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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
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Outline
Performance of pavements on expansive clays Roughness Cracking
Pavement monitoring program
Suction envelopes for design
Prediction of movement Edge of pavement Wheel path
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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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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
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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
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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