The Micro-CT Experimental Research On The Deformation Of The Soil Pore In Pressurization Process

As an important environmental problem,land subsidence influence the economic development has been widely recognized the whole society.In the past related research,the soil porosity change caused by the compressionin the process of sedimentation could only be given a qualitative understanding nor quantization;then the porosity and the hydraulic conductivity are usually default to the constant in many land subsidence and groundwater coupling models.But this assumption has a big discrepancy with the actual situation resulting in the final output to lack fidelity.This paper addresses the following aspects: The first is taking the soil samples besides the D7 ground fissure in Xi'an conducting the Saturation-compression test in order to simulate the effective stress increases during the exploitation of groundwater.Digital images were provided by X-ray computed tomography scanner and pores were recognized by digital picture processing technique.The second is analytical studies for the pore deformation via the change of pore parameters under different stress state.In the third part,it mainly studies the spatial variability for the parameters of pores and the rules of succession in the state of the increasing stress by introducing the multifractal theory.The forth is simulating the change of permeability and hydraulic conductivity with the chaging stress by the combination between the fractal theory and the Darcy's Law.The main conclusions were as following:(1)The number of soil pores has significant positive correlation with the increasing effective stress by the exploitation of groundwater,the increasing rate of the number in macropores and the elongated pores were lower than other types of pores.The macroporosity and porosity of elongated pores had significantly decrease as the increase of effective stress.The average area of macropores in the borehole of YGH-1 and YGH-2 declined 39.05% and 9.22% which would hinder the water movement between pores.While the coarse mesoporosity remained about the same and might even increase.So it had shown that a strong corrleration between the compression process of soil by the exploitation of groundwater and the decreasing area of macropores.(2)The pore circularity had a negative correlation with the pore diameter(area).The larger pores always had the more irregular shape.The pores which diameter larger than 1000?m were almost composed of elongated pores while irregular pores' diameter were between 200 and 500?m and the regular pores were dominant in the pores had a diameter smaller than 200?m.The percentage of pores with the diameter >1000?m declined as the increase of effective stress.(3)The multi-fractal parameter was simulated to find that only the 4-128 pixels pores can satisfy the multi-fractal theory.The dimension spectrum D(q)can reflect the pores' degree of variation,the curve of D(q)became flat with the larger pressure,indicated that the complexity of pore distribution gradually decreased.(4)The curve of ?(q)-f(?)was left-hook reflected the small pores dominated but uneven distributed while the macropores had a concentrated distribution.The width of multi-fractal spectrum(35)? was gradually smaller with the increasing pressure.The decrease extent had a rank of silt>silt clay>clay.(5)According to the fractal feature,coupled with the revised Hagen-poiseulle equation and the Darcy Law,the theoretical formula for permeability was established.The fractal dimension of area Df and the maximum pore diameter ?max had a positive correlation with the permeability and in inverse proportion of the fractal dimension of pore tortuosity DT.The larger value of DT enhanced the difficulty of the moisture movement.The rank of the parameter influence for permeability was DT>?max> Df through the parameter sensitivity analysis.The hydraulic conductivity fall off along with the increased pressure with a maximum drawdown of 71.08%.The multivariate linear regression analysis model was conducted between the parameters of the pore structure and the hydraulic conductivity.The results showed that the maximum pore area affected most and followed by the total porosity.