Civil Engineering Reference
In-Depth Information
5.9 Summary
Classification of soils requires a careful and detailed description of the soil
in situ
and
in samples together with some simple classification tests. The important characteristics
required for description of soils are:
1. The nature of the grains including the grading (i.e. the distribution of particle sizes)
and the mineralogy, particularly of clay soils. The Atterberg limits give indications
of clay mineralogy.
2. The state of a soil is given by the stresses, the history of deposition and erosion
and the water content. Important indicators of soil state are the liquidity index of
fine-grained soils or the relative density of coarse-grained soils.
3. Structure including bedding, layering, fissuring, jointing and bonding.
4. The method of formation of the soil, which may be deposited from water, wind or
ice, residual formed by weathering or compacted by rolling, vibration or impact.
Worked examples
Example 5.1: Grading of soils
Table 5.3 gives the results of particle size tests on four
different soils. The grading curves are shown in Fig. 5.6.
Soil A is predominantly medium sand; it is poorly (i.e. uniformly) graded with a
relatively small range of sizes. It was probably deposited from a relatively fast flowing
river.
Soil B is well graded with a very wide range of particle sizes from coarse gravel to
fine silt with a little clay. It was probably deposited from a glacier and has not been
sorted by wind or water.
Soil C is a silty clay. It could be deposited either in a shallow sea, in a lake or in an
estuary.
Soil D is gap graded; there is a gap in the grading between coarse silt and medium
sand. It possibly was originally a deposit with layers of fine to medium silt and
coarse sand.
Table 5.3
Results of particle size tests - Example 5.1
BS sieve
Size from
sedimentation
(mm)
% smaller
Soil A
Soil B
Soil C
Soil D
63 mm
100
20 mm
75
6.3 mm
100
66
100
2mm
96
60
74
600
µ
m
86
55
48
212
µ
m
10
45
100
45
63
µ
m
2
34
95
42
0.020
22
84
40
0.006
15
68
15
0.002
8
42
4
