Civil Engineering Reference
In-Depth Information
the most common soils in the region; you should look up the relevant standard for the
region you will work in. In the United Kingdom these are the British Standards for site
investigations (BS 5930:1999) and for soil testing (BS 1377:1990) but slightly different
schemes are used in other regions. A simple and universal scheme for soil description
is as follows:
1. The nature of the grains. The most important features of soil grains are their size
and the grading (i.e. the proportions of different sizes), together with the shape
and surface texture of the grains and their mineralogy.
2. The current state of the soil. The important indicators of the state of a soil are
the current stresses, the current water content and the history of loading and
unloading: these are reflected by the relative strengths and stiffnesses of samples
of the soil.
3. The structure of the soil. This consists of fabric and bonding. Natural soils are
rarely uniform and they contain fabric features, such as layers, which are seen in
small samples and in large exposures. In some natural soils the grains are weakly
bonded together. (If the grains are strongly bonded the material has become a
rock.) Soil structure will be discussed further in Chapter 16.
4. The formation of the soil. Soils are formed in different ways. They may be
deposited naturally from water, ice or wind; they may be the residual products
of rock weathering; they may be compacted by machines into embankments and
fills.
A more detailed scheme for description of soils is given in BS 5930:1999. This is
similar to the scheme described above but is more detailed and gives helpful quantitative
values for a number of visual observations.
The nature of a soil does not usually change during normal civil engineering works;
occasionally weak and brittle soil grains may fracture during loading so the grading
changes. On the other hand, the state of a soil does change as soils near foundations
and excavations are loaded or unloaded and compress or swell.
The manner of formation of a soil will influence both its nature, its initial state and
its structure (i.e. layering, fissuring and bonding). In this topic I will be examining the
basic behaviour of soils observed in remoulded and reconstituted samples where any
fabric and bonding has been removed by the preparation of the sample. Since most
natural soils have some structure it is important always to test some intact samples,
but their behaviour should be examined within the basic framework established for
reconstituted samples.
5.3 Soil particle sizes, shapes and gradings
The range of particle sizes in soils is very large and ranges from clay grains that are
smaller than 2
m (0.002 mm) to boulders that are larger than 200 mm. A particular
range of particle sizes is given a name, as in Fig. 5.l, so that, for example, in UK
practice medium sand is 0.2 to 0.6 mm. As a general guide, individual sand-sized and
coarser particles are visible to the naked eye while individual silt-sized particles are
visible using a
µ
10 hand lens. If you can wash fine grained soil off your boots it is
probably silt, but if you have to scrape it off it is probably clay; similarly, if silt dries
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