Cohesion of Clay

Cohesion, although thought to have only one meaning by many engineers, actually has two close meanings to each other. One meaning is in soil physics, which means, is the attraction of particles in soil, creating a sticking action. The other meaning comes for soil mechanics, and it means the shear strength of soil, when there is no normal stress applied to the soil.

We saw the relationship

τ = c + σ’sin Ø


if σ’ = 0

this means,

τ = c

Sticking action is a result of surface attraction forces between particles, and therefore applicable to only very fine particles, such as clay and some silt. Cohesive soils, as we have talked about before, are fine grained soils. They get their strength by this sticking action, but they are not big enough to generate frictional resistance between them, such as, internal friction.

So, in other words,

a purely cohesive, fine grained soil, should have zero internal friction (Ø=0), and its strength results only from cohesion as:

τ = c


a perfectly coarse grained soil such as sand, should have zero cohesion (c=0), and its strength results from internal friction only as:

τ = c + σ’sin Ø

In real life however, most soils have characteristics from both sides and they are called c-Ø soils.

So, the shear strength of a soil, has two components, the component coming from cohesion, and the component coming from friction.

At the start of this section, we said in soil mechanics, cohesion means, shear strength when applied normal stress is zero. But how do we measure strength of something when we do not apply load?  For this, we apply loads, and draw Mohr’s circles, and then we obtain the c value by intersecting the failure envelope tangent line by the vertical axis, as in figure Soil Strength 12, in the previous post of this series. In other words, cohesion c is an inferred value, and not measured directly.

In the next post of this series, we will discuss “Bearing Strength of Soil”

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