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

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

Effect of Drainage

Moisture-Related
Distresses

Moisture-Related
Distresses

Moisture-Related
Distresses

Moisture-Related
Distresses

Approaches to Drainage
Problems

Seal the pavement

Use moisture-insensitive materials

Install subsurface drainage

Types of Subsurface
Drainage

Daylighted bases

Longitudinal edge drain system

Drainable pavement system

Daylighted
Traffic Lane
Agg. Base
Subgrade
Traffic Lane
Aggregate Base
Carried
Out to Ditchline
Aggregate Base
Carried
Out to Ditchline

Longitudinal Edge Drain
System
Inner Shoulder Traveled Way
PCC
Slope
Slope
PCCP PCCP
Outer Shoulder
Slope
PCC
TPM
Base (LCB, ACB)
Filter Fabric, High Side of
Tangents and Superelevations
California Design
Filter
Fabric

Example Drainable
Pavement System
AC
Shoulder
PCC Mainline
AC
Shoulder
Geotextile
Aggregate Separator
Layer Permeable
Base
Geotextile

Components of a Drainable
Pavement System

Permeable base

Separator layer

Longitudinal collector pipe

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

Separator Layer

Prevent intrusion of fines into permeable base

Dense-graded aggregate

Geotextile

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)

Design and Analysis of
Drainage Systems

Define

Material properties

Roadway/pavement geometrics

Climatic data

Determine inflow/outflow characteristics

Determine drainage requirements

Consideration of Drainage in Slab Thickness Design

AASHTO Drainage Coefficient

Drainage , Thickness

Reduction in properties of paving materials

Determining Need for
Subsurface Drainage

Available free moisture

Material properties

Cross sectional design

Subgrade drainability

Traffic levels

NCHRP 1-32
Drainage Guidelines
ESALs,
WET CLIMATE
Weak Strong
DRY CLIMATE
Weak Strong millions Subgrade Subgrade Subgrade Subgrade
< 3.0
None
3.0 - 6.0 None/EDS None None None
> 6.0
EDS/DPS
EDS = Longitudinal Edge Drain System
DPS = Drainable Pavement System

Inclusion of
Subsurface Drainage

Local experience

Past performance

Cost effectiveness

Construction and maintenance practices

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

Video Inspection of Edge
Drains

Crushed Pipe and Silt
Buildup

Unwanted Inhabitants

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%

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