session 3 subgrade. the foundation upon which the pavement and base are constructed embankment...
TRANSCRIPT
Subgrade
The foundation upon which the pavement and base are constructed
Embankment
Natural soil
Rigid layer} Subgrade
Concrete slabBase
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
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
ASTM D 1195, AASHTO T221 k = slope of pressure to elastic deformation
760-mm (30 in) plate required
Pla
te p
ress
ure,
p
Deflection,
k = mean p / e
p e
Plate Bearing Tests
• Nonrepetitive loading test
ASTM D 1196, AASHTO T222 k = pressure/deformation ratio at 1.25 mm (0.05 in) 760-mm (30 in) plate required
Pla
te p
ress
ure,
p
Deflection,
= 1.25 mm (0.05 in)
k = p /
Correlation of k to Soil Properties
Soil Class Density CBR k
A-1-a, well graded 125 - 140 60 - 80 300 - 450
A-1-a, poorly graded 120 - 130 35 - 60 300 - 400
… … … ...
A-2-4 or 5, gravelly 130 - 145 40 - 80 300 - 500
A-2-4 or 5, sandy 120 - 135 20 - 40 300 - 400
… … … …
A-4, silt 90 - 105 4 - 8 25 - 165
A-4, mix 100 - 125 5 - 15 40 - 220
… … … …
Degree of Saturation Affectsk of Fine-Grained Soils
0
50
100
150
200
250
50 60 70 80 90 100
Degree of saturation (percent)
Su
bg
rad
e k
valu
e (p
si/i
n) A-6
A-7-6A-7-5A-5A-4
Backcalculation of k
Falling Weight Deflectometer (FWD)
existing pavement
new alignment on similar soil
Backcalculation of k
Westergaard’s interior deflection equation:
P
= ----- { ( a / ) } k 2
= radius of relative stiffness:
E h3
= 12 ( 1 - 2 ) k
4
Adjustments toBackcalculated 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
> 10 ft
< 10 ft
246
8
10
12 Thickness of fill (ft) Density of fill (lb/cu ft)
Depth torigid layer
200400600psi/in
200 400 600 psi/in
90 100 110 120 130 140 150
200
400
psi/in
Enter with k fornatural subgrade
Adjusted k value
1 ft = 0.305 m,1 psi/in = 0.27 kPa/mm,1 lb/cu ft = 159 N/cu m
Embankment and/or Rigid Layer
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, lime-flyash, asphalt)
• Reinforcement with geosynthetics
Frost Heave
• Formation of ice lenses in frost-susceptible 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-frost-susceptible 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