A study of effectiveness of ground improvement for liquefaction mitigation

摘要

Our ability to identify the existence of soil liquefaction potential is now better than our ability to know how to mitigate it economically and effectively. Based on a recent survey conducted by the Deep Foundation Institute (DFI) on liquefaction mitigation, remedial design methods and their effectiveness verification are regarded as \u22somewhat to highly non-uniform\u22 by the majority of the geotechnical engineering community in the U.S.Many recent reconnaissance reports also indicate that previous remediation design of ground improvement for liquefaction mitigation is not as reliable as expected. Hence, the lack of a uniform framework to evaluate and compare improvement effectiveness is an important factor leading to the insufficient or inefficient remedial design for liquefaction mitigation by ground improvement. An efficient, representative and comprehensive collection, evaluation and comparison of quantitative effectiveness data is a great challenge in liquefaction mitigation practice and the key issue is the establishment of a uniform evaluation framework. These expectations and objectives comply well with the requirements of an evolving evolutionary seismic design guideline termed Performance-Based Design (PBD). The process of establishing such an evaluation and comparison framework is also a process of re-evaluation of the improved performances and seismic design optimization, within the framework of PBD.To establish the uniform framework, a comprehensive numerical study is conducted to identify the failure mechanisms of a well-documented case history; an unimproved caisson quay wall in liquefiable soil reported in the Kobe earthquake in 1995. After the calibration of numerical model based on the case study, in the second step, various remedial methods including the stone column method, vibro-compaction method and deep soil mixing method are evaluated to improve the performance of this specific quay wall. For each analyzed countermeasure, a comprehensive parametric study is conducted to optimize the remedial design by determining the optimum design parameters. Eventually, all of the improved performance data or termed Engineering Demand Parameters (EDPs) in terms of quay wall seismic deformation and performance grades are plotted together to show the difference of improvement effectiveness achieved by various examined cases. These cases differ in the use of remedial methods and/or design parameters. The results are also used to rank the analyzed remedial methods and optimize their designs.In addition, as a stand-alone product of this study, a simplified chart method is proposed to estimate the improved deformation of caisson quay walls placed in liquefiable soil. Based on the method, the improved deformation of the walls after an earthquake can be reasonably estimated with the input of peak ground acceleration (PGA) of ground motion, improvement zone dimensions and improved soil properties.The results of this study show that failure of the examined caisson quay wall is induced by the deformation of the foundation and backfill soils. For all the analyzed remedial methods, improving the top 10 to 15 m of foundation soil under the wall and first 20 to 25 m of backfill soil behind the quay wall shows the best improvement efficiency. The improved performances of the quay wall are estimated to be acceptable, which only requires reasonable restoration effort to fully recover the damage under the earthquake motion with a probability of exceedance of 10 percent during its life-span. Different remedial designs using various methods are classified into three categories depending on their improved performance grades. Future research is recommended to include verification, implementation and updating of the proposed framework to advance the state-of-the-art of liquefaction mitigation using ground improvement.