Study On Treatment Technique Of Soft Loess Foundation Under High Embankment

Questioning the treatment technique of soft loess foundation under high embankment, the characteristics and diseases of soft loess foundation are analyzed through on-site survey and data statistics. Through in situ tests, static loading tests, static sounding, finite element and theoretical analysis, treatment technologies of soft loess such as heavy tamping, sand column of vibration diving casting and crowd dense, geocell cushion, cement DJM pile and pile-raft composite foundation are studied systematic.On-site survey and data statistics show that soft loess of Gansu province is mainly distributed in the area with low terrain, high level of ground water and poor drainage conditions. The thickness of soft loess is 4.5~22m. The soft loess shows complex properties of soft soil, loess and saline soil.Field tests of dynamic consolidation on soft loess foundation indicate that the best tamper number under the condition of 2000kN·m dynamic consolidation energy is 7~9. Horizontal influence range is 8m and effective treatment depth is 5m. As a result dynamic consolidation method is only suitable for foundation with thin soft loess.Field tests of foundation treatment with sand column of vibration diving casting and crowd dense state that the age of column has distinct impact on composite foundation bearing capacity and the length of pile and pile distance have less impact on bearing capacity. Pile distance has distinct impact on displacement and strength of soil among piles. Stress ratio of pile to soil is 1.5~2.5. As a result the treatment effect of soft loess foundation with sand column of vibration diving casting and crowd dense is better.The FEA and on-site tests of foundation treatment with geocell cushion indicate flexible geocell cushion restrains the lateral deformation of ground within the limits of 50 percentage of its thickness and reduces stress at the surface of roadbed by 40 percentage of original stress. Reinforcement effectiveness using geocell raft is better for superficial soft loess foundation with the thickness less than 4 meters. Economical thickness of geocell is 20 centimeters. At last according to the theory of Winkler the formula calculating bearing capacity of ground reinforced with flexible geocell cushion is derived.The FEA and on-site tests of foundation treatment with cement DJM pile show that cushion can harmonize deformation of pile and soil and reduce the stress in the soil under it, and equate damping rate of stress in soil under load plates with different area. The rational thickness and modulus of cushion is put forward. The length of pile within effective length has remarkable influence on settlement. Stress ratio of pile to soil is 3~8, which significantly related to the shape of loading plate. Under flexible foundation the soil among piles destroys firstly and leads into the destruction of composite ground. Quasi modulus of pile and soil among piles hardens at first, then softens and stabilizes at last. The formula of bearing capacity and differential settlement between pile and soil is proposed on the base of elastic theory. At last, design method as well as construction craft and quality control specification of the technology of reinforcing soft loess foundation under high embankment with pile-raft system is proposed.