Model tests on bearing behavior of pile groups in collapsible loess ground under water immersion

The artificially prepared collapsible loess is used as the similar material to carry out 2×2 pile group model tests under water immersion.The change law of collapsibility deformation and the load transfer mechanism are analyzed.The results show that with the increase of soaking time, the relationship between the cumulative collapse settlement and the time mainly includes the initial gentle section, immersion steep drop section, medium gentle section and gentle section after water cut.With the increase of collapse depth, the negative frictional resistance develops from top to bottom, and the depth of neutral point moves down gradually.When the settlement achieves stability, the neutral point depth ratio is 0.7.In large-thickness collapsible loess, when the depth of collapsible loess is determined by the laboratory experiments, and considering the working load on pile top, the neutral depth ratio of negative friction resistance can be determined by the recommended value of technical specification for building pile foundation.In the steep drop period of water immersion for collapsible deformation of soil layer, the negative friction resistance acting on the pile increases rapidly, which leads to the increase of downward load and pile end load and gradual increase of the settlement of pile foundation.Collapse of soil layer presents a synchronous trend with the negative friction of pile side and the occurrence and development of pile foundation settlement.