An Introduction To Chemical Soil Stabilization

Presented by:Marco Silvestre, P.E Gary Dukewits

Agenda

• Objective of Presentation

• Purpose of Chemical Treatment of Soils

• Typical Chemical Stabilizers

• Chemical Stabilization Processes

• Mix Design Considerations

• Construction Considerations

Objective of Presentation

• Overview of typical available chemical soil stabilizers

• A general understanding of chemical soil stabilization process

• Advantages and disadvantages in the use of each chemical soil stabilizer

• Considerations in specifying chemical soil stabilization method

Purpose of Chemical Treatment of Soils

Drying of Soil

Short term effect, reducing

the moisture content level of

wet soil to improve its

workability, keep project on

schedule, and extend

construction season.

Purpose of Chemical Treatment of Soils

Soil Modification

Soil enhancement, its effects

longer lasting than drying, but

not permanent. Speeds construction

with stable “weather proof” working

platform. Maximizes use of low cost,

on-site materials. Reduces plasticity.

Improves workability.

Purpose of Chemical Treatment of Soils

Soil Stabilization

All soil modification benefits, and

permanently increase compressive

strength, increase load-deformation

properties, and improve durability

during freeze-thaw and wet-dry

cycles, to be incorporated into

structural design.

Typical Chemical Soil Stabilizers

• Lime

• Fly Ash

• Cement Kiln Dust

• Portland Cement

• Lime Kiln Dust

• Bitumen

• Various Blends

Typical Chemical Soil Stabilizers

Lime

Quicklime: CaO, a caustic and alkaline

substance formed during calcination of

limestone in lime kiln.

Hydrated Lime: Ca[OH]2 formed by

exothermic reaction between CaO + H2O.

Lime Slurry: A suspension of quicklime

or hydrated lime in water.

Typical Chemical Soil Stabilizers

Fly Ash

Finely divided residue that results

from the combustion of pulverized

coal and is transported from the

combustion chamber by exhaust gases.

Class F - anthracite or bituminous coals

Class C - lignite or subbitumous coals

Fly ash is a pozzolan (contains AlO3, SiO2, and Fe2O3 ).

Class C fly ash self cementing due to significant amount of free lime.

Typical Chemical Soil Stabilizers

Cement Kiln Dust (CKD)

Fine-grained, solid, highly alkaline

material removed from the cement

kiln exhaust gas by scrubbers.

- Pre-calciner kiln dust

- Long-wet or Long-dry kiln dust

Chemical Soil Stabilization Processes

Lime

Initial Soil Drying:

CaO + H2O Ca(OH)2 + Heat (Hydration, occurs instantly).

Modification:

Flocculation and agglomeration process where calcium ions of hydrated lime replace sodium and hydrogen ions from surface of clay particles, resulting in a soil with friable and granular characterisics and reduced plasticity index. (Occurs within hours).

Stabilization:

Breaking down clay particles, releasing silica and alumina, which react with calcium from lime into CSH and CAH cementious matrix. (Occurs within hours and can continue for years).

Chemical Soil Stabilization Processes

Class C Fly Ash

Pozzolans react with lime and water to form cementious material.

CaO + H2O => Ca(OH)2

Ca(OH)2 => Ca++ + 2[OH]-

Ca++ + 2[OH]- + SiO2 => CSH

(silica) (gel)

Ca++ + 2[OH]- + Al2O3 => CAH

(alumina) (gel)

Chemical Soil Stabilization Processes

• Cement Kiln Dust

Both processes described for lime and Class C Fly Ash will occur for pre-calciner CKD due to presence of pozzolans and relative large amount of free lime.

Long-dry or Long-wet CKD does not contain significant amount of free lime; thus stabilization process primarily based on formation of cementitious material as described for Class C Fly Ash.

Mix Design Considerations

• Type of Soil– Lime use limited to plastic clay soils CH and

CL with PI > 10– Class C Fly Ash and CKD applied to wider

range of soil types from SM to CL-ML to CH

• Purpose of Stabilized Soil– Pavement Subgrade– Bridging Soft Subgrade Soils– Low Plasticity Structural Fill

Mix Design Considerations

• Type of Desired Improvement– Increased Strength– Reduced Plasticity and Shrink-Swell– Improved Freeze-Thaw or Wet-Dry Durability

• Required Strength and Durability Gain

• Readily Available Chemical Additives & Source Specific Chemical Analysis

• Cost

Mix Design Considerations

• Typical quantities of chemical additives, based on soil dry weight.– Quicklime: 3-6%– Hydrated Lime: 5-7%– Class C Fly Ash: 12-17%– Pre-calciner CKD: 5-8%– Other CKD: 8-12%

Construction Considerations

• Available Construction Standard Specifications– FAA

• P-155 – Lime Treated Subgrade• P-157 – CKD Treated Subgrade• P-158 – Fly Ash Treated Subgrade

– ODOT• Section 307 – Lime Treated Subgrade• Section 317 – Fly Ash Modified Subgrade

Construction Considerations

• Typical Equipment– Graders and Scarifyers– Spreaders– Mixers and Pulverizers– Sheepsfoot and Pneumatic Rollers– Sprinklers– Trucks

Construction Considerations

• Typical Mixing Process for Lime– Application and Spreading of Dry Additive– Mixing & Moisture Condition +3-5% omc

• 1st mixing within 6 hrs from application, lightly roll, and allow curing of 48-72 hours.

• Final mixing after curing and until 100% < 1-1/2” Sieve and 60% < #4 Sieve.

– Compaction & Moisture Condition• Typically -2 to +2% omc

– Finishing and Curing

Construction Considerations

• Typical Mixing Process for Fly Ash– Application and Spreading of Dry Additive– Mixing & Moisture Condition

• 1st Mixing of soil and fly ash on dry of omc until 100% < 2-1/2 inches

• Final mixing and add water +2 to 5% omc until 100% < 1-1/2” Sieve and 50% < ¾” Sieve.

– Compaction within 2 to 3 hours from application• Immediately following final mixing, and such that moisture

ranges -2 to +2% omc

– Finishing and Curing

Construction Considerations

• Typical Mixing Process for CKD– Application and Spreading of Dry Additive– Mixing & Moisture Condition

• Mixing and add water +0 to 6% omc, continue mixing until 100% < 1-1/2” Sieve.

– Compaction may be delayed 24 hrs after mixing• No extended curing time required as with lime and no narrow

time window between application and compaction as with fly ash.

– Finishing and Curing

THANK YOU!

• Marco Silvestre, P.E

• Gary Dukewits