pavement analysis and design zdrainage considerations
TRANSCRIPT
Pavement Analysis and Design
Drainage Considerations
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
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
This may lead to three main effects:1. fatigue failure 2. subgrade rutting3. potholing and collapsing
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
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
Sources of Water
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
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
Methods for Reducing Moisture Effects
There are two types of drainage Surface Subsurface
Surface Drainage
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 Open-graded hot mix can adequately drain water to the
sides. A transverse pipe may then be used to drain the water Roadside interceptor ditches
Curbs and gutters Storm sewers in areas where open channels are not appropriate.
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.
Ways to get moisture out of pavement
Drainage components Longitudinal drains Transverse drains Permeable bases Separator layers
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.
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.
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
Drainage Materials
Aggregates Drainage layer – permeability, filter criteria
French drains – permeability, filter criteria
Geotextiles (filter criteria)
Pipes Perforated
Slotted
Open-jointe
Drainage Materials
Geotextiles criteria Filter fabric Allow water to flow Retain soil Protect drainage layer from clogging Pumping resistance criteria Fine soil Granular material
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
Pipe Edge Drain
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
Permeable Base and Edge Drain
Outlet Pipe Design