All you need to know about road stabilisation

In some locations, the soil is naturally weak and unsuited to make a stable base for road construction. Traditionally, higher quality aggregates are brought in to upgrade the road base.  However, as this can be expensive, additives are also mixed in to improve the soil properties. This is not a new development in road construction, but a technology that has been common practice for many centuries. The Romans would commonly mix weak soils with pozzolana (volcanic ash containing alumina and silica) and lime to improve load-bearing capacity. Pozzolana-like materials and lime are still widely used today, but road stabilisation has also developed to offer tailor-made solutions better suited for the natural soil and the road requirements. 

With road stabilisation, the properties of the base layer of the road are improved. The objective is to increase strength, reduce plasticity and lower compressibility either by binding the soil particles, waterproofing them or a combination of both. With an improved road-base, you obtain roads with enhanced load-bearing capacity and stability, and the degradation of any overlying surface will be greatly reduced. Proper road stabilisation will, by minimising road wear, decrease maintenance needs which in turn leads to requiring less labour and a better economic solution.

In some cases, by upgrading the road base, the asphalt or concrete layers can be reduced while still maintaining the same structural integrity of the road.  This offers another way to save on road construction costs.

Mechanical vs. chemical stabilisation

There are several methods of stabilisation, which can be grouped into two categories, namely mechanical stabilisation and chemical stabilisation.

Mechanical stabilisation is the process of improving the properties of the road material by changing its gradation. This is accomplished either by mixing two or more soils or by placing a nonchemical, nongranular material in or on the soil to provide added strength (such as geonets or geotextiles).

Chemical stabilisation is the process of incorporating a binder such as cement, lime, fly ash, lignin-based biopolymers, foamed bitumen, or combinations of these materials, with the base to form a stronger composite material. Smaller amounts of chemical additives can “modify” properties so as to improve workability and/or reduce plasticity, whereas a larger addition of additive would improve strength and durability.

The different chemical stabilisation additives will be discussed more in detail in Section 6 below: “What binders exist for road stabilisation?”. The different binders used for road stabilisation and how to apply them is the focus of this document.

Learn more about Dustex® - Borregaard's cost-effective, sustainable and environmentally friendly binder for road stabilisation and full-depth reclamation (FDR)

A commonly used engineering technology for road stabilisation is Full Depth Reclamation (FDR). It is used for road rehabilitation where an old deteriorating asphalt road is milled or crushed in-situ, and the milled asphalt is intermixed with the underlying road base materials. This mix of old asphalt and old road base becomes the new road base - a process often called road-reconstruction or road rejuvenation. However, often this new road base still provides inadequate base strength or structure. So additional stabilisation by, for example, the addition of binders is required to meet specifications. After shaping and compaction, the stabilised base is ready for a new surface paving.

FDR is a sustainable road construction technology as it recycles the existing roadway. Instead of removing the old pavement and hauling in new granular material, the existing material is pulverised, homogenised and upgraded with stabilising additives and reused on site. The reduced trucking to and from the site saves materials, time and money compared to traditional remove-and-replace construction methods. With the reduced material replacement, FDR technology has low greenhouse gas emissions with fuel savings and less materials (i.e, asphalt) required to renew the road base.

Good quality aggregates may not always be available near the road construction site. Hauling supplementary aggregates from a long distance may not be economically feasible, and is a non-sustainable solution considering the fuel consumption and greenhouse gasses emitted from moving the materials. Using existing aggregates from the site also means you avoid the issue of landfilling of old materials. Where possible, locally sourced aggregates of inferior quality should be milled and upgraded with use of a stabilising agent. Road base stabilisation is a cost-effective and environmentally friendly method of converting below standard road material into good construction materials.

By applying proper road stabilisation when constructing the road, the maintenance need is greatly reduced. This saves transport of materials and equipment, reduces cost and is less labour intensive. Over time, the total emissions from road construction with a stabilised road base are greatly reduced.

Some road stabilisation additives, or road material itself, can have a negative environmental impact. Choosing a sustainably sourced road stabilising agent has become an important priority for many customers today. When making such a decision you should consider the Life Cycle Analysis (LCA) of the product and how it affects the surrounding ecosystem.

LCA: see how you can reduce your CO₂ emissions by more than 80% by replacing cement or bitumen with Borregaard's sustainable biopolymer Dustex

Road stabilisation is common practice in modern road construction and maintenance for public, private and commercial use. It is mostly used for roads with medium to low traffic volume. Both paved and unpaved roads benefit from a stabilised road base. Stabilisation is used on a spectrum of roads including: municipal; county; private; bike; haul; rural; forest; and service roads. It can also be used in hardstands and lay down areas.

When the base of the road has insufficient strength and high plasticity and/or compressibility, the load-bearing capacity of the road is weak. The porous, unstable base is also more subject to water infiltration, reducing the strength and stiffness of the base and making it more vulnerable to freeze/thaw and shrink/swell issues. The problems originating from the road base will eventually manifest themselves on the road surface. Cracking, potholes, frost heave, ravelling, rutting and washboarding are all common issues that are aggravated by an unstable road base. This causes a poor driving surface requiring reduced speed and affecting comfort. It is also a safety risk as traffic avoiding road damage instances can cause serious accidents.

By applying a stabilising agent, the strength of the road base is improved, and point loading of vehicle wheels is spread more uniformly. This provides for greater load-bearing capacity of the road, which is able to withstand greater stress and strain while resisting deformation. The strengthening of the road base gives a longer-lasting road with increased structural capacity and durability. Some stabilising agents also improve the road modulus of elasticity and are more resistant to freeze-thaw cycles. This will increase the life of the road and reduce the frequency of maintenance over time, saving resources, time, and money.  As the quality and longevity of the road improve through stabilisation, the stabilised roads are generally safer and provide more comfort for vehicles.

Chemical road stabilisation additives create a strong matrix of the road aggregates. Either the stabilising agent reacts chemically with the road material (e.g., lime additive reacting with clay), or they react on their own forming a cementing compound around the aggregates. The additive should provide sufficient improved strength and impart stability to the road. Too little stabilising agent will not create a strong enough matrix, and the sublayer will deform under stress and strain. Too much stabilising agent may cause a highly rigid matrix forming a brittle sublayer that can induce cracking in the overlaying pavement.

The most important parameter to consider when selecting a road binder is that it suits the soil type. Whether the soil consists of fine-, or coarse-grained particles or has a high content of clay, silicates or organic content are determinants in the selection.  In addition, weather conditions and seasonal variations, estimated traffic load, compliance with local laws and regulations and sustainability of the binder all influence the choice of road stabilising agent. Some commonly used stabilising agents are listed below. Each can be used on its own, or in combination with others to treat the road base material. For a more extensive guide on these and other road stabilisation additives, see this guide.

Lime is used primarily to treat fine-grained soils with a plasticity index of 20 or greater. The calcium oxide (CaO) component of the lime reacts with clay particles to reduce plasticity. Lime reacts chemically and hardens in the presence of water. Cementation takes place slowly. Common forms of lime include hydrated high-calcium lime, monohydrated dolomitic lime, calcitic quicklime, dolomitic quicklime and lime kiln dust (LKD), a by-product of lime production.

Cement is similar to lime but contains pozzolanic materials that cause rapid hardening, resulting in a solid, bound, impermeable layer. Fine-grained and sandy soils with a plasticity index below 20 are readily stabilised with cement. Cement should not be used for soils with high organic content or soils that contain sulphates. Cement can create an over-rigid matrix and is not a good choice for roads subjected to seasonal freezing.

Fly ash or coal ash are mineral residues from the combustion of pulverised fuel (usually coal).  When fly ash is mixed with lime and water, it creates a pozzolanic reaction and is cementitious. Coarse-grained soils with little to no fines can be stabilised using lime fly ash or lime-cement fly ash. Similar to cement, it works best for soils with a plasticity index below 20 and low organic content.

Bituminous materials such as coal tar, bituminous emulsions, foamed bitumen and cutback bitumen are widely used for road stabilisation. Bituminous stabilisation is not actually a chemical stabilisation as there is no chemical reaction between the binder and the soil. Rather, the strength gain is achieved by coating aggregate particles, both waterproofing the particles and aggregating them by the development of adhesive bonding. Since it does not depend on a reaction with soil particles, bituminous stabilisation works well for almost all types of soils. Ageing and oxidising of bitumen can make it brittle over time.

Lignin based products are biopolymers derived from the lignin that naturally binds cellulose fibres giving trees their stiffness. It forms physical and chemical bonds with itself and with the soil particles, binding the road materials together. Lignin based products can be used for a variety of road materials but are most effective for soils containing up to 30 percent fines and a plasticity index greater than 8. Some studies have shown little to no improvement for soils with a high plasticity index (i.e., greater than 20). They do not work as well for sandy, permeable soils which allow rapid leaching of the product. For soils with high clay contents, the treated soil tends to remain slightly plastic.

Lignin based products are considered a green alternative to bitumen/cement for road stabilisation. Lignosulfonates from Borregaard, manufactured from wood pulp production, are examples of such lignin-based products. Available LCA data shows that lignin-based biopolymer production has lower CO₂ emissions and consumption of fossil fuels compared to bitumen and cement. This natural biopolymer has low toxicity towards aquatic and terrestrial species and can, hence, be regarded as an eco-friendly alternative when used for road stabilisation.

Read more about Dustex, a lignin-based biopolymer for sustainable road stabilisation.

When making a stabilised road base, you should expect to see an improved road. By strengthening the foundation, the load-bearing capacity is improved as well as the resistance to stress and strain that could result in road deformations and loss of road materials. Proper selection and use of a binder, combined with good compaction and drainage will reduce the infiltration of water, mitigating the effects of seasonal variations and freeze-thaw cycles.

Chemically stabilised roads coupled with proper maintenance have a much greater lifespan before requiring rebuilding. Depending on the road and the type of binder, an unpaved road could remain viable for up to ten years. Paved roads can keep a high standard for much longer, and are less subject to frost-heaves, rutting, cracking and potholing. Because of the reduced maintenance needs and longer lifespan, a stabilised road will provide long term savings in labour, materials and the environment.

Information regarding the environmental impacts of the stabilising agent should be easily available for you. Make sure to choose an eco-friendly and sustainable road stabilising product for your road base stabilisation needs.

Learn about a road stabilisation study that was initiated in Norway in 2001, and see how roads stabilised with Dustex® still performed after fifteen years! 

The most important factor when selecting a stabilising agent is the type of soil to be stabilised. The uniformity, gradation, plasticity and texture of the soil aggregate material determines which binder is best suited. Often, soil quality can be assessed based on past experience in the same area or by conducting soil tests prior to treatment. The presence of clay, silt, organic materials, sands and gravels all affect the binder performance.

If significant quantities of clay and silt are present (plasticity index >20), the proper stabilising agents are hydrated lime, quicklime or a combination of lime and Portland cement. (lime reacts with the clay particles to reduce plasticity). If a much coarser material is present (plasticity index <20), additives such as Portland cement, fly ash and cement kiln dust are more suitable. Bituminous stabilisation is the best option for cleaner aggregate materials, such as sand and gravels that have a low percentage of silts and clays.

You should consider the purpose of the stabilised layer (paved surface or not), and the required strength and durability (depending on anticipated traffic volume). Climate conditions such as freeze-thaw cycles, heavy rainfall or drought periods can affect the stabiliser performance in extreme cases and should also be taken into consideration. Another important factor is, of course, the cost of the additive.

The ability of the binding agent to reactivate is important for unpaved gravel roads. Choosing a lignosulfonate to stabilise an unpaved road means the road can be reworked during later maintenance and regain its binding properties. Some products will set when cured and cannot be reactivated with normal ripping (scarifying) and reshaping (blading) with routine maintenance equipment. Examples include Portland cement, lime, and asphalt/ emulsions. Another benefit of lignosulfonates on unpaved gravel roads is that in addition to stabilising the road base, they also contribute to dust suppression and reduce the loss of fines from the road-surface.

Worker safety and equipment needs should be considered when picking a stabilising agent. Bituminous stabilisers are applied at high temperature, which can be hazardous for workers. Cement is caustic and abrasive. In contact with skin, it can cause irritation, allergic reaction and burns. The silica content in cement can cause silicosis of the lungs with prolonged exposure.   

Over the years, sustainable additives have been developed as alternatives to bitumen and cement, without compromising performance. Sustainability has become a priority for many users and is often a requirement in company and governmental guidelines. Lignin based biopolymers from Borregaard have been shown in independent research to have as much as 80% lower carbon emissions (in CO₂ equivalents) when used for road stabilisation compared to cement and bitumen.

Read more about Dustex, a lignin-based biopolymer for sustainable road stabilisation.

1. Preparing the site

The first step in road stabilisation is an assessment of the existing roadway. Examine the road surface and determine if there is a need for improving the border ditches, or if it is necessary to add gravel to improve the soil characteristics. Poor drainage is the main reason for road failure and is essential before continuing with road construction.

The existing soil is ground by a crusher or reclaimer, converting the road material into finer particles. Typically, the road is milled 20-30 cm. While crushing, the moisture content of the soil can be adjusted with damping either with a separate water truck spray or in the rotary mixer.

2. Mixing in the additives

Once the road has been crushed and homogenised, it is ready for the addition of a stabilising agent. The application method depends on the type of additive. Dry powders can be spread out. Liquid solutions are typically sprayed on. A stabilising machine is used to thoroughly mix the stabilising agent and the homogenised road material together. Some additives should be milled as soon as possible to facilitate even mixing as they set very quickly. Some additives require multiple applications and mixing to be fully incorporated.

3. Shaping and Grading

The roadway is shaped and levelled with a grader.  The thickness of the stabilised layer across the width and length of the road (grade and cross-slope adjustments) should be maintained.

4. Compaction and Finishing

Then compact with light vibration aiming to increase the maximum density of the soil for improved load-bearing capacity. Roller types include vibratory pad foot, a pneumatic compactor and tandem drum roller. Proper compaction is essential to achieve the required density and avoid saturated water content in the soil.  Both increased density and decreased moisture content potentially increase strength. 

After compaction, the road surface is shaped and trimmed to achieve a smooth surface, good crown, proper grade, etc. If there is not enough material to grade, the addition of new gravel might be needed to shape the road.

After shaping the road, it is often compacted again to ensure high density.

5. Curing

After the stabilised road is formed, it is important to allow time for drying and curing. The curing time depends on the type of additive being used and can be anything from a couple of days to a few weeks.

6. Wearing course

Many stabilised roads are kept unpaved. Sometimes a wearing course can be established on the road surface. This can be a paved asphalt surface, gravel or chip seal.

Learn how to apply Dustex®, a sustainable binder for road stabilisation and full-depth reclamation. The product is environmentally friendly, robust, cost-effective and easy to use