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  • Writer's pictureSi Shen

Atterberg limits – a theory to describe how watery the soil is

Soil can be very watery in some situations. Think of watery mud – it behaves a lot like water in a lot of ways, or can be understood to behave like soup. Its usefulness in engineering is next to nothing, because nothing can be built on top of it. As this previous blog explains, water has little shear capacity, and shear capacity is what makes soil a useful founding material.

Not all soil is watery. Some are dry and some others in between. The Atterberg Limit is the theory that provides classifications of how watery a soil is, and the dominating structural behaviour for each classification.

The soil can be ‘solid’ or ‘liquid’, which are the two extreme states of the soil. There is almost no water in the ‘solid’ state and almost all water in the ‘liquid’ state. The state of soil in between solid and liquid is called ‘plastic’. Within this ‘plastic’ state, the soil behaves as a jelly. It is able to deform rapidly without breaking or cracking, but it does not change shape as freely as water, as it can be moulded into shapes without collapsing straight away.

As water content gets reduced in an extremely watery soil, it gradually changes into a ‘plastic’ soil. The limit at which it changes from theoretically liquid into theoretically plastic is called the liquid limit – water content above that, it is liquid; below that, it is plastic. As water content reduces further down, there is also a plastic limit, below which the soil is treated as theoretically ‘semi-solid’. In the semi-solid state, the soil loses the majority of its cohesion and is therefore liable to cracks when moulded.

As the semi-solid soil continues to lose water, at some point called ‘shrinkage limit’, it reaches a complete solid. With water content above the shrinkage limit, soil expands in volume as it absorbs water; below it, its volume barely changes with the change in water content. Below the shrinkage limit, the soil is not theoretically ‘saturated’, which means air can enter the voids between solid particles.

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