SESSION 4
Drainage Design
Considerations
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
Traffic Lane
Agg. Base
Subgrade
Aggregate Base
Carried
Out to Ditchline
Aggregate Base
Carried
Out to Ditchline
Longitudinal Edge Drain
System
Inner Shoulder
Slope
PCC
Traveled Way
Slope
PCCP
Outer Shoulder
Slope
PCCP
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,
millions
WET CLIMATE
Weak
Strong
Subgrade Subgrade
< 3.0
3.0 - 6.0
> 6.0
DRY CLIMATE
Weak
Strong
Subgrade Subgrade
None
None/EDS
None
None
EDS/DPS
EDS = Longitudinal Edge Drain System
DPS = Drainable Pavement System
None
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
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