What is Soil Stabilisation?
Soil is one of nature’s most abundant construction materials. Almost all construction is built with or upon soil. When unsuitable construction conditions are encountered, a contractor has four options;
Find a new construction site
Redesign the structure so it can be constructed on the poor/weak soil.
Remove the poor soil and replace it with good soil or stone.
Improve the engineering properties of the site soils.
In general option 1 and 2 tends to be impracticable today’s competitive environment. While in the past, option 3 has been mostly commonly used method. However, due to improvement in technology coupled with increased transportation costs, option 4 is being used more often today and is expected to dramatically increase in the future.
Improving an on-site (in situ) soil’s engineering properties is referred to as either “soil modification” or “soil stabilisation.” The term “modification” implies a minor change in the properties of a soil, while stabilisation means that the engineering properties of the soil have been changed enough to allow field construction to take place.
There are two primary methods of soil stabilisation used today:
Mechanical
Chemical
Nearly every road construction project will utilize one or both of these stabilisation techniques. The most common form of “mechanical” soil stabilisation is compaction of the soil, while the addition of cement, lime, bituminous, or other agents is referred to as a “chemical” or “additive” method of soil stabilisation. There are two basic types of additives used during chemical soil stabilisation: mechanical additives and chemical additives. Mechanical additives, such as soil cement, mechanically alter the soil by adding a quantity of a material that has the engineering characteristics to upgrade the load-bearing capacity of the existing soil. Chemical additives, such as lime, chemically alter the soil itself, thereby improving the load-bearing capacity of the soil.
Why and when is it used?
Traditionally, stable sub-grades, sub-bases and /or bases have been constructed by using selected, well-graded aggregates, making it fairly easy to predict the load-bearing capacity of the constructed layers. By using select material, the engineer knows that the foundation will be able to support the design loading.
Gradation is an important soil characteristic to understand. A soil is considered either “well-graded” or “uniformly-graded” (also referred to as “poorly-graded”). This is a reference to the sizes of the particles in the materials. Uniformly-graded materials are made up of individual particles of roughly the same size. Well-graded materials are made up of an optimal range of different sized particles. With “uniformly-graded” soils, stabilisation can prove difficult especially during compaction, with the soils acting like a bag of marbles creating problems with binding and reducing density.
It is desirable from an engineering standpoint to build upon a foundation of ideal and consistent density. Thus, the goal of soil stabilisation is to provide a solid, stable foundation. “Density” is the measure of weight by volume of a material, and is one of the relied-upon measures of the suitability of a material, and is one of the relied-upon measures of the suitability of a material for construction purposes. The more density a material possesses, the fewer voids are present.
Voids are the enemy of road construction; voids provide a place for moisture to go, and make the material less stable by allowing it to shift under changing pressure, temperature (freeze/thaw) and moisture conditions.
Uniformly-graded materials, because of their uniform size, are much less dense than well-graded materials. The high proportion of voids per volume of uniformly-graded material makes it unsuitable for construction purposes. In well-graded materials, smaller particles pack in to the voids between the larger particles, enabling the material to achieve high degrees of density. Therefore, well-graded materials offer higher stability, and are in high demand for construction. With the increased global demand for energy and increasing local demand for aggregates, it has become expensive from a material cost and energy use (sustainability).
Traditional thinking; to remove inferior/weak soils and replace them with choice, well-graded aggregates, another way to reduce the amount of select material needed for base construction is to improve the existing soil enough to provide strength and still conform to engineering standards. This is where soil stabilisation has become a cost-effective alternative essentially; soil stabilisation allows engineers to distribute a larger load with less material over a longer life cycle.
Advantages of soil stabilisation:
Stabilised soil functions as a working platform for the project
Stabilisation waterproofs the soil
Stabilisation improves soil strength
Stabilisation helps reduce soil volume change due to temperature or moisture
Stabilisation improves soil workability
Stabilisation reduces dust in work environment
Stabilisation upgrades marginal materials
Stabilisation improves durability
Stabilisation dries wet soils
Stabilisation conserves aggregate materials
Stabilisation reduces cost
Stabilisation reduces trucking of materials
Stabilisation conserves energy
Need of Stabilisation
Aggregates are important ingredients of pavement structure. Good quality aggregates may not always be available nearby road construction site. Transporting the aggregates from long distance may not be economically feasible. Under such circumstances, locally available inferior quality material like soil or industrial waste material like fly-ash may be proposed to be used in pavement as base/sub-base material. These materials are of inferior quality, and hence may not satisfy the requirement as pavement material. Therefore, engineering properties of these materials are modified by means of a process, known as stabilisation. This is not only economic solution, but also offers a potential use of the industrial/domestic waste materials. Advantages of stabilisation are summarised in the following:
Improved stiffness and tensile strength of the material.
Reduction in pavement thickness.
Improved durability and resistance to the effect of water.
Reduction is swelling potential
Mechanism of stabilisation can be broadly divided into two categories, mechanical and chemical stabilisation. Mechanical stabilisation includes compaction, blending of aggregates to improve gradation, and addition of asphalt. Asphalt, as a stabilizer, does not generally react chemically with the materials being stabilised, but coats the particles and imparts adhesion and helps waterproofing. Chemical stabilisation includes addition of materials such as lime, cement or fly-ash in combination or alone. These materials either react chemically with materials being stabilized (for example, lime reacts with clays) or react on their own to form cementing compounds (for example, Portland cement).
Basic ingredients for cemented base/sub-base in pavement
Portland cement and lime are most commonly used cementing materials for construction of cemented base with soil, sand and aggregates for pavements of roads and runways. Low grade or marginal aggregates with suitable proportioning of coarse and fine fractions, lime, clay, lime-late rite-soil, lime-fly-ash, lime-granulated blast-furnace-slag-soil mixture, etc. can be used as a cemented base or sub-base.
Various types of materials are considered by several researchers to study the suitability of those materials as pavement base and sub-base layer. Natural soil, fly-ash, sand, stone dust, river bed materials, reclaimed asphalt pavement, low quality aggregates etc. are considered as ingredients of pavements with cemented base/sub-base.
Cemented materials may be classified in to three categories:
Traditional stabilisers: hydrated lime, Portland cement, and fly-ash
By-product stabilisers: cement kiln dust, lime kiln dust and other forms of by product lime and
Non-traditional stabilisers:
Many alternative additives have been developed and increasingly promoted for use in stabilisation.
Santoni and Tingle (2002) divided these products into several categories, i.e. salts, acids, enzymes, lignosulfonates, petroleum emulsions, polymers, and tree resins.
Many of these products are advertised as being low cost in use, less construction time required or higher durability or higher overall performance compared with traditional stabilisation additives (Tingle & Santoni, 2003).
The effect of these products on the stabilisation process has been evaluated in a few studies (Rauch et al., 2002; Tingle & Santoni, 2003; Santoni et al., 2005) which generally demonstrated that some products can provide some additional strength improvement, such as polymers, while some products failed to show observable changes or even resulted in a decrease of strength.
Despite the potential advantages of using these non-traditional additives, most highway authorities and engineers hesitate to specify the use of these products (Rauch et al, 2002). Rauch (2002) attributed this lack of acceptance to the following issues:
Principal concern is the lack of published, independent studies of these stabilisers, especially field performance data. Test results or field case studies provided by the product producers typically aim to demonstrate the benefits of these commercial products without showing data on the untreated control sections. The information provided by the stabiliser supplier is often not adequate.
For instance, many manufacturers consider the chemical composition of their product to be proprietary, which makes it difficult to understand well the stabilisation mechanism and forecast the potential field benefits.
One product RoadCem, manufactured in Holland by PowerCem Technologies BV stands out with a number of independent studies carried out and plenty of published field performance data available. RoadCem is also fully patented, certified and quality tested. (full details can be found on their website: www.powercem.com or in the UK: www.powercem.co.uk)