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Appendix C: New Zealand Geomechanics Society Terminology for Description of Soils

Soil name

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.

Table A2: Names for different particle sizes
Term Size
Boulders   > 200 mm
Very coarse gravel   60-200 mm
Gravel Coarse 20-60 mm
Medium 6-20 mm
Fine 2-6 mm
Sand Coarse 0.6-2.0 mm
Medium 0.2-0.6 mm
Fine 0.06-0.2 mm
Silt   2-60 µm
Clay   < 2 µm

 

Table A3: Proportions
  Term % of soil mass Example
Subordinate fraction (...)y 20-50 Sandy
Major fraction ...—... 35-50 Sand-gravel
Major constituent Gravel
Minor fraction With trace of < 5 With trace of sand
With minor 5-12 With minor sand
With some 12-20 With some sand

Strength

Table A4: Fine-grained soils (cohesive)
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

Coarse-grained soils

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.

Moisture condition

Table A5: Moisture condition
Term Description
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

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.

Grading qualifications

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

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.

Bedding

Table A6: Bedding characteristics
Term Inclination (from the horizontal) Term Bed thickness

Sub-horizontal

0-10°

Very thick

> 2 m

Gently inclined

10-30°

Thick

600 mm - 2 m

Moderately inclined

30-60°

Moderately thick

200-600 mm

Steeply inclined

80-90°

Moderately thin

60-200 mm

Sub-vertical

80-90°

Thin

20-60 mm

   

Very thin

6-20 mm

   

Laminated

2-6 mm

   

Thinly laminated

< 2 mm

Particle shape