Civil Engineering Reference
In-Depth Information
A highly simplified version of the stratigraphic column is shown in Table 4.1; this
gives the name and approximate age of the major divisions, the general nature of the
deposits and typical examples from the United Kingdom. In other parts of the world,
the major divisions and their ages are the same but the nature of the deposits may well
be different: for example, the Cretaceous Chalk in South East England is the same age
as the Deccan Lavas in India.
In order to describe the chronological history of the Earth geologists classify major
strata according to their age, not what they are. Notice that the initial letters of geo-
logical names are capitals (e.g. Old Red Sandstone, London Clay, etc.) whereas the
engineering descriptions (e.g. overconsolidated clay) have lower-case initial letters.
For example, the deposit called London Clay is of Eocene age and was deposited 40
to 60 million years ago. The deposit is found in South East England and is also found
in Belgium, where it is called Boom Clay. In the London region it is largely a marine
clay but to the west of London, in the Hampshire Basin, it is mostly silt and fine sand
with very little clay. Old Red Sandstone is of Devonian age and was deposited 350 to
400 million years ago. It is generally red in colour, unlike the Carboniferous rocks
above and the Silurian rocks below, which are both grey, but it is not all sandstone
and it contains thicknesses of mudstones and siltstones.
Generally soils and rocks become stiffer and stronger with age: London Clay is obvi-
ously stronger than the soils found in the English Fens and the slates in NorthWales are
stronger still. As a very rough guide, materials of Cenozoic age are generally regarded
as soils for engineering purposes; materials of Mesozoic age are generally regarded as
soft rocks and materials of Palaeozoic age are regarded as hard rocks. The soils and
rocks in the stratigraphic column contain fossils which are the most important indica-
tors of their age and provide a record of evolution on Earth. Cambrian and Ordovician
rocks contain mollusc shells and corals; land plants occur in the Devonian, reptiles in
the Carboniferous, amphibians in the Permian, dinosaurs in the Triassic and birds in
the Jurassic; the dinosaurs became extinct in the Cretaceous. Mammals, fishes, insects
and birds had evolved by the Eocene, but modern man did not evolve until the middle
of the Pleistocene, about 1 million years ago.
Since the engineering properties of sands, silts and clays and of sandstones, siltstones
and mudstones are likely to be different, the standard geological age-based classifica-
tions will only be of limited use in geotechnical engineering. Much better schemes for
engineering classifications of soils and rocks are based on the nature of the grains and
on the state of stress and water content. These are described in Chapter 5.
4.5 Depositional environments
The nature of the weathering and the mode of transport largely determine the nature
of a soil (i.e. the size and shape of the grains, the distribution of grain sizes and their
mineralogy). The environment into which it is deposited and the subsequent geological
events largely determine the state of the soil (i.e. the denseness or looseness of the
packing of the grains) and its structure (i.e. the presence of features such as fissuring,
bedding, bonding and so on).
As you move about the world you can see weathering, erosion, transportation and
deposition taking place. In the present day in the United Kingdom most of the trans-
portation is by water (rivers look dirty because they are carrying soil particles) and
