Design of Pavements on Expansive Clay Subgrades

Robert L. Lytton

Professor, Fred J. Benson Endowed Chair Zachry Department of Civil Engineering

Texas A&M University

Foundation Performance Association Houston, Texas

December 12, 2012

2

Outline

Performance of pavements on expansive clays Roughness Cracking

Pavement monitoring program

Suction envelopes for design

Prediction of movement Edge of pavement Wheel path

3

Outline, cont.

Prediction of roughness

Longitudinal cracking over expansive soils Design countermeasures Crack spacing

Features of design program WinPRES

WinPRES demonstration

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5

6

7

8

9

10

11

12

13

14

15

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20

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Exponential Suction Profile for Extreme Wetting and Drying Condition

0

2

4

6

8

10

12

14

2 3 4 5

Suction (pF)

Dept

h (ft

)

Wet

Equil.

Dry

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6

Volumetric Water Content

WetDry

0

2

4

6

8

10

12

14

0 1 2 3

Vertical Movement (in)

WetDry

Fort Worth Interstate 820

e onπ nπU(Z,t) = U +U exp - Z cos 2πnt - Zα α

e onπU(Z) = U +U exp - Zα

Mitchell (1979)

Moisture Active zone

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0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Dep

th, f

t.

Soil Suction, pF

Copyright John T. Bryant (2008)

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Figure 1 - Total Soil Suction Histogram for 2006

0

100

200

300

400

500

600

700

800

900

1000

2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 4.6 More

Total Soil Suction, pF

Freq

uenc

y

25

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Performance Criteria for Engineering Structures

Engineering Structure

Performance Criteria

Foundations • Differential movement: vertical and lateral and allowable stresses

• Differential movement and allowable stresses

• Total vertical and lateral movement; lateral pressure; allowable stresses

Pavements • Roughness spectrum, International Roughness Index, Longitudinal cracking

• Roughness spectrum, Pilot and Passenger acceleration

Retaining Walls • Lateral pressure and movement, allowable stresses

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Performance Criteria for Engineering Structures, cont.

Engineering Structure Performance Criteria

Pipelines • Roughness spectrum, allowable stress, fatigue criteria, corrosion

Slopes • Downhill movement, shallow slope failure, slope stability

Canals • Combination of the performance criteria of retaining walls, pipelines, and slopes; thermal and shrinkage cracking; permeability of the cracks and joints

Moisture Barriers • Reduction of the movement of water in the soil and of total vertical movement

Land Fill Covers and Liners

• Moisture and leachate transmission (including the effects of cracks)

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The Design Problem

How do you design a foundation to perform successfully when you have poor site conditions?

Vegetation Drainage Slopes

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Answer: Design for the worst that they can do

Site condition Problem Limiting Condition

Vegetation Drying shrinkage Wilting point pF=4.5

Poor drainage Swelling Clay wet limit pF>2.5

Slopes Downhill creep, shallow slides

Uphill offsets, drainage control

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Answer: use suction envelopes to determine the worst that they can do

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Dry Season

(-) Suction Ground Surface

Wet Season

Equilibrium

Dep

th

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(-) Suction Ground Surface

Wet Season

Equilibrium

Dry Season

Dep

th

Water Table Suction of the

Water

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Field Conditions

0

2

4

6

8

10

12

14

2.0 3.0 4.0 5.0

pFDe

pth

(ft

Root zone

Moisture active zone

Wilting point 4.5 pF Field capacity Equilibrium

πα

= ±

0( ) exp -e

nU Z U U Z

= −e 3.5633exp( 0.0051TMI)U

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Volume Change

Zone III (Covar and Lytton, 2001)

PI / %fc

LL /

%fc

(Lytton, 1994)

µγ γ −

= × − 0

% 2% .200h

mNo sieve

σγ γ

θθ

=+

∂ ∂

1

1h h

h

µ−=

−% 2%

% .200mfc

No sieve

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Volume Change

(Lytton, 1977)

Volume–Mean Principle Stress-Suction surface

10 10log logf fh

i i

hVV h σ

σγ γ

σ ∆

= − −

H VfH V

∆ ∆ =

0.67 0.330.5 ;0.8

f pFf when dryingf when wetting

= − ∆

= =

1

n

i ii i

Vf ZV=

∆ ∆ = ⋅∆ ∑

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Suction vs. Pressure vs. Volume Surface

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Formation of Suction vs. Pressure vs. Volume Surface

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Calculated Vertical Movement

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Transverse Distribution of Vertical Movements

-1.5

-0.5

0.5

1.5

2.5

3.5

0 10 20 30 40

Section ASection BSection C

d (ft) Swel

ling

Shrin

kage

(in)

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Predicted Roughness vs. Time; Fort Worth I-820 B

2.5

3.0

3.5

4.0

4.5

0 10 20 3060

80

100

120

140

0 10 20 30

Loss of Serviceability Increase in Roughness

Time (yrs) Time (yrs)

SI

IRI

Soil

Soil

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Predicting Changes in IRI (R)

Pavement categories: Moisture barriers with paved medians β1 = 0.619, β2 = 1.295 Moisture barriers with sodded medians β1 = 1.583, β2 = 2.011 Control section with and without medians β1 = 2.701, β2 = 4.015

( )1 2dR Hdt

β β= ∆ +

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IRI vs. PSI

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Roadside Drainage Conditions

Hill Slope

Valley

Cut

Flat

Fill

2.3 pF

Longitudinal Drainage

Lateral Slope

2.0 pF 2.0 pF

2.5 pF 2.2 pF 2.2 pF

2.6 pF 2.3 pF 2.3 pF

Thornthwaite Moisture Index (TMI, 1948)

Climatic Conditions

100 60

p

R DEFTMIE−

=R = runoff moisture depth DEF =deficit moisture depth Ep = evapotranspiration

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Acceptable Predicted Performance

2.0

2.5

3.0

3.5

4.0

4.5

0 10 20 30

Fort Worth I-820 A Flexible Pavement

60

100

140

180

0 10 20 30

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Acceptable Predicted Performance

60

100

140

180

0 10 20 302.0

2.5

3.0

3.5

4.0

4.5

5.0

0 10 20 30

Austin SR-1

Rigid Pavement

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Longitudinal Cracking over Expansive Soil

Expansive soil Experiences volumetric change when subjected to

moisture variation Longitudinal crack

Initiates in shrinking expansive subgrade Propagates to pavement surface

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Practice of Lime Treatment

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Without Geogrid Reinforcement…

Subgrade (Expansive soil)

Asphalt

CL

Base Crack

* Rong Luo, Texas A&M University

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With Geogrid Reinforcement…

Subgrade (Expansive soil)

Asphalt

CL

Base

Geogrid

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Transverse Stress Distribution in Pavement (Crack at Edge of Shoulder)

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0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

0.200

0 400 800 1600 3200 6400 12800

Geogrid Stiffness (kN/m)

Stre

ss In

tens

ity F

acto

r (M

Pa*m

^.5)

0

10

20

30

40

50

60

70

80

90

Geo

grid

Ten

sile

Str

engt

h (k

N/m

)

Number of Crack=1

Number of Crack=2

Number of Crack=3

Number of Crack=4

Number of Crack=5

Upper Tensile Strength

Lower Tensile Strength

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Transverse Distribution of Vertical Movements

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Edge Moisture Variation Distance, em

2uo

em

P =maximumpermissibleverticalstrain

pF Suction envelopes

Designsuctionrange

02ln( )mT ue

pFαπ

=∆

31 1h

ppFγ

∆ = − +

∆

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Longitudinal Crack Spacing

2u0

pF Tensile Strength of Soil∆ ∝

Initial Suction

Wilting Point

Pavement

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Shrinkage Strain

[ ]1 (1 ) ( )6s hpF pFε γ= + ∆ − ∆

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Distance to First Shrinkage Crack

01

0

2ln( )2

T uxu pF

απ

=− ∆

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Diffusivity

[ ]2

40.0029 0.000162( ) 0.0122( ) 10sec hm Sα γ −

= − − ×

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pFint

1

S

w, water content wsat

pF(w)

00

7

intercept( )Sw pF w pF= −

intercept 5.622 0.0041(%fine clay)pF = −

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Alternative

Use built-in empirical expression: α = 0.0029 – 0.000162 S – 0.0122 γh

where: S = −20.3 – 0.155 (LL) – 0.117 (PI) + 0.068 (%−No. 200)

h 0% - 2μm×

% -No.200sieveγ = γ

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Field to Laboratory Diffusion Coefficient Ratio

Field α/laboratory α0

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Program WinPRES

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Soil

Prop

ertie

s

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Lane

/Bar

rier c

onfig

urat

ion

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Initi

al S

ervi

ceab

ility

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Diff

usiv

ity

Slope of SWCC Suction Compression Index

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Traf

fic/R

elia

bilit

y

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WinPRES Demo

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Soil Survey of Harris County, Texas Lake Charles series

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Lake Charles series, cont.

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Soil Survey of Harris County, Texas

Engineering properties and classifications

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Soil Survey of Harris County, Texas

Engineering test data

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Soil Survey of Harris County, Texas

Profile of Lake Charles clay

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WinPRES Demo