Geotechnical Issues in Pavement Design

John Poullain, P.E.

Course Outline

This four hour online course covers the methods and procedures used to address the geotechnical issues in pavement design, construction and performance for new construction and reconstruction. It discusses the characterization of existing and constructed subgrades and unbound base and subbase materials. The influences of geotechnical factors are reviewed with respect to the current and earlier AASHTO design guides. Case histories are presented to show the successes and failures in support of the design concepts. Basic concepts of soil behavior are considered. Problem subgrade soils such as plastic and expansive soils, collapsible soils, and frost susceptible soils are discussed. The basics of AASHTO pavement guides are reviewed.

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:

• Review and discussion of the components of pavement structures and the  types of pavements;
• Review of the performance of the different types of pavement;
• Geotechnical issues and key parameters that are considered in pavement design and construction;
• Case histories of successes and failures to show the advantages and disadvantage of pavement design and construction concepts;
• Influence of climate, moisture, drainage, and unsuitable subgrades on  performance of the pavement;
• Review of subgrade problems and types of problem soils encountered during construction and remedial solutions;
• Determination of geotechnical inputs required for pavement design, construction, and performance;
• Geotechnical issues to consider for pavement specifications;
• How soils properties such as stiffness and swell and collapse potential are affected under imposed loads;
• Selection of remediation measures for unsuitable pavement subgrades such as collapsible and expansive soils;
• Considerations of mechanistic and empirical design concepts and history of pavement design;
• Advantages of mechanistic-empirical design of pavements; and
• The basics of the current and past AASHTO design guides.

Intended Audience

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

Benefit to Attendees

The student will become familiar with the methods and procedures used to address the geotechnical issues in pavement design, construction and performance.

Course Introduction

The design of a pavement, subgrade, excavation, and fill requires an understanding of soil strength; soil characteristics and consideration of problem soils and how soil behaves under imposed wheel loads. The course discusses the influence of geotechnical inputs on pavements with respect to empirical and mechanistic design methods. The components of a pavement structure are reviewed along with their reaction to subgrade materials, traffic loads and environment. The history of empirical and mechanistic methods of road design shows the assumptions and inputs as developed from the 1956 AASHTO Road Test.

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 included 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 and reduced road costs.

The required inputs for the design equation are:

W₁₈      predicted 18 kip loads, estimated single axle loads (ESAL) the pavement will experience over its design lifetime
Z_r        standard normal deviate , accounts for reliability
So        overall standard deviation, accounts for uncertainty
ΔPSI     difference between initial serviceability index, po, and design terminal serviceability index, pt
M r        subgrade resilient modulus in psi

SN        assumed structural number, the output that the AASHTO design equation solves for with the above 5 parameters      

The 1993 empirical equation has grown very complex to include the factors a pavement must withstand. Given the equations difficulty spreadsheets and software such as DARWIN are very beneficial for solving it, especially for the iteration process involved for optimizing the pavement design for layer thickness, materials, traffic and environmental conditions.

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 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
     
Error: equation C.5, page C-5, should be ΔPSI = Po – Pt

Course Content

The course is based on Chapters 1-3 and Appendix C of the US DOT Federal Highway Administration publication FHWA NHI-05-037, “Geotechnical Aspects of Pavements”, (2006 Edition, 92 pages), PDF file.

The link to the those document is:

“Geotechnical Aspects of Pavements”, Chapters 1-3 and Appendix C

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 for consideration in pavement design, construction and performance. Basic concepts of soil behavior are discussed to better understand the strength and stress values under imposed loads and other variables in pavement structures. This course should serve as a guide for selecting pavement structures to meet the site conditions and understanding the basic concepts of soil behavior. 

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.

Quiz

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

---

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.

---