Environmetal Soil Properties and Behaviour

Environmetal Soil Properties and Behaviour


DisciplinaControle e Remediação da Poluição dos Solos5 materiais18 seguidores
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felsites, durite, gabbro, basalt, and others 
basalts and granites are by far the greatest proportion of rocks that classify 
as igneous.
Sedimentary rocks are formed from the wind and water deposition of the 
material derived from chemical and physical breakdown of other source 
rocks, compressed under considerable geologic pressure and heat. The 
commonly found sedimentary rocks include sandstone, siltstone, dolomite, 
shale, and limestone. Metamorphic rocks are formed from metamorphos-
ing of igneous or sedimentary rocks. Since considerable heat and pressure 
are required in the process of metamorphic transformation of the igneous 
and sedimentary rocks, there will be transformation of the original miner-
als found in the rocks. The common metamorphic rocks include quartzite, 
schist, slate, gneiss, and marble.
1.2.2 Weathering of Rock
1.2.2.1 Natural Processes
The natural processes involved in the transformation of source rocks are 
generally referred to as weathering processes. General usage will refer to this 
as weathering, with or without inclusion of the term processes. In the context 
of soil formation from rocks, weathering is a spontaneous reaction involving 
geologic material (rocks) and energy. Since it is a change in the direction of a 
decrease in free energy of the system, it is possible to predict the thermody-
namic susceptibility of minerals to weathering. The assemblages of minerals 
in soils are reflective of the temporary equilibrium stages in the dynamic 
process of changes from source rocks to the final end product. The weath-
ering sequence in development of clay minerals from source rocks may be 
represented in terms of a reaction series portraying soils as an equilibrium 
stage in a continuous progression from, for example, igneous rocks such as 
granite and basalt to a final end product such as laterite.
The intensity of weathering is highest where interfaces between the atmo-
sphere, hydrosphere, biosphere, and lithosphere meet and overlap. A good 
example of this can be found in the upper soil zone or layer in temperate 
humid climate regions, and deeper soil zones in humid tropic regions. 
4 Environmental Soil Properties and Behaviour
Transformation of rock fragments to soil occurs primarily in the regolith: the 
region between solid rock and the soil.
The rate of weathering is influenced or controlled by several factors, not the 
least of which are the composition and size of the source rock (rock mass) or 
rock fragments, size and type of exposed surface areas of rocks, relative solu-
bilities of rock mass, fragments and weathered material, permeability of the 
rock, source and availability of water (e.g., position of water table or hydro-
geological setting), chemical composition of water, temperature, topography, 
oxygen content or availability, organic matter, cyclical freezing and thawing 
and wetting-drying cycles, climatic envelope, and others.
The primary types of actions responsible for the forces involved in frag-
menting source rocks are physical and chemical in nature. The agents 
responsible for these soil-forming forces can be physical, chemical, or biolog-
ical in nature. Strictly speaking, the actions of biological agents are actually 
biologically mediated chemical actions. In that sense, we see references in 
the literature to only two types of soil-forming actions or forces: physical and 
chemical. Under natural circumstances, that is, without any actions attrib-
utable directly or indirectly to anthropogenic input, these types of actions 
are often referred to as chemical weathering and physical weathering. In short, 
physical weathering and chemical weathering of rock are two of the natural 
processes of disintegration of rock that ultimately result in the production of 
soil. Figure 1.1 gives a summary illustration of the major weathering pro-
cesses involved.
1.2.2.2 Physical Weathering
Physical weathering of rocks involves forces and processes that result in 
fragmenting of the rocks. Particles of much lesser sizes are obtained as a 
result of the breakdown of the various rock fragments or minerals. The 
agents producing these forces and processes could be (a) thermal in nature 
such as cyclic freezing and thawing, and heating and cooling, since rock 
components have different coefficients of expansion; (b) expansion of water 
in rock fissures as a result of freezing; (c) physical movement of glaciers atop 
the rocks, resulting in application of abrasive forces, and water on/against 
rocks and in rock fissures; and (d) physical pressures and loads from super-
posed glaciers and displacements due to plant growth and activities of ani-
mals. Note that in physical weathering, there is generally no alteration of the 
chemical composition of the rock material.
1.2.2.3 Chemical Weathering
Chemical weathering of rocks involves processes leading to transforma-
tion of the constituents and even loss of some constituents, addition of 
protons, and rearrangement of constituents into new materials. It should 
5Origin and Function of Soils
be noted that chemical weathering will not occur without the presence of 
water; that is, water is an essential component in chemical weathering. The 
rates of chemical reactions in the weathering process are determined by 
temperature, moisture, composition, potential for leaching, and composition 
of the leaching solution. Chemical processes involved include hydrolysis, 
hydration, carbonation, oxidation, reduction, and solution. Protons in the 
leaching solution can be supplied from CO2 in the air or from decaying veg-
etation or other acids added to the system.
Both the amount of water available and heat are big factors in the chemical 
weathering of the parent rock mass. Warm and humid climates are therefore 
instrumental in producing correspondingly high weathering of the minerals 
in the parent rock mass. The presence and amount of organic matter are also 
very important factors in the control and rate of weathering. If organic mat-
ter is allowed to remain, as for example in cool environments, high humid-
ity would produce conditions that would successfully promote reactions 
between the organic acids and parent rock. However, if the environment is 
changed to a hot, humid environment, oxidation of the organic matter would 
preclude the formation of organic acids, resulting in little or no reactions 
between the organic matter and the parent rock.
Parent material
(unaltered source rock) Weathering envelope
climate, topography,
time, vegetation, etc.
Physical Weathering
	ermal\u2013cyclical freeze-thaw,
heating-cooling.
Pressures and stresses\u2013overburden
loading and load release.
Hydraulic\u2013fluid pressures & freezing of
water in fissures.
Abrasion\u2013abrasive stresses from movement
of overlying glaciers.
Chemical Weathering
Hydrolysis
Oxidation
Hydration
Carbonation
Solution 
SOIL
Residual soils (remaining in-place)
Glacial
Alluvial
Marine
Lacustrian
Aeolian 
Transported soils
FIguRE 1.1
Soil formation beginning with weathering of unaltered source-rock mass (parent material) 
and ending with final resting place\u2014that is, residual or transported.
6 Environmental Soil Properties and Behaviour
1.2.3 Processes of Chemical Weathering
The major processes involved in formation of soil by chemical weathering 
agents include
\u2022	 Hydrolysis: Hydrolysis is the reaction between the H+ and OH\u2c9 ions of 
water and the elements or ions of a mineral in the source rock or other 
type of parent material. It is considered to be an important reaction 
for silicates. These are the primary rock-forming minerals. For hydro-
lysis reactions to cause weathering of rocks, it is necessary for the 
weathering products to be removed continuously. This means that 
fresh application of water is necessary throughout the weathering 
process. A