Pavement Subdrains & Subgrade Improvements

John Poullain, P.E.

Course Outline

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.

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

Learning Objective

At the conclusion of this course, the student will have covered these topics:

• The types and key components of pavement structures;
• Geotechnical issues and key parameters considered in the design of pavements;
• 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;
• Design of edgedrains, drainage layers and pipe sizes;
• Hydraulic requirements for pavement drainage layers;
• Hydraulic design of separator layers, drainage filters and other components;
• Materials used for separator layers, permeable layers, and drainage filters;
• Pavement base and subbase layer requirements, and subgrade stabilization methods;
• Guidelines for stabilization methods for mitigating problem subgrades;
• How compaction effects soil properties, subgrades and pavement layers;
• Subgrade problems and types of problem soils encountered during construction and how soils properties such as stiffness, swell and collapse potential are altered;
• Geotechnical issues considered for pavement subdrainage specifications; and
• The basics of the AASHTO design guide.

Intended Audience

This course is intended for civil engineers, pavement design engineers, and geotechnical engineers and project engineers.

Benefit to Attendees

The student will become familiar with design of pavement subsurface drainage.

Course Introduction

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.

A majority of states use the 1993 AASHTO empirical equation, which has evolved from a 1956 Road Test. Over the years the Design Guides have been revised to include additional factors to better design pavements for the conditions traffic and materials used at specific road sites. This has allowed better optimization of pavement thickness, subdrainage systems, and reduced road costs.

The 1993 empirical equation has grown very complex to include more of the factors affecting the performance of the pavement. Given the complex equations spreadsheets and software such as DARWIN are very useful for selecting alternatives, especially for the iteration process involved for optimizing the pavement design for layer thickness, materials, traffic and climate conditions.

The hydraulic design of the pavement subdrainage system components is also complex. Designing for drainage pipe sizes, layer thickness etc requires a reiteration process. Fortunately the Federal Highway Administration (FHWA) provides a downloadable version of a software program they developed called Drainage Requirements in Pavements or DRIP. The software provides an easy solution for designing the components of a subdrainage system and can be downloaded from their web site:
 
http://www.fhwa.dot.gov/p avement/library.htm

Edgedrains retrofitted to existing pavements are an area of concern. Some studies showed that only about 1/3 is functioning as designed and the loss in effectiveness was probably caused from settling along the pavement edge. Other observations noted that properly doweled jointed concrete pavements were not helped greatly with permeable bases as far as reducing joint failures. However a permeable base significantly helped non-doweled jointed concrete pavements. Also permeable bases reduced D-cracking in concrete pavements.

Observations showed that properly designed and constructed subdrainage decreased distresses such as rutting and fatigue cracking of AC pavements and joint faulting of jointed PCC pavements. Edgedrains present problems as they become clogged over time and become ineffective in removing water as designed. Pumping and faulting then increases. Proper construction and maintenance are very important to keep edgedrains effective and therefore it is stressed that the O & M costs should be analyzed if edgedrains are considered.

Ground Improvement Methods

One of the earliest methods of subgrade improvement is by compaction in which soil particles are rearranged and densified to improve the soils’ engineering properties of strength, permeability and compressibility. The existing subgrade may have poor strength or instability problems due to excess clay, expansive clays, silts, and fine sands, voids, collapsing soils or high watertables. There are problem soils such as loess, and tailings, which have collapsing or low-density structures, and when saturated have large decreases in volume and loss of strength. Other soils, which contain clays such as bentonite, can expand and increase in volume when exposed to water. Expansive soils however can shrink or decrease in volume when water is not present. There are also dispersive clays so named because the soil particles are not structurally sound and can easily disperse or detach and erode in still water.

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 but handling and excavation of the existing soil would also have to be considered.  Certain soils because of their chemical nature, organic or high acid compounds, may not be responsive to these stabilization methods and may be corrosive to steel reinforcement

Unstable soils in the United States, which includes expansive clay soils cause billions of dollars of damage to property each year and may exceed the total costs of natural disasters. Expansive soils are a problem in over 30 states especially in Texas, Colorado, Virginia, North Dakota, Oklahoma and Montana. Expansive clay soils are affected by the shrink/swell cycles caused by changes in moisture content during the year. The amount of swell depends on the amount of clay, relative density, location of water table and overburden stress. Affected sites include building foundations, roads and highways, parking lots, building pads, driveways, houses, and pools. Damage may range from sticking doors and cracked walls to foundation failures and building condemnations.

Stabilization treatments for expansive soils include:

1. injection with lime, water, potassium chloride
2. lime, cement, fly ash and LKD admixtures
3. groundwater control with slurry wall drains at footings
4. surface water control
5. soil removal and replacement
6. dewatering and drain systems

Selection of the most suitable method for stabilization will depend on the type of soil, degree of improvement and depth and extent of treatment required. Another factor to consider is whether the ground improvement is required for a new or existing structure.

Other Ground Improvement Methods -Grouting

Grouting is a high-cost treatment method and should be used where there is adequate confinement to handle the injection pressures. The typical applications include control of groundwater during construction, filling voids to prevent larger amounts of settlement, soil strengthening, and stabilization of loose sands, foundation underpinning, filling voids in calcareous formations and strengthening soils for protection during excavation.

Grouting especially with some chemical grouts may present risks to the public health and environment which should be considered. Considerations for utilizing a treatment method include energy use, maintenance costs, requirements for excavation and adequate treatment performance. Environmental risks include mismanagement of surface and groundwater drainage and incomplete treatment. Leachates and migration of contaminants can contaminate subsoil, groundwater, water wells and nearby surface water unless properly managed. There are several ground barrier methods used to control seepage, which include slurry-trench cutoff walls and grout curtains.

The advantages of grouting include:

a. performed on almost any ground condition  
b. vibration is not induced and can be controlled to avoid structural damages
c. improvements to ground formations is measureable
d. very useful for confined spaces and low headroom applications
e. used for slab jacking to lift or level distorted foundations
f. can be performed adjacent to existing walls

Course Content

The course is based on Chapter 7 of the US DOT Federal Highway Administration publication FHWA NHI-05-037, “Geotechnical Aspects of Pavements”, (2006 Edition, 102 pages), PDF file.

The link to the those document is:

“Geotechnical Aspects of Pavements”, Chapter 7

Please click on the above underlined hypertext to view, download or print the document for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader. If you still experience any difficulty in downloading or opening this file, you may need to close some applications or reboot your computer to free up some memory.

Course Summary

This course should serve as a guide to assist engineers in understanding the geotechnical issues and hydraulic design considered in pavement subdrain design, construction and performance. Basic concepts of soil behavior are discussed to understand the strength and stress values imposed by loads and other variables in pavement structures. This course should also serve as a guide for designing and selecting pavement subdrains and subgrade improvements to meet various subgrade conditions. The detrimental effects of water infiltration in pavements and the basic concepts of soil behavior are stressed.  

Related Links

For additional technical information related to this subject, please refer to:

http://www.haywardbaker.com/
Information and applications describing construction methods for ground improvement, structural support and earth retention and necessary materials. Provides solution tools for problem soils and project applications.

http://www.fhwa.dot.gov/pavement/desi.cfm
FHWA site for design, construction, and maintenance of pavement and the design status of pavements in the US.

www.vulcanhammer.net/geotechnical/laboratory_field.php
Offers geotechnical downloads for various manuals from FHWA, Dept. of the Interior, US Army Corps of Engineers and papers from collages and PE’s. Case studies, recent developments and downloadable software are available.

http://www.fhwa.dotgov/pavement/library.htm
Software (DRIP) developed by FHWA for designing pavement subdrains.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

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DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.

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