Model Test Study On Bearing Capacity Of Single Pile Under Cyclic Loading In Loess Foundation

Pile foundation is a common form of foundation.Because of its many advantages,it is widely used in the loess area.When the pile foundation is used as the foundation of industrial workshops,roads and railway subgrades and bridges,it is often subjected to vertical cyclic loading.The influence mechanism of cyclic load on the bearing capacity of pile foundation is different from the influence mechanism of static load.In this paper,the indoor model test are used to study the bearing characteristics of the single pile of loess foundation under cyclic loading.In the laboratory,the model container,data acquisition system and loading and control system used for the model test were built,and the test model pile and the soil material around the pile were prepared,which is laid the foundation of the experiment.Then carried out the static load and cyclic load loading test under water non-soaking conditions,static load and cyclic load loading test under water-immersed conditions,collected test data of pile body strain,vertical displacement of pile top and pile top load,the control variables are the immersion condition,the number of cyclic load actions and the cyclic load ratio.Finally,the static load and cyclic load test data,water-immersed and non-immersed load test data were compared,analyzed and summarized,the conclusions are as follows:(1)After analyzing the data of the static load test,it is found that the static load bearing capacity of the saturated loess foundation single pile is significantly reduced compared with before the water immersion,and the ultimate bearing capacity is 40% of the ultimate bearing capacity under non-immersion;the immersion condition will change the ratio of the pile end resistance and the pile side friction resistance to bear the load.The resistance at the pile end increases faster under immersed conditions than under non-immersed condition,and the proportion of the load on the side friction resistance of the pile after saturation is less than before immersion.After saturated with water,the ultimate frictional resistance of the pile side only accounts for 37% of that before water.(2)After analyzing the data of the cyclic load test,it is found that the cumulative settlement of the pile top increases with the increase of the number of cycles,but the speed of settlement development will gradually slow down.The displacement amplitude of the pile top cycle will change with the increase of the number of cycles.Under non-immersion conditions,the first 10 cycles will increase slightly and then decrease;under the condition of saturated water immersion,the displacement amplitude of the pile top cycle gradually decreases in the first 10 cycles and then increases,but none exceeds the displacement amplitude of the first cycle.With the increase of the cyclic load amplitude,the final cumulative settlement,the ratio of the cyclic displacement amplitude to the dynamic and static displacement ratios increase accordingly,which shows that the cyclic load amplitude is the key factor affecting the vertical displacement of the pile top.(3)After comparing and analyzing the results of static load test and cyclic load test,it was found that in the three cyclic load ratio tests,the dynamic and static displacement ratios of pile foundations in loess under non-immersed conditions are all less than 1;and the dynamic and static displacements of pile foundations in saturated loess,the minimum ratio is 1.76,and the cumulative settlement of single pile in saturated loess is more affected by the cyclic load.Compared with the static load,the axial force of the pile body is redistributed under the continuous action of the cyclic load.(4)Comparing and analyzing the results of the cyclic load test under immersed and nonimmersed conditions,it is found that there is a ratchet effect of single pile under cyclic load,and the ratchet effect of the pile foundation in saturated loess is more obvious than that in unsaturated loess.The redistribution of the axial force of the pile body after immersion is more obvious than before immersion.The cumulative displacement of the pile top after immersion in water is 11.4 to 12 times that before immersion,and the maximum cyclic displacement amplitude is 1.99 to 2.3 times;the settlement of the pile top after being submerged is more difficult to stabilize.The cumulative displacement of the pile top before immersion is mainly generated in the first 1000 cycles,and the cumulative displacement of the pile top after immersion is mainly generated in the first 3000 cycles;the cumulative displacement of the pile top after immersion in water needs more cycles and longer time to slow down the growth.