Experimental Study On Vertical Bearing Capacity Of Single Pile In Coral Sand Foundation Under Cyclic Loading

Coral sand is a special geotechnical medium mainly developed in the tropical Marine environment.It is widely distributed in the coastal areas from 30° N to 30° S latitude and on the islands of the Pacific and Indian Ocean,especially in the South China Sea.Coral sand is mainly composed of coral remains and other Marine biological debris deposited in situ or after transport,with high content of calcium carbonate.The special development environment,material composition and structure lead to the special physical and mechanical properties of coral sand,such as high porosity ratio,high compressibility,high friction Angle and easy breakage.With the increasing demand for energy and the increasingly tense political situation around the South China Sea,it is expected that China will continue to vigorously develop and construct wind power generation,airports,docks,housing and other military and civilian facilities in the South China Sea in the future.These structures will inevitably encounter calcareous sand foundations.Pile foundation,as an ancient foundation form,has many advantages such as high bearing capacity,strong adaptability to geological environment and small post-construction settlement,and has been widely used.For wharves,offshore oil platforms and other offshore off-shore structures,will be subjected to wave load;Airport runways and road foundations are subjected to traffic loads.Under the action of these cyclic loads,the coral sand around the pile may have particle breakage,leading to a decline in the bearing capacity of the pile foundation,which may further cause the destruction of the super-structure.A large number of field engineering cases show that the traditional pile foundation bearing theory is not applicable to coral sand.Through model test and numerical simulation,this paper explores the vertical cyclic bearing characteristics of piles in coral sand foundation under cyclic load.The main research contents and relevant conclusions are as follows:(1)The repeated loading and unloading tests of pile foundation in coral sand foundation were carried out.Under the repeated loading and unloading,the axial force of pile body would redistribute.Under the unloading condition,the pile segment near the top would produce negative friction resistance,and the negative friction resistance area would expand downward with the increase of loading and unloading amplitude.Repeated loading and unloading will make the axial force distribution of pile foundation "steepen" in the loading state,that is,the pile side resistance near the top will decrease in the process of repeated loading and unloading.When the load is small,the pile side resistance will play ahead of the pile tip resistance.With the increase of the load,the ratio of the pile side resistance decreases gradually,and the ratio of the pile tip resistance increases gradually.Compared with the static load condition,under the same load action,the ratio of pile tip resistance bearing load is higher in the condition that the load has undergone 20 times of loading and unloading for each stage.The ultimate side resistance under repeated loading and unloading condition is 12.27 k Pa,which is less than that under static load condition 13.03 k Pa.(2)The model test of bearing capacity of single pile in Coral Reef Sand Foundation under cyclic dynamic load was carried out.The results show that: under the cyclic load,the load is shared by the pile side and the pile end,the load will transfer to the pile end in the process of cyclic loading,that is,the pile side resistance will weaken,and the corresponding pile end resistance will increase.The weakening of side resistance mainly occurs in the first 500 cycles,and then the side resistance tends to be stable.In addition,for the first loading pile,due to the velocity effect of cyclic load,the average lateral resistance of the pile will weaken from a value greater than the static load condition,and stabilize at a value less than the static load condition.The development of cyclic cumulative settlement is affected by the magnitude of static bias load and cyclic amplitude: under the same static bias load,the larger the amplitude of cyclic load,the faster the development of cyclic cumulative settlement,and the more difficult it is to achieve stability;when the amplitude of cyclic load is less than static bias load,the existence of static bias load is conducive to restraining the occurrence of cyclic cumulative settlement.(3)The pile shaft axial force will gradually shift downward and laterally with the increase of the number of cycles.It indicates that the lateral resistance weakens at the pile segment near the pile top,and the weakening area will gradually expand downward with the increase of the number of cycles.The change of pile axial force is more obvious at the initial stage of cyclic loading,and the redistribution of axial force tends to be stable gradually with the increase of cyclic loading times.From the meso-mechanism point of view,the weakening of lateral resistance is mainly caused by the crushing and migrating redistribution of coral sand near the pile-soil interface during loading and unloading process,and the effective horizontal stress is reduced due to the soil shear shrinkage.The weakening of pile side resistance,the depth effect and the increase of soil modulus of pile bottom make the pile tip resistance increase in the process of cyclic loading.Continue static load test on pile foundation after cyclic load,and it will be found that the pile top will obtain greater stiffness,and the ultimate static bearing capacity of pile foundation will also be increased to a certain extent.(4)The bearing characteristics of pile foundation under cyclic loading are simulated by finite element method.The results show that: the magnitude of the cyclic load amplitude will affect the distribution of axial force and lateral resistance after the cycle,the maximum negative friction area and the neutral point position will shift downward with the increase of the cyclic amplitude;the maximum negative friction value will also increase between 20 k Pa and 30 k Pa with the increase of the cyclic amplitude.When the loading frequency is less than or equal to 1 Hz,the accumulated settlement during 20 cycles is almost not affected by the loading frequency;once the loading frequency is more than 1 Hz,the accumulated settlement will increase significantly with the increase of frequency.Increasing or decreasing the stiffness of pile has little influence on the distribution of axial force and lateral resistance after cyclic loading.Through the static load test of the pile foundation after the cyclic load,it is found that the pile top obtains greater stiffness,and the ultimate static bearing capacity of the pile foundation will also be improved to a certain extent.