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Stiffness degradation, G/G0, curves of Champlain clay at St-Adelphe, Quebec, and the associated variation of its damping ratio with shear strain are constructed in this study using the new combined triaxial simple shear (TxSS) apparatus. The apparatus offers the ability to obtain the stiffness (G/G0) and damping ratio of soils over a wide strain spectrum from 0.001% to 10%. The value of the small-strain stiffness of the tested clay is further confirmed through another series of piezoelectric ring-actuator technique (P-RAT) tests. Although, the stiffness degradation curve of the tested clay follows to some extent traditional curves suggested in the literature, the examined Champlain clay exhibits a different trend with respect to hysteresis damping, especially at large strains (>1%), and available analytical models could not successfully predict the damping behavior of the Champlain clay at such a strain level. A new constitutive model is therefore presented as a modification of the original Sig4 model considering the pore-water pressure built up with shear strain. Stiffness degradation and damping ratio versus shear strain curves of Champlain clays estimated using the proposed soil model are similar to their experimentally determined counterparts even at large shear strains where other models tend to misjudge the damping behavior of the clay.

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On May 21st, 2003, the north Algeria was stricken by a 6.8 magnitude earthquake which was felt over a distance of 250 km from the epicenter, which is localized in Mediterranean Sea at 10 km from the coast. During this event, several ground accelerations were recorded by the instrumentation network of the National Center of Applied Research in Earthquake Engineering (CGS). The records analysis revealed an important difference in peak ground acceleration (PGA) between two close stations (0.58g and 0.33g, respectively, in East-West direction) at about 20 km from the earthquake epicenter. Also, two other record stations, located in the Mitidja basin, at about 29 km and 86 km from the earthquake epicenter, respectively, showed a high level of acceleration: PGAs of 0.54g and 0.16g. So, the authors attempt in this paper to analyze these records through the characteristics of strong ground motions, the effects of different parameters such as damping ratios, soil conditions and epicentral distance on normalized response spectra. Also, the quantification of site effects during this earthquake is analyzed. Then, the authors compare the near-field mean response spectra obtained during this earthquake with the Algerian seismic design spectra (RPA99 – 2003 version) and with two other well-known design spectra: Eurocode 8 and UBC97 in order to contribute to the future revision of RPA99.

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