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