Study On Soft Soil Microstructure And Mechanism Of Seepage And Consolidation In Pearl River Delta (PRD)

The holocene marine soft soil with a thickness of several meters to tens of meters iswidely distributed in the Pearl River Delta region, which is mainly including of the silt, siltymud and clay. The soft soil has many unfavorable project properties such as high watercontent, high void ratio, high compressibility, low strength, poor permeability, remarkablestructural and rheological characteristics and so on, therefore the serious engineeringaccidents such as foundation settlement, instability occur frequently. In order to improve softsoil foundation’s engineering quality and reduce construction accidents, it is needed to furtherstudy the engineering properties of soft soil, raise the level of design and construction of softfoundation. But for a long time, people mainly focuses on the study of engineeringcharacteristics of soft soil on macroscopic level, which is difficult to catch the essentialfactors to make deep research on its engineering properties. Therefore, the soil microstructureanalysis on the engineering properties of soft soil is a kind of "tracing" method forunderstanding the engineering properties of soft soil in essence.With the natural or reconstituted soft soil in the Pearl River Delta(PRD) as the researchobject, this dissertation studies domain features and engineering properties of PRD’s soft soilfrom the statistical point of view, and analyses variation characteristics and fractalcharacteristics of some microstructure factors in seepage consolidation process of soft soil,such as particle shape, particle distribution, arrangement and connection, polymer morphology,pore number, pore shape, scale and distribution, connectivity and twists, etc. by the mineralcomposition test, ESEM test and mercury injection test. Furthermore, the microscopicparameters and macroscopic engineering properties of soft soil have been linked forexplaining the micromechanism that influences the soft soil characteristics. Finally, modifiedconsolidation model based on microscopic test is established, which is confirmed reasonablyand effectively by test results.The major achievements and conclusions of this dissertation are listed as follows:(1) Through the analysis of the cause of regional characteristics on PRD’s soft soil,which contains geological and hydrological environment, mineral composition, particlecharacteristics, microscopic characteristics and so on, it is found that micro-factors are decisive factors to cause special macro engineering characteristics. The laws of physical andmechanical indexes are studied by using "skewness and kurtosis method", in the future, whichwill provide the reference for the soft soil engineering in the region.(2) Local threshold segmentation method’s result of void ratio is more consistent withthe practice when compared with the global threshold segmentation method. As thecoordination of threshold selection in quantitative analysis of microstructure binarization,micropore characteristics test and three-phase diagram calculation is performed, it will solvethe transitional discreteness and human disturbance problems in microscopic quantitativeanalysis of test results to improve the reliability and accuracy of existing microscopic test.(3) In the process of consolidation, microstructure types of soft soil will change gradually,from the cellular structure (sponge structure) to the skeleton structure, turbulence structureand compact structure gradually. The connection mode transforms edge to edge, edge tosurface into face to face with structure unit area increasing and porosity decreasing; while dueto the lack of structuredness, particles of remolded soil blend together with relative density inthe form of polymer and clot structure, with the increase of load, the connection of particleswill be mainly transformed into face to face connection.(4)The microstructure quantitative parameters of structure units or pores has differentrules in different stages of consolidation. In the early stage of consolidation, the effectivestress is small, the microscopic structure is in a stable period of adjustment owing to silt’scertain structural strength, at this time, the small number of pores number, high void ratio anddominated by major and medium pores, the size, shape and orientation of changes of structureunits and pores are relatively smaller; When the effective stress increases to the structure yieldstress, structure is gradually destroyed and aggregate is crushed, microstructure reengineeringviolently, equivalent diameter of structure units increases significantly, the roundness and theaverage shape factor decreases significantly, change trend in narrow and perpendicular to thedirection of pressure form obvious orientation. At the moment, the number of pores increasessignificantly, the equivalent diameter becomes smaller, patterns become more complex, thepore distribution by the small pores, micropores to small, even ultrafine poredevelopment,,but its orientation has no obvious change; While effective stress continue toincrease, the new microstructure adjusts slightly and quantitative parameters of structure units or pores change is not obvious.(5) The structure unit and pore of natural or remolded soft soil in PRD has obviousfractal structure characteristics. Pore structure influence factor, which can reflectcomprehensive reflection of pore size, shape, distribution and orientation characteristics ofpore structure, has a good correlation with the load. The study shows that there is a closerelationship between macroengineering properties of soft soil and the microscopic parameters.Compressibility and permeability of consolidated soil can be made of natural andconsolidation change parts. Among them, natural part is related to the initial structuralcharacteristics (mainly pore characteristics), generally is also associated with multiplestructure parameters;while consolidation change part is related to changes in structuralcharacteristics (mainly pore characteristics)which is caused by consolidation pressure,eventually, consolidation change part can be expressed as the relationship with theconsolidation pressure.(6) Revised consolidation model is established on the basis of the micro test (empiricalformula) results, and its rationality and validity is verified by experimental results. Revisedconsolidation model based on soil microstructure, especially on the physical mechanism offine-pore structure characteristics, can reflect the nonlinear relationship between the soilconsolidation and the load.