Experimental Study On Deformation Characteristics Of The Deeply Pile-tip Soil Under Cyclic Loading

Beijing to Zhangjiakou high-speed railway is an important traffic link connecting the site of the 2022 Winter Olympic Games.Controlled by the regional sedimentary environment,there are thick sediment lacustrine muddy soil in Guanting section of Beijing Zhangjiang high-speed railway.During the operation of high-speed railway in th furture,the long-term cyclic loading is likely to cause large deformation and settlement of the soil at the pile-tip,thus laying hidden dangers for the operation of high-speed railway.The paper relies on the geological survey project of the geological survey of China,"Beijing-Tianjin-Hebei Cooperative Development Zone,Zhangjiakou to Bohai Bay active structure and regional crustal stability survey",through the data and field investigation,combined with the field investigation,the regional engineering geological conditions in study area and the engineering geological conditions of the Guanting section of Beijing to Zhangjiakou high-speed railway are systematically summarized.Based on the experiment results of the laboratory,the engineering characteristics of the silty soil in the deep buried section of Guanting area with different depths are studied,and the correlation between the composition of clay minerals and the physical and mechanical parameters of the soil is analyzed.The GDS Cyclic dynamic tri-axial test was conducted to study the dynamic characteristics of silt soil with deep embedded pile tip.Using the ABAQUS numerical simulation software,the tri-axial experiment is simulated,and the settlement of the s soil at the pile-tip under the long-term high-speed railway dynamic load is calculated.The main achievements of the study are as follows:(1)The geological structure in Beijing to Zhangjiakou area is complex and the fault is very developed.The fault activity is extremely strong and the historical earthquakes occur frequently in this area.The region engineering geological conditions are relatively poor.There are many active faults along the Guanting section of Beijing to Zhangjiakou high-speed railway,which pose a potential strong earthquake threat to the high-speed railway.At the same time,rainfall and human engineering activities have certain effects on slope stability along the high-speed railway.(2)Thick layer lacustrine silty soil is deposited in Guanting section of Beijing to Zhangjiakou high-speed railway,and the engineering properties are relatively poor.Because of the long history of consolidation and low compressibility,the deep buried lacustrine soil in Guanting area has better engineering properties compared with the surface muddy soil.But after being disturbed,the strength of soil may be greatly lost.Especially when the content of clay minerals in illite and montmorillonite mixed layer is high in lacustrine muddy soil,the engineering properties vary greatly.(3)The Cyclic dynamic tri-axial test study on dynamic characteristics of deep buried pile-tip soil,especially on dynamic deformation characteristics,is carried out.According to the results of the dynamic tri-axial test,first,for the same depth of soil loading the same frequency load,the greater the dynamic stress ratio is,the more obvious the dynamic strain is.Second,under the same dynamic stress ratio,low frequency load has more obvious deformation effect on the same layer of soil than high frequency load.Third,under the same dynamic stress ratio,the deeper the soil consolidation is,the more difficult to produce large deformation.The results of numerical simulation of dynamic tri-axial test are basically consistent with the test results of stable samples.It is shown that the boundary layer model can be used to simulate the deformation characteristics of stable soils.However,the simulation results of the destructive and critical soil samples are different from the experimental results.The long-term settlement of deep buried pile-tip prediction simulation results show that under the action of high-speed railway load,the soil of the 30 m layer is more prone to experience deformation,and the displacement of the soil at the end of the pile should be observed to ensure the safety of high-speed trains.