Application Study Of High Energy Level Dynamic Compaction In Thick Collapsible Loess Regions

Dynamic compaction method is widely used because of its simple equipment for construction, fast construction and less construction cost, as well as good reinforcement effect. But there is no complete theory to guide practice due to the various nature of the soil and the complexity of mechanism of dynamic compaction。Current research work is mostly concentrated on saturated soft clay and the low level of dynamic compaction.But for loess which is widely distributed in northwest China, especially for the deep thickness of unsaturated collapsible loess, the work lacks further research.The high energy level dynamic compaction of 15000kN.m was rarely used in domestic and the related standards have not provided the projections of reinforcement depth when energy of dynamic compaction was more than 8500kN.m.The trial ramming condition of dynamic compaction with energy of 15000kN.m in thick collapsible loess regions was introduced based on an engineering practice. Some reasonable construction parameters, curve fitting equations of ramming pit settlement with the number of ramming and the attenuation of vibration velocity peak with distance, as well as various buildings safe distance were put forward when dynamic compaction with energy of 15000kN.m was used in such loess field.The changes of the major physical and mechanical indexes of loess before and after the trial ramming were analysed through the laboratory test, cone penetration test and plate loading test. The interval time, reinforcement depth, the characteristic value of subgrade bearing capacity, deformation modulus and compression modulus, etc, was determined when high energy level dynamic compaction was used in the eastern gansu loess regions. The reinforcement mechanism was analysed and reasonable determination method of the compression modulus and deformation modulus were put forward.Based on doing consolidated-undrained triaxial test with the loess which was rammed with 15000kN.m and 8000kN.m, the types of loess stress-strain curve, the change rule of strength and deformation under different confining pressures were analysed, and the inhomogeneity of dynamic foundation soil was analysed. Test results show that it is not the higher levels of dynamic compaction the harder of the shallow foundation soil, but the thicker of soil be reinforced.Simulation for the first ramming process of high enegy level of dynamic compaction was done preliminarily by the finite element program of ADINA,and the problems which were found in the process of numerical simulation was put forward.This test has important significance for perfecting the consolidation mechanism of the collapsible loess with high energy level dynamic compaction and adding standard of its effective treatment depth. Also can provide according and reference for design and construction in dynamic compaction projects.