| If clay minerals(montmorillonite,illite,kaolinite,etc.)interact with water,bound water of certain thickness will form around their grains.It is because of bound water that clay' s permeability is totally different from that of other types of soil.Thus,clay becomes an indispensable natural permeation barrier material for water conservancy and hydro electricity engineering,landfill engineering,and nuclear waste disposal engineering.Hence,it is of great theoretical significance and practical value to make in-depth study on the permeation mechanism of clay.In this paper,the influence of bound water on the permeability of clay was analyzed first,an effective void ratio formula was derived on this basis,and an empirical formula of permeability coefficient suitable for clay was obtained by modifying the commonly-used empirical formula of permeability coefficient of coarse-grained soil;a series of experimental analyses were carried out on cohesive loess from Qingyang and clay from Hefei,having verified the rationality of the empirical formula of permeability coefficient of clay;then,the micro pore structures of Qingyang cohesive loess and Hefei clay were observed with scanning electron microscope,and relevant pore parameters were obtained by processing the images;the influence of the micro parameters on.the permeability of clay was discussed in combination with the fractal theory.What we have done and the achievements are as below:(1)A two-dimensional ideal model of soil granule with 0.12?m thick bound water was built.Calculations show that:different from coarse-grained soil,most pores of clay are occupied by bound water;thanks to the presence of these ineffective pores,the void ratio of clay is large,but its permeability coefficient is low.To make a quantitative analysis of effective pores,four formulas for calculating the effective void ratio were derived on the basis of the three-dimensional theoretical models of different grain sizes and the macroparameters of soil,ana an empirical tormuia of permeability coefficient of clay was obtainea by modifying the typical empirical formula of permeability coefficient of coarse-grained soil.According to the comparison of advantages and disadvantages of the four calculation methods,it was recommended to calculate the effective void ratio of clay with the consistency factor method.(2)Four soil samples of Qingyang cohesive loess and Hefei clay of maximum liquid limit and minimum liquid limits were taken for thermal-gravimetric experiment,and the coefficients of the proportion of the content of bound water to the liquid limit of the four samples were calculated in combination with the experimental results.After ?0 was determined,the permeability coefficients of all samples were calculated out with the empirical formula of permeability coefficient of clay.Comparison showed that there was little difference between the calculations and the experimental values,having verified the rationality of the empirical formula of permeability coefficient of clay.Relevant physical parameters of different types of clay were extracted from literatures of other researchers and put into the unmodified and the modified empirical formulas to calculate the values of permeability coefficient,the values were compared with the average experimental values,and the comparison results also had verified the rationality of the empirical formula of permeability coefficient of clay.Since the deduction process of the Kozeny-Carman permeability coefficient formula is relatively strict,and the physical meaning of the parameters in the formula are clearly defined,it is recommended to adopt the Kozeny-Carman formula that is modified with the consistency factor method as the empirical formula of permeability coefficient of clay.(3)Observation on the microstructure of 14 soil samples was carried out with scanning electron microscope,and a lot of information contained in the SEM images was extracted.Through data processing,the two-dimensional void ratio and three-dimensional void ratio of the samples in the SEM images were calculated out.In the event that the SEM images were scaled up by 1000 times,the calculation results had little difference from the values of void ratio obtained via macroscopic experimental method.Some of the parameters of the pore area were extracted with IPP(an image processing software)by separating the soil granules from pores in the SEM images that were scaled up by 1000 times,and the effective void ratio of the observed clay was finally calculated out.The relative error between the results and the values of effective void ratio calculated with the consistency factor method is small.(4)The size and distribution of effective pores of clay satisfy the fractal scale and the probability density function.Thus,the formulas of the fractal dimension Df of total pore space,that Daw of bound water,and that De of effective pores were derived according to the fractal theory,of which all parameters have definite physical meaning.The analytical solutions of average sinuosity calculated by Yu and Li were modified on the basis of the effective void ratio,and a formula for calculating the average sinuosity of effective pores of clay was obtained on this basis.Connection between the sinuosity of effective pores and the fractal dimension Dt of sinuosity was established.The parameter relationships established as above were utilized to modify the recognized Hagen-Poiseulle equation,and a fractal expression for calculating the permeability coefficient of clay was built on this basis.The 6 parameters in the model have definite physical meaning,and don't include other empirical coefficients.(5)The fractal model of length distribution of effective pores in clay was deduced and verified,a program was developed with fish language,the effective pore network model was imported into UDEC with the program to calculate the equivalent permeability coefficient,and the empirical equations of the correlation between the permeability coefficient and the fractal dimensions Dl,Dt and D? were developed respectively.The mathematical expressions of the correlation between Dl and the length,mass and density of effective pores,the correlation between D? and the effective void ratio,and the correlation between LD and Dl were derived.168 smaller effective pore network models with different rotation angles were extracted from the original model,numerical calculation was performed,and an ellipse of equivalent permeability coefficients of effective pore network models in correspondence with different Dl values was obtained on this basis.The influence of the fractal dimension Dl and the side length of model on the direction of the equivalent permeability coefficient was calculated and analyzed.The ellipse of equivalent permeability coefficients can be used for estimating and calculating the directions and values of the maximum and minimum equivalent permeability coefficients of a clay region. |
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