Sedimentation-compression curve down to a depth of 45 cm in sediment core C10 from the Cachí reservoir, Costa Rica. F = probable flushing periods with subaerial exposure of the sediment surface, dehydration, and formation of mud crusts. From Axelsson 1992 (UNGI Rapport Nr 81).

The sedimentation-compression curves, showing the vertical variation in void ratio in relation to the effective overburden pressure, are used to illustrate sediment compaction. In water-saturated sediments the void ratio is the ratio of the volume of water (= the volume of the pores) to the volume of sediment particles forming the given mass. With effective overburden pressure is meant the total overburden pressure minus acting pore pressure. These curves may also be used for the prediction of sediment compaction with increasing depth of burial.

The initial porosity of a sediment layer generally increases with decreasing grain size. Organic content, bioturbation, and gas turbation also affect the porosity. Due to formation of tube burrows or gas expansion holes the porosity may be very high in surficial muds, and the uppermost part of these deposits are often underconsolidated. In the uppermost cm of clay-rich lake deposits the porosity may be higher than 90%. The initial porosity of sands is in general lower than 50%. The porosity decreases with time and with increasing depth of burial. Sandy deposits decrease rather slowly in volume even during the early stages of compaction while clays decrease rather quickly, though the rate of porosity loss decelerates with increasing effective overburden pressure.

The rate of sedimentation is often expressed in terms of thickness per unit of time. However, as discussed above, the porosity, the water content, and thus the original thickness of a sediment layer will normally decrease with increasing depth of burial because of gravitational compaction. The thickness of a sediment layer may also decrease due to dewatering during subaerial exposure. Therefore the X-ray densitometric technique is used to express the rate of sedimentation not only in terms of thickness per unit of time but also in weight of solids per unit of area and time, necessary especially when comparing sediment accumulation in a water reservoir with the inflow of sediment from its drainage area.

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