Bearing Capacity Characteristics And Differential Settlement Of Composite Subgrade In Collapsible Loess Region

The characteristics of sub-grade bearing capacity and differential settlement are the main problems in high-speed railway research.The bearing capacity and differential settlement of sub-grade are greatly influenced by the form of sub-grade treatment,considering the particularity of loess and the different thickness of loess in the collapsible loess area.The rational selection of sub-grade treatment methods can improve the bearing capacity of sub-grade and reduce the settlement of sub-grade.Pile network composite sub-grade is one of the most widely used treatment methods in high speed railway sub-grade treatment.The thickness of mattress layer,pile diameter,pile length,pile spacing,pile pattern and pile cap are the main factors affecting the bearing capacity and differential settlement of the sub-grade.There are many studies on the bearing capacity and settlement of the pile network composite sub-grade,but less on the bearing capacity and differential settlement of the composite sub-grade in the collapsible loess area.In this paper,bearing capacity characteristics and differential settlement of composite subgrade was studied based on a case study of Quzi town test section in Yinchuan-Xian high speed railway in collapsible loess region.Based on current norms and field tests,the characteristics and differential settlement of the bearing capacity of CFG-soil-cement compaction pile composite sub-grade and pile thrusted expanded in column-hammer composite sub-grade are studied in this paper.The main research results are as follows.(1)Based on the current code,the bearing capacity eigenvalue and settlement of two kinds of composite sub-grade in the test section of Quzi town are calculated.The finite element model(FEM)of two kinds of composite sub-grade was established,and the pile stress,pile soil stress ratio,pile side friction resistance and soil arch effect were analyzed by simulating the embankment filling of two kinds of composite sub-grade.The load test of two kinds of composite sub-grade single pile is carried out by the method of slow speed maintenance load,and the load settlement of different piles,the logarithm of settlement load and the sedimentation time logarithm are obtained.The reliability of the finite element model is verified.The variation law of the bearing capacity characteristics of two types of composite sub-grade is obtained.The neutral point of the friction resistance of the pile is at 2/3 of the pile body,and the pile-soil stress ratio from the pile top to the height of 2/3 the CFG pile is relatively larger.The pile-soil stress ratio of soil cement compaction pile and pilethrusted-expanded in column-hammer is close to 1.The effective height of soil arch is about 0.8m,which is close to 0.8 times of pile spacing.(2)Through the current norms,FEM and field test,the settlement and change law of CFG-soil-cement compacted pile composite sub-grade and pile thrusted-expanded in column-hammer are analyzed.When the composite roadbed single pile is loaded to464 k Pa,the settlement of pile thrusted-expanded in column-hammer is the largest,the CFG pile is the smallest,and the soil-cement compaction pile is between them.Namely,under the same load,the settlement of the composite sub-grade with CFG-soil-cement compaction pile is less than the pile thrusted-expanded in column-hammer(3)The train action principle of CFG-soil-cement compacted pile composite sub-grade in Quzi Test section is artificial number-setting excitation method,the vertical dynamic stress of lime-soil compaction pile is obtained about twice times the vertical dynamic stress of CFG pile while simulating the load amplitude of 4s;The neutral point of friction resistance of CFG pile is located at 2/3 of pile body;The effective height of soil arch between soil cement compaction pile is 1 time times of pile spacing,and there is no cylindrical soil arch between CFG pile,but vertical stress is equivalent in the top and 1m of CFG pile bottom.The vertical dynamic displacement of CFG pile is 8.8692mm-8.8699 mm,the vertical dynamic displacement of the soil-cement compaction pile is 18.0292mm-18.0304 mm,and the vertical dynamic displacement between the piles is 18.9048mm-18.9060 mm.That is,the load action time of train is 4s,and CFG pile,lime-soil compaction pile and vertical displacement of soil pile are relatively small when the embankment is completed.