic increases, the effectiveness of the stabilization decreases. Due to the potential for leaching, application of chlorides should not be done if rain is expected within 24-hours. To further protect against potential leaching, a buffer zone of 25–30 feet should be provided between the treated road and any nearby bodies of water or sensitive tree species.
Clay additives are typically used to stabilize non-cohesive, non-plastic crushed aggregate. However, adding clay is risky and not likely to be economical in New York State.
Electrolyte emulsions are used for soil stabilization and dust suppression. Electro-chemical bonding within the soil replaces water molecules, and results in the soil rejecting water and allowing the soil particles to bond, reducing dust. Most electrolyte emulsions are proprietary in nature and exactly how they work is not completely known. They are considered to be effective on most soils provided there is a clay content of greater than ten percent (which is rare in New York State).
Enzymatic emulsions, also typically proprietary, are protein molecules (enzymes) that react with the soil to bind the soil particles. They work well in reducing dust when applied at low application rates. At higher application rates, they can help stabilize soils. They are found to be most effective in soils with a clay content between 12 and 24 percent and a plasticity index between 12 and 35 percent. Again, such soils are rare along highways in New York State.
Lignosulfonates are derived from the plant resins that naturally bind the plant fibers together. They draw moisture from the air to keep the road surface moist which makes them effective for dust suppression when applied at low application rates directly to the surface. Lignosulfonates can also be mixed into the base to help stabilize and increase compressive strength and load capacity. Lignosulfonates can be used with most soil types but are most effective on soils with 8–30 percent fines and a plasticity index of greater than 8. Lignosulfonates are water soluble; they should not be used with highly permeable soils since they can leach out easily. Because of this they do not have a long life expectancy and may require multiple applications during the year.
Synthetic Polymer Emulsions
Synthetic polymer emulsions consist of acrylic and acetate polymers that form a bond with the soil particles. These polymers are often by-products of the adhesive or paint manufacturing industries. These polymers can be used on most soils, but application should be specific to the existing soil type for best results. They are most effective on silty sand materials with fines content between 8 and 20 percent and a plasticity index of greater than 8. Synthetic polymers require additional time to cure, it is therefore recommended that they should not be applied unless there is a minimum of 48-hours of no rain after application.
Tree Resin Emulsions
Tree resin emulsions are made from spruce, pine, and fir tree resin. These resins are combined with other additives to form an emulsion that can be used for dust suppression and soil stabilization. Tree resins work best in silty sands with a fines content between 5 and 30 percent and a plasticity index of below 8.
Deciding What to Do
Chemical stabilization methods are very dependent on the type of soil to be treated. Test your subgrade or base to determine the amount of fines and soil type. Knowing this information can help you determine the correct treatment and application. Application rates will vary by soil, location, and the reason for material application. Typically a heavier application is used when the product is mixed into the material to provide base stabilization. Traditional stabilizers, with the exception of bitumen, focus more on base stabilization while the non-traditional methods focus on both dust suppression and base stabilization. The application of any dust suppressant should provide an adequate window for the material to stabilize within the soil, typically a minimum of 24-hours but in some cases even longer.
The following table should help you decide which method to use:
If your fines content is between...
No stabilizer needed
Asphalt emulsion or foamed asphalt
*Sand equivalent greater than 35 and plasticity index less than 2
**Sand equivalent greater than 30 and plasticity index less than 6
This has been a very quick overview of chemical stabilization techniques. The take home message is that there are many choices to stabilize your highway and help it handle the loads that use it.
Soil Stabilization. Industrial Resources Council (IRC).
Stabilization Selection Guide for Aggregate and Native-Surfaced Low Volume Roads. US Forest Service.
Dust Palliative Selection and Application Guide. USDA Forest Service.
Upgrading Your Roads. Cornell Local Roads Program.
Asphalt Paving Principles. Cornell Local Roads Program.