Session 68 Björn Birgisson

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Session 68 Björn Birgisson

  1. 1. UNSATURATED FLOW OF WATER IN PAVEMENTS Prof. Björn BirgissonThe Royal Institute of Technology (KTH) Transportforum 2009
  2. 2. Problem Statement• Water in pavement systems can lead to detrimental effects• Complete prevention is not possible. Quick removal of the water should be enhanced before any damage can be initiated• There is a need to develop an improved understanding of the mechanics of water flow through pavement systems• Current drainage criteria is based on saturated flow theory
  3. 3. Objectives• How water moves through pavements• How long the water stays in a pavement structure.• What material properties control how long water stays in a given structure• What boundary and structure conditions (water table, shoulder construction, edge drains, layering, etc.) most affect the moisture conditions in the pavement
  4. 4. Saturated Vs. Unsaturated• Below water table • Above water table• Volumetric water • Volumetric water content (θ) = porosity content (θ) < porosity, and f(ψ)• No suction (negative • Suction (ψ < 0) pressure, ψ > 0)• Hydraulic • Hydraulic conductivity is conductivity is a constant (k = ksat) function of ψ.• Faster Drainage • Slower Drainage
  5. 5. s Volumetric Water Content (%) 20.0 Air entry = 10 kPa 15.0Soil Water 10.0 Saturated condition Unsaturated conditionCharacteristic Curve 5.0 0.0 0.01 0.10 1.00 10.00 100.00 1000.00 Suction (kPa) 1.0E-04 Air entry = 10 kPa Unsaturated condition 1.0E-06Hydraulic Saturated condition k (m/s)Conductivity Curve 1.0E-08 1.0E-10 0.01 0.10 1.00 10.00 100.00 1000.00 Suction (kPa)
  6. 6. Calibration of cells 33, 34, 35In order to understand the behavior of waterflow through flexible pavements underunsaturated conditions, actual Mn/ROADpavement geometries and materialcharacteristics were used along with resultsfrom automated time domain reflectometry(TDR) probes placed in the base layers of thesections studied
  7. 7. Cells 33, 34, 354.04’’ 3.92’’ 3.96’’ 12’’ 12’’ 12’’Cell 33 Cell 34 Cell 35 HMA Class 6 Special
  8. 8. Cells geometry 3.05 m 4.27 m 4.27 m 1.83 m CL 4:1 4:1 0.1 m Hot Mix Asphalt 0.3 m Class 6 Special4.0 m R-70 silty clay 16.5 m
  9. 9. Finite Element Model Extended Subgrade Extended SubgradeH=0m H=0mMaterial characterization: Mn/DOT dataImpervious HMASame model for all cellsInfiltration (q [m/s]) on shoulders and subgradeInitial water tableTotal Head = 0 m at bottom to induce drainage
  10. 10. TDR Locations Offset Centerline (-1.83 m) 0.25m 0.13 m 0.10 m0.38m 101 102 0.30 m 103 HMA 3.6 m Class 6 Special R-70 silty clay subgrade Automated *TDR
  11. 11. TDR in FEM CL Hot Mix Asphalt Base Subgrade
  12. 12. Initial Results (Cell 33-Location 101) 36.0 sVolumetric Water Content (%) 32.0 28.0 24.0 20.0 16.0 12.0 8.0 Measured 4.0 Predicted 0.0 210 220 230 240 250 260 270 Time (Julian day)
  13. 13. Precipitation Adjustment 12.0 fVolumetric Water Content (%) 11.0 10.0 9.0 8.0 7.0 Measured Predicted 6.0 210 220 230 240 250 260 270 Time (Julian day)
  14. 14. Location 102 20.0Volumetric Water Content (%) f 18.0 16.0 14.0 12.0 10.0 Measured 8.0 Predicted 6.0 210 220 230 240 250 260 270 Time (Julian day) Location 103 Volumetric Water Content (%) 28.0 24.0 20.0 f 16.0 12.0 Measured Predicted 8.0 210 220 230 240 250 260 270 Time (Julian day)
  15. 15. Density Adjustments 24.0 fVolumetric Water Content (%) 22.0 20.0 18.0 16.0 14.0 12.0 10.0 Measured 8.0 Predicted 6.0 210 220 230 240 250 260 270 Time (Julian day) 24.0 f Volumetric Water Content (%) 22.0 20.0 18.0 16.0 14.0 12.0 Measured 10.0 Predicted 8.0 210 220 230 240 250 260 270 Time (Julian day)
  16. 16. Parametric Study• Purpose: Identify the effects of certain material properties and boundary conditions (Ground Water Table) on the water flow through typical flexible pavement configurations.• Original conditions: Cell 33 was selected as a representative pavement configuration, with TDR location 101.
  17. 17. Parametric Study (cont…)• Air entry potential Base Material Volumetric Water Content (%)f 22.0 18.0 14.0 10.0 Predicted - 3 kPa 6.0 Predicted - 4kPa Predicted - 5 kPa 2.0 210 220 230 240 250 260 270 Time (Julian day)
  18. 18. Parametric Study (cont…)• Ksat Base Material 9.90 9.85 Volumetric Water Content (%)f 9.80 9.75 9.70 9.65 9.60 9.55 9.50 9.45 9.40 9.35 Predicted - 1.55E-06 m/s 9.30 Predicted - 1.55E-05 m/s 9.25 Predicted - 1.55E-04 m/s 9.20 210 220 230 240 250 260 270 Time (Julian day)
  19. 19. Parametric Study (cont…)• Air entry potential Subgrade 9.8 Volumetric Water Content (%)f 9.7 9.6 9.5 9.4 9.3 9.2 Predicted - 0 kPa 9.1 Predicted - 5 k Pa Predicted - 10 kPa 9.0 210 220 230 240 250 260 270 Time (days)
  20. 20. Parametric Study (cont…)• Ksat Subgrade Volumetric Water Content (%)f 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 Predicted -2.75E-8 m/s 7.0 Predicted - 2.75E-7 m/s 6.5 Predicted - 2.75E-6 m/s 6.0 210 220 230 240 250 260 270 Time ( Julian day)
  21. 21. Parametric Study (cont…)• Infiltration event 12.5 Volumetric Water Content (%)f 12.0 11.5 11.0 10.5 10.0 9.5 9.0 Predicted -100% Predicted - 70% 8.5 Predicted -70% and 30% 8.0 210 220 230 240 250 260 270 Time (Julian day)
  22. 22. Parametric Study (cont…)• Water table position 10.0 Volumetric Water Content (%)f 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 Predicted -3.20 m 5.0 Predicted - 3.00 m 4.5 Predicted - 2.85 m 4.0 210 220 230 240 250 260 270 Time (Julian day)
  23. 23. Drainage Systems Comparison• Edge Drain Hot Mix Asphalt Base Subgrade Edgedrain Pressure head = 0 m
  24. 24. Drainage Systems Comparison (cont…)• Under Drain Hot Mix Asphalt Base Under Drain Subgrade Pressure head = 0 m
  25. 25. Drainage Systems Comparison (cont…) 24.0 Volumetric Water Content (%)s 20.0 16.0 12.0 8.0 Original case 4.0 Case 1:Under Drain Case 2: Edgedrain 0.0 210 220 230 240 250 260 270 Time (Julian day)
  26. 26. Conclusions and Recommendations• Saturated flow assumptions may not adequately represent the physics of flow through pavement systems• Unsaturated material properties are needed to simulate the drainage performance of a pavement system. SWCC and hydraulic conductivity curves allow us to evaluate when and how fast pavement layers can drain
  27. 27. Conclusions and Recommendations (cont…)• Due to the installation procedures for the TDRs, the density around the TDR probes in the field is likely different from that in the laboratory.• The SWCC tend to be sensitive to density and gradation. These differences can result in a variation in both the air entry value and the slope of the soil water characteristic curve in the unsaturated region.
  28. 28. Conclusions and Recommendations (cont…)• The air entry potential determines the transition of a material from saturated to unsaturated conditions - the higher the air entry potential the longer the material will retain water.• The higher the hydraulic conductivity, the faster the material will drain.• If the water table is set at different elevations, the system will be under different initial suction and volumetric moisture conditions.
  29. 29. Conclusions and Recommendations (cont…)• Under Drain systems provide a faster drainage than Edge Drains. However, both systems keep the water table really close to the base layer. – Material with high air entry potential may not drain well in the presence of “positive” drainage systems• An improvement in the use of TDRs is suggested. It would be more helpful having more measurement points.
  30. 30. QUESTIONS ?

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