Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 4 LABORATORY TESTING
FOR SOILS
4.6.1
Classification Testss
Soil classification, although introducing a further stage of data acquisition
into site investigation, has
an important role to play in reducing the costs and increasing the cost-effectiveness of laboratory
testing. Together with
detailed sample description, classification tests allow the soils on a site to be
divided into a limited number of arbitrary groups, each of which is estimated to contain materials of
similar geotechnical properties. Subsequent more expensive and time-consuming testss carried out to
determine geotechnical parameters for design purposes may then be made on limited numbers of
samples which are selected to be representative
e of the soil group in question.
Particle Size Distribution Tests
BS 1377:1990 gives four methods for determining the particle size distribution of
soils (part 2,
clauses 9.2—9.5). The coarse fraction of the soil (>0.06mmm approximately) is tested
by passing it
through a series of sieves with diminishing apertures. The particle size distribution is
obtained from
records of the weight of soil particles retained
on each sieve and is usually shown as a graph of
‘percentage passing by weight’ as a function of particle size (Fig. 4.1).
Figure 4.1 Typical Particle Size Distribution
Two methods of sieving are defined in BS 13777 (part 2, clauses 9.2, 9.3) . Dry sieving is only suitable
for sandss and gravels
which do not contain any clay: the British Standard discourages its use, and
since the
exact composition of a soil will not be
known before testing, it is not often requested. Wet
sieving requires a complex procedure to separate the fine clayey particles
from the coarse fraction of
the soil which is suitable for sieving, as summarized below.
1. Select representative test specimen by quartering and
riffling.
2. Oven dry specimen at 105— —110°C, and weigh.
3. Place on 20mmm sieve.
4. Wirebrush each
particle retained on the 20mm sieve to remove fines.
5. Sieve particles coarser than 20 mm. Record weights retained on each sieve.
6. Riffle particles finer than 20mm to reducee specimen mass to 2kg (approx.) weigh.
March 2009
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