پاسخ سیستمی رسوبات روانگرا
Abstract
1. Introduction
2. Investigated Christchurch sites
3. Representative soil profiles
4. Effective stress analyses
5. Analyses results for YY-profiles
6. Analyses results for NN-profiles
7. Results from simplified analyses
8. Conclusions
Acknowledgements
References
Abstract
Detailed geotechnical characterization and in-depth liquefaction assessment using seismic effective stress analyses are presented for 55 liquefaction case histories (sites) from Christchurch. Fifteen of the sites manifested liquefaction in the two major earthquakes during the 2010–۲۰۱۱ Canterbury earthquakes (YY-sites), while 17 sites did not manifest liquefaction in either event (NN-sites). The YY- and NN-sites are shown to have practically identical critical layer characteristics, with low CPT tip resistance (qc1Ncs = 80–۸۶), and shallow location of the critica.l layer at approximately 2 m depth. However, there are significant differences between the YY- and NNsites with regard to their deposit characteristics including the thickness and vertical continuity of their critical zones and liquefiable materials. Effective stress analyses are used to demonstrate key mechanisms of system-response of liquefying deposits that either intensify (for the YY-sites) or mitigate (for the NN-sites) liquefaction manifestation at the ground surface. The study illustrates the need to consider system-response of liquefying soils in the assessment of liquefaction manifestation and severity of liquefaction-induced damage.
Introduction
Simplified liquefaction triggering procedures used in current engineering practice have been developed based on case histories in which liquefaction was manifested at the ground surface during past earthquakes. Such liquefaction case histories essentially reflect the overall response of soil deposits during earthquakes, and their key characteristics with regard to the severity of liquefaction manifestation. However, despite the intent to capture the overall performance of the deposit at a given site, in the simplified liquefaction evaluation procedures each layer is considered in isolation, and a factor of safety against liquefaction triggering, maximum shear and volumetric strains are estimated separately, and independently, for each layer. In these calculations, interactions between different layers in the dynamic response, and through excess pore water pressures and water flow are ignored. Hence, principal mechanisms of interaction or system-response effects of liquefying deposits that potentially contribute to the severity of liquefaction manifestation are not accounted for in the simplified procedures. Liquefaction damage indices, such as LSN [22] and LPI [14,15] use specific weighting functions to quantify the damage potential of liquefying layers depending on their proximity to the ground surface, but still they do not account for cross-interactions between different layers during the development of liquefaction and post-liquefaction triggering.