Laboratory Simulation Test And PFC^3D Numerical Analysis Of High Clay Dredger Fill In The Consolidation Process Of Step Vacuum Preloading

Dredger fill as a kind of fill deposit is pumped from waters to the shore or the construction site by dredger and mud pump. The sedimentary formation of dredger fill is gradually consolidated by different methods. And reclaiming land is a popular method with a long history all over the world. However, the special soil has many characteristics of high moisture content, high porosity ratio, high compressibility, low strength and so on. Drainage consolidation method still exists problems as follows. First, the surface of the dredger fill would become hard shell commonly in 2 or 3 years. So it is hard to apply with the engineering loading which is lack of time. Second, under the powerful vacuum suction effect, particles move with the water to drain pipes. And the large plastic pipes are surrounded with large fine clay of dredger fill and then are formed mud membrane, which cause lower consolidation efficiency. Clearly, traditional vacuum preloading method cannot meet engineering construction of high clay content dredger fill.In order to improve consolidation velocity of high clay dredger fill, this thesis researched on it step by step. First, compared several sample soils from different coastal regions to study the basic properties. And geology engineering characteristics of "three-high and two-low" of high clay dredger fill were definitely concluded, which means high moisture content, high porosity content, large compressibility, low permeability, low strength and so on. At the same time, geology engineering analysis showed other particular properties, like high clay content (high plasticity index), high liquid limited moisture content. Meanwhile, this provided the material basis and theoretical basis for further research on consolidation properties of high clay dredger fill.With the help of China Communication Water Transportation Planning and Design Institute Co.Ltd, straight-line vacuum preloading method was used to reinforce high clay dredger fill. In physical model test, making sure that pipes type and distance were main purpose in straight-line vacuum preloading process. Laboratory simulation test was taken to deeply analyzed on experimental data through monitoring basic physical properties, chemical properties and mechanical properties. During high clay dredger fill consolidation process, the distance from high clay dredger fill to drain pipes was shorter, the soil strength was higher. On the other hand, the settlement was smaller. Soil consolidation produced horizontal migration, which caused uneven settlement. Meanwhile, under the action of straight-line vacuum preloading method, high clay dredger fill produced consolidation drainage, greatly reduced the vacuum pressure relay frictional cost. Compression deformation and strength were increased, construction period was shortened, vacuum transfer efficiency and reinforcement effect were improved, that all reduced reinforcement expenses.Above experiments shortened the period of consolidation, but the phenomenon of drainage plugging was still serious. In order to improve efficiency of high clay dredger fill continually, the idea of step vacuum preloading (SVP for short) method was come ture based on the straight-line vacuum preloading method. And SVP method was proved staged and stepped vacuum degree with high clay dredger fill. So it was taken laboratory test, measurement and simulation research, deeply analyzed with experimental data through monitoring basic physical properties, chemical properties and mechanical properties, expounded advantage. Furthermore, it was improved SVP method significant which played important role in high clay dredger fill consolidation. Based on the results of SVP test, the soil moisture content continually reduced and liquid soluble migrated. Then, organic content decreased, cation exchange capacity reduced, PH value changed, soluble salt content declined, and sodium content reduced.Meanwhile, grey correlation method analyzed physical, chemical and granulometric composition indexes. It was found that the clay content changed with soil strength, which was the largest correlation in soil. Moisture content and organic content changed with soil strength were the less important correlation. That explained the growth of soil strength closely related with clay content in high clay dredger fill, confirmed feasibility of fine particles as the consolidation degree index. So, this conclusion assisted in strength increasing rule with high clay dredger fill.By two kinds of consolidation methods above, microstructure analysis was taken to research dredger fill. Scanning electron microscope and mercury porosimeter were used to analyze qualitative and quantitative characteristics of microstructure. High clay dredger fill has high moisture content and large void ratio at the beginning of consolidation, so the pore boundary value of microstructure were defined 40μm, 4μm, 0.4μm, 0.04μm respectively. Especially, it was analyzed from big pore to small pore, whose intervals were gradually changed from large to small. At the same time, fractal theory was used to calculate the fractal dimension of structural. It was found that the distribution fractal dimension of structural cell was in a growing trend, while the distribution fractal dimension of the pore was in reducing trend, which reflect a regularly structure changing rule of high clay dredger fill.Finally, in the view of practical size characteristics and drainage construction characteristics of high clay dredger fill, PFC3D numerical model was set up, under the action of straight-line vacuum preloading for the first time. It was reflected drainage consolidation characteristics under the joint action of different distances of two drain pipes. At the beginning of consolidation, fine particles discharged with flow water. The porosity of the dredger fill was larger at the side of drain pipes than the porosity in the middle of two pipes. Under these circumstances, the consolidation velocity decreased with the porosity increased. Meanwhile, another PFC3D numerical model was set up under the action of SVP for the first time, reflecting the advantage of SVP method for high clay dredger fill. Furthermore, combined with numerical simulation test, the porosity of the dredger fill was gradually decreased. And high clay dredger fill was consolidated with it. So, consolidation mechanism was explained and SVP method was confirmed feasibly.Above all, based on physical simulation experiment and numerical simulation analysis, a new method of SVP accelerated high clay dredger fill consolidation. The theory of SVP method was consummated to reinforce high clay dredger fill. An exploratory study was analyzed to improve high clay dredger fill consolidation efficiency. Furthermore, the mechanism of SVP method was illustrated, provided a theory basis and feasibility suggestions for engineering practice.