Sand-steel interface behaviour under large-displacement and cyclic shear

Large-displacement shear appears between pile shaft and its surrounding soil during the penetration process, and cyclic shear is the typical feature in subsequent service load condition. In this paper, a series of interface shear tests have been conducted to investigate the sand-steel interface behaviors under large-displacement and cyclic shear. Three types of particles (quartz sand, carbonate sand and steel bead) and two normal boundary conditions (constant normal load, CNL; constant normal stiffness, CNS) were used in these tests. To simulate the large-displacement shear process for monotonic installed pile, the large-displacement shear is performed using monotonic shear method. It is found that under the large-displacement shear with CNL condition, the quartz sand and carbonate sand show different breakage degrees and modes. Obvious particle breakage changes both the particle gradation and sample densification, which are the main reasons for the increase of interface friction angle. While for the cyclic shear with CNS condition, the interface strength shows a quick degradation feature, which is mainly affected by the initial shape of particles. It is also found that the previous interface shear plays a key role in this kind of degradation. When the shear displacement is small, the degradation is promoted with only a small amount of sand breakage. But if the sand breakage is significant under the large-displacement shear, abundant broken fine particles fill into the void and make the sand sample much denser. So, the degradation trend of interface strength will be suppressed. It is suggested that the previous large-displacement shear process for displacement pile should be adequately considered in the design. •Under large-displacement interface shear, quartz sand and carbonate sand show different breakage modes and degrees.•The cyclic degradation of interface strength can be obviously affected by the previous large-displacement shear.