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C363
Pavement Subdrains & Subgrade Improvements

John Poullain, PE

This five-hour online course covers design of pavement subsurface drainage. The key geotechnical components of subdrain systems - permeable bases, edgedrains, deep drains, and separators - are presented. An overview of the potential subgrade problems such as subsurface water and saturated soils, problem soils such as collapsible, swelling, expansive and frost susceptible soils are covered. Guidelines for typical stabilization methods and geosynthetics used to improve pavement subgrade problems are also covered. Also discussed are the basic concepts of soil behavior and problem subgrade soils, and how pavement performance deteriorates due to problem subgrade soils. The influences of geotechnical factors are reviewed with respect to the AASHTO design guides.

Excess water causes many types of pavement failures and improved subdrainage minimizes its damage. Pavement and subgrade drainage design, excavation, and fill require an understanding of soil strength and soil characteristics. Consideration of problem soils and how soil behaves under water infiltration and imposed wheel loads. The course discusses the influence of pavement subdrainage systems with respect to the NHWA design methods. It describes the sources of water infiltration, effects on pavement layers and the hydraulic design of permeable layers, drain pipe sizing and other components. The components of a pavement structure are reviewed along with their reaction to subgrade materials, saturated soils, traffic loads and climate.

Compaction or mechanical stabilization is one of the oldest means of soil stabilization. Mechanical stabilization may achieve the desired results by blending two soils and/or mixing with admixtures. If suitable soil was located within a feasible haul distance, blending the soils together could effect an improvement in the existing soil. Using chemical or bitumen additives to improve a soil is another possibility. Some of the topics covered:

  • How climate, excess moisture, subsurface water, saturated soils and unsuitable subgrades affect the performance of a pavement.
  • Sources of water infiltration into a pavement.
  • Moisture-related and other distresses in AC and PCC pavements and the probable sources.
  • Key geotechnical components of subsurface drainage systems.
  • Hydraulic requirements for pavement drainage layers.
  • Hydraulic design of separator layers, drainage filters and other components.
  • Guidelines for stabilization methods for mitigating problem subgrades.


  • This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.


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    NY PE & PLS: You must choose courses that are technical in nature or related to matters of laws and ethics contributing to the health and welfare of the public. NY Board does not accept courses related to office management, risk management, leadership, marketing, accounting, financial planning, real estate, and basic CAD. Specific course topics that are on the borderline and are not acceptable by the NY Board have been noted under the course description on our website.

    AIA Members: You must take the courses listed under the category "AIA/CES Registered Courses" if you want us to report your Learning Units (LUs) to AIA/CES. If you take courses not registered with AIA/CES, you need to report the earned Learning Units (not qualified for HSW credits) using Self Report Form provided by AIA/CES.

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