SESSION 3 Subgrade. The foundation upon which the pavement and base are constructed Embankment...

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Transcript of SESSION 3 Subgrade. The foundation upon which the pavement and base are constructed Embankment...

  • Slide 1
  • SESSION 3 Subgrade
  • Slide 2
  • The foundation upon which the pavement and base are constructed Embankment Natural soil Rigid layer } Subgrade Concrete slab Base
  • Slide 3
  • 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
  • Slide 4
  • Subgrade Models Dense liquid ( k ) model Elastic solid ( E ) model Real soil
  • Slide 5
  • 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
  • Slide 6
  • Static vs. Dynamic k Static k: the elastic portion of a soils response to a static load Dynamic k: the elastic response to a dynamic load - a fast-moving wheel load - an FWD load
  • Slide 7
  • 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
  • Slide 8
  • 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
  • Slide 9
  • Plate Bearing Tests Direct measurement of static elastic k value new alignment on subgrade soil on test embankment existing alignment remove slab and base
  • Slide 10
  • Plate Bearing Tests Repetitive loading test ASTM D 1195, AASHTO T221 k = slope of pressure to elastic deformation 760-mm (30 in) plate required Plate pressure, p Deflection, k = mean p / e pp ee
  • Slide 11
  • 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 Plate pressure, p Deflection, = 1.25 mm (0.05 in) k = p /
  • Slide 12
  • 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
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  • Degree of Saturation Affects k of Fine-Grained Soils 0 50 100 150 200 250 5060708090100 Degree of saturation (percent) Subgrade k value (psi/in) A-6 A-7-6 A-7-5 A-5 A-4
  • Slide 14
  • Dynamic Cone Penetrometer (DCP)
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  • Backcalculation of k Falling Weight Deflectometer (FWD) existing pavement new alignment on similar soil
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  • Backcalculation of k
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  • Westergaards interior deflection equation: P = ----- { ( a / ) } k 2 = radius of relative stiffness: E h 3 = 12 ( 1 - 2 ) k 4
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  • Backcalculation of k Deflection, Load, P radius, a AREA = f ( ), for given sensor configuration
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  • 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
  • Slide 20
  • Embankment and/or Rigid Layer
  • Slide 21
  • 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?
  • Slide 22
  • 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
  • Slide 23
  • 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
  • Slide 24
  • 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
  • Slide 25
  • 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
  • Slide 26
  • 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
  • Slide 27
  • 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)
  • Slide 28
  • 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
  • Slide 29
  • 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