Seismic behaviour of micropile systems I. Shahrour* and I. Juran y *Laboratoire de Me ´canique de Lille, Universite ´ des Sciences et Technologies de Lille, France; y Urban Utility Center, Polytechnic University, Brooklyn, USA This paper includes analysis of the performance of micro- pile systems under seismic loading. It presents results of both centrifuge model testing and three-dimensional finite element modelling carried out within the FOREVER research programme. The experimental and numerical results were analysed to study the effect of the main controlling parameters of the micropile–soil–structure system, such as the kinematic interaction, the group effect, the influence of micropile inclination, and performance assessment of micropile systems in liquefiable soils. Analysis shows that micropile systems offer effective engineering solutions for construction in seismic areas. Keywords : centrifuge; finite element; inclined; lique- faction; micropiles; seismic Cette e ´tude est constitue ´e d’analyses de performances pour des syste `mes a ` micropiles soumis a ` des charges sismiques. Elle pre ´sente les re ´sultats d’essais sur mode `le centrifuge ainsi que les re ´sultats d’une mode ´lisation en trois dimen- sions d’e ´le ´ments finis, le tout effectue ´ dans le cadre du programme de recherche FOREVER. Nous avons analyse ´ les re ´sultats expe ´rimentaux et nume ´riques afin d’e ´tudier l’effet des principaux parame `tres de contro ˆle du syste `me micropile-sol-structure, comme l’interaction cine ´matique, l’effet de groupe, l’influence de l’inclinaison des micro- piles et afin d’e ´valuer la performance des syste `mes a ` micropiles dans des sols lique ´fiables. Les analyses mon- trent que les syste `mes a ` micropiles offrent des solutions d’inge ´nierie efficaces pour la construction dans des zones sujettes aux se ´ismes. Notation a acceleration D diameter E Young’s modulus f 1 fundamental frequency of the soil layer f st frequency of the superstructure f load frequency of the seismic H horizontal loading L micropile length M bending moment M* normalised bending moment [ M=(r p D 4 p a g )] m st mass of the superstructure N axial force p pore pressure S micropiles spacing S/D spacing ratio T shearing force T* normalised shearing force [T=(r p D 3 p a g ] u lateral displacement z depth Æ micropiles inclination í Poisson’s ratio r density æ amping factor Æ9 v0 nitial vertical effective stress cap relative to the cap g relative to the ground movement head relative to the micropiles head max maximum value p relative to the pile s relative to the soil st relative to the superstructure Introduction Micropiles are used as foundation systems for new construc- tion and for underpinning of existing structures. Their great interest lies in the ease of installation, in particular in an access-restrictive environment. They can be used in all soil types and ground conditions. Micropiles are also used for ground reinforcement. In this case, both vertical and inclined micropiles are used to create in-situ a geocomposite material and thereby to improve the mechanical properties of the natural soils, in particular soil stiffness and resistance. Analysis of foundation damage induced by the Kobe earthquake in 1995 showed that steel piles with small diameter presented a better resistance to seismic loads than large diameter reinforced concrete piles (Tokimatsu et al., 1996). In addition, post-earthquake observations in Loma Prieta (Bardet et al., 1996) and Kobe (Tokimatsu et al., 1996; Gazetas and Mylonakis, 1998) showed that damage to structures founded on inclined piles was significantly less than for structures with vertical pile group foundations. These observations suggest that the use of micropile groups in seismic areas, in particular in sites presenting liquefaction risks, such as the site of Kobe, presents the advantages of flexibility and ductility. Further, the use of micropile networks as ground reinfor- cement is of particular interest for poor liquefiable soils. The reinforcement results in an apparent ground cohesion and increased soil stiffness, reducing the soil movement and, consequently, the risk of liquefaction. Herbst (1994) reported on the seismic performance of micropiles in Italy and Ground Improvement (2004) 8, No. 3, 109–120 109 1365-781X # 2004 Thomas Telford Ltd (GI 3185) Paper received 8 August 2003; last revised 4 February 2004; accepted 24 February 2004