Soil classification - Geotechnical Investigation and Design Tables - page 26

- Cemented is shown in the Table, as an extension to what is shown in most
references.
- N - Values

> 50 often considered as rock.

- Table applies to medium grain size sand. Material finer or coarser may have

a different value. Correction factors also need to be applied. Refer Tables 5.4
and 5.5.

2.16 Moisture content
•

This is separate from the water level observations. There are cases of a soil
described as wet above the water table and dry below the water table.

•

The assessor must distinguish between natural moisture content and moisture
content due to drilling fluids used.

Table 2.16 Moisture content.

Term Symbol Field assessment

Cohesive soils Granular soils

Dry D Hard and friable or powdery Runs freely through hands

Moist M Feels cool, darkened in colour

Can be moulded Tend to cohere

Wet W Feels cool, darkened in colour

Free water forms on hands when handling Tend to cohere

- Some reports provide the moisture content in terms of the plastic limit.

This however introduces the possibility of 2 errors in the one assessment,
Refer Table 10.2 for inherent variability in soil measurement for the moisture
content and plastic limit.

2.17 Origin
•

This can be obtained from geology maps as well as from site and material
observations.

•

Soils are usually classified broadly as transported and residual soils.

Table 2.17 Classification according to origin.

Classification Process of formation and nature of deposit

Residual Chemical weathering of parent rock. More stony and less weathering with increasing
depth.
Alluvial Materials transported and deposited by water. Usually pronounced stratification.
Gravels are rounded.

Colluvial Material transported by gravity. Heterogenous with a large range of particle sizes.

Glacial Material transported by glacial ice. Broad gradings. Gravels are typically angular.

Aeolian Material transported by wind. Highly uniform gradings. Typically silts or fine sands.

Organic Formed in place by growth and decay of plants. Peats are dark coloured.

Volcanic Ash and pumice deposited in volcanic eruptions. Highly angular. Weathering produces

a highly plastic, sometimes expansive clay.

Evaporites Materials precipitated or evaporated from solutions of high salt contents. Evaporites

form as a hard crust just below the surface in arid regions.