For coarse-grained soils (> 65% sand and gravel) the soil name is based on the particle sizes present. For fine-grained soils (> 35% silt and clay sizes) it is based on behavioural characteristics.
|Boulders||> 200 mm|
|Very coarse gravel||60-200 mm|
|Clay||< 2 µm|
|Term||% of soil mass||Example|
|Minor fraction||With trace of||< 5||With trace of sand|
|With minor||5-12||With minor sand|
|With some||12-20||With some sand|
|Term||Diagnostic features||Undrained comprehensive strength (kPa)|
|Very soft||Exudes between fingers when squeezed||< 25|
|Soft||Easily indented by fingers||25-50|
|Firm||Indented only by strong finger pressure||50-100|
|Stiff||Indented by thumb pressure||100-200|
|Very stiff||Indented by thumb nail||200-400|
|Hard||Difficult to indent by thumb nail||400-1000|
A visual assessment is based on:
- loosely packed: can be removed from exposure by hand or removed easily by shovel
- tightly packed: requires pick for removal, either as lumps or as disaggregated material.
|Dry||Soil looks and feels dry: cohesive soils are usually hard, powdery or friable while granular soils run freely through the hands.|
|Moist||Soil feels cool, darkened in colour: granular soils tend to cohere, while cohesive soils are usually weakened by moisture presence, but no free water forms on hands when remoulding.|
|Wet||Soil feels cool, darkened in colour: granular soils tend to cohere, while cohesive soils are usually weakened and free water forms on hands when handling.|
|Saturated||Soil feels cool, darkened in colour and free water is present in the sample. 'Fully saturated' refers to the case where the soil is below the water table.|
Plasticity of clays and silts is determined from the results of Atterburg limit tests. In the field the characteristics of fine-grained soils are identified using dilatancy (reaction to shaking), dry strength (crushing) and toughness (consistency near the plastic limit) behaviour. The most characteristic test of plasticity in a soil is dilatancy, where on rapid shaking water appears and similar shaking gives no reaction for a plastic soil.
The grading of gravels and sands may be qualified in the field as well graded (good representation of all particle sizes from largest to smallest). Poorly graded materials may be further divided into uniformly graded (most particles about the same size) and gap graded (absence of one or more intermediate sizes).
Weathering of soils is more relevant to coarse-grained soils, and where weathering does not have an influence on the properties of a soil the term may be omitted.
|Term||Inclination (from the horizontal)||Term||Bed thickness|
> 2 m
600 mm - 2 m
< 2 mm