The Basic Road Stabilization Process

Many engineers are paying increasing attention to road stabilization in order to build quality roads that can hold up against rough and heavy use.

The objective is to improve strength, reduce soil plasticity and lower compressibility either by binding the soil particles or waterproofing them or a combination of both. It involves the application of mechanical processes and chemical additives to increase the density of the soil. Higher the density of a material fewer the voids present in it.

Voids are most damaging to the integrity of a road or for that matter any type of construction. As they trap moisture and air, they become less stable and disintegrate under pressure and changing temperature and moisture conditions. Soft soils such as silt, organic soils and clayey peat are the type of soils that need to be first stabilized before they can be used because of their high porosity and organic content. Road stabilisation methods and appropriate applications improve material properties and performance. It is very important to consider how a pavement performs either loaded or unloaded over its intended design life which solely depends on selection and use of appropriate pavement materials, pavement structural design and drainage as well as the construction process that satisfies the design intent. The types of stabilisation such as subgrade, granular, modified, lightly bound and bound also governs the type of binder that is adopted and used in the road stabilization procedure. 

The steps constituting its process vary based on the location, time requirements, available machinery, budget and environmental conditions. However, the following described steps are common to most processes.

1. SOIL TESTING

This is the key to successful soil stabilization. There is no standard method and the additives that can be used vary based on the soil type. Strength, compressibility, permeability, durability and volume stability are some of the properties that are of chief concern in road stabilization.

Soils found across the globe are of different varieties and the same can be said about their engineering properties. Thus, soil samples are first tested to determine their engineering and environmental properties. Based on the assessment, a particular additive is selected and used to cure the soil sample. The cured sample is then tested again to determine whether or not it produces the desired results. Understanding the material is very important and in more cases than the other practitioners rely on local knowledge when it comes to assessment of suitability of pavement materials in borrow pits than what is obtained in laboratory tests. It is often attributed to unavailability of material laboratories and consideration of costs. 

2. PREPARING THE SITE

There is ex-situ and in-situ stabilization. In-situ stabilization involves stabilizing the soils at its original site while ex-situ stabilization involves curing the soil by removing the bulk of soil and transporting it to another treatment site. In-situ it is the preferred option because it is cost effective and time efficient.

In any case, the existing or parent soil is pulverized using a rotary mixer, this means it is crushed into fine particles. Additional base materials and aggregates are added during this stage. The moisture content of the soil should be at an optimum level for the reaction to take place successfully when the additive is added. Hence, dry soils are made damp while soils with high water content are made dry through water drainage. 

3. INTRODUCTION OF ADDITIVES

After the soil is prepared, the additives are introduced. They are either applied dry or sprayed. The method of application varies based on the additive being used. Selected additives require several applications and repeated mixing. These additives improve material properties such as strength, permeability, volume stability and durability. A spreader truck or tanker for liquid agents can be used to apply the required dosage rate or through a plant mix pugmill operation. 

4. MIXING

A stabilizing machine makes several passes over the soil so that it mixes the soil and the additives homogenously. Deep soil mixing serves to stabilise weak soils using augers at a great depth with columns used for boring through the ground at a considerable depth in a way similar to piling techniques. Selected additives require to be mixed immediately after they are introduced as they set very quickly. Time is of the essence in the mixing procedure and the sensitivity of the chemistry of setting requires careful consideration before the mixing operation is performed. Therefore since time-dependent chemical reactions occur spontaneously the mixing is not regarded as time-intensive. Procedurally, mixing is achieved in the following ways:

• Use of laboratory mix design and analysis to choose the type of additive required and the most effective quantity
• Determination of the ratio of water to additive for which mixing can achieve optimal performance 
• Selection of the mixing column geometry using inference from the laboratory tests 
• Installation method and column configuration ready to mix 

5. COMPACTION AND TRIMMING

Compaction increases the density of the soils and also moves it towards achieving a smooth surface and optimum moisture content during construction. In the process of compaction, different machines are employed namely vibratory pad foot, a pneumatic compactor and tandem drum roller. The last two are used to shape and trim the surface to remove depression marks and achieve a proper crown and grade and a smooth surface. The size of pavement materials also determines the type of compaction that has to be performed on the material. A well compacted layer tends to have greater stability and is less susceptible to deformation under traffic load which contributes to longevity in the pavement performance during its in-service life. In essence, comprehending the moisture density relationship of a soil, the specification density can be targeted through the addition of a specific amount of moisture during compaction works. 

It is recommended in practice that selection of a heavier roller can achieve the desired density at a lower moisture content which can be a benefit in areas where construction water is scarce. 

Compaction specifications critically consider the possibility of prevent entrapment of moisture in the pavement during construction of pavement layers. As a matter of principle, each layer must be allowed to dry back from the optimum moisture content before the next layer is placed. Dry-back increases stability by generating suction forces within a pavement materials which is often reflected in the percentage of optimum moisture content or as a degree of saturation. Lack of meeting dry-back specifications can lead to compromised stability and long-term strength of the granular layer of the pavement which increases risk of premature failure of the pavement. 

6. CURING

This is the final step in the process. It is what helps the additive to achieve its full engineering potential. A period of seven days is sufficient for proper curing. However, again the time varies based on the additive used to stabilize road soils. Common cure intervals are seven days and twenty-eight days. Chemical constituents of additives go through both instant and prolonged time-dependent chemical reactions with the soil and other additives. These result in overall enhancement of the soil matrix with swell reduction, shear strength improvement and resistance to influence of wetting and drying. Processes such as cation exchange, flocculation, agglomeration, pozzolanic reactions and carbonate cementation are typical in the mechanisms of the reactions.  

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      Lin Yang(Ms.)

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