Soil Mechanics

Roberto Nova

• Publisher: Wiley-ISTE
• Year: 2012
• ISBN:
  9781118586983
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Ingénierie civile

Knowledge of the behavior of soil mechanics is essential for forecasting the internal displacements and actions of any construction. This book, although theoretical at first glance, also offers a more practical scope, giving readers adequate tools to plan geotechnical projects correctly.

Chapter 1

Introduction: Basic Concepts

1.1. Soils and rocks

The term “soil” is used in civil engineering to describe a material composed of a natural accumulation of mineral particles, whose sizes range between specified limits, according to a conventional classification system.

Soil is the result of the chemical-physical alteration of rocks due to atmospheric agents (weathering), rocks being the primary element that constitutes the Earth’s crust. Soil particles can be completely uncemented or weakly cemented, depending on the degree of alteration of the parent rock. On the other hand, soil that is exposed to atmospheric agents for a long period of time undergoes chemical reactions that cement the particles, so that deposits that were originally composed of uncemented particles are gradually transformed into sedimentary rocks (diagenesis).

Since the processes of weathering and diagenesis are gradual, the distinction between soil and rock is to a certain extent arbitrary. To the geotechnical engineer soil is any accumulation of mineral particles with weak chemical bonds, such that the stress levels typical of civil engineering applications can easily exceed their strength. On the other hand, rock is defined as a material with strong chemical bonds. The deformation and failure of rock masses are governed by the mechanical behavior of the pre-existing geometric discontinuities (faults or joints) rather than by the intrinsic characteristics of the rock itself.

Several geological materials (e.g. tuff, clay stone, marble, limestone, etc.) have an intermediate behavior. These materials behave as rocks if subjected to relatively low stresses, and as soil if subjected to stresses high enough to break the chemical bonds cementing the particles.

Soil grains are mainly composed of silica minerals (e.g. silicon dioxide and other silica-based minerals), which are more resistant to chemical-physical attack by weathering than other minerals. Quartz (SiO₂) is almost insoluble in water, is relatively acid proof, and is a very stable mineral. It is primarily composed of rounded or prismatic particles of the order of a millimeter or less and is the main mineral of silica sands, followed by feldspars.

Feldspars are chemically altered by water, oxygen and carbon dioxide. The gradual breakdown of feldspar crystals forms microcolloidal particles of kaolin. Similarly, phillosillicates, existing in large quantities in igneous rocks, delaminate along their basal plane, due to their mineralogical foil structure, and form illite and smectite. Kaolin, illite and smectite are the primary minerals appearing in clay; they are characterised by plate-like particles with length and width in the order of a micron.

Soil particles are also composed of calcite and gypsum, as well as of minerals of volcanic origin (pyroclasts). Particles formed by these types of minerals are usually weaker than those formed by silica minerals; therefore, they have a greater influence on the strain behavior of these materials.

The shape of the particles and their structural arrangement depends on the materials that compose them and on their geological history.

For example, on the one hand, rounded shape sand grains with faces and angles bevelled by abrasion are typical of sand deposits formed after wind or water transportation. On the other hand, sand grains that remain in their original location, where weathering of the parent rock took place, are angular and have an irregular shape.

The chemical environment in which the particles are deposited has a significant influence on the structure of clay that can aggregate in different ways. If clay particles align in the same direction (face-to-face orientation, Figure 1.1a) it is referred to as dispersed structure, while a structure similar to a card house (edge-toface or edge-to-edge orientation) is referred to as flocculated structure (Figure 1.1b) and is much more unstable than the former. With the change in the deposit chemical conditions, the structure can pass from dispersed to flocculated and vice versa.

Figure 1.1. a) Dispersed clay structure; b) flocculated clay structure (“cardhouse”)

1.2. Engineering properties of soils

As seen in the previous section, several types of minerals compose a soil, its “solid skeleton”, and its fabric are influenced by its geological history and by the chemical environment. However, for the majority of engineering aims, different types of soil can be initially classified according to the size of the constituent particles. The classification of the different types of soils is somewhat arbitrary. Examples of classifications adopted by British Standards (BS), Italian Geotechnical Association (AGI) and American Association of State Highway Officials (AASHO) are listed in Table 1.1.

Table 1.1. Classification of different types of soils. Sizes are in mm

Note that in the proposed classifications there is no direct reference to the grain chemical composition, to the type of parent rock or to the formation process of the deposit (for transport or in situ alteration). This type of classification has two advantages. Firstly, it is a quantitative classification, and hence it is almost free from the subjectivity of the operator. Secondly, it allows for the direct identification of a property that has a fundamental influence on the soil mechanical behavior. The range of possible particle sizes is enormous. Soil particle sizes range from sub microscopic clay particles, discernible only by a scanning electron microscope, to rounded sand grains with a diameter a thousand times larger, to cobbles with a diameter a hundred times larger.

On a single particle, both body forces (weight) and surface forces (electrostatic forces) have effect. The former, are proportional to the volume of the particle, while the latter are proportional to the external surface. An initial difference between fine and coarse particles consists of the different role interplayed by the electrostatic forces on their surface. An indicator of the relative role played by the two types of forces is the specific surface, S_s, defined as the ratio of the area of the surface of the particle to the mass of the particle, ρV:

[1.1]

where ρ is the density and V is the volume of the particle.

In the case of a rounded particle of silica sand, the specific surface is inversely proportional to the diameter of the grain, d_g.

[1.2]

Quartz density is equal to 2.65 g/cm³; hence, for a rounded ...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Chapter 1: Introduction: Basic Concepts
• Chapter 2: Field Equations for a Porous Medium
• Chapter 3: Seepage: Stationary Conditions
• Chapter 4: Seepage: Transient Conditions
• Chapter 5: The Constitutive Relationship: Tests and Experimental Results
• Chapter 6: The Constitutive Relationship: Mathematical Modeling of the Experimental Behavior
• Chapter 7: Numerical Solution to Boundary Value Problems
• Postscript: From Soil Mechanics to Geotechnical Engineering
• Index