An Introduction To Chemical
Soil Stabilization
Presented by:
Marco Silvestre, P.E
Gary Dukewits
Agenda
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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
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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
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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