Study of Preconsolidation Pressure Values
Derived by the Modified Casagrande Method


Ioannis N. Grammatikopoulos

Assistant Professor,
Laboratory of Soil Mechanics and Foundation Engineering, Geotechnical Engineering Division, Department of Civil Engineering, Aristotle University of Thessaloniki, Thesssaloniki, Greece


The degree of settling occurring to a clay layer bearing a certain stress level strongly depends on the amount of maximum pressure the soil had previously been subjected to, the pre-consolidation pressure. Therefore, laboratory investigation of pre-consolidation pressure       s'v-max values for soil settling estimation is of vital importance.

It is known that the value of s'v-max is derived from the cross plot of the final void ratio versus logs'v, for each load increment during consolidation trials. Such time intervals take into account initial, primary and secondary consolidation, probably caused by gradual soil structure resettling or by progressive inter-particle bond failure due to the pressure applied. Actually, in practice, compression characteristics of in situ soils differ from those obtained from oeodometer tests conducted on soil specimens; in many cases, secondary consolidation in the field appears to be negligible in relation to the value derived from laboratory measurements. In such cases, the final void ratio for each load increment corresponds to the volume change at the end of primary consolidation (U=100%), which results in a total change of the e-logs'v and, consequently, of the s'v-max value.

The main objective of this laboratory research study was to investigate the difference between s'v-max values derived from volume change at 24 hours and from volume change at the end of primary consolidation. A significant number of reconstituted soil specimens were used in the experiments, containing different clay to fine-sand ratios. All soil specimens were subjected to pre-consolidation pressure before initiating oedometer tests.

Analysis of experimental results revealed that s'v-max values calculated on the basis of volume change at the end of primary consolidation are higher than those calculated on the basis of volume change at the end of 24 hr loading. In some cases this difference was as high as 20%.

Keywords: over-consolidation, clay, fine sand, oedometer test.

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