Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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the data obtained, leading one to conclude 
that the determination of SSA is operationally defined.
2.6 Soil Structure
2.6.1 Soil Fabric and Structure
We study soil fabric and soil structure because they constitute, in essence, 
the structural framework of a soil. We define soil structure as that property 
of a soil that establishes the integrity of the soil\u2013water system. As a prop-
erty, soil structure includes the distribution and arrangement of particles 
and the bonding mechanisms between particles, together with their mutual 
interactions and reactions with the porewater. The distribution and arrange-
ment of soil particles can be viewed as the skeletal framework of the soil 
and is generally defined as the soil fabric. It is important to recognize this 
as a separate property or as a subproperty of soil structure. The scanning 
electron microscope (SEM) picture of the network of microstructural units 
of kaolinite shown in the bottom right of Figure 2.13 is a good example of a 
soil fabric picture.
The role and influence of soil structure on the properties and performance 
of soils can be studied from many different perspectives, depending upon 
end-purpose use or goals. The importance of the microstructural compo-
nents and their contribution to the overall structure of a soil has long been 
recognized in the field of soil science because of their need to deal with such 
problems as soil moisture movement, soil tillage, water-holding capacity of 
soil, soil\u2013water potentials, and water uptake of plants. In the field of soil engi-
neering, attention has historically been directed more toward the physical 
and mechanical performance of soils. Early studies on clay structure in geo-
technical engineering, for example, provided us with descriptions of floc-
culent, honeycomb, and \u201ccardhouse\u201d structures (Terzaghi and Peck, 1948). 
Studies such as those reported by Lambe (1953, 1958), Pusch (1966), and Yong 
and Warkentin (1966) called attention to the contributions made by the dif-
ferent clay minerals in combination with other clay fractions on the engi-
neering properties and performance of clays.
Besides the significant contribution of soil structure to the strength and 
compaction properties of soils, it is abundantly clear that elements of soil 
structure such as packing of granular particles, aggregation of particles, and 
arrangement of clay minerals are central to the formation of the intrinsic 
60 Environmental Soil Properties and Behaviour
pore structure for soils. The pore spaces in soils hold and allow flow of water 
and gas. In effect, soil structure plays a significant role in the water retention 
characteristics of soils, the permeability of soils to water and gas, and the 
rheological property of soils.
2.6.2 granular Soil Packing
The packing of granular soil particles, that is, grains of soil, is very strongly 
dependent on the sizes, shapes, and distribution of sizes of the soil. For soil 
engineering purposes where stability and maximum resistance to shearing 
and shear displacement are required, optimum packing of the soil grains 
will provide the optimum density of the soil. This requires a specific range of 
sizes of the soil grains to fill the in-between voids created by larger particles 
in contact with each other, as shown in the left-side sketch in Figure 2.15 for 
idealized rounded soil grains. It is clear from the illustration shown in the 
left-hand sketch in the figure that it would be very unlikely that one would 
be able to find a distribution of soil grains that would completely fill all the 
voids in volume element depicted in the diagram. Idealized grain-size distri-
butions of soil grains do not exist in natural soils. Note that the angularity\u2014
especially the sharp angularity of soil grains such as those obtained from 
crushed rock\u2014will create more difficult conditions to obtain optimum den-
sity. In part, this will be due not only to the roughness of the surfaces of the 
particles, but also to length (L):width (W) ratios greater than one (L/W > 1).
The number of adjacent particles in contact with any particle is an impor-
tant factor in obtaining optimum density and, hence, minimum porosity n. 
Granular soil particle Simple cubic packing
Tetrahedral rhombic packing
FIguRE 2.15
The left-hand drawing gives an example of an ideal particle-size distribution for optimum 
packing. The right-hand illustrations show a simple cubic packing of equal-sized spheres at the 
top and tetrahedral rhombic packing of the same kind of spheres at the bottom.
61Nature of Soils
We define porosity n as the volume of voids in a representative elementary 
volume (REV) of soil divided by the total volume, that is, the REV. The num-
ber of particles in contact with any given particle is defined as the coordination 
number N. We can demonstrate the importance of obtaining the maximum 
coordination number to achieve optimum density by using the example of 
packing of spheres of equal radii R. The right-side sketches in Figure 2.15 
show the maximum (top) and minimum (bottom) porosities obtained for the 
packing modes shown. There are five different packing modes that can be 
obtained with spheres with equal radii. These are
\u2022	 Simple cubic: This packing mode is shown in the top sketch in the 
right-side illustration in Figure 2.15. The coordination number N is 
6, meaning that any one sphere is in contact with 6 other spheres, 
and the porosity n obtained from this packing mode is n = 47%.
\u2022	 Cubic tetrahedral: The coordination number N for this mode of pack-
ing is 8 and the porosity obtained is n = 39%.
\u2022	 Tetragonal sphenoidal: For this mode of packing, N = 10 and n = 30%.
\u2022	 Pyramidal: For this packing mode, N = 12 and n = 26%.
\u2022	 Tetrahedral rhombic: This is shown in the bottom sketch in the right-
side illustration in Figure 2.15. The coordination number N and the 
porosity n are identical to those obtained for pyramidal packing, 
that is, N = 12 and n = 26%.
The comparison of N and n values obtained with the five different packing 
modes for the spheres of equal radii show that the simple cubic mode with 
the lowest N value has the highest porosity n. The highest numbers of N 
values obtained are with the pyramidal and tetrahedral rhombic packing 
modes, meaning that these two modes of packing gave one the optimum 
densities and minimum porosities for these kinds of spheres.
The study of packing of spheres with unequal radii, which is more rep-
resentative of real soils, can be done mathematically. To do so, one needs to 
prescribe a limiting set of conditions, as demonstrated by Wise (1952) with 
one large sphere and all other spheres of smaller sizes placed such that they 
would all be placed on the surface of the large sphere. The first two spheres 
should not only touch each other but should also touch the surface of the 
large sphere, and each new subsequent sphere to be added must touch both 
the large sphere and at least two other spheres.
In natural sandy soils, the surface of soil grains is more or less coated or cov-
ered with organic matter and precipitated chemical compounds such as cal-
cium carbonate and Al- and Fe-hydroxide and others. The soil grains of such 
soils contact each other or combine with clay minerals to form soil structures 
similar to aggregates because of the gluing action and cementation by the coat-
ing materials. To determine the distribution of particle sizes in sandy soils in 
62 Environmental Soil Properties and Behaviour
experiments and laboratory tests, removal of gluing and cementing materials 
from soil particle surfaces and contact points between particles is necessary.
The aggregate size distribution is also an index of the structure of granular 
soil packing in natural soils. It is especially important to have information 
of water-bearing aggregates in relation to