weathering and its significance in engineering
construction
Weathering is defined as a
process of decay, disintegration and decomposition of rocks under the influence
of certain physical and chemical agencies.
Disintegration:
It may be defined as the process
of breaking up of rocks into small pieces by the mechanical agencies of
physical agents.
Decomposition:
It may be defined as the process of breaking up of mineral
constituents to form new components by the chemical actions of the physical
agents.
Denudation:
It is a general term used when
the surface of the earth is worn away by the chemical as well as mechanical
actions of physical agents and the lower layers are exposed.
The process of weathering depends upon the following three
factors:
i) Nature of
rocks
ii)
Length of
time
iii) Climate
Two Chief types of weathering are
commonly distinguished on the basis of type of agency involved in the process
and nature of the end product. They are:
i) |
Physical
or mechanical |
weathering ii) |
Chemical weathering |
Physical weathering:
It is the physical breakdown of
rock masses under the attack of certain atmospheric agents. A single rock block
is broken gradually into smaller irregular fragments and then into particles of
still smaller dimensions. It is the most active in cold, dry and higher areas
of the earth surface Temperature variations are responsible to a great extent
of physical weathering.
Thermal effects:
The effect of change of
temperature on rocks is of considerable importance in arid and semi arid
regions where difference between daytime and nighttime temperature is often
very high. Such temperature fluctuations produce physical disintegration in a
normally expected manner. Expansion on heating followed by contraction on
cooling. When the rock mass is layered and good thickness additional disturbing
stresses may be developed into by unequal expansion and contraction from
surface to the lower regions. The rock sometimes is found to break off into
concentric shells. This process is known as exfoliation.
When weathering occurs part of
the disintegrated rock material is carried away by running water or any other
transporting agent. Some of them are left on the surface of the bedrock as
residual boulders. It is often seen that boulders have an onion like structure.
This kind of weathering is called spheroidal weathering.
Chemical weathering:
The
chemical decomposition of the rock is called chemical weathering which is
nothing but chemical reaction between gases of the atmosphere and minerals of
the rocks.
The chemical changes invariably
take place in the presence of water generally rainwater -in which
are dissolved many active gases from the atmosphere like C02, nitrogen,
Hydrogen etc.These conditions are defined primarily by chemical composition of
the rocks humidity and the environmental surrounding the rock under attack.
Chemical weathering is
essentially a process of chemical reactions between gases of the atmosphere and
the surface rocks. For example:
1) 2CaCO3 +
H2O + CO2 ------------------ 2 Ca (HCO3) 2
2) CaSO4 +
2H2O -------------------- CaSO42.H2O
Engineering importance of rock weathering:
As engineer is directly or
indirectly interested in rock weathering specially when he has to select a
suitable quarry for the extraction of stones for structural and decorative
purposes. The process of weathering always causes a lose in the strength of the
rocks or soil.
For the construction engineer it is always necessary to see
that:
To what extent the area under consideration for a proposed
project has been affected by weathering and
What may
be possible effects of weathering processes typical of the area on the
construction materials.
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