Civil Engineering Reference
In-Depth Information
Natural soils
16.1 Characteristics of natural soils
In previous chapters I described the basic mechanics of soils and in later chapters these
simple theories for soil behaviour will be used to investigate the performance of soil
structures such as slopes, retaining walls and foundations. The behaviour described
and the theories developed are largely idealizations for the behaviour of reconsti-
tuted soils, but natural soils differ from reconstituted soils in a number of important
aspects.
A reconstituted soil is manufactured in the laboratory by mixing it with water to
form a slurry at a very high water content, pouring it into a mould and loading and
unloading it by one-dimensional compression or consolidation in a test apparatus to
the required initial state. Coarse grains are not bonded to each other; fine grains,
especially clays, may be very slightly bonded due to small surface forces. If the soil is
well graded the grains of different sizes are distributed randomly. The sample is tested
soon after it has been manufactured. Soil properties measured in tests on reconsituted
samples depend only on the nature of the grains and they are material parameters.
Natural soils have features which are known as
structure
, which is a combination of
fabric
and
bonding
and these features arise from their formation and age.
Natural soils are either sedimented, usually through water, or they are residual soils
which are the end products of weathering in situ. In reconstituted soil the grains are
distributed randomly: in structured soil, fabric is the way in which grains are arranged
in a non-random way. It includes layers or lenses of poorly graded soil within a body
of soil with different grading and collections or flocs of fine grained soil which appear
as larger grains. (Go and look carefully at freshly excavated slopes in soils and you
will almost always be able to see layering; occasionally you can find thick beds of
nearly uniform clay deposited in an unchanging environment but these are rare.) Soil
fabric is the non-random arrangement of soil grains and it is created largely during
deposition.
Natural soils are compressed and swelled by further deposition and erosion of over-
lying sediment, by loading from glaciers and ice sheets and by changes in groundwater.
With geological time all soils age and their properties change. Most natural soils are
very old. London Clay is about 60 million years old and even recent glacial soils are
over 10,000 years old. Occasionally you may come across natural soils like Mississippi
delta muds or the soils of the Fens of East Anglia, which are only decades or centuries
old, but these are the exception.
