Chemical Stabilization of Subgrades

Chemical Stabilization of Subgrades

Section Engineers’ Meeting Lake Cumberland State Resort Park

March 5-7, 2013

Why should soil subgrades be stabilized?

Improve Bearing Capacity

CBR of Clays Soils are 1 to 5

CBR of Silts 4 to 10

Silts Very Susceptible to Changes in Moisture

Stabilized Subgrade Will Last for Many Years

85% of Soils in Kentucky are Silts and Clays

Types of chemical stabilization

Lime: Lean to Fat clays Plastic Index Predominantly > 15

Cement: Sandy and Silty Soils Plastic Index <20PI 15 to 25: Lime or Cement may be used

Lime Kiln Dust: Clays

Cement Kiln Dust: Sandy Soils

Kiln Dust is byproduct from cement and lime manufacturingCheaper but more may be neededGood for drying wet soils (modification)

Types of chemical stabilization

Lime: takes longer (mellowing period initial and final mixing)

Cement: Mixed and compacted within a few hours

Kiln Dust: Lower Cost; not as much active calcium available

When should soil subgrades be stabilized?

CBR of Clays Soils are 6 or less

CBR Values KY 1303 Section 2

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10

CBR

# of

Sam

ples

15 samples

When should soil subgrades be stabilized?

CBR of Clays Soils are 6 or lessKY 1303 Section 2

0102030405060708090

1001 2 3 4 5 6 7 8

CBR

Perc

entil

e

CBR = 2.6 @ 85th percentile

How much stabilizer should be used

Eads and Grim Test (ASTM D 6276)The lowest amount needed to reach a pH of 12.4

pH Versus Percent Stabilizer

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8 9 10 11

Percent Stabilizer

pH

Unconfined Compressive Strength, Qu, test on lime stabilized sample

How much stabilizer should be used

How much stabilizer should be used?

The lowest amount needed to reach a specified strength

UCS Versus Percent Stabilizer

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9 10 11

Percent Stabilizer

Unco

nfin

ed C

ompr

essi

ve S

treng

th

Procedures (Lime Stabilization)

Construct subgrade to plan elevation

Sampling before starting

• Send samples (1 per 1,000 feet min.) to Geotechnical Branch at least 3 weeks before starting stabilization

• Proctor tests may take 2 weeks• If you think a soil needs stabilization call

Geotech

Procedures (Lime Stabilization)

Quick Lime is delivered and transferred to mixing (slaking tank)CaO + H2o Ca(OH)2 Quick Lime is converted to Hydrated Lime

Procedures (Quick Lime Stabilization)

Lime Slurry is transferred to dispensing truck

Procedures (Quick Lime Stabilization)

Scarify Subgrade

Procedures ( Quick Lime Stabilization)

Apply lime slurry to scarified subgrade

Procedures (Quick Lime Stabilization)

Apply lime slurry to scarified subgrade

Procedures (Lime Stabilization)

Mix slurry and soil to specified depth

Procedures (Lime Stabilization quick lime)

Mix slurry and soil to specified depth

#4 Sieve, 1 Inch Sieve, 2 Inch Sieve

Procedures (Lime Stabilization)

Lightly Compact

Procedures (Lime Stabilization)

Mellow for one day and mix again Clay will have a silty texture

Procedures (Lime Stabilization)

Mellow for one day and mix again

Procedures (Lime Stabilization)

Final Compaction (24- 72 hours)

Dry (hydrated lime) application

Ca(OH)2 Hydrated lime is delivered in tanker Transported to spreader truck

Procedures (Hydrated Lime Stabilization)

Procedures (Hydrated Lime Stabilization)

Dry (hydrated lime) application

Ca(OH)2 Hydrated lime spread on subgrade

Dry (hydrated lime) application

Water added during mixingCompaction same as for quick lime

Procedures (Lime Stabilization)

Check compaction

Procedures (Lime Stabilization)

Cut to final grade

Procedures (Lime Stabilization)

Cut to final grade

Procedures (Lime Stabilization)

Check Depth of chemical stabilization

Lime Testing

• Soil sampler

Depth Testing

• Phenolthalein lime testing solution

Procedures (Lime Stabilization)

Depth of chemical stabilization

Procedures (Lime Stabilization)

Depth of chemical stabilization

Procedures (Lime Stabilization)

Keep moist

Procedures (Lime Stabilization)

Apply an asphalt seal No additional moisture is needed after sealingTypical cure time is seven days maximum

Why is quick lime paid at 1.25 times actual quantity?

Quick lime is delivered and transferred to mixing (slaking tank)CaO + H2o Ca(OH)2 quick lime is converted to hydrated lime

CaO + H2O Ca(OH)2

Molecular Weight CaOCa 40.08 x 1 = 40.08O 16.00 x 1 = 16.00

Total 56.08

Molecular Weight Ca(OH2)Ca 40.08 x 1 = 40.08O 16.00 x 2 = 32.00H 1.00 x 2 = 1.02Total 74.10

74.10/56.08 = 1.32 Close to 1.25?

Truck being filled with Cement

Procedures (Cement Stabilization)

Mix one time and compact soon

Dry application

Water Applied to Soil

Procedures (Cement Stabilization)

Water can be applied to mixer directlyTypical cure time sometimes shortened

Procedures (Cement Stabilization)

Water applied to mixer directly

Field testing to determine long-term strengthsAnd durability of chemically stabilized subgrades

Field testing to determine long-term strengthsAnd durability of chemically stabilized subgrades

SPT TESTS

Coring a stabilized subgrade using compressed air as cooling medium

Core of stabilized subgrade and asphalt about 30 years old

Shelby Tubes of stabilized subgrade (special tubes made)and layer below the stabilized subgrade

Field CBR test on stabilized subgrade and below stabilization

Perc

entil

e Te

st V

alue

In Situ CBRFrom University of Kentucky Transportation Center

0

10

20

30

40

50

60

70

80

90

1000 20 40 60 80 100

120

140

160

180

CBR

Perc

entil

e

Lime Cement Cement&Lime Kiln Design

DCP Tests on stabilized subgrade

Implementation

• Stabilization or modification of all new highways with CBR’s of 6 or less is recommended by Kentucky Transportation Cabinet

• Chemical stabilization is now a standard in highway construction

• Many industries and businesses use chemical stabilization

Benefits

• Economical• Prevents pavement failures during

construction• Structural credit can and is be given to

stabilized subgrade• Reduces subgrade swell• Long-term 30 years or more

Spreads Loads

Unstabilized Granular Base

Stabilized Base

100 psi

15 psi

100 psi

4 psi

Eliminates Rutting Below Surface

Rutting can occur in surface, base and subgrade of unstabilized bases due to repeated wheel loading

Stabilized bases resistconsolidation and movement,thus virtually eliminating rutting in all layers but the asphalt surface.

Unstabilized BaseStabilized

Base

Reduced Moisture SusceptibilityHigh water table

Unstabilized Granular Base Stabilized Base

Thank You