Subsurface Investigation

A good knowledge about a site including its subsurface conditions is very important in its safe and economical development. It is therefore an essential preliminary to the construction of any civil engineering work. This chapter outlines the objectives of site characterization and the general objectives of geotechnical investigation. It discusses the phases of field investigation and the stages of a full exploratory program including methods of sample recovery and field tests and sampling methods. Geophysical techniques can contribute very greatly to the process of ground investigation by allowing an assessment, in qualitative terms, of the lateral variability and vertical profiling of the near-surface materials beneath a site. Some of these geophysical techniques are discussed in the chapter. Laboratory examination/verification and testing should be made of representative portions of the samples to establish appropriate soil parameters. Some soil parameters may be estimated by correlations. The results of the subsurface investigation and related testing, together with interpretations, discussions, and foundation recommendations, are usually presented in the form of a detailed soil report.

Stability of Earth Masses and Slopes

The design of open-cut slopes and embankments, foundations, levees, and earth-dam cross-sections is based primarily on stability considerations. There are many causes and types of earth instability. There are also many ways of analyzing the stability of slopes. The chapter considers the limit equilibrium approach, which aims essentially to determine a factor of safety, F, that would ensure a slope does not fail. The chapter considers the analysis of stability of infinite slopes based on translational type of failure and the analysis of finite slopes using the Swedish Method, Method of Slices, Bishop Simplified Method, Friction Circle Method, and the Translational Method. The solution of equations developed for the analysis of stability of slopes can be tedious and time consuming. A way of reducing the amount of calculation required in slope stability studies is by use of charts based on geometric similarity. The chapter discusses how Taylor (1948) and Janbu (1964) charts are used in stability analysis of slopes. Finally, the chapter discusses ways to reduce the risk of instability in slopes.

Seepage and Groundwater Flow

Water in soil exists in a variety of forms, affects its engineering properties, and plays a very important role in all soil mechanics problems. The effects caused by the presence of water whether at rest or when moving through the pores of the soil must therefore be properly understood. Capillarity and both saturated and unsaturated flows are considered. Methods of measuring soil suction and the hydraulic conductivity of soils in the laboratory and in-situ are highlighted. The chapter considers the permeability of stratified deposits, effective stress, and seepage pressures. Using Darcy's law and other assumptions, the basic flow equation is derived. Analytical and graphical (flow nets) methods for solving the Laplace equation are developed. Kozeny's analysis of seepage through earth dam sections using the basic parabola is introduced and the Casagrande constructions are illustrated for some homogeneous earth dams with different discharge slopes. Seepage in soils with transverse isotropy and through soils with nonhomogenous sections are analyzed. The chapter ends with a consideration of the mechanics of piping, filter, and its design.

Introduction to Geotechnical Engineering

Geotechnical Engineering is a branch of Civil Engineering that deals with soil both as a foundation material upon which all types of structures rest and with soil as a structural material. This chapter traces the genesis of Geotechnical Engineering and its development, practice, and importance as a subdivision of Civil Engineering. The chapter further explains the nature, origin, and types of soils, weathering and its agents, and factors affecting it with particular emphasis on tropical weathering and laterization and ends with a brief discussion of soil maps and geotechnical mapping of project sites. The type of maps that may be prepared for engineering or environmental purposes are many and varied and can be categorized on the basis of purpose, content, and scale.

Clay Mineralogy and Soil Structure

Most civil engineering projects are built on soil or rock and are constructed solely or partly of these materials. This chapter provides engineers with a good knowledge of the type and characteristics of the terrain on which such projects are to be constructed in order to achieve optimum safety and economic performance. The earth's crust, which is of interest to geotechnical engineers, is made up of rocks and the so-called unconsolidated sediments composed chiefly of solid mineral particles derived primarily from the physical and chemical weathering of rocks. The concepts of plate tectonics and geologic and soil structures are used to explain the geological processes in the earth. Mineralogy is the primary factor controlling the size, shape, and properties of soil particles. It also determines the possible ranges of physical and chemical properties of any given soil; therefore, a priori knowledge of what minerals are in a soil provides intuitive insight as to its behavior.

Soil Improvement and Stabilization

It is often necessary to improve the properties of the soil whether as a foundation material or as a construction material because it is not suitable for its intended purpose. The fundamental techniques for improving the properties of natural materials are compaction, modification, stabilization, drainage, precompression, vibrocompaction, soil reinforcement, which includes soil nailing, and the use of geotextiles. The principles and methods of compaction are discussed: compaction parameters like maximum dry density and the optimum moisture content, zero-air-voids curve for different degrees of saturation, factors affecting compaction. The chapter further discusses field compaction, compaction control, and compaction equipment. The chapter also considers the other fundamental techniques for improving the properties of soils. Finally, the chapter briefly considers the types and requirements of a pavement and the two essential methods of design namely semi-empirical method and rational method of design.

Stresses in Soils Due to External Loading

Stress analysis is often necessary in the design of foundations of all types of structures, particularly buildings, retaining structures, dams, highway pavements, and embankments. In this chapter, the mathematical definitions of stress and strain and the elasticity of an isotropic material are first treated. This is followed by the classical theory of Boussinesq for the stress in a semi-infinite, elastic, isotropic, and homogeneous continuum loaded normally on its upper plane surface by a concentrated load. The Boussinesq solution is later extended to analyze the stresses produced by a uniformly distributed load over a flexible circular foundation, rectangular loading, strip loading, line loading, triangular loading, and embankment loading. The case of irregular loading using the Newmark's Chart is also considered. The settlement of a foundation under external loadings by the use of both the Boussinesq theory and the semi-empirical strain influence factor method proposed by Schmertmann et al. (1978) are considered.

Bearing Capacity and Foundations

Foundations are structural elements that transmit loads from structures to the underlying soil. The choice of the appropriate type of foundation is governed by some important factors such as the nature of the structure, the loads exerted by the structure, the subsoil characteristics, and the allotted cost of foundations. The primary design concerns of foundations are settlement and bearing capacity. The design must also take into consideration the requirements of safety, dependability, serviceability, functional utility, and economy. The chapter considers the modes of failure and several methods of determining the ultimate bearing capacity of foundations. The procedure and considerations in the design of shallow foundation are discussed. The chapter examines the types, situations calling for the use, advantages and disadvantages, load-carrying capacity, and design of deep foundations. The efficiency of the group of deep foundations is discussed. The group capacity can be determined by the use of empirical formulas and by the rational/equivalent method. Negative skin friction, its causes, capacity, and ways of reducing its effect are considered.

Earth Pressure Analysis and Retaining Walls

Retaining walls are structures used not only to retain earth but also water and other materials such as coal, ore, etc. where conditions do not permit the mass to assume its natural slope. In this chapter, after considering the types of retaining wall, earth pressure theories are developed in estimating the lateral pressure exerted by the soil on a retaining structure for at-rest, active, and passive cases. The effect of sloping backfill, wall friction, surcharge load, point loads, line loads, and strip loads are analyzed. Karl Culmann's graphical method can be used for determining both active and passive earth pressures. The analysis of braced excavations, sheet piles, and anchored sheet pile walls are considered and practical considerations in the design of retaining walls are treated. They include saturated backfill, wall friction, stability both external and internal, bearing capacity, and proportioning the dimensions of the retaining wall. Finally, a brief treatment of earth pressure on underground structures is included.

Compressibility and Consolidation of Soils

The total compression of soil under load is composed of three components (i.e. elastic settlement, primary consolidation settlement, and secondary compression). The consolidation component is time-dependent and its analysis is usually based on Terzaghi's theory. The chapter considers the consolidation characteristics of a soil and their experimental determination. The coefficient of consolidation can be determined by the Casagrande Logarithm-of-Time Fitting Method or the Taylor Square-Root of Time Method. The concepts of preconsolidation and overconsolidation are discussed while ways of determining the preconsolidation pressure, compression index, precompression index, and the coefficient of volume compressibility are explained. Ways to compute the settlement using coefficient of volume compressibility and e-logs methods for both normally consolidated and overconsolidated soils are provided. The chapter also explains Schmertmann (1955) graphical procedure for approximating the field compression index from the laboratory curve. It includes the derivation of Terzaghi's 1-D theory of consolidation and its solution using both analytical and graphical methods. Finally, the phenomenon and way of computing the secondary compression index are treated.