Study On Dynamic Compaction To Reinforce Foundation In The Yellow River Alluvial Plain

In the Yellow River alluvial plain, silt and silty clay are widespread. Severe differential settlement of freeway foundation would happen without reinforcement. Recently dynamic compaction has been widely used in ground reinforcement because of its economical efficiency. But the design parameters and construction methods of dynamic compaction should be based on quite many factors, such as the soil type and underground water level. So it is of great importance to study the reasonable dynamic compaction methods for the Yellow River alluvial plain, which will be useful for the design and construction of freeway in this area.Based on the Jile freeway construction engineering, two sections were chosen to do in-situ test to determine the reasonable parameters for a better design. Foundation settlement calculation was done by FORTRAN and FLAC3D. Then the calculation results were compared with the test data to analyze the effect of dynamic compaction on reducing settlement. Some field tests (static penetration, dynamic penetration) were also done after the reinforcement to evaluate the soil improvement. FLAC3D was used to analyze the effect of water table on dynamic compaction in the silt groud. We also studied the influence of water content on clay foundation during dynamic compaction. Then the treatment measures for dealing with water were summarized.The following results were found in the research:With the effect of1500kN-m dynamic compaction, the effective reinforcement depths of the two sites were6m and6.5m respectively and the effective reinforcement depth ratio were0.155and0.168respectively. In terms of the reinforcement width,1500kN-m dynamic compaction could affect more than6m, but the effective reinforcement width was between2-4m. When hammering one point, it had very little effect on the adjacent one. If one single point was hammered continuously, the liquefication possibility of surface soil is larger than that of deep soil. In order to avoid liquefication caused by dynamic compaction, detection of underground water level should be conducted. In general, the smaller the depth, the more the excess pore water pressure increased and the slower the excess pore water pressure dissipated. In the two sites, excess pore water pressure dissipated completely48hours after hammering. Settlement analysis showed that dynamic compaction could reduce more than40%of the total settlement. Comparing numerical results with the field settlement observation data, the settlement of reinforced ground is much less than that of natural foundation. Static penetration and dynamic penetration test showed that foundation bearing capacity was significantly enhanced after reinforcement within the the effective reinforcement depth. For silt foundation, the higher the underground water level, the severer liquefaction would happen. The equation y=0.449x0.280could reflect the relationship of critical water table depth and tamping energy. In order to ensure the good effect of dynamic compaction, well-points dewatering, dynamic compaction replacement and lime soil disposal method will be helpful if there is much unfavourable water. The ram pits should also be pluged-back immediately after hammering to avoid water filling.