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In [[soil mechanics]], '''consolidation''' refers to the process by which [[soil]] changes volume gradually in response to a change in [[pressure]]. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually [[groundwater]]. When soil [[soil saturation|saturated with water]] is subject to an increase in pressure, the high [[bulk modulus|volumetric stiffness]] of water compared to the soil matrix means that the water initially absorbs all the change in pressure without changing volume, creating excess [[pore water pressure]]. As water diffuses away from regions of high pressure due to [[seepage]], the soil matrix gradually takes up the pressure change and shrinks in volume. The theoretical framework of consolidation is therefore closely related to the [[diffusion equation]], the concept of [[effective stress]], and [[hydraulic conductivity]]. |
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In the narrow sense, "consolidation" refers strictly to this delayed volumetric response to pressure change due to gradual movement of water. Some publications also use "consolidation" in the broad sense, to refer to any process by which soil changes volume due to a change in applied pressure. This broader definition encompasses the overall concept of [[soil compaction]], [[subsidence]], and [[heave (soil mechanics)|heave]]. Some types of soil, mainly those rich in [[organic matter]], show significant [[creep]], whereby the soil changes volume slowly at constant effective stress over a longer time-scale than consolidation due to the diffusion of water. To distinguish between the two mechanisms, "primary consolidation" refers to consolidation due to dissipation of excess water pressure, while "secondary consolidation" refers to the creep process. |
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The effects of consolidation are most conspicuous where a [[building]] sits over a layer of soil with low [[stiffness]] and low permeability, such as [[marine clay]], leading to large [[subsidence|settlement]] over many years. Types of construction project where consolidation often poses technical risk include [[land reclamation]], the construction of [[levee|embankments]], and [[tunnel]] and [[basement]] excavation in clay. |
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[[Geotechnical engineer]]s use [[oedometer]]s to quantify the effects of consolidation. In an oedometer test, a series of known pressures are applied to a thin disc of soil sample, and the change of sample thickness with time is recorded. This allows the consolidation characteristics of the soil to be quantified in terms of the coefficient of consolidation (<math>C_{v}</math>) and hydraulic conductivity (<math>K</math>). |
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