Analysis and Design of Geotechnical Structures

Manuel Matos Fernandes

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9780429676154
• Edition: 1
• Topic: Computer Science
• Subtopic: Information Technology
• Index: Computer Science

Analysis and design of geotechnical structures combines, in a single endeavor, a textbook to assist students in understanding the behavior of the main geotechnical works and a guide for practising geotechnical engineers, designers, and consultants. The subjects are treated in line with limit state design, which underpins the Eurocodes and most North America design codes.

Instructors and students will value innovative approaches to numerous issues refined by the experience of the author in teaching generations of enthusiastic students. Professionals will gain from its comprehensive treatment of the topics covered in each chapter, supplemented by a plethora of informative material used by consultants and designers.

For the benefit of both academics and professionals, conceptual exercises and practical geotechnical design problems are proposed at the end of most chapters. A final annex includes detailed resolutions of the exercises and problems.

Chapter 1

Geotechnical characterization

In situ testing

1.1 Introduction

Geotechnical structures are the constructions whose conception and design are essentially controlled by the mechanical and hydraulic behavior of the soil or rock masses present at the site. Examples of geotechnical structures include foundations, earth-retaining structures, embankments, cuts and slopes, and underground works, to name but a few.

This chapter is dedicated to the operations carried out at a construction site for its geotechnical characterization. This usually involves:

1. (i) the identification, in geological and geotechnical terms, of the sequence of layers or strata that may affect the behavior of the structure;
2. (ii) the physical, mechanical, and hydraulic characterization of the soils that form these layers or strata;
3. (iii) the characterization of water conditions in the ground.

The physical, mechanical, and hydraulic characterization of soils requires laboratory and in situ tests. Laboratory tests have been studied in soil mechanics, such as triaxial tests, direct and simple shear tests, confined and isotropic compression tests, and other more sophisticated tests, namely the hollow cylinder and the resonant column tests.

The importance assigned to laboratory tests in theoretical soil mechanics is understandable, since these address well-defined and controllable stress, strain, and drainage conditions. These conditions enable the interpretation of test results and, using the principles of soil mechanics, the derivation of soil parameters.

This practice has been of critical importance for the development of soil mechanics and it continues to be invaluable for the design of geotechnical structures. However, when dealing with conventional types of structures and foundations, with no exceptional soil or loading conditions, the design is essentially based on field tests.¹ This explains the importance assigned to in situ tests in this introductory chapter of the book, after some brief considerations on the preliminary operations for site characterization.

The final part of the chapter is devoted to soil stiffness characterization. This is one of the most challenging topics in geotechnical engineering, one that has seen outstanding progress in recent decades, and treatment of which requires a special combination of laboratory and in situ tests.

Readers particularly interested in studying the theme of this chapter are advised to study the “Manual on Subsurface Investigations” (Mayne et al., 2001) published by the National Highway Institute of the United States of America, available online (http://geosystems.ce.gatech.edu/Faculty/Mayne/papers/NHI%202002%20Subsurface%20Investigations.pdf).

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¹ A discussion on the advantages and limitations of laboratory and in situ tests is presented in Section 1.4.2.

1.2 Geotechnical investigation

1.2.1 Preliminary surface survey

The geotechnical investigation is preceded by the so-called preliminary geological-geotechnical survey. This consists of an on-site superficial survey, sometimes involving small investigation activities, such as the opening of shallow pits and shafts. Usually, the field visit is preceded by a desk study comprising the collection of existing written and drawn information about the site, in particular, topographic and geological charts and, if available, geotechnical charts. Currently, the use of two- and three-dimensional aerial images obtained from satellite and free web navigation tools is also very useful, particularly for large works outside urban areas, such as roads, dams, and for the stabilization of natural slopes. When dealing with densely populated areas, geotechnical characterization reports for nearby constructions can generally be found, which represent important sources to be collected and checked.

Data analysis of all this information, as well as the preliminary site survey data, is compiled in a report. This will form the basis of the preliminary design or viability studies stage of the construction works. This report will also allow definition of the most appropriate geotechnical investigation program for the project. In general, the geotechnical investigation comprises geophysical and geomechanical investigations.

1.2.2 Geophysical investigation

1.2.2.1 Introduction

In the past few decades, the application of geophysical methods to geotechnical engineering has expanded considerably. This has resulted from a combination of several factors:

1. (i) the advent of “new” methods, such as the surface seismic wave method;
2. (ii) the combination of conventional geomechanical in situ tests with geophysical methods (such as the seismic cone penetration test and the seismic flat dilatometer test);
3. (iii) the comparative application of geophysical methods in the field and in the laboratory on samples under well-defined test conditions;
4. (iv) the strong...