Subgrade Improvement Options -

Performance Study and Cost Comparison

• Remove-and-Replace

• Lime Treatment

Larry Cole, P.E.

Carmeuse Lime and Stone

Pittsburgh, PA

2

Subgrade Soil on Gateway Project (Detroit, MI)

3

Cut-and-Fill

4

Lime Treatment

5

Lime’s Effect on Soil

Drying up wet soil: –Drying

Modification: –Plasticity and swell reduction

–Improved stability & compaction

–Solid working platform

Stabilization: –Permanent strength increase

–Long-term strength gains

–Less moisture absorption

–Freeze-thaw durability

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6

Lime Reacts Chemically with Clays

to Alter Particle Interactions

Clay Particle Water Layer

7

• Increased Strength

• Reduced Plasticity

Lime Modified Soil:

• Reduced Expansion

8

Soil Before And After Lime Treatment

9

CBR Increases with Lime Modification

Soil Classification

State

Untreated

CBR

Treated CBR

(Immediate)

Treated

CBR

(Cured) AASHTO UCS

A6 CL MI 4 40 68

A-7- 6 CH MI 3 40 80

A-7- 6 CL MI 4 22 40

A-2- 6 SC MI 4 40 85

A6 CL VA 6 30 140

A-7- 6 CH GA 3 28 66

A-7- 6 CH TN 5 34 84

A-7- 6 CH TN 8 25 74

A6 CH TN 2 26 109

A6 CL OH 4 21 62

A-2-6 SC IL 9 27 100

10

Lime Treatment or Cut-and Fill?

Which technique:

• Works better for construction loads?

• Cost less?

11

The decision depends on a number of factors, including:

• Cost

• Project size

• Contractor capabilities and preferences

• Material availability

• Disposal options (for remove and replace)

• Equipment access or operating constraints

• Existing soil types and soil conditions

and should be evaluated on a case-by-case basis by a

competent geotechnical engineer, site work contractor and

soil stabilization specialty contractor.

Neither solution is always best in all situations.

Lime Treatment or Cut-and Fill?

12

North Carolina State University Study

13

NCSU Subgrade Testing Apparatus

14

Subgrade Installation

15

Unsoaked CBR of Subgrade Soil

16

Subgrade Test Sections

Subgrade -

CL Soil

% Passing #200 = 58

PI = 16

OMC = 15.6%

Agg Base Course - GW

(12” - 20”)

or Select Fill - SW

(17” - 36”)

Subgrade -

CL Soil

% Passing #200 = 58

PI = 16

OMC = 15.6%

Lime Treated Soil

(3% lime) (9” )

4” - 5” Agg Base Course

17

Subgrade Load Plate

18

Construction Load (Cat 631 Scraper)

80 psi

25-30 mph

19

Displacement After 200 Loading Cycles

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

9" LSS

with 4"

ABC

20" ABC 36" Select

Fill & 3"

ABC

16" ABC

with

Geogrid

14" ABC 12" ABC

with

Geotextile

Inch

es

20

Displacement After 10,000 Loading Cycles

0

0.5

1

1.5

2

2.5

3

9" LSS

with 4"

ABC

20" ABC 36" Select

Fill & 3"

ABC

16" ABC

with

Geogrid

14" ABC 12" ABC

with

Geotextile

200 Cycles

10,000 Cycles

Inch

es

21

Number of Loading Cycles to 0.5” Displacement

0

100

200

300

400

500

600

700

800

900

1000

9" LSS

with 4"

ABC

20" ABC 36" Select

Fill & 3"

ABC

16" ABC

with

Geogrid

14" ABC 12" ABC

with

Geotextile

Lo

ad

ing

s

Over

10,0

00

22

Unit Costs for Subgrade Improvement Options

Item Avg Cost ($) Units

Undercut Excavation $10.04 /yd3

Select Granular Fill $24.59 /yd3

Aggregate Base Course $24.12 /ton

Lime Treated Subgrade $2.54 /yd2

Lime $195.36 /ton

NCDOT Statewide Averages - 2008

23

Unit Costs for Subgrade Improvement Options

Item Small Quantities Large Quantities

Geotextile $4.57

Geogrid A $4.50 $3.04

Geogrid B $2.25 $1.28

Manufacturer Cost Estimates ($/SY)

24

Cost Comparison Subgrade Improvement Options

Test Configuration Unit Cost

($/SY)

9” Lime Stabilized Soil with 4” ABC $10.44

14” Agg Base Course $20.18

12” Agg Base Course with Geotextile $22.59

16” Agg Base Course with Geogrid $24.34

20” Agg Base Course $28.82

36” Select Fill and 3” ABC $38.95

12” of Lime Stabilized Soil: $5.40 /SY

$5.40 will buy < 4” of cut and fill with ABC

or

< 2” of cut and fill with ABC and geogrid (based on NCDOT State-wide average bid costs)

25

Performance and Cost

Low Displacement

Low Cost Low Displacement

High Cost

High Displacement

Low Cost

High Displacement

High Cost

1¾

40 Cost ($/SY)

Dis

pla

cem

en

t (i

n)

12” ABC w/Geotextile

14” ABC

20” ABC

18” ABC

16” ABC

w/ Geogrid

9” LSS

w/ 4” ABC 12”

LSS

19” ABC

with

Geotextile

26

Comparison Subgrade Improvement Options

NCDOT Comparative Costs (2008/2010) Unit Cost ($/SY)

12” Lime Stabilized Soil $5.44 / $5.00

14” Agg Base Course $20.18 / $15.43

12” Agg Base Course with Geotextile $22.59 / $18.54

16” Agg Base Course with Geogrid $24.34 / $19.43

20” Agg Base Course $28.82 / $22.04

36” Select Fill and 3” ABC $38.95 / $18.55

Compared to Cut-and Fill –

Lime-treated soil gives better results

at less than 1/3 the cost

27

Unit Costs for Subgrade Improvement Options

Item Avg Cost ($) Units

Excavation $7.46 /yd3

Geotextile (Subgrade Stab) $1.19 /yd2

Aggregate Base $37.46 /yd3

Lime Treated Subgrade,

12 inches deep $2.30 /yd2

Lime $128.82 /ton

ODOT Statewide Averages

2011

28

Comparison Subgrade Improvement Options

ODOT Comparative Costs - 2011 Unit Cost

($/SY)

12” Lime Treated Soil $4.26

14” Agg Base Course $17.46

12” Agg Base Course with Geotextile $16.17

16” Agg Base Course with Geotextile $21.16

20” Agg Base Course $24.97

Compared to Cut-and-Fill with ABC–

Lime-treatment is less than 1/3 of the

cost of remove-and-replace with

aggregate base course

29

Comparison Subgrade Improvement Options

GDOT - Comparative Costs Unit Cost

($/SY)

8” Lime Stabilized Soil $4.43

8” Agg Base Course $10.61

8” Agg Base Course with Geogrid $13.26

Compared to Cut-and-Fill with ABC–

Lime-treatment is less than 1/3 of the

cost of remove-and-replace with

aggregate base course & geogrid

31

Unit Costs for Subgrade Improvement Options

Item Avg Cost ($) Units

Regular Excavation $10.69 /yd3

Geotextile (Subgrade Stab) $1.61 /yd2

Agg Material 21B $24.70 /ton

Lime Treated Subgrade

Manipulation $2.90 /yd2

Lime $202.65 /ton

VDOT Statewide Averages

July 2009 to Aug 2011

32

Comparison Subgrade Improvement Options

VDOT - Comparative Costs Unit Cost

($/SY)

12” Lime Stabilized Soil $6.00

14” Agg Base Course $21.96

12” Agg Base Course with Geotextile $20.43

16” Agg Base Course with Geotextile $26.74

20” Agg Base Course $28.82

Compared to Cut-and-Fill with ABC–

Lime-treatment is less than 1/3 of the

cost of remove-and-replace with

aggregate base course

33

Unit Costs for Subgrade Improvement Options

Item Avg Cost ($) Units

Regular Excavation $3.47 /yd3

Geogrid $2.10 /yd2

Class 2 Base (rock or PCC) $25.23 yd2, 12” deep

Lime Treatment $315 ton

LADOTD Statewide Averages

Q4 - 2012

34

Comparison Subgrade Improvement Options

LADODT - Comparative Costs Unit Cost

($/SY)

12” Lime Treated Soil $7.10

12” Class 2 Base (rock) with geogrid $28.40

12” Sand $15.65

Compared to Cut-and-Fill with Rock or Sand –

Lime-treatment is less than ¼ - ½

of the cost

35

Lime Treatment or Cut-and Fill?

Which technique:

• Works better for construction loads?

• Cost less?

Subgrade Improvement Options -

Performance Study and Cost Comparison

• Remove-and-Replace

• Lime Treatment

Larry Cole, P.E.

Carmeuse Lime and Stone

Pittsburgh, PA