Civil Engineering Reference
In-Depth Information
ments are typically deposited in layers or beds called strata and when compacted and cemented together
(lithification) they form sedimentary rocks.
Examples of sedimentary rocks:
shale, sandstone, chalk.
Metamorphic rocks
Metamorphism through high temperatures and pressures acting on sedimentary or igneous rocks pro-
duces metamorphic rocks. The original rock undergoes both chemical and physical alterations.
Examples of metamorphic rocks:
slate, quartzite, marble.
1.2.2 Soil
The actions of frost, temperature, gravity, wind, rain and chemical weathering are continually forming
rock particles that eventually become soils. There are three types of soil when considering modes of
formation.
Transported soil (gravels, sands, silts and clays)
Most soils have been transported by water. As a stream or river loses its velocity it tends to deposit some
of the particles that it is carrying, dropping the larger, heavier particles first. Hence, on the higher reaches
of a river, gravel and sand are found whilst on the lower or older parts, silts and clays predominate, espe-
cially where the river enters the sea or a lake and loses its velocity. Ice has been another important trans-
portation agent, and large deposits of boulder clay and moraine are often encountered.
In arid parts of the world, wind is continually forming sand deposits in the form of ridges. The sand
particles in these ridges have been more or less rolled along and are invariably rounded and fairly uniform
in size. Light brown, wind-blown deposits of silt-size particles, known as loess, are often encountered in
thin layers, the particles having sometimes travelled considerable distances.
Residual soil (topsoil, laterites)
These soils are formed
in situ
by chemical weathering and may be found on level rock surfaces where the
action of the elements has produced a soil with little tendency to move. Residual soils can also occur
whenever the rate of break up of the rock exceeds the rate of removal. If the parent rock is igneous or
metamorphic the resulting soil sizes range from silt to gravel.
Laterites are formed by chemical weathering under warm, humid tropical conditions when the rainwater
leaches out of the soluble rock material leaving behind the insoluble hydroxides of iron and aluminium,
giving them their characteristic red-brown colour.
Organic soil
These soils contain large amounts of decomposed animal and vegetable matter. They are usually dark in
colour and give off a distinctive odour. Deposits of organic silts and clays have usually been created from
river or lake sediments. Peat is a special form of organic soil and is a dark brown spongy material which
almost entirely consists of lightly to fully decomposed vegetable matter. It exists in one of three forms:
●
Fibrous
: Non-plastic with a firm structure only slightly altered by decay.
●
Pseudo-fibrous:
: Peat in this form still has a fibrous appearance but is much softer and more plastic
than fibrous peat. The change is due more to prolonged submergence in airless water than to
decomposition.
●
Amorphous
: With this type of peat, decomposition has destroyed the original fibrous vegetable struc-
ture so that it has virtually become an organic clay.
