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What BS5930 says about the c-word. . .

'THE WORDS cohesive and non-cohesive, or granular, are often used to distinguish soils that contain a significant proportion of fine grains and behave in a cohesive manner when subjected to quick loading, from coarse grained soils, which have no apparent cohesion.

'The word cohesive usually describes a soil which has an undrained compression test.

In this test the strength arises from a combination of friction and negative pore pressure; cohesion therefore describes the ability of a soil to sustain a pore water suction when unconfined.

'All soils can be considered granular so, in terms of effective stress, true cohesion in any uncemented soil is very small (most uncemented soils slake when immersed in water). Moreover any uncemented soil can behave in either a cohesive manner or in a noncohesive manner depending on the response of the pore pressure to loading.

'For example, saturated clean sand is noncohesive and can be poured, but unsaturated sand can behave in a cohesive manner and a cohesive strength can be measured.

'Under long term, fully drained conditions, clays can behave in a frictional, non-cohesive manner and the long term stability of clay slopes is governed by the friction angle of the soil.

'To avoid these contradictions soils are described as coarse or fine. Coarse soils are gravels and sands.

When saturated and unconfined, they cannot sustain negative pore pressures, so these soils do not have any undrained strength or apparent cohesion.

'Fine soils are clays and silts. When saturated they can sustain suctions in unconfined tests and so have an apparent cohesion.

If well-graded soils contain sufficient fine grains to fill the spaces between the coarse grains, they are described as fine soils; if well-graded soils contain insufficient fine grains, they are described as coarse soils.

'These distinctions based on grain size apply equally to natural and compacted soils. Reference is made to 'material' and 'mass' characteristics; these can be divided into nature, state and structure.'

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