Study On The Characteristics Of Bound Water And Secondary Consolidation Of Cohesive Soil In The Southeast Of Chongming Island

Shanghai is not only one of the core cities in China,but also the economic center of China.With the rapid economic development of Shanghai and the influx of a large number of external population,its land resources are becoming increasingly tense.With the needs of coastal city construction and opening-up and exchange development,hydraulic reclamation has become the main way of land expansion.After the dredger fill becomes land,its engineering nature is poor.At the same time,it will further compress the underlying soil layer and aggravate the settlement of the lower soil layer,which is easy to cause engineering disasters such as land settlement and damage of upper buildings.According to the existing research results of the research group,it is found that there are a large number of uneven settlement phenomena in the southeast of Chongming Island,and there are large and thick sandy silt layer,muddy clay layer,clay layer and silty clay layer under the dredger fill,and the viscous soil layer is the main compressible layer.In order to further understand the settlement and deformation mechanism in this area,this paper takes the cohesive soil layer underlying the dredger fill in the southeast of Chongming Island,Shanghai as the research object,carries out the test from the perspective of soil bound water and microstructure,carries out the secondary consolidation test,and analyzes the characteristics of soil bound water and secondary consolidation.The main research contents and conclusions of this paper are as follows:(1)The engineering geological conditions in Chongming area of Shanghai are known through consulting data and field investigation and sampling.The composition characteristics and physicochemical properties of soil samples of muddy clay layer,clay layer and silty clay layer at different sampling points are obtained through laboratory tests.It is found that the natural water content of cohesive soil layer is high and the void ratio is also high;The content of silt is the highest in soil particles,followed by clay;The minerals in each soil layer are mainly primary minerals,in which the content of quartz is the most,and the absolute content of clay minerals is: Clay > muddy clay >silty clay.(2)Through mercury injection test,it is found that the content of pores in the samples of muddy clay layer,clay layer and silty clay layer is very high,indicating that the pores of cohesive soil in the southeast of Chongming Island are concentrated in 0.4-4?m aperture interval.Through the electron microscope photos,it is found that the soil samples of muddy clay layer are mainly flocculation agglomeration structure,the soil samples of clay layer are mainly matrix agglomeration structure,and the soil samples of silty clay layer are mainly skeleton agglomeration structure.The soil quality of the clay layer is relatively uniform,and the micro parameters of the structural unit body change little;For mucky clay layer,the structural unit of soil samples from holes CG30 and CG31 is closer to circular,while the soil samples from holes CG29 and CG32 are closer to strip shape and more regular in shape;For the silty clay layer,the structural unit of soil samples in the early land forming area is closer to the circle than that in the new land forming area,but it is more irregular and the average diameter is smaller.(3)Based on the principle that the freezing point of bound water is different from that of free water,the unfrozen water content of soil sample in the process of freezing and thawing is measured by nuclear magnetic resonance method,and the freezing law of bound water is found.The unfrozen water content when the temperature of soil sample is-0.5? in the process of thawing is taken as the natural bound water content of soil sample.It is found that the bound water content of clay layer is the highest,that of muddy clay layer and that of silty clay layer is the lowest.For the same soil layer,the bound water content of soil sample is related to the material composition,microstructure and the properties of pore water solution.(4)The secondary consolidation characteristics of cohesive soil are analyzed through one-dimensional consolidation creep test.The secondary consolidation coefficient of clay layer is larger as a whole,and that of muddy clay layer is the lowest.Taking the average value of the secondary consolidation coefficient under all levels of pressure from the soil samples of the clay layer,it is found that the bound water content of the clay layer has a strong positive correlation with the secondary consolidation coefficient,a negative correlation with the average diameter of the pores,and a positive correlation with the average shape coefficient of the pores.This is because the pores in the old land area are smaller and more complex,and the bound water content is higher,so the secondary consolidation deformation is greater.With the profile from west to East,the time required for the completion of the main consolidation gradually decreases,indicating that the clay in the newly formed land area has completed the main consolidation and entered the secondary consolidation stage in a short time,and the proportion of the secondary consolidation in the newly formed land area is also higher.This is due to the different microstructure between the newly formed land area and the old formed land area.