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  • Writer's pictureSi Shen

Consolidation settlement and heave of clay

Updated: Nov 27, 2021

Foundation settlement is a well understood concept. Heave is the exact opposite of settlement – the ground comes up rather than down. There could be many reasons for heave, such as the chemical content of the ground, change in ground water table, moisture content, vegetation growth, loading conditions and so on. Settlement and heave can be equally damaging to a structure. This blog particularly explains the mechanism of heave due to consolidation from unloading the ground.

Consolidation settlement of clay

As this previous blog explained, the settlement of foundation comes from three sources: immediate settlement, consolidation settlement and creep settlement. For foundations founded on top of clay, the immediate settlement is typically negligible. This is because the pore water is incompressible and therefore doing the vast majority of the work in holding up the pressure from the loading increase. This state immediately after the additional loading is applied is called the ‘undrained’ condition. Most of the settlement will come from consolidation and happen over long time. This is because water slowly gets squeezed out of the soil and achieves consolidation in the end, during which process the work from the pore water reduces gradually down to zero (called the ‘drained’ condition). Both the deformation of the soil skeleton and water squeezed out contribute towards the long term settlement of the ground.

This process is analogous to a ‘spring-water’ model. The spring represents the soil skeleton, acting elastically from pressure. The spring is compressible whereas the water is incompressible. In an ‘undrained’ condition, the box is closed, not letting water to exchange with the surrounding ground, and any pressure from the top will be taken 100% by the water, since its effective stiffness is infinite. If we poke a little whole in the box, water will leak gradually out of the box and the spring will gradually come into action. Eventually, the spring will be doing all the work, whereas water does nothing at all. This is a ‘drained’ condition.

The spongy ground

Therefore it could be conceptually understood that soil acts as a sponge, in that it pushes out water when squeezed, and sucks in water when let go. Adding pressure is the squeeze and removing pressure is the let-go. Clay has very small voids and water channels in between particles and therefore is a very slow-acting sponge.

When a soil is unloaded, the slow-reacting sponge operates in the exact opposite direction. The water is holding back the spring from relaxing, and therefore is in negative pressure (called ‘negative pore water pressure’). This is a suction effect that increases the effective pressure of the soil, which means that it makes the stress in the soil skeleton (the ‘spring’) such that it is (temporarily) higher than what it should be after the removal of loading. Due to the presence of the leaky hole, the suction effect diminishes over time until the soil becomes fully consolidated again. Therefore, immediately after the soil is unloaded, it seems as if the soil barely moves at all, as the bounce-back is held up by negative pore water pressure. Over time, as consolidation progresses, the spring bounces back. This is seen as heave that happens over time, but is really just a manifestation of the slow-acting nature of clay in response to the change of loadings.

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