Traffic Characterization

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SESSION 4
Drainage Design Considerations
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Objectives

• Identify types of subsurface drainage
• Determine need for subsurface drainage
• Recognize importance of maintenance
• Identify surface drainage design factors

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Introduction

• Moisture is a major cause of distress
• Drainage long recognized as key design
  consideration
• Focus on drainage of surface infiltration water

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Effect of Drainage
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Moisture-Related Distresses
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Moisture-Related Distresses
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Moisture-Related Distresses
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Moisture-Related Distresses
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Approaches to Drainage Problems

• Seal the pavement
• Use moisture-insensitive materials
• Install subsurface drainage

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Types of Subsurface Drainage

• Daylighted bases
• Longitudinal edge drain system
• Drainable pavement system

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Daylighted
Traffic Lane
Traffic Lane
Shoulder
Shoulder
PCC Slab
Agg. Base
Subgrade
Aggregate Base Carried Out to Ditchline
Aggregate Base Carried Out to Ditchline
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Longitudinal Edge Drain System
Inner Shoulder
Traveled Way
Outer Shoulder
Slope
Slope
Slope
PCC
PCCP
PCCP
PCC
TPM
Base (LCB, ACB)
Filter Fabric
Filter Fabric, High Side of Tangents and
Superelevations
California Design
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Example Drainable Pavement System
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Components of a Drainable Pavement System

• Permeable base
• Separator layer
• Longitudinal collector pipe

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Permeable Base

• Permeability of 300 to 3,000 m/day (1,000 to
  10,000 ft/day)
• 100 to150 mm (4 to 6 in) thick
• Treated or untreated

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Separator Layer

• Prevent intrusion of fines into permeable base
• Dense-graded aggregate
• Geotextile

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Longitudinal Collector Pipe

• Collects water and outlets to ditches
• Typically 100 to 150 mm (4 to 6 in) corrugated
  plastic pipe
• Outlet spacing 76 to 150 m (250 to 500 ft)

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Design and Analysis of Drainage Systems

• Define
• Material properties
• Roadway/pavement geometrics
• Climatic data
• Determine inflow/outflow characteristics
• Determine drainage requirements

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Consideration of Drainage in Slab Thickness Design

• AASHTO Drainage Coefficient
• Drainage , Thickness
• Reduction in properties of paving materials

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Determining Need for Subsurface Drainage

• Available free moisture
• Material properties
• Cross sectional design
• Subgrade drainability
• Traffic levels

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NCHRP 1-32Drainage Guidelines
WET CLIMATE DRY CLIMATE ESALs,
Weak Strong Weak Strong
millions Subgrade Subgrade Subgrade
Subgrade
lt 3.0 None 3.0 - 6.0
None/EDS None None
None gt 6.0 EDS/DPS
EDS Longitudinal Edge Drain System DPS
Drainable Pavement System
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Inclusion ofSubsurface Drainage

• Local experience
• Past performance
• Cost effectiveness
• Construction and maintenance practices

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Maintenance of Drainage Installations

• Ensures functionality of drainage system
• Often a forgotten maintenance item
• Activities include
• Reference markers
• Cleaning outlets
• Flushing/rodding edge drains
• Cleaning/reestablishing ditches
• Periodic video inspection

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Video Inspection of Edge Drains
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Crushed Pipe and Silt Buildup
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Unwanted Inhabitants
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Surface Drainage Considerations

• Pavement cross slope (tangent section)
• 2 recommended
• Shoulder cross slope
• 3 recommended
• Ditches
• Width 0.9 to 1.2 m (3 to 4 ft)
• Depth 1.2 m (4 ft) beneath pavement Grade 1

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Summary

• Moisture-related distresses
• Subsurface drainage types
• Determining need for subsurface drainage
• Maintenance considerations
• Surface drainage considerations