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Geotechnical Issues of Road Construction & On Site Monitoring

John Poullain, P.E.


Course Outline

This four-hour online course covers the methods and procedures used to address the geotechnical issues in construction and performance for new construction and reconstruction of pavements. It discusses the characterization of existing and constructed subgrades and unbound base and subbase materials and the basic concepts of soil behavior are considered. The influences of geotechnical factors are reviewed with respect to AASHTO design guides. Problem subgrades such as plastic, expansive, frost susceptible, and collapsible soils, and karst, and caves are discussed. The types of construction specifications and QA/QC requirements are reviewed.
The construction of a pavement, subgrade, excavation, and fill requires an understanding of soil strength; soil characteristics and consideration of problem soils. The course discusses the influence of geotechnical inputs on pavements. The basic components of a pavement structure are reviewed along with their reaction to subgrade materials, traffic loads and environment.

Among the topics covered in this course are:

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:

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 the geotechnical issues in construction and performance for new construction and reconstruction of pavements.

Course Introduction

The construction 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. The basic components of a pavement structure are reviewed along with their reaction to subgrade materials, traffic loads and environment.

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.

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.

Excess water causes many types of pavement failures and improved subdrainage minimizes its damage. 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 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. 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 Chapters 1 (pages 1-14) and Chapter 8 of the US DOT Federal Highway Administration publication FHWA NHI-05-037, “Geotechnical Aspects of Pavements”, (2006 Edition, 70 pages), PDF file.

The links to the those documents are:

“Geotechnical Aspects of Pavements”, Chapter 1

“Geotechnical Aspects of Pavements”, Chapter 8

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 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 methods used for constructing durable pavements, correcting problematic subgrades, and practicing QA/QC measures effectively.  

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.

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.