Bituminous (Cementing) Stabilization

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Bituminous soil stabilisation is an effective method which is being widely used. Bituminous materials are : bitumen, asphalt and tar. Bitumens are nonaqueous systems of hy drocarbons which are completely soluble in carbondisulphide. Asphalts are materials in which the primary components are natural or refined petroleum bitumens or combine- tions thereof. Tars aie bituminous condensates produced by the destructive distillation of organic materials such as coal, oil, lignite, peat and wood.

Bituminous material stabilizes the soil either by binding the particles together or protecting the soil from the deleterious effects of water (i.e., waterproofing) or both these effects may occur together. The first mechanism takes place in cohesionless soils and the second one in cohesive soils. Among the bituminous materials, most of bitumen stabilization has been with asphalt. Therefore, soil stabilised by asphalt may be referred to as soil-asphalt. Asphalts are produced by three processes:

Vacuum distillation producing straight-run asphalt
High-temperature pyrolysis of refinery heavies, producing cracked asphalt
High- temperature air blowing straight-run asphalt, producing blown asphalt.

As the straight-run asphalt has low softening temperature and low melt viscosity, it is commonly used in soil stabilization. Asphalt can not be directly added to the soil because it is too viscous. Its fluidity can be increased by (i)heating, (ii) emulsifying in water (emulsions), or (iii) cut back with some solvent like gasoline (cutbacks). Both emulsions and cutbacks are used in soil stabilization. Although soil-asphalt has varied applications, it is mostly used in bases for highway and airfield pavements.

Nature of Soil

All inorganic soils with which asphalt (emulsion or cutback) can be mixed can be stabilized. Soils satisfying the following requirements yield the best results:

Maximum particle size less than one-third the compacted thickness of the treated soil layer.
Greater than 50% finer than 4.76 mm size.
Thirty five to 100% finer than 0.42 mm size.
Greater than 10%, but less than 50% finer than 0.074 mm size.
Liquid limit less than 40%.
Plasticity index less than 18%.

Organic matter of acid origin is detrimental to soil-asphalt. Asphalt stabilization cannot be effective in fine grained soils with high pH and dissolved salts. It is difficult to handle plastic clays because of mixing problem.

Amount of Asphalt

An increase in asphalt content gives better results. In fine-grained soils addition of asphalt does not increase the strength but tremendously improves the waterproofing property and thereby yielding a better stabilized soil. Asphalt also should be added optimally otherwise results in a gooey mixture which cannot be properly compacted.

Mixing

A thorough incorporation of the additive with the soil yields a better stabilized soil

Compaction Conditions

The density of a mixture of soil and asphalt is governed by the volatiles content and amount and type of compaction. In general lower the volatiles content, the higher the strength. Further, samples which were cured and then immersed in water showed a maximum strength at a moulding volatiles content near or slightly above that for maximum compacted density and the water pickup and thus strength loss, is least at this moulding volatiles content (Lambe, 1962). In plastic soils the volatiles content which gives maximum cured strength and that which gives optimum density can be quite different and the difference can vary with type of compaction.

Cure Conditions

The following behaviour have been reported to be true (Lambe, 1962) : (i) the longer the period of cure and warmer the temperature of cure, the greater the volatiles lost; (ii) the longer the period of immersion, the greater the water pickup. The strength of a soil-asphalt is inversely proportional to the volatiles content at the time of test. A general strength-volatiles content relationship was obtained regardless of the formulation employed.

Construction of Soil-Asphalt

The conventional sequence of construction operation is as follows (Lambe, 1962) : (i) Pulverisation of the soil to be treated, (ii) Addition of water for proper mixing, (Hi) Adding and mixing of the bitumen, (iv) Aeration to the proper volatiles content for compaction, (i;) Compaction, (ui) Finishing, (vii) Aerating and curing, and (viii) Application of surface cover. The important items to ensure proper stabilization which need control are mixing, compacting, drying and applying the surface protection. The mixing plants used for soil-cement can be used for soil asphalt also. The necessary field control tests are moisture content determination before and during processing, bitumen content determination after mixing and density determination after compaction.

The optimum moisture content for stability is usually below that for compaction. As good mixing is generally considered to be most easily obtained at fairly high moisture contents, it is often found necessary, except with sands, to allow a period for the mix to dry between the mixing process and compaction. In practice, treated sands are placed at about 3 to 5% volatiles content whereas cohesive soils are placed at about the optimum volatiles content for compaction. Compared to cutbacks or tars, emulsions provide more latitude in the stabilization of fine-grained soils.

Chemical Stabilization: stabilization consists of bonding the soil particles with a cementing agent e primary a itive is a chemical that is produced by a chemical reaction within the soil.

Lime

Lime has been used as a soil stabilizer for roads from olden days. Lime is produced from natural limestone. The type of lime formed is based upon the parent material and production process.

ion exchange of calcium for the ion naturally carried by the soil,
A depression of the double layer on the soil colloids because of the increase in cation concentration in the pore water, and
an expansion of the double layer of the soil colloids from the high pH of the lime.

The second reaction takes considerable time in a cementing action. The cementing action, also called pozzolanic action, is not completely understood, but is thought to be a reaction between the calcium from the lime with the available reactive alumina or silica from the soil (Lambe, 1962). Soil plasticity, density and strength are changed by the addition of lime to soil. Lime generally increases the plasticity index of low plasticity soils and decreases the plasticity index of highly plastic soils. Because of reduction in the plasticity of plastic soils, due to addition of lime, the soil becomes more friable and easy for handling in the field. Addition of lime causes a reduction in the maximum compacted density and an increase in the optimum moulding water content. In general, lime increases the strength of almost all types of soil.

Construction procedure of lime-stabilized soil bases are similar to those employed for soil-cement with a difference that more time is allowed for placement operations for lime. This relaxation is possible as the lime-soil cementation reaction is a relatively slow one. Adequate care should be taken to prevent carbonation of the lime. The normal construction sequence for lime-stabilized bases is as follows : (i) Scarify the base, (ii) Pulverise the soil, (iii) Spread the lime, (iv) Mix the lime and soil, (v) Add water if necessary to bring to optimum moisture content, (ui) Compact the mixture, (vii) Shape the stabilized base, (viii) Cure-keep moist and traffic-free for at least 5 days, and (ix) Add wearing surface.

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