A Study On Plastic-Drainage And Dynamic Compaction Composite Technique For Improving Deep Soft Soil Ground

Deep soft clay grounds formed from sea-phased sediment are extensively distributed along coastal lines of China. The soft soils are characterized by high moisture content, high compressibility, low shear strength and low permeability. These bad features results in many difficulties in design and construction of groundwork. Drainage consolidation method is usually applied to improvement of such a type of foundations. In order to reduce construction duration and speed up consolidation of foundation, sometimes dynamic compaction technique is used after preloading. As one of the key issues in design and construction of dynamic consolidation-drainage method, the pore water pressure during consolidation and drainage should be evaluated. However many technical problems remains to be unsolved well. Therefore in this thesis, a study on plastic-drainage and dynamic compaction composite technique for improving deep soft soil ground is made.Firstly, the existing calculation methods of ground consolidation with vertical drains are systematically summarized. The general patterns of pore water pressure dissipation during preloading with plastic drainage plates and post dynamic compaction is analyzed. By using existing consolidation theory of ground with vertical drains under instantaneous loading, the pore water pressure solution for ground consolidation with vertical drains is worked out. According to consistency of average consolidation degree, the ground with partially penetrated vertical drains is turned into an equivalent double-layered ground under varying load and then one-dimensional consolidation analytical solution can be adopted. The computed results are compared with numerical FEM solution obtained by using universal FEM computer package ABAQUS and the rationality of equivalent-double-layered ground method for calculating ground consolidation with partially penetrated vertical drains is demonstrated. It is shown through numerical example that multi-step loading can be simplified as single-step loading in preliminary analysis.Secondly, the mechanism of dynamic consolidation by drainage and the consolidation calculation methods during and after dynamic compaction are summarized and analyzed. Through analyses for a practical example, it is found that during dynamic drainage consolidation, the consolidation degree of ground after each dynamic compaction can be calculated by using the solution of the pore water pressure dissipation obtained under staticloading, in which the reconsolidation coefficient should be used. Such a simplified procedure can be used for construction control of dynamic compaction.