It has been discussed earlier that soil
is formed by the process of physical and chemical weathering. The
individual size of the constituent parts of even the weathered rock
might range from the smallest state (colloidal) to the largest
possible (boulders). This implies that all the weathered constituents
of a parent rock cannot be termed soil.
According to their grain size, soil
particles are classified as cobbles, gravel, sand, silt and clay.
Grains having diameters in the range of 4.75 to 76.2 mm are called
gravel. If the grains are visible to the naked eye, but are less than
about 4.75 mm in size the soil is described as sand.
The lower limit of visibility of grains
for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075
to 0.002 mm are termed as silt and those that are finer than 0.002 mm
as clay. This classification is purely based on size which does not
indicate the properties of fine grained materials.
Residual and Transported Soils
On the basis of origin of their
constituents, soils can be divided into two large groups:
1. Residual soils, and
2. Transported soils.
Residual soils are those that remain at
the place of their formation as a result of the weathering of parent
rocks. The depth of residual soils depends primarily on climatic
conditions and the time of exposure. In some areas, this depth might
be considerable. In temperate zones residual soils are commonly stiff
and stable.
An important characteristic of residual
soil is that the sizes of grains are indefinite. For example, when a
residual sample is sieved, the amount passing any given sieve size
depends greatly on the time and energy expended in shaking, because
of the partially disintegrated condition.
Transported soils are soils that are
found at locations far removed from their place of formation. The
transporting agencies of such soils are glaciers, wind and water. The
soils are named according to the mode of transportation. Alluvial
soils are those that have been transported by running water. The
soils that have been deposited in quiet lakes, are lacustrine soils.
Marine soils are those deposited in sea
water. The soils transported and deposited by wind are aeolian soils.
Those deposited primarily through the action of gravitational force,
as in land slides, are colluvial soils.
Glacial soils are those deposited by
glaciers. Many of these transported soils are loose and soft to a
depth of several hundred feet. Therefore, difficulties with
foundations and other types of construction are generally associated
with transported soils.
Organic and Inorganic Soils
Soils in general are further classified
as organic or inorganic. Soils of organic origin are chiefly formed
either by growth and subsequent decay of plants such as peat, or by
the accumulation of fragments of the inorganic skeletons or shells of
organisms. Hence a soil of organic origin can be either organic or
inorganic. The term organic soil ordinarily refers to a transported
soil consisting of the products of rock weathering with a more or
less conspicuous admixture of decayed vegetable matter.
Names of Some Soils that are
Generally Used in Practice
Bentonite is a clay formed by the
decomposition of volcanic ash with a high content of montmorillonite.
It exhibits the properties of clay to an extreme degree. Varved Clays
consist of thin alternating layers of silt and fat clays of glacial
origin. They possess the undesirable properties of both silt and
clay.
The constituents of varved clays were
transported into fresh water lakes by the melted ice at the close of
the ice age. Kaolin, China Clay are very pure forms of white clay
used in the ceramic industry. Boulder Clay is a mixture of an
unstratified sedimented deposit of glacial clay, containing unsorted
rock fragments of all sizes ranging from boulders, cobbles, and
gravel to finely pulverized clay material.
Calcareous Soil is a soil containing
calcium carbonate. Such soil effervesces when tested with weak
hydrochloric acid. Marl consists of a mixture of calcareous sands,
clays, or loam. Hardpan is a relatively hard, densely cemented soil
layer, like rock which does not soften when wet. Boulder clays or
glacial till is also sometimes named as hardpan. Caliche is an
admixture of clay, sand, and gravel cemented by calcium carbonate
deposited from ground water.
Peat is a fibrous aggregate of finer
fragments of decayed vegetable matter. Peat is very compressible and
one should be cautious when using it for supporting foundations of
structures. Loam is a mixture of sand, silt and clay.
Loess is a fine-grained, air-borne
deposit characterized by a very uniform grain size, and high void
ratio. The size of particles ranges between about 0.01 to 0.05 mm.
The soil can stand deep vertical cuts because of slight cementation
between particles. It is formed in dry continental regions and its
color is yellowish light brown. Shale is a material in the state of
transition from clay to slate. Shale itself is sometimes considered a
rock but, when it is exposed to the air or has a chance to take in
water it may rapidly decompose.
