Behavior of a single micropile in sand under cyclic axial loads

Numerical pile segment analysis is developed in this study with an advanced soil model to investigate skin friction behavior of a micropile installed in sand and subjected to both static and cyclic axial loading. Soil parameters used in the soil model are obtained from results of laboratory soil tests under both static and cyclic loading. Pile installation effects are also considered in this analysis.; Results of static numerical pile segment analyses for CIP piles including micropiles are compared with those from pile loading tests and design methods. Results of the static numerical analyses show good agreement with design methods which are based on data of pile load tests and experiences for various relative densities of sand and pile diameters.; Results of cyclic pile segment analysis are compared with those of cyclic CNS interface simple shear tests and a cyclic model pile test. Direct comparison between pile segment analysis and these tests are impossible due to different boundary conditions between them and limitations of pile segment analysis. However, same trends of cyclic shear behavior from pile segment analysis and these tests are obtained.; Results of numerical analyses for a micropile are compared with those for conventional CIP piles to obtain distinctive features of a micropile under both static and cyclic loading. Two advantageous features of a micropile are observed under both static and cyclic analyses. These are stiff static and cyclic shear stress-displacement relationship from its small diameter (around 0.2 m), and high static maximum side resistance and remarkable improvement of cyclic resistance from its grouting pressure regardless of characteristics of cyclic loading and relative densities of sand. Therefore, a micropile, with its characteristic features, may be useful for a seismic retrofit for existing foundations and it may be a very useful foundation for conditions in which control of settlement is the main issue under both axial static and cyclic loading.; Mechanisms causing cyclic degradation of skin friction and accumulated permanent displacement are found from investigation of changes in effective stresses and volumetric strains during cyclic loading.