| The unique sedimentary environment and climatic characteristics of the arid and semi-arid areas since the Quaternary have created its unique loess landform features and hydraulic properties.Malan loess（Q3）is at the top of the Pleistocene loess-palaeosol sequences.The permeability and micro-mechanism of Malan loess is unique compared with other types of loess,and Malan loess is often used as a natural material for agricultural production and engineering construction in loess areas.It has important theoretical and practical significance to systematically study its permeability of Malan loess and the microstructure changes in the process of penetration.At present,the methods for measuring the hydraulic conductivity of Malan loess are mainly divided into two aspects:laboratory and in-situ test.Different methods have similarities and differences,and the results between different measurement scales of the same method may also have differences.In order to systematically study the permeability of undisturbed Malan loess and its micro-mechanism,in this study we have measured the hydraulic conductivity of Malan loess through single and double ring tests of different sizes and laboratory seepage tests,and analyzed the influence of the size of infiltration meter on the results of in-situ test and compared the difference between the in-situ and laboratory tests.On this basis,the scanning electron microscopy（SEM）imaging were conducted on the soil samples of the horizontal and vertical surfaces before and after the laboratory seepage test,and the microstructure characteristics of Malan loess were qualitatively and quantitatively analyzed.The anisotropy and its penetration mechanism of undisturbed Malan loess were explored from a microscopic perspective.The results of this study are as follows:（1）Within the size of the infiltration meter in in-situ test,there is no significant correlation between test results and the size of the infiltration meter.The saturated hydraulic conductivity measured by the double ring method is between 0.0929 and 0.1672 m/d,with an average value of 0.1198 m/d.The saturated hydraulic conductivity measured by the single ring under the same seepage diameter is about twice of the saturated hydraulic conductivity measured by the double ring method.A double-ring infiltration meter with an inner ring diameter of 25 cm and an outer ring diameter of 50 cm is recommended to be used for in-situ tests in loess areas where the water transportation is difficult.（2）The results of the laboratory saturated seepage test show that the vertical saturated hydraulic conductivity of the undisturbed Malan loess is 4.13×10-2～6.31×10-2m/d,with an average value of 5.06×10-2m/d.The horizontal saturated hydraulic conductivity is 1.22×10-2～1.95×10-2m/d,and the average value is 1.48×10-2m/d.The vertical saturated hydraulic conductivity is about 3.45 times of the horizontal saturated hydraulic conductivity.Affected by the boundary,size and heterogeneity of the soil samples,test conditions and calculation methods,the vertical saturated hydraulic conductivity measured by the laboratory test is one order of magnitude smaller than that of the in-situ test,which is about 0.49 times the result of the double-ring method.（3）Under the combined influence of physical and chemical effects,the permeability of the undisturbed Malan loess showed a significant deterioration trend under the laboratory long-term constant head seepage conditions,and the deterioration trend was the most obvious in the early stage of seepage（the first 4-5 days）.The saturated hydraulic conductivity of the soil sample has a power function relationship with the seepage time as a whole.（4）Compared with the vertical plane,the area of overhead pores and large pores in the horizontal plane of Malan loess is larger and the horizontal plane has better pore connectivity,which is reflected by anisotropy of permeability.Under the penetration of water,the pore area ratio and pore size distribution range of the horizontal and vertical planes were significantly reduced.（5）Compared with the skeleton particles of Malan loess before the seepage test,the contact mode changed from the support contact to scattered contact.The type of cementation gradually changed from macropores micro-cement structure of scaffold to the flocculating gel structure.The type of pores has changed from overhead pores to inlay pores and cement pores,and the large pores are transformed into small pores,elongated pores are transformed into irregular pores and circular pores. |
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