Soil properties and state of the soil - Geotechnical Investigation and Design Tables - page 77

Soil properties and state of the soil

7.1 Soil behaviour
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A geotechnical model is often based on its behaviour as a sand (granular) or a clay
(fine grained), with many variations in between these 2 models.

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A sand with a fine content of 20% to 30% (depending on the gradation and size
of the coarse material) will likely behave as fine grained material, although it has
over 50% granular material.

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The table provides the likely behaviour for these 2 models.

Table 7.1 Comparison of behaviour between sands and clays.

Property Sands Clays Comments

Permeability (k) High k. Drains quickly Low K. Drains slowly Permeability affects the

(assumes 30% (assumes non fissured long term (drained) and <

fines). or no lensing in clay). short term (undrained)

properties.

Effect of time Drained and undrained Drained and undrained Settlement and strength

responses are response needs to be changes are immediate in

comparable. considered separately. sands, while these occur

over time in clays.

Water Strength is reduced by Relatively unaffected by In the long term the

half when submerged. short term change in effects of consolidation,

water. or drying and wetting

behaviour may affect
the clay.

Loading Immediate response. Slow response. 30% See Table 21.4 for N c

Not sensitive to shape. change in strength from a bearing capacity factor

strip to a square/circular (shape influenced).

footing.

Strength Frictional strength Cohesion in the short In clay materials both

governs. term often dominates, long term and short term

while cohesion and analysis are required,

friction to be considered while only one analysis is

in the long term. required for sands.

Confinement Strength increases with Little dependence on If overburden is removed

confining pressure, and the confining pressure. in sands a considerable loss

depth of embedment. However, some strain in strength may occur at

( Continued )