Performance of screw piles in thick collapsible loess

The bearing capacity of the composite foundation would be significantly increased by the squeezing effect of soil displacement screw pile. To investigate the effect of soil displacement screw pile treatment on the large-thickness collapsible loess foundation, full-scale field tests and pore expansion theory were used to compare the physical and mechanical properties of the soil around the pile before and after soil displacement screw pile treatment. It was found that the maximum influence range of a single pile in loess foundation was 3D, and the effective influence range was 2.5D. Compared with natural foundation, the average dry density of soil between two and three piles increased by 23% and 15%, and the compression coefficient decreased by 57% and 38%, respectively, when the pile spacing was 2.0D and 2.5D. The collapsibility coefficient and self-weight collapsibility of soil between two and three piles were far less than 0.015 when the pile spacing was less than or equal to 2.5D. The soil deformation caused by compaction was mainly plastic deformation. The plastic zone radius calculated by pore expansion theory can be used to estimate the compaction range of soil displacement screw piles in large-thickness loess regions. When the pile spacing was 2D and 2.5D, compared with the natural foundation, the characteristic values of soil bearing capacity between piles increased by 60% and 30%, that soil displacement screw pile of plain soil composite foundation increased by 220% and 200%, and that of plain concrete soil displacement screw pile increases by 450% and 400%, respectively. It shows that a soil displacement screw pile can significantly increase the bearing capacity of a large-thickness collapsible loess foundation. When the area replacement rate was 0.145-0.23, the pile-soil stress ratio of soil displacement screw piles in large-thickness collapsible loess region ranged from 2.6 to 4.7.