1. Pavement Analysis and Design
Drainage Considerations

2. Introduction Moisture is a major cause of distress
accounts for 60 percent of all pavement failures
parking lots, truck parks, etc., have greater problem
Many sources of moisture in pavements

3. Moisture Related Problems
Pavement strength is reduced when wet. Reasons:
Increased pore pressure lowers internal friction and shear resistance.
Buoyancy of particles reduces effective weight and lowers inter-particle friction.
Expansive soils cause differential heave.
Frost heave, due to freeze-thaw cycles, occurs

4. This may lead to three main effects:
1. fatigue failure
2. subgrade rutting
3. potholing and collapsing

5. Detrimental Effects of Water
Causes pumping of PCC pavements leading to faulting, cracking and general shoulder deterioration
Reduces the strength of unbound granular material and subgrade
Pumping of fines in base course may also occur leading to loss of support
Continuous contact with water causes stripping of AC mix and durability cracking of concretes

6. Effect of Moisture on Fine-Grained Soils
Weakening of pavement layers
Degradation of pavement material (stripping and erosion of AC, erosion of other materials, D-cracking of PCC
Loss of bond between layers

7. Sources of Water

8. Moisture Control Methods
Prevention
Intercept groundwater
Seal pavement surface
Provide good surface drainage
Removal (most economical)
Drainage blanket
Longitudinal drains
Transverse drains
Build strong enough pavement

9. Movement of Water By Gravity, capillary action and vapor pressure
Granular materials => gravity
v = ki (Darcy’s law)
v is discharge velocity, k is coeff. permeability
i is hydraulic gradient
Q = vA
To resist combined effect of moisture and load
Fine grain materials => capillary action

10. Methods for Reducing Moisture Effects
There are two types of drainage
Surface
Subsurface

11. Surface Drainage Curbs and gutters
Open-graded hot mix can adequately drain water to the sides.
A transverse pipe may then be used to drain the water
Crown/ Cross slopes
Roadside interceptor ditches
Curbs and gutters
Storm sewers in areas where open channels are not
appropriate.

12. Pavement Surface Hydraulics
It is desirable for pavement to be sloped, both in the Cross sectional as well as in the longitudinal sense of the Pavement. Typical minimum
values for pavement Slope are as follows:
Cross sectional slope : 2% for the first two travel lanes
3% for the third travel lane
*when the road exceeds 3 lanes, pavement should be sloped inward towards the median using the same criteria as above
Longitudinal slope : Minimum longitudinal slope allowed
When using a curb and gutter section = 0.3 %. Otherwise
Flat longitudinal slopes are allowed.

17. Ways to get moisture out of pavement
Drainage components
Longitudinal drains
Transverse drains
Permeable bases
Separator layers

18. Subsurface Drainage Pavement and shoulders
Poorly drained subgrades can cause a layer of water to form at the base and subgrade interface. This then forms a mud slurry under traffic that will fill voids in the base material. The pavement should be maintained without cracks or holes.

19. Subgrade drains are used to drain the soil
The design of subsurface systems is aimed at lowering thewater table, eliminating active springs, seepage and other water sources, and collecting discharge from other drainage systems. It must have adequate capacity, and maintain this capacity, during the life of the pavement.

20. Internal Drainage Factors
Subgrade soil
The drainability of the subgrade soil is a function of:
Soil grain size
Depth of the water table
Soil plasticity and topography

24. Drainage Materials Aggregates Geotextiles (filter criteria)
Drainage layer – permeability, filter criteria
French drains – permeability, filter criteria
Geotextiles (filter criteria)
Pipes
Perforated
Slotted
Open-jointe

26. Drainage Materials Geotextiles criteria Filter fabric
Allow water to flow
Retain soil
Protect drainage layer from clogging
Pumping resistance criteria
Fine soil
Granular material

27. Geotextiles criteria (Cont.)
Permeability criteria
The filter material must be coarse enough to carry water without any significant resistance
Clogging criteria
The filter material must be fine enough to prevent the adjacent finer material from piping or migrating into the filter material

30. Pipe Edge Drain

31. Pipe Edge Drain Pipe diameter
Function of flow rate, grade, and outlet spacing >= 100 mm (4 in) recommended
Longitudinal slope
> 1% for smooth pipes
> 2% for corrugated pipes
Top of pipe should be 50 mm (2 in) below subgrade

32. Permeable Base and Edge Drain

33. Outlet Pipe Design