Soil stabilization is the process of improving the shear strength parameters of soil and thus increasing its bearing capacity in road construction.
It is required when the soil available for construction is not suitable to carry structural load. Generally, soils exhibit undesirable engineering properties unless they are treated to enhance their physical properties.
Stabilization can increase the shear strength of a soil and control its shrink-swell properties, thereby improving the load bearing capacity of a sub-grade to support pavement and its foundation140.
Soil stabilization is used to reduce permeability and compressibility of the soil mass in earth structures and to increase its shear strength.
An embankment refers to a volume of earthen material that is placed and compacted for the purpose of raising the grade of a roadway (or railway) above the level of the existing surrounding ground surface. Highway embankments made of heavy materials are often built over soft ground and this can sometimes leads to settlement and instability of the embankment.
There are three broad types of soil stabilization, they are;
1. Biological soil stabilization :- This is achieved through afforestation or planting and its main purpose is erosion control. Root traits such as architectural, morphology, physiological and biotic play an important role in both the physical and chemical development of soils enabling structural stability of soil.
2. Physical stabilization :- It is the modification of soil particle size distribution and plasticity by the addition or subtraction of different soil fractions in order to modify its physical properties.
3. Chemical types of soil stabilization :- It can be achieved through use of traditional and non-traditional agents.
Soil Stabilization • Mechanical stabilization: Improvement of soil properties can be achieved by changing particle gradation. The engineering performance depends mainly on grain size distribution and plasticity. Significant changes in the properties can be made by the addition or removal of suitable soil fractions. Granular fraction imparts strength and hardness. Fine fraction imparts cohesion, water, retention capacity and it acts as filler. • Extreme caution is required for adding fine to roadbases and subgrades. • The common technique of soil alteration is the addition of sodium montmorillonite (bentonite) to reduce the permeability of the soil. Addition of 10% of
bentonite reduces permeability coefficient k. Asphalt have been also tried.
Lime Stabilization : Hydrated (or slacked) lime is very effective in treating heavy plastic clayey soils. Lime may be used alone or in combination with cement, bitumen, or flash. Lime has been mainly used for stabilizing the road bases and subgrades. • On addition of lime to the soil, two main types of chemical reactions occur i) alteration in the nature of the adsorbed layer through base exchange phenomenon and ii) cementing or pozzolanic action. Lime reduces P.I. of highly plastic soils making them more friable and easy to be handled and pulverized. The P.I. of low plasticity generally increases. There is generally an increase in the O.M.C. and decrease in the compacted density, by strength and durability increases. • Normally 2 10 8 % of lime may be required for coarse-grained soils and 5 to IO % for plastic soil.
Cement Stabilization : The soil stabilized with cement (Portland) is known as soil cement. The cementing action is believed to be the result of chemical reactions of cement with silicious soil during hydration. The binding action of individual particles through cement may be possible only in coarse-grained soils. In cohesive soils, only some of the particles can be excepted to have cement bonds and the rest will be bonded through natural cohesion.
Bitumen Stabilization : • Earth roads can be improved by spraying the dry soil surface with a stabilizer such as aspheric and cut back bitumen oil, tars, or emulsions in small quantities. The bituminous materials when added to soil impact cohesion or binding action and reduce water absorption. About 4 to 7 % by weight of bitumen is required. Any inorganic soil can be stabilized well-graded soils give goods results from 10^3 mm/s to 10^8 mm/s in case of silty sands. Local clays may be used.