Drainage characteristics should be noted during a visit to the project site. Items such as the general terrain drainage, the highway drainage (including cross slopes, condition of existing culverts, and ditch depth/capacity), and any existing internal pavement drainage features should be noted.

Another drainage item to consider is bridge-class drainage structures. The number of bridges and how the existing pavement terminates at the bridge ends is important to note. Also, note if the bridges have bridge approach (rigid pavement) slabs. The condition of the bridge end/approach slab and the approach slab/pavement interface conditions are of special interest where concrete pavement is present. These pavement interfaces often provide a location for surface runoff to enter the pavement structure and may lead to wash outs of fill material behind bridge head walls, MSE embankments, and rip-rapped slopes.

Moisture intrusion into a pavement structure has been a known source of reduced service life since the earliest roads were constructed. The principles of drainage management for pavement structures have not changed radically since AASHTO published the 1986

Guide for Design of Pavement Structures

.

In Vol. 2, App. AA of that reference, internal drainage systems are advocated particularly for “problem areas” as determined from experience and a drainage analysis of the particular project right-of-way. Federal-Aid Policy Guide’s supplemental materials, 23 CFR Part 626, stresses:

". . . inadequate subsurface drainage continues to be a significant cause of pavement distress, particularly in Portland cement concrete pavements." And “Where the drainage analysis or past performance indicates the potential for reduced service life due to saturated structural layers or pumping, the design needs to include positive measures to minimize that potential.”

Positive drainage measures are defined as permeable bases and the gathering and discharge system required for these bases. They are generally synonymous with internal or subsurface pavement drainage (underdrain) features. When internal drainage is contemplated for use within the pavement structure, the department philosophy since 1994 on conducting a full drainage analysis has been restricted to:

The following is a list of exceptions:

The department’s overall philosophy on pavement drainage design has been more focused on minimizing surface moisture infiltration or the effects of surface infiltration through construction and maintenance techniques such as constructing an adequate surface cross slope, maintaining proper ditch depth, using non-moisture susceptible materials, and aggressive use of seal coats and crack sealing, rather than on establishing internal drainage features.

Department Policy. Aspects of the department’s policy are evident in many ways, such as, establishing a non-erosive base beneath rigid pavements and establishing QC/QA density requirements and anti-stripping evaluation for HMA. A substantial concern has existed over the maintainability of the internal drainage systems, including clogging of the permeable layer with fines, crushing of the permeable layer during construction, clogging of edge drains through rodent activity or sedimentation, crushing of edge drains, etc. A clogged drainage system is worse than no drainage system; it can keep the pavement in a state of saturation for a prolonged period.

As stated above, there is potential for back-flow if ditches cannot be constructed with enough depth. However, there are occasions where positive pavement drainage may be considered a viable alternative, especially in cases where non-uniform cross sections exist or are planned. In particular, sections that have a “bath tub” nature where the outside edge of the structure is fairly impervious and will not allow lateral exodus of trapped moisture, positive drainage systems may be a good solution.

The case of retro-fitting edge drains on old rigid pavements with flexible pavement shoulders is one example. These structures tend to have a highly pervious longitudinal joint at the PCC-HMA interface that resists long-lasting maintenance solutions. An edge drain trenched into the shoulder at the interface with laterals to carry the water away from the structure can be effective in reducing or eliminating pumping under the slab. A similar situation can exist in flexible pavements widened using full-depth HMA or deep structures with impervious backfilled side slopes. Diligence in ensuring adequate compaction and thickness of pavement materials to support truck wheel loads above the retro-fitted drain is needed to prevent pavement failure at the lane/shoulder interface for traffic that may occasionally wander from the driving lane.

A typical retro-fitted edge drain system is shown below in Figure 2-17. An example of positive drainage using a permeable base layer is shown in Figure 2-18. Variations can exist where laterals will empty into a storm drainage system.

Another major source of free moisture into the pavement structure is ground water. The department’s policy has been that ground water should be intercepted outside of the pavement structure to eliminate the impact of this source. This should be pursued when seepage from higher ground is a problem. Moisture migration from capillary action or vapor movements can be addressed using the drainage options pictured above.

‘Appendix AA’ of the

1993 AASHTO Guide

and the

Drainable Pavement Systems Participant Notebook

for the FHWA Demonstration Project 87 (1992) are useful references for designing internal pavement drainage systems.

Other references:

1993 AASHTO Guide, NCHRP Synthesis 285

(2000), and the

NCHRP Mechanistic-Empirical Pavement Design Guide (NCHRP Project 1-37A)

, ‘Appendix SS, Hydraulic Design, Maintenance, and Construction Details of Subsurface Drainage Systems.’

Cited references are available upon request from MNT – Pavement Asset Management.