Experimental Study On Mechanical Properties And Microstructure Of Nansha Soft Soil

Soft soil is the most common porous medium material in engineering construction,which is widely distributed in coastal areas,middle and lower reaches of rivers,river mouth impact plain and other areas.Its physical and mechanical properties are very complex,with obvious structure,outstanding regional characteristics,high water content,large porosity,low permeability,small strength and other characteristics.It is mainly composed of solid clay particles,organic matters,minerals and liquid water.Nansha is located in the front of the alluvial plain of the Pearl River Delta,which is widely covered with soft soil layer with thickness of tens of meters.In September 2012,Nansha became the Sixth National New Area approved by the State Council.With its development and construction rising to the level of national strategy,various large-scale municipal engineering and building complex are constantly planning and construction.In order to study quantitatively the influence of the structure of Nansha soft soil on its macro properties from the micro perspective,and further deepen the understanding of the structure of Nansha soft soil,this paper adopts the method of combining macro experiment and micro experiment to analyze the influence mechanism of the micro structure characteristics of soft soil on the macro strength index of soil,and the influence mechanism of the micro pore characteristics of soft soil on the macro permeability characteristics of soil A modified Darcy seepage model considering the micro pore characteristics of soft soil is established to fit and verify the seepage model.The main research work and results of this paper are as follows:(1)Through direct shear test,triaxial compression test and unidirectional dynamic triaxial test,it can be seen that the shear strength indexes of Nansha undisturbed soil and remolded soil with different structures,such as cohesion c,internal friction angle ?,shear stress,main stress difference and dynamic stress change range,are quite different,which mainly shows that the strength indexes of undisturbed soil are significantly higher than those of the two remolded soil.(2)Through the analysis of the microstructure characteristics of soft soil,it is found that there are differences between undisturbed soil and remolded soil in the directionality,shape characteristics of soil particle units and the microstructure formed by stacking of soil particle units.The soil particle units of undisturbed soil have more concentrated directionality and more complex shape outline,and the soil force skeleton with higher ultimate strength will be formed after stacking The strength index of undisturbed soil is higher than that of remolded soil.(3)According to the seepage test and seepage consolidation test,there are obvious differences in the permeability characteristics of the same soft soil with different structure,among which the permeability coefficient of Nansha undisturbed soil is only 17%-29% of that of its remolded soil;compared with the permeability coefficient of soft soil under different consolidation pressure,it can be seen that with the increase of consolidation pressure,the permeability coefficient of soft soil will gradually decrease,but the permeability of soft soil with different structure will gradually decrease The change of permeability coefficient is also different,which shows that the decrease of permeability coefficient of undisturbed soil is much smaller than that of remolded soil.(4)Through the analysis of micro pore characteristics of soft soil,it's found that there are great differences in pore size distribution and pore complexity between undisturbed soil and remolded soil,the smaller pore accounts for more in undisturbed soil,and the pore complexity of undisturbed soil is higher,while the above micro pore characteristics of soft soil will affect the difficulty of pore fluid flowing through the pore to varying degrees.Degree the higher the proportion of small pores,the smaller the pore diameter,the more tortuous and complex the pore channel,the more difficult it will be for the pore fluid to flow through the soil,resulting in the phenomenon that the permeability coefficient of undisturbed soil is far less than that of remolded soil.(5)A modified Darcy seepage model considering the pore size distribution of soft soil and the barrier effect of water film on pore fluid is established.The rationality of the model is verified by fitting the permeability coefficient of soft soil under various consolidation pressures.It is proved that the modified Darcy seepage model is more consistent with the actual situation of low permeability soft soil.