An intensive experimental study was carried out to investigate the causes of the overconsolidation phenomena in the Namak New City site, Korea. Geological events in conjunction with sea level changes, groundwater fluctuation, weathering, leaching and cementation were examined as possible causes of overconsolidation. In addition to oedometer and triaxial tests, radiocarbon dating, geochemical components and pore water chemistry were analysed. It was shown that the overconsolidation in the upper marine clay deposit was not caused by stress reductions such as erosion and groundwater fluctuation. Furthermore, weathering or leaching seemed unlikely. The apparent overconsolidation can be explained by cementation, which is indicated by profiles of the sulphate and potassium ion concentrations in the pore water. At the early stage of the Holocene marine deposition, the rate of deposition was 4·4 mm/yr; thereafter it was reduced to 1·4 mm/yr, controlling chemical processes such as sulphate reduction and ion exchanges and, in turn, apparent overconsolidation. Some mechanical and chemical properties of the apparent overconsolidated clay were compared with the geologically overconsolidated Palaeosol layer in the site.

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A stress-path testing program was proposed and performed to investigate shear strength and deformation behaviors due to an open cut excavation in overconsolidated clay. Stress changes due to the staged excavation for different locations of the slope were estimated and applied on undisturbed specimens using a triaxial testing system. Axial load, chamber pressure, and back pressure were actively controlled to implement the various stress paths. Vertical and horizontal deformations were monitored during the stress paths, and then an undrained compression or extension shearing stage followed. Test results for three cases from the testing program: the crest and the toe of the slope, and the base of the excavation are presented. The clay deposit was found to be non-expansive, based on the deformation behaviors. Effective stresses appeared to govern the vertical and horizontal strains. Failure envelopes for compression and extension were established from the stress path testing. The mobilized undrained shear strength from the stress path approach is substantially lower than one from the conventional method.