| The Quaternary brand-new soft soil layer with thickness of tens of meters or even hundreds of meters is widely distributed in the Yangtze River Delta area.Because of its characteristics of high water content,large void ratio,high compressibility,low bearing capacity,poor permeability and rheology,it will not only produce large settlement after land formation,but also cause different degrees of compression of the underlying under consolidated sea land soft soil layer It causes a series of serious engineering problems,such as the uneven settlement of the ground,the loss of the filling elevation,and the secondary geological disasters.For a long time,people mainly evaluate the engineering characteristics of soft soil from the macro level in view of the huge economic loss brought by the land subsidence in the actual engineering,but this can not explain the essential law of the poor engineering performance of soft soil from the perspective of mechanism.Therefore,in view of the above problems,this paper takes the dredger fill and the underlying cohesive soil in the east beach of Chongming,Shanghai as the research object,based on the macro permeability test,combined with the micro mercury injection test and the scanning electron microscope test,analyzes the changes of the microstructure parameters of particles and pores in the consolidation and permeability process of the soft soil after the reclamation,and explains the micro mechanism affecting the permeability of the soft soil.Finally,the grey correlation theory is used to verify the correlation between the macro permeability coefficient and the microstructure parameters of the structural unit and pore,and the rationality and validity of the model are verified.The main research results and conclusions are as follows:(1)This paper introduces the physical and geographical conditions of Chongming area in Shanghai.Through the indoor test of soil samples of different depths in the east beach of Chongming,the composition characteristics and physicochemical properties of different sampling points and soil layers of different depths from west to East are obtained.The test results show that the soil in this area is weakly alkaline with PH value between 7.41 and 8.88;the primary mineral content of the soil at the sampling point is greater than that of the secondary mineral content;the primary mineral content of the shallow dredger fill and sandy silt layer is significantly higher than that of the deep clayey soil layer;the primary mineral content is the most,and the proportion of the illite and illite in the secondary mineral is the largest,and the content of the kaolinite is relatively high Less;the silt content is the main one,followed by the clay content,with the depth of 8m as the boundary,and the particle size composition of soil samples is different.When the depth is less than 8m,the dredger fill and sandy silt are mainly silty subsoil,belonging to low liquid limit clay CL;the clayey soil with the depth greater than 8m is mainly silty light subsoil and silty heavy subsoil,belonging to low liquid limit clay Cl and high liquid limit clay ch.(2)Based on the mercury intrusion test and SEM scanning electron microscopy,the microstructure characteristics of soil layers with different land forming time and depth were studied,and qualitative and quantitative analysis were carried out.The results of mercury injection test show that the pore size of soil samples at the sampling points is mostly distributed in the range of 0.4-4 ? m and 0.04-0.4 ? m,mainly in the range of 0.04-0.4 ? m,and less in the range of > 40 ? M,There are more macropores;SEM images show that the microstructure of soil layers in different depths is mainly agglomerated and flocculated,and in the longitudinal depth,the content of large particles in the dredger fill layer and sandy silt layer is more,and the structure is more loose;in the transverse section from west to East,with the increase of land forming time,the structural units and pores form an obvious directional arrangement in the vertical vertical vertical pressure direction At the same time,the small pores change to the big ones,the abundance and roundness increase,and the pores tend to be flat and round.(3)With the increase of consolidation time,the permeability coefficient of soil layers from west to East increases gradually.The statistical analysis of the relationship between the characteristic parameters of soil pore structure and the macro permeability coefficient obtained from the mercury intrusion test shows that there is a strong correlation between the size of permeability coefficient and the percentage content of macropores and the average pore size,but only a weak correlation between the size of permeability coefficient and the ratio of pores and the content of medium pores.(4)A grey correlation model based on microstructure parameters(including average diameter,average abundance,average roundness,average shape coefficient and average fractal dimension of structural unit and pore)and permeability coefficient is established.According to the quantitative index of microstructure parameters and permeability coefficient,the correctness and reliability of the model are verified by the measured results. |
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