The Structutal Strength Formation Mechanism Of Seashore Dredger Fill And Key Technique Of Vacuum Preload Method

Tianjin Binhai New Area is another important economic development area following Pearl River Delta and Yangtz River Delta. With the change of investment strategy, it will be a new point of economic growth in China. Because the basic condition is sufficient, Reclamation has been the main method for Tianjin Binhai New Area to obtain land resource. The land resource obtained by reclamation may produce great economic efficiency more easily since it is close to harbor where the economy develops quickly.With the fast development of the Tianjin Binhai New Area city scale, the area of land resources obtained through filling sea increases continuously. Because fill material in Tianjin Binhai New Area is seabed silt,the hydraulic fill has a longer drained consolidation time,higher sensitivity, lower strength and larger settlement,which must be treated before engineering beginning. Vacuum preloading is a kind of appropriate hydraulic fill soft clay treatment method. But the method has such shortcoming as long treat period,and it must form crust on the surface of mud before beginning. At the same time,the research on the basic mechanism and the treatment process are still not clear.Taking the constructive hydraulic fill in Tianjin Binhai New Area as an example,The Structutal Strength Formation Mechanism of Dredger Fill in Tianjin Binhai Area and Key Technique of Vacuum Preload Method are studied in detail. The research result makes the theory of Structutal Strength Formation Mechanism of Dredger Fill clearly,and has important practical value to the construtive hydraulic fill treatment in Tianjin Binhai New Area.(1) The physical property test,chemical composition test,mechanical property test and original test of hydraulic fill in Tianjin Binhai New Area has been done,the research result shows that the clay pellet content of the hydraulic fill in Tianjin Binhai New Area reachs 45.5～50.1%,the hydraulic fill in Tianjin Binhai New Area has strong compressibility.low strength and poor permeability.(2) For evaluating of soil structural feature, referring to study results existed.soil structure potential index defined by the ratio of soil strength difference between before structure failing and after structure failing to strength before structure failing is proposed as the parameter to evaluating soil structural feature.At the same time, the classification standard of soil structure is put forward,it provides basis for soil structure classification.(3) For reseaching structural character of hydraulic fills in Tianjin Binhai New Area,according to sensitivity test, compression consolidation,shear and in situ cross sheer test result, structure parameters of different test condition are calculate,the result show the range of structure potential index of hydraulic fills in Tianjin Binhai New Area is 0.77~0.86,so hydraulic fills in Tianjin Binhai New Area is strong structure constitution.(4) For studing the water separate mechanism and deposition regulation of hydraulic mud,the settlement simulation test and micro-structure test are done.Micro-structure analysis result shows that the flocculent form clay pellet distribute at the top of settlement columniation,grain diameter is less than 0.005mm,the contact of of the grain isn't very obvious,the big void is seldom,In addition to clay particle,silt particle which diameter is bigger than 0.005mm is existed at the middle of settlement columniation,clay particle is adhere to silt particle,the void and cranny is extensive. the silt particle distribute at the bottom of settlement columniation, the void and cranny is very extensive.So it can be find that the silt particle sedimentation rate is quicker than clay particle. Grain sedimentation is accord with Stocke law,rate is proportion with the square of grain diameter namely.It can be gotten from microstructure parameter analysis that with the sedimentation depth Increasing,the quivalent diameter,perimeter and area of void and grain become larger of void,the number,shape ratio and roundness of void and grain become less, the orientation of the middle of settlement columniation is worse than the orientation of the top and bottom of settlement columniation.(5) For researching microstrcture change regulation of hydraulic fill treated by vacuum preloading and untreated,electronic scanning microscope is used to obtain the microscopic scanning picture of both original hydraulic fill and treated with vacauum preload,at the same time,the microstructure parameter is analyzied.the result shows that before treatment with vacauum preload, the hydraulic fill is very loose under the natural condition,void is is extensive.Flocculent structure is obvious, clay particle is existed in pellet,the contact of clay particles is blurry.After vacauum preload treatment,the structure of hydraulic fill becomes compact,void becomes fewer, clay particle stack each other, the contact of clay particles is very clear,the contact between grains is surface to surface.It can be found from microstructure parameter analysis that after treatment with vacauum preload, the quivalent diameter, area, perimeter,shape ratio and roundnes of void and grain become less,the numberof void and grain become larger,with the depth increasing,the same law exists.(6) The relationship between equivalent load ,effective stress increment, water contain, pore ratio and Structure strength is studied for researching the influence factor of structure strength during vacuum preloading,the result shows that effective stress increment and equivalent load are larger, Structure strength of hydraulic fill stronger, the water contain or pore ratio are larger, structure strength of hydraulic fill is lower.it is because that the effective stress increment is come into being under the equivalent loading.Under the effective stress, the soil structure is formed , and the connect between pellets is more and more close, gradually cementation and coupling come into being on the contact joint or interface, so stronger structure strength is formed.(7) The structural strength increase mechanism of hydraulic fill is researched. It is believed that the formation of structure strength of hydraulic can be divided into two part: hydraulic fill mud removing water and hydraulic fill concretion drainage, hydraulic fill mud removing water process includes pellets flocculate subsidence and fill mud deadweights concretion. The hydraulic fill concretion drainage process includes forepart air discharge, penetrate concretion in vacuum seepage field and water gasification discharge under the vacuum condition. The forepart air discharge stage develops very quickly, and the surface sedimentation rate is quick too, but the increase of structure strength of hydraulic fill is limited. The penetrate concretion rate in vacuum seepage and field the surface sedimentation rate is rapid. Under the vacuum seepage field coming from strong vacuum pressure, soil consolidates and the structure strength increases. The water gasification discharge under the vacuum condition develops very slow, the rate is controlled by environment temperature and vacuurm degree. The water is gasified and gets away, the water contain of hydraulic fill drops off, so ...the structure strength increases significantly. But this process is controlled by gasification process and develops slowly. The proportion of the time spending on the water gasification discharge process to the whole time that vacuum preload needed is great, so the hydraulic fill treatment period by vacuum preload is very long, and the same time, it's cost is very high.(8) For quickly treating freshly hydraulic fill by vacuum preloading, the improving thinking of three key technique including surface crust substitute layer, drainage shutter insert technique and level drainage system is provided. Surface crust is artificially built,mending drainage shutter insert machine existed through Lower height, minishing weigh,improving dynamical equipment and mending level drainage system through replacing sand layer by other material.thus, level drainage system can be used as surface crust substitute layer.(9) For validating the feasibility of vacuum preloding in treating freshly hydraulic fill, surface crust substitute layer and level drainage system is mended and optimized.The result shows that no-weave sting cloth combining with engineering gridding is fully feasible to substitute surface crust, the drainage shutter insert depth can't exceed 10 meters, and combination no-weave sting cloth with high strength sponge is the best level drainage system. So vacuum preloading can be used to treat with freshly hydraulic fill. Comparing with vacuum preloding now used ,mended vacuum preloding can save about 40% early constrction cost of ,and shorten constrction time 6 month.