Ground Improvement Technics And Field Test Of Down Hole Dynamic Compaction Pile Composite Foundation

The passenger delicated railway from Zhengzhou toXi'an is newly constructed with two lanes and a speed of 350 km/h. It is characterized by high speed in operation, high standard in technology and strict requirements for subgrade, and the settlement control of subgrade has been one of the most difficult technologies. A composite foundation called down hole dynamic compaction pile is introduced to conquer this problem. Since it is firstly used in the high-speed railway to improve the foundation, its relative calculation theory and parameters, and its treatment effect are not all clear, so making researches on its relative calculation of settlement and its field test has highly practical significance.Combined with the practice of the passenger delicated railway from Zhengzhou toXi'an, the methods including theory analysis, numerical simulation analysis,long-term field test are adopted to investigate the settlement calculation, properties in practice and its improvement effect systematically.Firstly, the theory of settlement for down hole dynamic compaction pile composite foundation is introduced, including calculation methods of reinforced area, substratum and additional stress of pile's end. Then analysis is made on effect of settlement of subgrade with the change of height of subgrade and modulus of pile.Numerical analysis is adopted to study the cement-soil down hole dynamic compaction pile, to investigate the influence of different reinforced cushions, pile spacing, pile modulus and the pile aspect ratio on controlling the settlement of foundation, and the corresponding influence law is obtained.Secondly, long-term field test is practiced, the specific property of settlement and stress are received. The technique of settlement prediction are studied,through which the settlement after civil works is calculated.Meanwhile, it is verified that the settlement control is effective by the cement-soil down hole dynamic compaction pile in Zhengzhou-Xi'an railway.Finally, the construction technology and improvement effect of cement-soil down hole dynamic compaction pile are introduced, and by comparson with other methods in improving loess ground, this kind of composite foundation is technically feasible and economical in improving collapsible loess ground in Zhengzhou-Xi'an high-speed railway.