(1. Department of Geotechnical Engineering，Tongji University，Shanghai 200092，China；2. Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education，Tongji University，Shanghai 200092，China；3. Shanghai Harbour Soft Soil Treatment Engineering Co.，Ltd.，Shanghai 200092，China)
Abstract：Using technique of image visual tracing and non-target measuring，the meso-model tests of the rough nail-soil during pull-out in dense sand were carried out. The displacement field of the nail-soil contacts，trajectories of the sand particles，and shear strain of sand particles of contacts were analyzed during the process of pull-out of nail-soil. The result shows that in the first instance，the displacement motion of the particles is vertical and then was horizontal；also the particles have shear expansion action. The soil shear strain around the nail-soil changes in radial and regional role from top to bottom of the nail is symmetry. Based on the model test，the particle flow code in 3D(PFC3D) method was introduced to simulate the pull-out process of nail-soil，Firstly，the law of displacement field of contacts between soil and nail in model test was verified in meso-scale. Then，the PFC3D method is used to discover some changes regular of meso-mechanism，such as the porosity and coordination numbers during the process of the pull-out of nail-soil. It is revealed that the porosity of the sand around nail-soil reduced with the increase of the distance to the nail-soil；and the porosity contour of the sand in the end of nail-soil was gradually outward proliferation，which was irregular concentric circles；coordination numbers were the smallest around the nail and soil contacts，and gradually increasing to the outside. The study is expected to contribute to disclose the mechanism of soil nail pull-out and the development of contacts.
