Introduction: what is soil mechanics?

Soil mechanics may be defined as the study of the engineering behaviour of soils, with reference to the design of civil engineering structures made from or in the earth. Examples of these structures include embankments and cuttings, dams, earth retaining walls, tunnels, basements, sub-surface waste repositories, and the foundations of buildings and bridges.

An embankment, cutting or retaining wall often represents a major component, if not the whole, of a civil engineering structure, and is usually (for better or for worse) clearly vis-ible in its finished form (Figure 1.1). Tunnels and basements are generally only visvis-ible from inside the structure, while foundations and underground waste repositories—once completed—are not usually visible at all. By definition, foundations form only a part of the structure which they support. Although out of sight, the foundation is nonetheless important: if it is deficient in its design or construction, the entire building may be at risk (Figure 1.2).

Problems in soil mechanics had begun to be identified and addressed analytically by the beginning of the eighteenth century (Heyman, 1972). Despite this, the growth of soil mechanics as a core discipline within civil engineering, taught at universities with almost the same emphasis as structures and hydraulics, has taken place largely within the last fifty years or so. The expansion of the subject during this time has been very rapid, and the term geotechnical engineering has been introduced to describe the application of soil mechan-ics principles to the analysis, design and construction of civil engineering structures which are in some way related to the earth.

The development of geotechnical processes and techniques has been led primarily by innovation in construction practice. The terms ground engineering and geotechnology are often used to describe the study of geotechnical processes and practical issues, includ-ing techniques for which the only available methods of assessment are either qualitative or empirical.

If these somewhat arbitrary definitions are accepted, the various terms cover a spectrum from soil mechanics (at the theoretical end), through geotechnical engineering (which is analytical but applied) to ground engineering and geotechnology, where the methods used in design may be largely empirical. This book is concerned primarily with soil mechanics and its application to geotechnical engineering (although section 11.4, on ground improvement techniques, could probably be classed as ground engineering or geotechnology). It describes the mechanical (e.g. strength and stress-strain) behaviour of soils in general terms, and shows how this knowledge may be used in the analysis of geotechnical engineering structures.

2 Soil

Figure 1.1 A visible and, at the time of its construction, controversial road cutting (the M3 motorway at Twyford Down, near Winchester, Hampshire, England). (Photograph courtesy of Mott MacDonald.)

The book does not (apart from the very brief overview given in section 1.3) cover engineer-ing geology; nor does it examine the mineralogy, physics, chemistry or materials science of soils. The book takes a macroscopic view, and does not address at the microscopic level the issues which constitute what Mitchell (1993) calls the why aspect of soil behaviour. This is not to say that these issues are unimportant. A study of engineering geology, and the geo-logical history of an individual site, will give an invaluable understanding of the structure and characteristics of the soil and rock formations present. It might also lead the engineer to anticipate the presence of potentially troublesome features, such as buried river beds which form preferential groundwater flow paths, and historic landslips which give rise to pre-existing planes of weakness in the ground. At least a basic knowledge of soil mineral-ogy and soil chemistry is essential for anyone involved in the increasingly important issue of the movement of contaminants (e.g. from landfill sites) through the ground.

These subjects are covered in more detail by Blyth and de Freitas (1984: engineering geology); Marshall et al. (1996: soil physics); and Mitchell (1993: mineralogy and soil chemistry). Full references to these works are given at the end of this chapter.

Origins and classifi cation of soils 3

Figure 1.2 A well-known building with an inadequate foundation (Pisa, Italy). (Photograph courtesy of Professor J.B.Burland.)

Objectives

After having worked through this chapter, you should have gained an appreciation of:

• the origin, nature and mineralogy of soils (sections 1.2–1.4)

• the influence of depositional and transport mechanisms and soil mineralogy on soil type, structure and behaviour (sections 1.3 and 1.4)

• the principles and objectives of a site investigation (section 1.14).

You should understand:

• the three-phase nature of soil, including the relationships between the phases and how these are quantified (sections 1.5 and 1.6)

• the need to separate the total stress σ into the component carried by the soil skeleton as effective stress σ′ and the pore water pressure u, by means of Terzaghi’s equa-tion, σ=σ′+u (section 1.7)

4 Soil

• the importance of soil description, and classification with reference to particle size and index tests (sections 1.8, 1.10 and 1.11).

You should be able to:

• manipulate the phase relationships to obtain expressions for the unit weight of the soil (section 1.6)

• determine water content, unit weight, grain specific gravity, saturation ratio, liquid and plastic limits, and optimum water content from laboratory test data (sections 1.5, 1.6, 1.11 and 1.12)

• calculate the vertical total stress at a given depth in a soil deposit and, given the pore water pressure, the vertical effective stress (section 1.7)

• construct a particle size distribution curve from sieve and sedimentation test data (section 1.8)

• design a granular filter (section 1.9)

• apply the phase relationships to the practical situations of compaction of fill and the settlement of houses founded on clay soils (sections 1.12 and 1.13).