air". More generally, consolidation refers to the process by which soils change volume in response to a change in pressure, encompassing both compaction and swelling. Magnitude of volume change Consolidation is the process in which reduction in volume takes place by the gradual expulsion or absorption of water under long-term static loads. When stress is applied to a soil, it causes the soil particles to pack together more tightly. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil. The magnitude of consolidation can be predicted by many different methods. In the classical method developed by Terzaghi, soils are tested with an oedometer test to determine their compressibility. In most theoretical formulations, a logarithmic relationship is assumed between the volume of the soil sample and the effective stress carried by the soil particles. The constant of proportionality (change in void ratio per order of magnitude change in effective stress) is known as the compression index, given the symbol when calculated in natural logarithm and when calculated in base-10 logarithm. This can be expressed in the following equation, which is used to estimate the volume change of a soil layer: where δc is the settlement due to consolidation. Cc is the compression index. e0 is the initial void ratio. H is the height of the compressible soil. σzf is the final vertical stress. σz0 is the initial vertical stress. When stress is removed from a consolidated soil, the soil will rebound, regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will consolidate again along a recompression curve, defined by the recompression index. The gradient of the swelling and recompression lines on a plot of void ratio against the logarithm of effective stress often idealised to take the same value, known as the "swelling index" (given the symbol when calculated in natural logarithm and when calculated in base-10 logarithm). Cc can be replaced by Cr (the recompression index) for use in overconsolidated soils where the final effective stress is less than the preconsolidation stress. When the final effective stress is greater than the preconsolidation stress, the two equations must be used in combination to model both the recompression portion and the virgin compression portion of the consolidation processes, as follows, where σzc is the preconsolidation stress of the soil. This method assumes consolidation