Session 68 Björn Birgisson

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)

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nt (%

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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

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Time (Julian day)

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MeasuredPredicted

Precipitation Adjustment

6.0

7.0

8.0

9.0

10.0

11.0

12.0

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Measured

Predicted

Location 102

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

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Measured

Predicted

Location 103

8.0

12.0

16.0

20.0

24.0

28.0

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Measured

Predicted

Density Adjustments

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

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Measured

Predicted

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

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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

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Predicted - 3 kPaPredicted - 4kPaPredicted - 5 kPa


• Ksat Base Material

9.209.259.309.359.409.459.509.559.609.659.709.759.809.859.90

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Time (Julian day)

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Predicted - 1.55E-06 m/sPredicted - 1.55E-05 m/sPredicted - 1.55E-04 m/s


• Air entry potential Subgrade

9.0

9.1

9.2

9.3

9.4

9.5

9.6

9.7

9.8

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Time (days)

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Predicted - 0 kPaPredicted - 5 k PaPredicted - 10 kPa


• Ksat Subgrade

6.06.57.07.58.08.59.09.5

10.010.511.011.512.0

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Time ( Julian day)

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nt (%

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Predicted -2.75E-8 m/sPredicted - 2.75E-7 m/sPredicted - 2.75E-6 m/s


• Infiltration event

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

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Predicted -100%Predicted - 70%Predicted -70% and 30%


• 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)

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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

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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.


• 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.


• 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 ?

Session 68 Björn Birgisson

Business

Transcript of Session 68 Björn Birgisson