Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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interaction with water since water-
bearing aggregates affect the properties and behaviour of soils, for example, 
water retention, transport phenomena, and rheology.
2.6.3 Microstructure of Clays
The early studies of soil behaviour, as a formal discipline in the 1950s, pro-
vided the impetus for targeted research into the role of soil structure in 
the development of soil properties, behaviour, and response performance. 
It has long been understood that for many soil engineering applications, it 
was acceptable to consider soil as a continuum instead of a complex sys-
tem of particulate media involving interacting soil solids and porewater. 
Accordingly, laboratory and field test procedures measured or determined 
the macroscopic performance of the soil, using testing-analytical tools that 
relied on data-reduction models developed for uniform and homogeneous 
media. Even today, the majority of analytical tools used for assessment of 
soil performance are still based on continuum mechanics principles.
Whilst deterministic considerations and analyses of soil as a continuum can 
be used successfully for many soil engineering applications, problems arise in 
predicting soil behaviour when the soils under consideration are (a) heteroge-
neous and structured and therefore require consideration of the structure of 
the medium such as the sensitive marine clays, and (2) such that the behaviour 
of the soils responds to both gravitational and molecular forces, and hence 
requires analyses that incorporate intermolecular forces. This is particularly 
true for clay soils with active clay minerals, that is, clay minerals with active 
and reactive particle surfaces (smectite is a good example of this type of clay).
The importance of soil structure in clay properties has long been recog-
nized. Taylor (1948) classified soils into three types of fundamental struc-
tures: single-grained structure (granular soils), honeycombed structure, and 
flocculent structure. In addition, Taylor recognized the role of intermolecular 
forces, stating for example: \u201cThe finer the grains (of soil), the more notice-
able becomes the effect of intermolecular forces\u201d in reference to honeycomb-
structured soils. In regard to flocculent-structured soils, he states that \u201cin 
addition to the force of gravity, the molecular impact forces must be given 
consideration in the study of the action of these small particles.\u201d Present-day 
understanding of clays readily recognizes that clays are generally composed 
of soil\u2013particle structural units grouped together in some coherent fashion 
to make up the macrostructure of the clay.
Figure 2.13 shows the basic elements that make up the microstructural 
units (msu) and their aggregation into the macrostructure of a soil. These 
63Nature of Soils
microstructural units range from individual mineral particles to what is 
known in the various disciplines and interest groups as domains, flocs, 
clusters, peds, and aggregate groups, to cite a few of the names used in 
the literature. To determine the kinds of microstructural units, there are at 
least three broad groups of techniques available, depending on the sizes of 
the particles involved. The macroscopic technique that constitutes the first 
level of study of msu relies on recognition of msu with the naked eye. This 
is confined to granular particles. It is possible to see aggregate groups of 
particles generally called peds, or crumbs, consisting of a large number of 
indistinguishable (with the naked eye) clay particles. The two other groups 
of techniques for identifying microstructural units are microscopic and 
ultramicroscopic in nature. As the name implies, microscopic techniques 
rely on light microscopes for determination of flocs, and ultramicroscopic 
techniques use various forms of electron microscopic viewing to study 
domains, and so forth. Note that regardless of the technique used for deter-
mination of msu, the results obtained will be operationally defined since 
(a) the technique used will establish the nature of msu observed, and (b) 
the size of specimen studied is nothing more than a very miniscule portion 
of any laboratory or field soil sample and hence cannot be confidentially 
assumed to be totally representative of the soil under consideration.
We can also determine the presence or formation of msu by less direct meth-
ods, using the energies of interactions between particles. These will be more 
qualitative in nature than quantitative. Interactions between adjacent parti-
cles and interactions with water are by and large controlled by intermolecu-
lar forces since particle sizes are in the micron range, and since the specific 
surface areas of these particles and structural units are on the order of ten 
to hundreds of square metres per gram of soil. The total interaction energy 
between particles can be calculated from zeta potential test measurements 
relative to distance between surfaces for various particle arrangements. (The 
subject of particle interaction energies will be discussed in the next chapter.) 
Taking note that the zeta potential is directly related to the total interaction 
energy between particles, Yong and Sethi (1977) studied clay dispersibility 
using refiltration experiments as described by LaMer and Healy (1963a, 1963b). 
Measurements of refiltration rate relative to zeta potential provided informa-
tion on the dispersibility of the clay studied by Yong and Sethi. The results 
showed that the higher the refiltration rate, the more flocculated the clay. The 
results obtained in conjunction with the corresponding zeta potential values 
are shown in Figure 2.16. The illustration shows that as the zeta potential \u3b6 
increases, the degree of dispersibility decreases, indicating that single particle 
interaction becomes less and less. At some point, flocculation of particles will 
occur, and interactions between the clay particles will be in relation to flocs 
(i.e., microstructural units). This technique allows one to obtain a mechanis-
tic picture of not only the formation of microstructural units, but also their 
interactions.
64 Environmental Soil Properties and Behaviour
2.7 Interparticle Bonds
It is important to remember that when one speaks of soil structure and 
microstructural units, one is referring to material that exhibits inherent 
strength (resistance to shear displacement) and other properties such as 
transmissibility of fluid, heat, and so forth. In other words, microstructural 
units have physical, mechanical, physicochemical, and chemical properties. 
When such units are combined into a macrostructure that defines a clay, 
their integrated properties define the properties of that clay. It follows that 
the physical integrity of a clay is directly related to the nature, distribu-
tion of the microstructural units, and the bonding and interaction forces 
not only between the clay particles within the msu, but also between the 
various msus themselves. There are two groups of bonds that exist between 
particles in microstructural units and between these units themselves, 
both of which are responsible for the development of the properties and 
characteristics of the msu and the macrostructure itself. The first group of 
forces and bonds deals directly with particle-to-particle interaction, that is, 
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