Study On Bearing Capacity And Settlement Characteristics Of DDC Pile Reinforced Loess Foundation For High-speed Railway

With the continuous improvement of China’s high-speed railway running speed, the requirements of roadbed settlement and foundation bearing capacity is more strict. Especially passenger dedicated line constructed in the loess area, it has been eliminated the collapsibility of loess to meet the requirement of high standards about the roadbed settlement after construction. As a new foundation treatment method, Down hole deep compaction which evolved from soil compaction pile method and dynamic consolidation. The process of the method in the treatment of the collapsible loess foundation can effectively reduce the porosity,eliminate collapsibility foundation, and engineering application is very extensive. The method uses a special high kinetic energy hammer for deep dynamic ram, smashing and forced lateral extrusion effect on brusque to make the pile very dense, the soil between piles under the strong lateral compacting stress also begins to release on the pile. The pile has great lateral restraint, largely enhancing the pile side friction to make the reinforced piles and the soil between piles to form a dense overall. The bearing behavior of composite foundation improves significantly after treatment, which can reach general effect that the flexible pile foundation reinforcement difficult to obtain.Although DDC piles has been widely used in engineering practice, but its theoretical research has been fall behind the engineering practice. Based on the foundation treatment of Borland Passenger Line in Lanzhou hub, bearing capacity and settlement characteristics of DDC pile that reinforced loess foundation is studied, main aspect is as follows:(1) Discussed the reinforcement mechanism of DDC pile composite foundation and the influence factors on bearing characteristics, the current settlement calculation theory of DDC composite foundation is summarized.(2) Experimental study on the improvement of the pile filler of DDC is done. By compaction test, optimum water content and biggest dry density of improved loess are determined. By compression test, the improved loess compression level is mastered. By the unconfined compressive strength test, the factors that affect the strength of improved loess is obtained. By triaxial shear test, shear strength index of the improved loess is studied.(3) Judging from the results of DDC field test, the size of pile composite foundation bearing capacity and load of composite foundation settlement curve are determined by the plate loading test. The blow count of SPT on piles and the soil between piles is got by the standard penetration test. The situation of composite foundation pile-soil stress ratio is mastered by pile-soil stress ratio test. By the load test in water immersion, the deformation of composite foundation settlement under flooding conditions is studied and different range moisture content of the soil samples around piles after without water is measured, calculatingmoisture penetration range in test pit surrounding. Through the test results of piles and the soil between piles, many important indicators include compaction factor at different depths of the pile, compaction coefficient and the average dry density are obtained.(4) By the finite element software MIDAS/GTS, the model of DDC single pile composite foundation and pile group composite foundation are respectively established. The settlement deformation and load transfer law of composite foundation in the vertical loads are simulated, and the distribution curves of axial and pile side friction is obtained. Through deeply analysis of these curves, the load transfer mechanism of DDC pile composite foundation is concluded. Finally, the influence of imposed load, pile length, pile modulus and cushion thickness to the bearing capacity and settlement characteristics DDC pile composite foundation are studied.