Numerical modeling of skirted foundation subjected to earthquake loading W. R. Azzam Tanta University, Tanta, Egypt Abstract. This paper explores the possibilities of using skirted foundation system to mitigate the liquefaction hazards, control the horizontal soil movement and decrease the pore water pressure underneath the foundation during the earthquake. This technique is investigated numerically using finite element analysis. Four stories reinforced concrete building resting on raft foundation is idealized as two- dimensional model with and without skirts. In the present study, two dimensional plane strain program PLAXIS, dynamic version is used. A series of models for the problem under investigation was run under different skirts depth below the foundation level. The results showed improved effectiveness in liquefaction mitigation due to confinement effect and decreasing horizontal soil movement. The presence of such skirts can modify and decrease the induced excess pore water pressure. In addition, the skirts can significantly reduce the foundation acceleration and the ground motion amplification is reversed. Keywords. Liquefaction mitigation, Foundation, Skirts, Lateral confinement Introduction It's recognized that the liquefaction is process involving energy dissipation due to frictional loss along grain contacts during cyclic loading as a result of increases in pore water pressure, that leading to collapse the soil structure and this information can be applied to develop methods for liquefaction mitigation. The energy required to cause liquefaction depends on the density of packing grains. Extensive damage to foundation and structures in areas of liquefaction and lateral spreading has been observed in many earthquakes around the world as presented by Chu et al., (2004). The factors that lead to liquefaction in the subsoil are important as reported by different authors e.g. Hatanaka et al. (1987) and Ishihara, (1993). Liquefaction mitigation can be done using a variety of soil improvement technique as dynamic compaction, vibro stone columns and drainage wicks (Dise et al., (1994) and Luehring et al., (2001)). Improving liquefaction potential strength by using micropiles and inclined reinforcement is also investigated by McManus et al. (2005); Naein and Moaye (2006). These techniques of soil improvement against liquefaction aim to increase the soil density and develop the ideal densifications that avoid large increases in pore water pressure. Based on this approach of densification, lateral confinement technique of soil underneath the foundation was adopted using structural skirts that fixed rigidly to the foundation edges. This technique was used in improving the bearing capacity and the settlement characteristics (Martinez et al. (2008); Azzam and Nazer (2010). Nevertheless, their effects in liquefaction mitigation cannot be scrupulously investigated. Consequently, a new alternative technique is suggested for liquefaction mitigation by lateral Proceedings of the 15th African Regional Conference on Soil Mechanics and Geotechnical Engineering C. Quadros and S.W. Jacobsz (Eds.) IOS Press, 2011 © 2011 The authors and IOS Press. All rights reserved. doi:10.3233/978-1-60750-778-9-113 113