CHARACTERISTICS OF SOILS - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 25

TABLE 2.2b SPECIFIC (SOLID) GRAVITY G S

sand or silt clay

quartz 2.65 kaolinite 2.65

K-feldspars 2.54 - 2.57 montmorillonite 2.75

Na-Ca-feldspars 2.62 - 2.76 illite (mica) 2.84

Particle size distribution
A useful method for classification of granular soils is by particle size distribution
obtained by sieving oven-dried (disturbed) soil following a standard procedure.
The amount of material retained in each of the sieves is expressed in a percentage
of the total weight of the original dried sample and represented in a graph on
logarithmic diameter scale. The presentation of a well-graded and a poorly graded
soil by a standard particle size distribution is shown in Fig 2.1.

100

80

poorly graded

standard sieve

60

well graded

silt sand

40

sand gravel

20

0
0.01 0.1 1 10

D

D 10

2

0.0625 60

Figure 2.1 Particle grain-size distribution graph (D in mm)

By definition, D x represents the diameter for which x% of the graded material is
less than that diameter. In Fig 2.1, D 60 of the poorly graded material is 0.25 mm,
and D 10 is 0.13 mm (i.e. 10% of the grains has a diameter less than 0.13 mm). A
soil is considered uniformly graded when the uniformity coefficient U = D 60 /D 10 is
less than 2. For the poorly graded material in Fig 2.1, U = 0.25/0.13 = 1.92, and
thus uniformly graded. Fines less than 16 m are determined with sedimentation

tests (Hydrometer) after organic material such as peat has been removed.
Internationally accepted soil classification by particle-size ranges is shown in
Table 2.3.

TABLE 2.3 CLASSIFICATION BY PARTICLE SIZE D 50

fine-graded
clay silt
colloids fine medium coarse
2 6 20 50
m