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Admixtures are occasionally used to stabilize soils in the field，particularly fine-grained soils. The most common admixtures are lime，cement，and lime-fly ash. The main purposes of soil stabilization are to ① modify the soil，② expedite construction，and ③ improve the strength and durability of the soil.
The types of lime commonly used for stabilization of fine-grained soils are hydrated high- calcium lime ( Ca( OH)2) ， calcitic quick lime ( CaO ) ， monohydrated dolomitic lime ( Ca( OH)2·MgO) ，and dolomitic quick lime. The quantity of lime used for stabilization of most soils usually is in the range of 5% － 10% . When lime is added to clayey soils，several chemical reactions occur，cation exchange and flocculation-agglomeration，and they are also pozzolanic. In the cation exchange and flocculation-agglomeration reactions，the monovalent cations generally associated with clays are replaced by the divalent calcium ions. Based on their affinity for exchange，the cations can be arranged in a series:
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Any cation can replace the ions to its right. For example，calcium ions can replace potassium and sodium ions from a clay. Flocculation and agglomeration produce a change in the texture of clay soils. The clay particles tend to clump together to form larger particles. These reactions tend to ① decrease the liquid limit，② increase the plastic limit，③ decrease the plasticity index，④ increase the shrinkage limit，⑤ increase the workability，and ⑥ improve the strength and deformation properties of a soil.Pozzolanic reaction between soil and lime involves a reaction between lime and the silica and alumina of soil to form cementing material. For example，
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where C represents CaO; S represents SiO2; H represents H2O.
The pozzolanic reaction may continue for a long period of time.
Figure 10. 7 shows the variation of the liquid limit，the plasticity index，and the shrinkage limit of a clay with percentage of lime admixture. The first 2% － 3% lime ( on the dry weight basis) substantially influences the workability and the property ( such as plasticity) of the soil. The addition of lime to clayey soils affects their compaction characteristics.
Figure 10. 7 Variation of liquid limit，plasticity index，and shrinkage of clay with lime additive
Lime stabilization in the field can be done in three ways:
1) The in situ material and / or the borrowed material can be mixed with the proper amount of lime at the site and then compacted after the addition of moisture.
2) The soil can be mixed with the proper amount of lime and water at a plant and then hauled back to the site for compaction.
3) lime slurry can be injected into the soil to a depth of 12 to 16 ft ( 3. 6 － 4. 9 m ) . A common injection unit is a hydraulic-lift mast with cross beams that contain the injection rods. The injection rods are pushed into the ground by the action of the lift mast beams. The slurry is generally mixed in a batching tank about 10 ft ( 3 m) in diameter and 36 ft ( 11 m) long and is pumped at high pressure to the injection rods.
Because the addition of hydrated lime to soft clayey soils immediately increases the plastic limit，thus changing the soil from plastic to solid and making it appear to “dry up”，limited amounts of it can be thrown on muddy and troublesome construction sites. This action improves trafficability and may save money and time. Quick limes have also been successfully used in drill holes having diameters of 4 to 6 in ( 102 － 152 mm ) for stabilization of subgrades and slopes. For this type of work，holes are drilled in a grid pattern and then filled with quick lime.
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