Civil Engineering Reference
In-Depth Information
7.2 Standard tests and specifications
Many of the routine soil tests are very carefully and precisely specified in a number
of national standards and codes of practice. In the United Kingdom the standard is
BS 1377:1990
Methods of Test for Soils for Civil Engineering Purposes
, and similar
standards exist in other countries and regions. You should certainly look at a copy of
the standards for soil testing relevant to your region to see exactly what they cover.
Most of these standards follow what might be called a cookery book method: you
do this, you do that and you serve up the result in this or in that way. There are,
however, difficulties with the cookery book approach for soil testing which arise from
the characteristics of soil strength and stiffness.
The values obtained from a particular test will obviously depend to a greater or lesser
extent on details of the equipment and procedures used and for some tests, particularly
those that measure the nature and state of a soil, it is essential that the tests follow
standard procedures. This is because the parameters being measured (e.g. grading and
Atterberg limits) are material parameters (they depend only on the nature of the grains)
and different laboratories and different workers should obtain identical results for the
same soil.
While it is possible and desirable to set standards for construction of equipment and
for calibration of instruments to ensure that the accuracy of any observation is accept-
able (or at least known), it is not so easy to specify tests that measure soil strength and
stiffness because of the many important factors that affect these parameters. Instead,
engineers should determine what parameters are required for a particular analysis,
determine what factors will influence these within the theories described in Chapters 8
to 13 and then devise tests that take account of these. The engineer will need to specify
not only the loading path applied in the test but also, equally importantly, the loads
applied to the sample before the test starts.
I am not going to describe the standard equipment and soil tests in detail. Most of
the standard apparatus and routine tests are described at length in a three-volume book
by Head (1980, 1982 and 1986) and in various standards and codes of practice. All
engineers concerned with groundworks should carry out simple classification, consol-
idation, shear and triaxial tests for themselves at least once in their career; they should
also carry out simple foundation, slope stability and retaining wall experiments. The
emphasis of this work should be on handling equipment and soil samples, good scien-
tific practice and analysis and interpretation of test results within simple theories. They
should also play around with soils and soil-like materials at home, in their garden and
at the beach.
7.3 Basic classification tests
As discussed in Chapter 5, the nature of a soil is described principally by its grading (i.e.
the distribution of particle sizes) and the mineralogy of the grains. The state is described
by the current water content and unit weight together with the current stresses.
(a) Measurement of grading
The distribution of particle sizes in a soil is found by sieving and sedimentation. Soil
is first passed through a set of sieves with decreasing aperture size and the weight
