Mechanical stabilisation involves the mixing of two, or more, selected materials to modify particle size distribution or plasticity. Mixing can be carried out on-site, or at a quarry or pit before transportation. An alternative is to use rock crushers or grid rollers on-site to crush existing material. Static or portable vibrating screens can be used to separate material into appropriate sizes and grades before delivery and mixing. Laboratory tests of both the material to be treated, and the stabilising material, ensures that the desired end result will be achieved. An example of mechanical stabilisation is the blending of a granular material lacking in fines with a sand/clay mix. The finished material will have improved strength, abrasive resistance, imperviousness and compaction.
The purpose of chemical stabilisation is to improve material properties of the subgrade and/or aggregate layers. This can, consequently, reduce the quantity of aggregate required, or allow low quality aggregate sources to be used. Stabilisation can also reduce maintenance costs, and provide a better wearing all-weather road pavement. Unsuitable materials can be modified to overcome their deficiencies using stabilisation techniques and products. Their addition produces a layer within the road with increased strength and rigidity.
The properties that are usually altered by stabilisation include:
- Increased strength (bearing capacity)
- Increased stiffness (reduced flexibility) of soil
- Volume stability, with changes in moisture content
- Reduced soil plasticity
- Increased durability of soil particles
- Decreased permeability.
Most binder materials work by effectively dehydrating the subgrade or aggregate material. This reduces the shrink and swell effect caused by water entering or leaving the material.
Materials stabilised by modification do not always increase in strength. Cement stabilisation will increase strength and stiffness, allowing, in some cases, a reduction in pavement thickness. However, pavements which are stabilised by cementing action cannot be easily maintained by grading.
Soils and aggregates have a wide range of properties. Consequently, the reaction of a specific material with any particular stabiliser cannot be determined by simple observations, or by defining the soil type. Laboratory analysis is required before using chemical stabilisers, since an incorrect binder material or application rate will not produce the desired outcome. Insufficient quantity of chemical stabiliser can have negligible benefit, while too much chemical stabiliser is expensive and, in some cases, may worsen the properties of the treated soil or aggregate. Consultation with industry specialists should be carried out prior to using these products. The following table provides a guide.
6.9.1 Chemical stabilisation methods
Common types of chemical stabilisation materials include burnt lime Ca(O) and hydrated lime Ca(OH)2, bitumen and cement.
Application of stabilisation (after NAASRA 1986)
Lime can be applied to either the subgrade, sub-base or base course layers. The term ‘lime’ is commonly used to describe a number of products, including burnt lime (quick lime), and hydrated lime (slaked lime). Burnt lime and hydrated lime are the only types of lime which can be used for stabilising soils or aggregates for road pavement construction. Agricultural lime is insufficiently reactive, and will provide no benefits if used for roading applications.
Lime stabilisation requires the minerals present in most, but not all, clays for a reaction to occur. Consequently, lime treatment is ineffective on non-clay soils, and on many coarse aggregates. Organic soils, or soils containing significant percentages of organic material, do not react to lime. Subgrade stabilisation with lime is not recommended where heaving occurs due to freeze-thaw action.
Liming works to improve a soil in three ways:
- The lime reacts with clay minerals and water in an exothermic (heat producing) reaction. This reaction can be exploited to rapidly dry wet subgrade soils that would be otherwise unworkable
- The lime reacts with clay minerals to produce cementing bonds. The cementing action increases inter-particle bonding, and can increase soil strength and reduce shrinkage and swelling actions
- The lime modifies the structure of clay particles to make the soil more friable and porous. This produces a soil that is easier to work with and has improved drainage.
Lime stabilisation improves the durability and smoothness of unsealed roads, and can also help reduce dust problems. For forest roads, a stabilisation depth of 150 mm may be appropriate. Additional information on lime stabilisation is available in TR2 Lime Stabilisation for NZ Roads, (Dunlop, 2001), Transit NZ (now the NZ Transport Agency).
For each pavement material type, there is an optimum quantity of lime content beyond which the addition of further quantities will provide little, or no, additional benefit. The correct amount of lime to be used (percent by mass), depends on the amount and type of clay mineral predominating in the material. Small quantities of lime (1 to 3%) may reduce the plasticity index, and be sufficient to stabilise some materials, such as clayey gravel, which have good grading but moderately high plasticity. Lime reacts with most plastic materials; however, testing is necessary to determine the reactivity of the material to lime. It has been found that some poorly graded clayey sand and gravels, when treated with small percentages of lime, can become friable, and even completely non-cohesive, leading to failures.
The procedure for lime-stabilised construction consists of spreading lime onto the pavement at the required rate, mixing and adding water to improve compaction; compact the material to seal the surface; leave to completely cure before either allowing traffic to pass over the material or the placement of subsequent pavement layers.
Cementation is the formation of cementitious hydrates, which increases the cohesion between soil particles. The mechanical properties of cement-stabilised materials improve both with cement content (up to a point), and with time. The effect of cement on granular material is to ‘glue’ the particles together to form a stronger mass. The cement effectively reduces the material’s susceptibility to moisture, thereby reducing shrinkage and swelling. A typical quantity for the treatment of gravel pavement is 1 to 3% by weight. Additional cement can be detrimental leading to cracking of the surface layer which allows water to enter the pavement.
Cement stabilisation procedures are not commonly used for unsealed road pavement designs, as the bonds formed between particles are weak and unable to resist traffic action. Cement does have an effect on clay soil, but in most cases the improvement is not as great as for lime. Using cement stabilisation for the running/wearing course restricts the maintenance practices because reshaping etc breaks the cemented bonds. Cement stabilisation is, however, suited to sub-base stabilisation, or to upgrade poor quality rock, gravel or sand. Technical and construction details are contained in NZTA 2018: Best Practice Guidelines for Pavement Stabilisation for New Zealand roads.
Cement blends with lime, slag or bitumen are commonly used to make the process more workable (that is, less susceptible to delay in compaction, less likely to crack over time), and to reduce the cost of using large quantities of cement. Fly ash mixtures can reduce the optimum moisture content for compaction. Lime and cement mixes can be used to stabilise clay-based, or high clay content gravels.
Unlike lime stabilisation, cement cannot be reworked following initial mixing and there is only a limited time before setting of the cement takes place. Mixing of the cement and aggregate should be completed before water is applied. After the addition of moisture, adequate compaction must take place to ensure that the material is compacted and shaped before the cement sets.
Bitumen stabilisation is essentially limited to stabilisation of the wearing/running courses of unsealed pavements. The bitumen binder improves stability and strength through cohesion of non-plastic materials, waterproofs the pavement and provides dust suppression. Bitumen stabilisation is best suited to granular materials such as gravels, sandy loam, sand-clays and crushed rock. Construction practices are similar to other stabilisation products, except that compaction is generally delayed allowing the mix to aerate, and excess water to evaporate.
Bitumen stabilisation involves the use of a grader for scarifying and mixing the aggregate layer. A water tanker, with or without pressure spray, is used to apply the emulsion and then the materials are mixed, laid out, shaped and compacted. Depths of from 0 to 150 mm can be treated. Applications of thin films of bitumen produce a stronger material, and thicker films create a weaker, less permeable material. Applying emulsion to an existing pavement is a simple and cost-effective operation; however, it will need to be repeated at intervals of approximately six months.
There are various other products on the market which claim to be effective in reducing dust and stabilising pavements. These include:
- Enzymes: These are natural, biological products. They are becoming increasingly common for use as the basis of a variety of stabilisation and dust suppressant products. The mechanism by which enzymes stabilise a material is a very complex molecular process. Essentially, when the enzymes of a soil stabiliser are mixed with water and applied to the soil, they act by breaking down the clay lattice (that is, breaking down the clods), or by combining clay particles with organic molecules. Enzyme stabilisers are especially designed to stabilise clay-based materials
- Chlorides: Calcium chloride is a salt which acts to bind the material and form a hard surface. Good mixing of the chemical with the material provides for a good effect, and two separate light applications at different times provides a better service than one single application. Sodium chloride has a similar action to calcium chloride. The other option is magnesium chloride, which is not often used in New Zealand.
6.9.2 Equipment used in chemical stabilisation
A variety of machinery and equipment can be used to apply chemical stabilisation products. Various purpose-built machines have been designed to mix the material and add the stabiliser at the same time, generally giving uniformity of mixing. However, in most forestry situations, the addition and mixing of stabilisation is usually completed using graders, rotary hoes, water tankers and spreaders.
The choice of machinery used for a project is determined by a number of factors, including:
- Source of material to be stabilised – in-situ or imported
- Size of the project
- Type of material to be stabilised
- Availability of machinery
- Availability of trained personnel
- Type of stabiliser to be used.