Study On The Influence Mechanism Of Meso-microscopic Pore Structure On Seepage Characteristics Of Shallow Loess

In this paper,taking the shallow loess from the southern tableland of Jingyang as the research object,the saturation-unsaturated permeability law of the undisturbed loess was obtained through the saturated permeability test and soil-water characteristic test,and the distribution characteristics of meso-microscopic pores of the undisturbed loess were identified.On this basis,the influence mechanism of the pore structure on the seepage characteristics of the shallow loess is studied.Finally,the seepage simulation under different scale pores is carried out.The main conclusions are as follows:(1)The saturated permeability coefficients of different loess samples with the same depth were different due to the structural anisotropy.The saturated permeability coefficient of the same sample gradually decreases with the continuous saturated seepage.On the whole,the saturation permeability coefficient decreases gradually with the increase of depth.The SWCC curve of loess can be divided into three parts: near saturation zone,transition zone and residual zone.With the increase of depth,the saturated water content of loess increases gradually,the range of near saturation zone increases gradually,and the air intake value,characteristic matric suction and residua l matric suction of transition zone also increase gradually.(2)In the micro-pores of loess,the proportion and distribution density of micro-pores increased gradually with the increase of depth,showing a sudden increase at 5m;The proportion and density of small pores first increased and then decreased,while the proportion and density of middle pores and large pores gradually decreased.Based on the characteristics of mesomicroscopic pores,the loess pores were classified and quantitatively defined.Through the analysis of various parameters,it was found that the proportion of various microscopical pores in the loess did not change significantly with the depth,and the distribution density gradually decreased with the increase of depth.The pore structure became more and more simple,and the connectivity became worse and worse.(3)The micro-pores in the microscopic pores of loess mainly affect the residual stage of SWCC curve and control the loss of bound water;Small and medium-sized pores mainly affect the transition stage and control the flow of capillary water.Macropores mainly affect the near saturation stage and control the flow of gravity water,while mesoscopic pores have little influence on unsaturated seepage.All kinds of connected pore densities are significantly correlated with the changes of saturated permeability coefficients.Among them,connected pores with coordination number of 2,pore-throat ratio of 1-3,and pore-throat length of 1-3mm have the highest pore distribution density,and have the strongest influence on the saturated seepage process.The increase of the proportion and distribution density of micro-pores will inhibit the saturated seepage,and the saturation permeability coefficient will decrease.(4)The overall flow velocity of water in micro-scale pores is slow,but the flow velocity is relatively fast at the corner of pores and the middle part of pore pipes.In the pore connectivity is poor,the flow rate is slow in the pores are narrow.Similar to the flow law of water in mesoscale pores,water flow in mesoscale pores has better uniformity,more regular form,and faster migration speed in longer pores.In the pores with poor connectivity,the phenomenon of "sticking" is easy to occur.The variation trend of the simulated permeability coefficient at meso-scale is similar to that of the measured saturation permeability coefficient,but it is much larger than the measured data,which may be related to the proportion of mesoscopic pores.