Soil properties and state of the soil - Geotechnical Investigation and Design Tables - page 81

7.6 Weighted plasticity index
•

The plasticity index by itself can be misleading, as the test is carried out on the %
passing the 425 micron sieve, ie any sizes greater than 425

m is discarded. There
have been cases when a predominantly “rocky/granular'' site has a high PI test
results with over 75% of the material discarded.

µ

•

The weighted plasticity index (WPI) considers the % of material used in the test.

•

WPI = PI × % passing the 425 micron sieve.

Table 7.6 Weighted plasticity index classification (Look, 1994).

Volume change classification Weighted plasticity index %

Very low 1200 <

Low 1200-2200

Moderate 2200-3200

High 3200-5000

Very high 5000 >

7.7 Effect of grading
•

The grading affects the strength, permeability and density of soils.

•

Different grading requirements apply to different applications.

Table 7.7 Effect of grading.

Grading Benefits Application Comments

Well graded Low porosity with a

low permeability.

Structural concrete,
to minimize cement
content

Well graded U 5 and >

C

=

1to3

Uniformly graded Single sized or open -

graded aggregate has
high porosity with a
high permeability.

Preferred for
drainage

Uniform grading U

<

2

Moderate grading:
2 5. Open graded

identified by their nominal
size through which all of
nearly all of material (D 90 )

U

< <

P (%) (D/D max ) n 100 Maximum density Road base/sub - base

specification grading

n 0.5 (Fuller's curves) = × =

P - % passing size
D (mm)

D max

=

maximum particle

size

Well graded Increased friction

angle

Higher bearing
capacity

Most common
application

•

D 90 = 19mm is often referred to as 20mm drainage gravel.

•

=

D 90 9.5mm is often referred to as 10mm drainage gravel.

7.8 Effective friction of granular soils
•

The friction depends on the size and type of material, its degree of compaction
and grading.