Soil properties and state of the soil - Geotechnical Investigation and Design Tables - page 80

Table 7.4 Effect of colour.

Colour effect Significance

Light to dark Increasing moisture content. Dry soils are

generally lighter than a wet soil

Black, dark shades of brown and grey Organic matter likely

Bright shades of brown and grey. Red, Inorganic soils

yellow and whites
Mottled colours Poor drainage

Red, yellow - brown Presence of iron oxides

Figure 7.1 Soil plasticity chart.

7.5 Plasticity characteristics of common clay minerals
•

Soils used to develop the plasticity chart tended to plot parallel to the A - Line
(Refer Figure).

•

A - Line divides the clays from the silt in the chart.

•

A - Line: PI = 0.73 (LL - 20).

•

The upper limit line U - line represents the upper boundary of test data.

•

U - Line: PI = 0.9 (LL - 8).

Table 7.5 Plasticity characteristics of common clay minerals (from Holtz and Kovacs,1981).

Clay mineral Plot on the plasticity chart

Montmorillonites Close to the U - Line. LL 30% to Very High LL 100% = >

Illites Parallel and just above the A - Line at LL 60% 30% = ±

Kaolinites Parallel and at or just below the A - Line at LL 50% 20%

= ±

Halloysites In the general region below the A - Line and at or just above LL 50%

=

•

Volcanic and Bentonite clays plot close to the U Line at very high LL.