ABSTRACT
An extensive geotechnical investigation was performed on an oil terminal site located in a seismically active field. Combination of in situ and laboratory tests were performed including Piezocone Penetration Tests (CPTu), some of which were adjacent to the borehole locations, providing a detailed stratigraphic soil profile and different physical and mechanical parameters. In this paper, the application of the CPTu test for analyzing cyclic liquefaction is evaluated and discussed. Both “liquefaction” and “cyclic softening” potentials are evaluated depending on the magnitude of Ic factor. A clayey silt layer was identified having an Ic factor of about 2.5 between 12 and 19 m of depth. The average qc of this layer was 6 MPa and friction ratio (RF) was 2.5. The induced pore pressure, u, was measured between 300 to 1800 kPa from 12 to 19 m of depth. The equivalent SPT counts from correlations with CPT were 15 while the real SPT counts were about 50 or higher. According to CPT data, the factor of safety of liquefaction was calculated having an average of 0.56 for a 0.49g earthquake with M=7. Undisturbed samples were taken from the layer onto which cyclic undrained liquefaction tests were carried out. The results, however, revealed that specimens were resisting the cyclic liquefaction with sufficiently high factor of safety. The soil classification in the laboratory showed that the soil was clay and not silty clay. Detailed analyses and interpretations are carried out to find out the reasons of the differences between CPTu correlations and real response of the specimens to cyclic loading. The results show that modifications might be required at times in interpretations of soil classification as well as liquefaction potential evaluation for soils at the borderline of low plasticity clay and silty clay.
