Soil Stabilization Techniques & Benefits

Stabilization

• Soil stabilization involves physical or chemical treatments to maintain, or increase a soil's stability and improve its engineering properties.
• Soil stabilization may be applied to subgrade, subbase, or base soils.
• Low-quality materials may be treated to improve their properties like Atterberg limits and frost susceptibility.
• Chemical agents used in stabilization interact with clay minerals in fine-grained soils.
• Mechanical Agents used in stabilization include compaction, geosynthetic reinforcement, and drainage.

Benefits of Stabilization

• Stabilization enables the use of available materials
• Stabilization reduces the need to borrow materials and decreases the quantity of rejected materials, conserving resources in the process.
• It contributes to overall pavement design requirements and allows for thinner pavement sections.
• Stabilization facilitates construction by accelerating compaction.
• Stabilization increases overall value by proving cost-effective solutions.
• Reduces moisture susceptibility of soil and frost susceptibility of soil.
• Improves drainage and increases stability
• Stabilization also reduces the pumping of fines.

Mechanical Stabilization

• Mechanical stabilization treatments include compaction, adding/blending aggregate such as clear stone to clay or silt soils, and adding geosynthetic reinforcement material like geogrids or geotextiles.
• Mechanical stabilization improves bearing capacity and decreases influence of plasticity.

Chemical Stabilization

• Chemical stabilizing agents include traditional agents, viscoelastic materials, industrial co-products, and other agents.
• Traditional agents include lime, Portland cement, and fly ash.
• Lime acts as a stabilizer by binding soil particles to improve stability and strength.
• Lime increases the workability of clay soils, absorbs moisture, and reduces plasticity.
• Viscoelastic materials include bitumen or asphalt and acts as waterproofing agents and reduces permeability of coarse-grained soils.
• Industrial co-products include cement kiln dust (CKD) and lime kiln dust (LKD).
• Other agents include bio-enzymes, polymers, sulfonated oils, ammonium chlorides, and potassium compounds.

Lime

• Lime is Calcium (Ca) carbonates, oxides, hydroxides.
• Lime is able to modify almost all fine-grained soils (silt and clay).
• Lime improves plasticity, workability, and volume change characteristics.
• The most dramatic improvement with lime occurs in clay soils of moderate to high plasticity (PI>10).
• Small quantities of lime affect short-term modification of soil properties.
• Increased quantities of lime can also provide long-term soil stabilization.

Types of Lime

• Quicklime (CaO) is produced when CaCO3 + heat --> CaO + CO2
• CaO reacts with water to form hydrated lime (slaked lime): CaO + H2O --> Ca(OH)2 + heat.
• Quicklime is very effective in drying and modifying soil.
• Agricultural lime is crushed limestone (CaCO3) or dolostone (Mg, Ca)(CO3).
• Agricultural lime increases the alkalinity of soil (pH) to aid plant growth, but is not effective in modifying or cementing soil.

Action of Lime on Soil

• The action of lime on soil can be divided into three stages: drying(immediate), soil modification(short term), and soil cementation(long-term stabilization).
• The degree of effectiveness from lime is dependent on soil type, lime type, lime quantity, and curing conditions.
• Main factors that can affect the action of lime are time, temperature, moisture, and pH.

Soil-Lime Reaction: Drying

• When quicklime (CaO) is applied to wet soil, CaO reacts with available water forming hydrated lime Ca(OH)2 + heat.
• A soil-lime reaction results in an immediate reduction in soil moisture content.
• Longer-term water evaporation occurs as a result of the heat of a soil-lime reaction.
• Evaporation is affected by ambient temperature, relative humidity, and wind.

Clay Minerals

• Mica group (illite) formula: (Al, Mg,Fe)2(Si,Al)4O10[(OH)2, (H2O)]
• Mica group is a significant rock-forming mineral that is dominant in shales, slates, and common in soils.
• Chlorite formula: (Mg, Fe,Al)3(AI,Mg,Fe)2(Si,AI)4O10(OH)2 * (Mg,Fe,Al)3(OH)6
• Chlorite is weathered metamorphic rock, marine sediments, and sedimentary rocks.
• Kaolinite formula: Al2Si2O5(OH)4
• Kaolinite is a highly weathered soil.
• Smectite group (Montmorillonite) formula: Cao.17(Al, Mg,Fe)2(Si,Al)4O10(OH)2 * nH2O
• Smectite group is weathered from volcanic rocks, easily absorbs water, and demonstrates swelling behavior.
• Clay minerals have a high surface area and small particle size with a diameter < 2µm.
• Surface areas vary, Kaolinite has between 10-20 m²/g, Illite has between 65-100 m²/g, and Montmorillonite has between 50-120 m²/g.

Adsorption

• Adsorption is the adhesion of molecules and ions to a surface.
• Negative surface charge of clay attracts water molecules and cations.
• Water molecules and cations are adsorbed onto the clay mineral surface.
• The adsorbed water layer varies with moisture content and clay mineral type.

Soil-Lime Reaction: Modification

• Hydrated lime Ca(OH)2 produces Ca2+ ions (in solution): Ca(OH)2 --> Ca2+ + 2OH-
• Ca2+ ions react with clay minerals.
• Exchange of adsorbed cations occurs on the surface of clay minerals.
• Ca2+ displaces cations K+ and N+.
• Exchange is promoted by the high alkaline environment of the saturated lime-water system (pH=12.4).

CaO vs Ca(OH)2

• CaO has a higher density than Ca(OH)2.
• CaO has a higher available Ca content per unit mass than Ca(OH)2.
• CaO is less dusty than Ca(OH)2.
• CaO is beneficial to wet soils.
• CaO can accelerate strength gain.

Portland Cement Stabilization

• Portland cement is composed of calcium-silicate (CS) and calcium-aluminate (CA).
• CS/CA hydrates and forms cementing compounds of calcium-silicate-hydrate (CSH), calcium-aluminate-hydrate (CAH), and excess calcium hydroxide (CH).
• Portland cement can be successful in stabilizing both coarse and fine-grained soils due to cementitious material and the CH formed.
• Portland cement bonds soil particles to improve stability and strength but does not fill all the voids in soils.
• Well-graded materials require less cement content.
• Fine-grained soil requires larger cement content because of the amount of surface area that needs to be bonded.
• Cement stabilization is unsuitable for soils containing a large portion of clay or organic soils.

Construction Methods Using Cement

• Mixed in Place: involves spreading Portland cement then mixing it in, applying water and then re-mixing.
• Plant Mixed: involves mixing soil, cement, and water, then transporting the mixed material to the place area

By-Product Stablilizers

• Cement kiln dust (CKD) and lime kiln dust (LKD) can be used as activators.
• A combination of kiln dust and fly ash can act as the cementitious component of the mixture.
• CKD lime content is variable up to 40% Ca.
• CKD also contains 20-25% pozzolanic material and other oxides, sharing a composition similar to portland cement.
• CKD can be used alone as a cementitious material in stabilized base mixtures.
• LKD lime content varies depending on the lime production process, dolomitic lime or high C lime.
• Quicklime production can contain up to 30-40% CaO, pozzolanic material, MgO, and trace elements.

Fly Ash (FA) Stabilization

• Fly ash is a residual product from the combustion of coal, it is composed of silica and alumina compounds.
• Class C fly ash has a high lime content and is self-reactive in the presence of water.
• Class F fly ash has a low lime content and requires the addition of lime to form pozzolanic reactions.
• Fly ash is capable of high compressive strengths.
• Fly ash is suitable for granular base/subbase.

Bituminous Stabilizers

• Petroleum-based products, such as low viscosity liquid asphalts, asphalt emulsions, and asphalt primers, can be used as bituminous stabilizers.
• Surface coating of individual aggregate particles with asphalt cement acts as a binder.
• The asphalt cement binder provides adhesive binding between particles.

Bituminous Stabilization

• Bituminous stabilization binds particles together increasing stability and strength of non-cohesive soil.
• Excessive amounts of bituminous stabilization can reduce interparticle friction, lowering stability and providing erosion control.
• Bituminous stabilization blocks interparticle pores, reducing permeability and drainage, reducing water penetration into soil or pavement, and reduces capillary action.
• Bituminous stabilization of soil waterproofs particles protecting aggregate from moisture, reduces water adsorption, freeze-thaw effects.

Geotextiles

• Geotextile materials are often used to improve soil characteristics before building roads, pipelines, embankments, and other structures.
• Geotextiles have many functions, including waterproofing, reinforcement, and separation.
• Geotextile fabrics come in three basic forms: woven geotextiles, non-woven geotextiles, and (structural woven) heat-bonded geotextiles.
• Geotextiles can be produced from petroleum-based polymer products like polyester, polyethylene, and nylon.
• Composed of fibers form mainly three types of fabrics:
  • Woven: continuous fibers (arranged in longitudinal and transverse directions by weaving)
  • Non-Woven: randomly oriented fibers (staple fibers), bound together through mechanical interlocking, heat fusing
  • Heat-bonded geotextiles (structural woven)

Geosynthetics

• Manufactured from polymeric material and used with soil, rock, earth, or other geotechnical engineering-related material as an integral part of any project.
• Geosynthetics include geotextiles/geonets, geogrids/geocells, and geomembranes.

Geosynthetic Functions

• Geotextiles/geonets are used for separation, filtration, drainage, and reinforcement.
• Geogrids/geocells are used for stabilization and reinforcement.
• Geomembranes are used for separation.

Geotextile Applications

• Drains
  • French drains (geotextile encased graded stone)
  • Subdrains (geotextile encased pipe)
  • Wick drains
  • Strip drains
• Seepage control is used beneath rip rap, gabions, rock protection, and ditch armor.
• Geotextiles are used for erosion control using silt barriers/silt fences.
• Geotextiles are used for separation between subgrade/subbase and as rockfill/earth cuts.