Apparent Overconsolidation

Effect of geological history on the compressibility of NC clays. Note (1) o is equivalent to (2) a, is equivalent to (After Bjerrum, L., Embankments on soft ground. Proceedings of the ASCE Conference on Performance of Earth-Supported Structures, 2,1-54,1972. Reprinted with permission of ASCE.) 

Elder and Sills (1984) conducted a series of laboratory experiments to investigate the consolidation process. They also showed that if a sedimenf has experienced a reduction in volume due to creep, then it will appear to be apparently OC. In more general terms. 

In addition to the above, the volumetric expansion of sea wafer from a depth of 4268 m to the surface due to pressure release is approximately 2%. 

The effect of water depth on the effective stress has been discussed by Skempton (1960). He showed that the effective stress was a function of several variables (1) compressibility of the soil particles (Cf), (2) compressibility of soil sfrucfure (C), (3) water pressure (p), and 

Category Description Stress History Profile In Situ Stress Condition Remark s/Reference 

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Bjerrum (1972) presented Figure 8.19 in which he proposed that the secondary consolidation over a protracted period of time results in an apparent overconsolidation. In the example that he showed, the apparent overconsolidation would therefore be the difference between and as shown in the following equation ... 

A schematic representation of the apparent overconsolidation effect on otherwise NC sediments is shown in Figure 8.20. 

Schematic representation of apparent overconsolidation effect on otherwise NC sediments. Note o is equivalent to o v (From Chaney, R., and Fang, H.Y., Static and dynamic properties of marine sediments A state of the art. In Marine Geotechnology and Nearshore/Offshore Structures. STP 923, Chaney, R.C., and Fang, F1.Y., eds., ASTM Press, Philadelphia, PA, pp. 74-111,1986. Reprinted with permission. Copyright ASTM.)...

Although it cannot be concluded based on the above limited sample that all deep ocean sediments exhibit this apparent overconsolidation effect, there are some indications that this might be the case. Certainly, anomalies will be found to exist, particularly in areas of submarine canyons and their associated sediment fans, where substantial variation in consolidation characterization should be expected. 

Summary of Apparent Overconsolidation Behavior in Marine Sediments... 

A summary of typical overconsolidation behavior in marine sediments is presented in Table 8.6. A review of this table shows that the available data are variable and no definitive statements can be made on the parameters (i.e., sedimentation, depth of apparent OCR)... 

The consolidation state of the sediment below this cap of apparent of overconsolidation material appears to be dependent on the sedimentation rate if data from the East Bermuda Rise are deleted. In areas of low sedimentation such as the North Pacific, the sediments below the cap are NC. By contrast, for areas of high sedimentation such as the Eastern SOHM Abyssal Plain and the North Bermuda Rise Plateau, the underlying sediments are UC due to the presence of the cap restricting drainage (Silva et al., 1976). The decrease in the thickness of the apparent OC zone in the areas of high sedimentation is possibly due to the development of excess pore pressures. 

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