1.1 Introduction
Geotechnical engineering is the systematic application of principles and practices which allow construction on, in, or with earthen material. Virtually all civil infrastructure is in direct contact with soil and as such is dependent on the geotechnical properties. Throughout civilization, there has been the need for constructing buildings, roads, dams, bridges, and other structures. Foundation design was historically a trial and error enterprise where no effort was made to quantify or predict soil behavior. A common example of the consequence of this approach is given by the Leaning Tower of Pisa, which prior to recent corrections, was tilted at 5.5° from the vertical due to unanticipated differential settlement. The first rational approach to working with soils came from Charles Coulomb who worked with soils in retaining wall applications for the French army in the latter part of the eighteenth century. A more comprehensive contribution to the field, and what is often noted as the birth of geotechnical engineering, is Karl Terzaghi’s 1925 text, named in part “Erdbaumechanik,” which may be thought of as the first geotechnical textbook. Still, there were many more whose efforts and work have made the profession what it is today.
Currently, geotechnical engineering has emerged as a well-developed field that interfaces with many other engineers and professionals. Clearly, the work of the geotechnical engineer in estimating settlements and designing foundations is of interest to the structural engineer and the architect in connection with building construction. Similarly, geotechnical work performed to retrieve soil samples and characterize sub-surface properties is important for groundwater quality and control where interaction with environmental engineers and hydrogeologists is likely. Other projects for which the services of a geotechnical engineer are needed include designing dams, embankments, landfills, and assessing the stability of slopes. There are many opportunities for geotechnical engineers to find work with private consulting companies as well as state agencies and academia. In short, there will always be a need for understanding and designing with soil.
Although significant advances have been made in geotechnical engineering since the days of Terzaghi, many solutions are at best an approximation, mostly because of the heterogeneous nature of both the soil and prevailing environmental conditions. The word “Environmental” has come to mean many things to different groups. Applied herein, it refers to ambient conditions that are reflected by such variables as temperature, pressure, groundwater composition, microbial population, etc. Soils do not exist in a vacuum, and they are the product of a variety of ongoing physical and chemical weathering phenomena. While some properties remain constant, others are subject to change as a function of mineralogy and environmental conditions.
In addition to being inherently complex, soil is more sensitive to the local environment than other construction materials such as steel or concrete. When soil is combined with water to varying degrees above or below the groundwater table, the result is a multiphase soil–water–gas system. This system may be thought of as a miniature reactor wherein a variety of physical and chemical processes occur within these phases. More details of the relevant reactions and specific properties will be presented in subsequent chapters, however at this point it suffices to note that soil is an engineering material that can change dramatically with time and space. As such, we must make an effort to understand as much as possible about soil and its response to the local environment if we are to make accurate predictions of the engineering behavior during the service life of a particular project.
1.2 Need to study geotechnical engineering from an environmental perspective
In recent years, due to population growth, progressive living standards, and industrial progress, soils that are of good quality (e.g. in terms of strength, compressibility, or permeability) and clean (e.g. free of contamination by metals or organics) are becoming harder to find. Thus, the geotechnical engineer is called upon more frequently to work with sites that would otherwise be rejected because of some deficiency. To work with soils that are physically or chemically deficient requires a broader, environmental perspective.
Geotechnical engineering is actually an interdisciplinary science and one that requires an assessment of mechanical (loading) as well as the response to fluctuations in the local environment. These fluctuations may be summarized as chemical, physico-chemical, and microbiological including such processes as (1) ion exchange reactions (Sec. 4.7) in the soil–water system that can change the arrangement of soil particles; (2) crack formation which fragments the soil surface and arises from an energy imbalance caused by natural variations in moisture or temperature as well as variations in compaction energy during construction. The cracking patterns (Sec. 8.3) have a significant effect on prefailure (Sec. 10.4) characteristics of soil as well as the flow through saturated and unsaturated (Sec. 5.11) fine-grained soils; (3) For a given soil under in situ conditions, the stress–strain behavior can change from elastic to plastic, or from a softening or hardening process, if certain local environmental conditions change; and (4) Bacteria (Sec. 4.12) can influence the character of the pore fluid and can also impact particle contacts through the production of exocellular substances.
In analyzing the soil behavior for practical application at present, most project designs use the test results following American Society for Testing and Materials (ASTM) and American Association of State Highway and Transportation Officials (AASHTO) standards. These standards are important and will be discussed in subsequent chapters. However, many of them are based on controlled conditions at room temperature, often with distilled water or low concentration electrolyte (e.g. CaSO4) as the pore fluid, in part to insure uniformity of results and test repeatability. Also, many analyses concentrate on loading conditions tested under short-term duration conditions but projected into long-term performance. Since field conditions and the standard control condition are significantly different, many premature or progressive failures are difficult to predict on the basis of controlled tests alone.
1.3 Environmental geotechnology and geoenvironmental engineering
Those new to the field or even rigidly trained in geotechnical engineering may be confused by the “environmental perspective” proposed herein as it relates to other rapidly emerging areas, namely environmental geotechnology and geoenvironmental engineering. In particular, geotechnical engineering was defined at the beginning of the chapter in terms of engineering with soil and soil–structure interaction. An environmental perspective simply interprets and modifies these results in light of the relevant site-specific and time-dependent environmental influences, that is, it attempts to reflect more accurately the actual in situ behavior of soil. This is in contrast to environmental geotechnology or geoenvironmental engineering, which are discussed as appropriate in the text and summarized as follows.
1.3.1 Environmental geotechnology
Environmental geotechnology has been defined as an interdisciplinary science which includes soil and rock and their interaction with various environmental cycles, including the atmosphere, biosphere, hydrosphere, lithosphere, and geomicrobiosphere (Fang, 1986, 1997). The latter includes trees, vegetation, and bacteria as they influence soil behavior. By definition, the emphasis in geotechnology is broad in scope and includes elements of fields beyond civil or geotechnical engineering such as soil science, material science, and geology. Environmental geotechnology has grown quickly since the first international symposium was organized in 1986 at Lehigh University. Environmental geotechnology is not only of relevance to traditional geotechnical problems but also has been expanded to include (a) hazardous/toxic waste control; (b) wetlands, coastal margins, dredging and marine deposits; (c) arid and desert regions; and (d) sensitive ecological and geological environments as well as archaeological science and technologies.
1.3.2 Geoenvironmental engineering
Geoenvironmental engineering may be considered the part of environmental geotechnology that deals with geological, geohydrological, and geotechnical aspects of environmental engineering problems. Common examples relate to the containment and remediation of municipal, hazardous, and nuclear waste in soil and groundwater, including: (a) hazardous/toxic waste controlling systems such as hydraulic barriers and various types of containment systems; (b) various aspects of landfill problems including selection of landfill sites, compaction control, stability analysis, settlement prediction of landfill, and design and construction of barrier, top seal (cover, cap) and bottom seal (liners); (c) geological and hydrogeological considerations of pollution control systems of groundwater aquifers; (d) soil and groundwater remediation technologies including immobilization and in situ treatment such as solidification, stabilization, and vitrification; and (e) utilization of waste materials in civil engineering construction. Some of these aspects will be discussed in Sections 15.5, 15.6, and Chapter 16.
