SESSION 3
Subgrade
Subgrade
The foundation upon which the pavement
and base are constructed
Concrete slab
Base
Embankment
Natural soil
Rigid layer
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Subgrade
Objectives
• Characterize subgrade for concrete
pavement design purposes
• Select appropriate subgrade
preparation methods
• Identify subgrade remediation measures
for protection against frost heave and
soil swelling
Subgrade Models
Dense liquid ( k ) model
Real soil
Elastic solid ( E ) model
Soil Behavior
• Elastic response (k or E)
• Plastic (permanent) deformation
• Time-dependent response
• Standardized tests have been developed
to differentiate the elastic response from
the plastic and time-dependent
components
Static vs. Dynamic k
• Static k: the elastic portion of a soil’s
response to a static load
• Dynamic k: the elastic response to a
dynamic load
- a fast-moving wheel load
- an FWD load
K value steps,
1986/1993 AASHTO Guide
• K of unprotected subgrade soil
• Composite (top-of-the-base) k
• Adjustment for rigid layer
• Seasonal adjustment
• Loss-of-support adjustment
K value steps,
1998 AASHTO Supplement
• K value methods
correlation with soil type and properties
backcalculation
plate bearing tests
• Adjustment for fill and/or rigid layer
• Seasonal adjustment
Plate Bearing Tests
Direct measurement of
static elastic k value
new alignment
on subgrade soil
on test embankment
existing alignment
remove slab and base
Plate Bearing Tests
• Repetitive loading test
Plate pressure, p
ASTM D 1195, AASHTO T221
k = slope of pressure to elastic deformation
760-mm (30 in) plate required
k = mean p / De
Dp
De
Deflection, D
Plate Bearing Tests
• Nonrepetitive loading test
Plate pressure, p
ASTM D 1196, AASHTO T222
k = pressure/deformation ratio at 1.25 mm (0.05 in)
760-mm (30 in) plate required
k=p/D
D = 1.25 mm (0.05 in)
Deflection, D
Correlation of k
to Soil Properties
Soil Class
Density
CBR
k
A-1-a, well graded
A-1-a, poorly graded
…
125 - 140
120 - 130
…
60 - 80
35 - 60
…
300 - 450
300 - 400
...
A-2-4 or 5, gravelly
A-2-4 or 5, sandy
…
130 - 145
120 - 135
…
40 - 80
20 - 40
…
300 - 500
300 - 400
…
A-4, silt
A-4, mix
…
90 - 105
100 - 125
…
4- 8
5 - 15
…
25 - 165
40 - 220
…
Degree of Saturation Affects
k of Fine-Grained Soils
Subgrade k value (psi/in)
250
A-6
A-7-6
A-7-5
A-5
A-4
200
150
100
50
0
50
60
70
80
Degree of saturation (percent)
90
100
Dynamic Cone Penetrometer
(DCP)
Backcalculation of k
Falling Weight
Deflectometer (FWD)
existing pavement
new alignment on
similar soil
Backcalculation of k
Backcalculation of k
Westergaard’s interior deflection equation:
D =
P
----- {  ( a /  ) }
k 2
 = radius of relative stiffness:

4

=
E h3
12 ( 1 - 2 ) k
Deflection, D
Backcalculation of k
Load, P
radius, a
AREA = f (), for given sensor configuration
Adjustments to
Backcalculated k Value
• Slab size adjustment usually needed
• Static k value needed for design:
approximately = dynamic k / 2
• Different backcalculation equations for
deflections measured on AC-overlaid
PCC
• Variations in embankment thickness
and/or rigid layer depth affect k
Embankment and/or
Rigid Layer
D e n s ity o f fill (lb /c u ft)
T h ic k n e s s o f fill (ft)
12
90
100 110 120
130
140
150
10
8
6
4
2
p si/in
600
400
200
200
600
400
p si/in
A d ju s te d k v a lu e
200
E n te r w ith k fo r
n a tu ra l s u b g ra d e
< 1 0 ft
D e p th to
rig id la y e r
400
> 1 0 ft
p si/in
1 ft = 0 .3 0 5 m ,
1 p s i/in = 0 .2 7 k P a /m m ,
1 lb /c u ft = 1 5 9 N /c u m
Seasonal Adjustment
• 1998 AASHTO Supplement:
- seasonal movement of water table
- seasonal precipitation levels
- winter frost depths
- freeze-thaw cycles
- frost protection
• 1986/1993 AASHTO Guide
- annual average, or springtime?
Subgrade Preparation
• Foundation must provide:
– Assumed stiffness
– Uniformity
– Long-term stability
– Stable construction platform
• Has significant influence on smoothness
• Typically achieved by monitoring density
and moisture content during compaction
Subgrade Improvement
• Excavation and recompaction with
moisture density control
• Mechanical improvement (mixing in
coarser material)
• Excavation and replacement with select fill
• Stabilization (with lime, cement, limeflyash, asphalt)
• Reinforcement with geosynthetics
Frost Heave
• Formation of ice lenses in frostsusceptible soils
- fine sands and silts
- low-plasticity clays
• Both winter frost heave and subsequent
spring thaw can cause pavement
cracking
Frost Protection
• Replacing frost-susceptible soil with
non-frost-susceptible within depth of
frost penetration
• Covering frost-susceptible soil with
sufficient thickness of non-frostsusceptible soil
• Factors to consider: drainage, change
of grade, side slopes and ditches
Swelling Soils
• Some clays and shales are susceptible to
swelling (significant volume increase) when
sufficient moisture is available, especially when
an overburden pressure is removed
- southern and western US
- dry climates, low soil moisture contents
- pavement inhibits evaporation from soil
- excavation reduces overburden
• Swelling causes heaving and cracking
Swelling Protection
• Avoid cut sections in soils with known
swelling potential
• Avoid overcompaction on dry side of
optimum moisture content
• Lime stabilization to adequate depth
may be useful
• Minimize moisture variation (moisture
barriers or geomembranes may help)
Collapsing Soils
• Soils experiencing large decrease in volume
with increases in water content
• Treatment methods
– Modest depths: compaction with rollers,
wetting or inundation, and overexcavation
and recompaction (with lime or cement)
– Thicker deposits: ponding, flooding,
dynamic compaction
Summary
• Foundation: soil, embankment, rigid layer
• k value model works well for concrete
pavements
• Real soils exhibit some shear strength, elastic
and plastic behavior, time-dependent response
• Various methods for determining design k
• Prepare subgrade to achieve stiffness,
uniformity, long-term stability, stable
construction platform, protection against frost
and swelling