Geotechnical Exploration and Testing for Roads

John Poullain, P.E.

Course Outline

This five hour online course covers the latest methods for planning a geotechnical exploration for road pavement projects. The course provides guidelines for a well-planned exploration program including sampling and testing issues and the exploration requirements for borrow and subgrades. It reviews the geophysical methods, FWD, SR, GPR, EM, their advantages and disadvantages. It discusses the characterization of existing and constructed subgrades, base and subbase materials. Problem subgrade soils such as expansive soils, and frost susceptible soils are discussed. A flow chart outlines the steps in planning a geotechnical investigation.

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:

• Geotechnical issues and key parameters that are considered in pavement design and construction;
• Influence of climate, moisture, drainage, and unsuitable subgrades on the 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;
• How soils properties such as stiffness, swell and collapse potential is affected under imposed loads;
• Geotechnical exploration required for borrow, base, subbase and subgrade materials;
• Levels of geotechnical exploration required for type of pavement projects - new construction, reconstruction or rehabilitation;
• Review of geophysical tests, mechanical waves and electromagnetic techniques, used for stratigraphic profiling subsurface conditions;
• Methods for locating subsurface rock and rippability with construction equipment;
• Use of geophysical tests to assist in determining soil boring locations, for interpolating between borings and the advantages and disadvantages;
• Requirements for the minimum number of borings, depths and spacing of borings, samplings and testing pertaining to different types of construction;
• Handling, transporting and protection of disturbed and undisturbed samples;
• Essential data for presentation in a report for boring logs, plan views and subsurface profiles;
• Frequently used AASHTO and ASTM standards for field investigations;
• References, sources of existing data and web sites for planning geophysical exploration and testing for highway projects; and
• Difficult subsurface conditions such as boulders, high P.I. and high ground water tables and their effect on subsurface investigations.

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 geotechnical exploration for road pavement projects.

Course Introduction

It has been shown that a majority of all construction claims were related to inadequate subsurface information. 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 of pavements with respect to methods and techniques used for subsurface exploration. How the locations and number of borings and samples are selected and types of exploratory methods employed are discussed. 

Additional costs may result if the investigations are inadequate and the presentation or interpretation of subsurface conditions is incomplete. The design of foundations for buildings, roadways, excavations, and embankments requires a complete understanding of soil strength; soil characteristics, problem soils and how soil behaves under imposed loads. Drilling and coring provide the necessary samples for laboratory tests, for in-situ field-testing and for the detailed subsurface records, the boring logs and profiles.

From the initiation of a project, subsurface investigations progress through several phases from the concept to construction and to operation and maintenance. Initially the investigations are general and cover data gathering for subsurface conditions. As the project develops from the planning and concept phases to plans and specifications for construction the reports are more detailed for design and interpretation of subsurface conditions. It is important to follow established criteria and guidelines in order to select the appropriate drilling, sampling and testing methods given the expense of subsurface exploration.  

Planning for subsurface investigations can be broken into these steps:

a. Historical and Geological Studies.

Geological maps, topographical maps, aerial photographs, site histories and adjacent property developments are reviewed to gather data. Proposed construction features are studied for access and to plan investigations. Land use, problems areas, subsoil information; terrain conditions will indicate the necessary subsurface investigations.

b. Site Reconnaissance.

Existing surface features and topography of the site with the data gathered from prior historical and geological studies are compared. Telltale signs of problem areas, presence of soft terrain, wet areas, outcrops and changes in topography like cut or fill developments will be noted.

c. Subsurface Investigations Requirements.

Extent of investigations will depend on available subsurface data, geological data, groundwater variability, adjacent development and the proposed construction. Investigations progress from generalized and large areas to more specific and detailed studies. Conceptual or preliminary subsurface investigations require limited laboratory and sampling work and are used to prepare possible routes or structures layout for evaluation. More detailed subsurface investigations including borings, samplings and testing are performed in order to design for the selected route or structure location.

Boring logs are prepared from subsurface information encountered while drilling and sampling. Information is derived from measurements such as the energy required for drilling per foot of borehole, monitoring rock and soil debris, drilling mud and return water pumped from the borehole, sample recovery percentages and loss of drilling mud, to mention a few. It is very important to complete the logs in the field and not the laboratory and that the field observations and lab test results are differentiated to make clear the source of information. When more comprehensive information is desired, downhole logging may be performed.

 Soil and rock samples must be handled and stored with care and should be inventoried, examined and tested as soon as they are received. If samples are stored for a long time the undisturbed samples should be protected against damage or changes in water content by maintaining at temperatures close to those required for the project. Rewaxing and relabeling may be required. Nevertheless the stored samples may undergo physical and chemical changes when stored too long no matter how carefully stored and resealed.

Downhole Logging

Downhole logging uses tools, such as wireline logging, where electronic instruments are lowered down the borehole or logging-while-drilling (LWD) where the instruments are in the drill pipe behind the drill bit itself. Other tools take measurements while drilling, measure inclinations, tool temperatures, and optimize casing selection. Although they may be used to complement standard boring measurements and sampling methods, they have these advantages:

1. downhole logs give a continuous record of subsurface formations, especially if sample recovery is limited
2. downhole logs can reveal a more realistic stratigraphy of fractures, fissures etc and in real time
3. LWD’s tools record the logs in memory devices which are downloaded for assessment when returned to the surface after a drilling run 
4. downhole log measurements present in-situ conditions as compared to recovered cores
5. material cored may physically swell or slake when no longer under pressure at depth and give an erroneous presentation of the actual underground conditions

ther devises such as borehole cameras and TV cameras are used to prepare logs and serve to record subsurface conditions that may not have been obvious while boring and therefore not recorded on the log. They can be reviewed many times to assure the subsurface conditions are as presented on the original exploration.  

Course Content

The course is based on Chapters 1 (pages 16-28) and 4 of the US DOT Federal Highway Administration publication FHWA NHI-05-037, “Geotechnical Aspects of Pavements”, (2006 Edition, 88 pages), PDF file.

The links to the those documents are:

“Geotechnical Aspects of Pavements”, Chapter 1

“Geotechnical Aspects of Pavements”, Chapter 4

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 planning subsurface investigations for pavement projects and for determining the spacing and number of borings and samples. Basic concepts of soil behavior are discussed to better understand the strength and stress values under imposed loads and other variables in pavement structures. The basics of geophysical tests, advantages and disadvantages for GPR, FWD, CPT and EM techniques are reviewed. This course should serve as a guide for selecting methods and techniques for subsurface exploration and sampling to meet the road design and site conditions. 

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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