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 rutting 3. 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 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.

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 Geotextiles (filter criteria) 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