session 68 björn birgisson
DESCRIPTION
TRANSCRIPT
UNSATURATED FLOW OF WATER IN PAVEMENTS
Prof. Björn BirgissonThe Royal Institute of Technology (KTH)
Transportforum 2009
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
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
Saturated Vs. Unsaturated
• Below water table• Volumetric water
content (θ) = porosity
• No suction (negative pressure, ψ > 0)
• Hydraulic conductivity is constant (k = ksat)
• Faster Drainage
• Above water table• Volumetric water
content (θ) < porosity, and f(ψ)
• Suction (ψ < 0)
• Hydraulic conductivity is a function of ψ.
• Slower Drainage
Soil Water Characteristic Curve
0.0
5.0
10.0
15.0
20.0
0.01 0.10 1.00 10.00 100.00 1000.00
Suction (kPa)
Vol
umet
ric
Wat
er C
onte
nt (%
) s
Air entry = 10 kPa
Saturated condition
Unsaturated condition
1.0E-10
1.0E-08
1.0E-06
1.0E-04
0.01 0.10 1.00 10.00 100.00 1000.00Suction (kPa)
k (m
/s)
Air entry = 10 kPa
Saturated conditionUnsaturated condition
Hydraulic Conductivity Curve
Calibration of cells 33, 34, 35
In order to understand the behavior of water flow through flexible pavements under unsaturated conditions, actual Mn/ROAD pavement geometries and material characteristics were used along with results from automated time domain reflectometry (TDR) probes placed in the base layers of the sections studied
Cells 33, 34, 35
12’’
Cell 33
4.04’’
12’’
Cell 34
3.92’’
12’’
Cell 35
3.96’’
HMA
Class 6 Special
Cells geometry
1.83 m4.27 m 4.27 m3.05 m
CL
0.3 m Class 6 Special0.1 m Hot Mix Asphalt
R-70 silty clay
4:1 4:1
4.0 m
16.5 m
Finite Element ModelExtended Subgrade
H = 0 m
Extended Subgrade
H = 0 m
Material 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
TDR Locations
Offset(-1.83 m)
Centerline
0.13 m0.25m0.38m
0.10 m
0.30 m
3.6 m
Automated*TDR
HMA
Class 6 Special
R-70 silty clay subgrade
101102103
TDR in FEM
Hot Mix Asphalt
Base
Subgrade
CL
Initial Results (Cell 33-Location 101)
0.0
4.0
8.0
12.0
16.0
20.0
24.0
28.0
32.0
36.0
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) s
MeasuredPredicted
Precipitation Adjustment
6.0
7.0
8.0
9.0
10.0
11.0
12.0
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Measured
Predicted
Location 102
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
210 220 230 240 250 260 270Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Measured
Predicted
Location 103
8.0
12.0
16.0
20.0
24.0
28.0
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
)f
Measured
Predicted
Density Adjustments
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Measured
Predicted
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Measured
Predicted
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.
Parametric Study (cont…)
• Air entry potential Base Material
2.0
6.0
10.0
14.0
18.0
22.0
210 220 230 240 250 260 270Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Predicted - 3 kPaPredicted - 4kPaPredicted - 5 kPa
Parametric Study (cont…)
• Ksat Base Material
9.209.259.309.359.409.459.509.559.609.659.709.759.809.859.90
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Predicted - 1.55E-06 m/sPredicted - 1.55E-05 m/sPredicted - 1.55E-04 m/s
Parametric Study (cont…)
• Air entry potential Subgrade
9.0
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
210 220 230 240 250 260 270
Time (days)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Predicted - 0 kPaPredicted - 5 k PaPredicted - 10 kPa
Parametric Study (cont…)
• Ksat Subgrade
6.06.57.07.58.08.59.09.5
10.010.511.011.512.0
210 220 230 240 250 260 270
Time ( Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Predicted -2.75E-8 m/sPredicted - 2.75E-7 m/sPredicted - 2.75E-6 m/s
Parametric Study (cont…)
• Infiltration event
8.0
8.5
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
210 220 230 240 250 260 270Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Predicted -100%Predicted - 70%Predicted -70% and 30%
Parametric Study (cont…)
• Water table position
4.04.55.05.56.06.57.07.58.08.59.09.5
10.0
210 220 230 240 250 260 270Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) f
Predicted -3.20 mPredicted - 3.00 mPredicted - 2.85 m
Drainage Systems Comparison
• Edge Drain
Pressure head = 0 m
Hot Mix Asphalt
Base
Subgrade
Edgedrain
Drainage Systems Comparison (cont…)
• Under Drain
Pressure head = 0 m
Hot Mix Asphalt
Base
Subgrade
Under Drain
Drainage Systems Comparison (cont…)
0.0
4.0
8.0
12.0
16.0
20.0
24.0
210 220 230 240 250 260 270
Time (Julian day)
Vol
umet
ric
Wat
er C
onte
nt (%
) s
Original caseCase 1:Under DrainCase 2: Edgedrain
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
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.
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.
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.
QUESTIONS ?QUESTIONS ?