| Qian’an County lie to the northwest of Jilin Province in China,The region is geographically located in the hinterland of the Songnen Plain.As a typical seasonal frozen soil area in China,it is also located in the semi-arid grassland climate zone.The annual evaporation is far more than the annual rainfall,and the freezing period is as long as 6 months.This extreme climate condition with high temperature difference,high evaporation and high cold makes the soil salinization serious and strong dispersion in this area.Therefore,engineering problems such as road grouting,frost heaving and thawing settlement,shallow sliding and instability of slope,foundation erosion and cracking failure often occur in engineering construction.The occurrence of these engineering problems is often related to soil-water interaction,and the pores in saline soil as the occurrence space of water,which closely affects the engineering geology of soil.Therefore,it is of great significance to study the variation law of pore characteristics of saline soil under different conditions for mastering the engineering geological properties of saline soil,providing experimental basis for water conservancy and geotechnical engineering construction,and serving social production activities.In this paper,the saline soil in Qian’an area is taken as the research object.The pore characteristics of undisturbed soil in the study area are tested by mercury intrusion test,and the variation law of pore characteristics with depth is obtained.On this basis,in order to better meet the needs of practical engineering,remolded soil samples with different moisture contents and compactness were prepared to study the compressibility characteristics and pore variation law of soil.The main research results of this paper are as follows:（1）The pore characteristics of Qian’an undisturbed saline soil were tested by mercury intrusion method,and the distribution characteristics of pores at all levels in the profile were obtained.With 0.04μm,0.4μm,4μm and 40μm as the boundary aperture,the pores of saline soil are divided into micro,small,medium,large and ultra-large pores according to the aperture interval.The test results show that the soil pores in the freezing depth range are generally dominated by large pores,followed by medium pores.The content of ultra-large pores is only high in the surface layer and is lower than that of medium pores as a whole.The content of micro and small pores is low and fluctuates in a small range.The porosity of 40 cm soil shows significant difference,and the middle porosity content of this layer is as high as 56%.Macropores and ultra-macropores are also significantly lower than other depths of soil.The results of field investigation,indoor particle analysis test and mercury intrusion test show that the apparent morphology,particle size composition and pore distribution characteristics of the undisturbed profile of saline soil show the same three-layer layered characteristics.With the increase of depth,the number of peaks of the pore distribution curve of the three layers of soil decreases.The first layer is the soil in the depth range of 0-80 cm,and there are three peak intervals.The second layer is the soil in the depth range of 80–150 cm,and there are two peak intervals.The third layer is the soil in the depth range of 150–200 cm,and there is only one main peak interval.The analysis shows that the compaction effect of gravity stress on soil structure,the migration of water and fine particles caused by evaporation and leaching,and the freeze-thaw cycle caused by climate change are the three major factors affecting the pore distribution characteristics in the profile of the study area.（2）Based on the data of mercury intrusion pressure and volume obtained from mercury intrusion test,Menger sponge fractal model and thermodynamic fractal model were used to quantitatively characterize the complexity of pore structure.By comparing the results of the two fractal dimensions,it is found that:（1）Thermodynamic fractal dimension can characterize the complexity of pores more comprehensively in the larger pore size range,and the fractal dimension has a strong positive correlation with the content of micro,small and medium pores.That is,with the increase of the content of micro,small and medium pores,the fractal dimension of soil pores becomes larger and the pore structure becomes more complex.（2）The fractal dimension increases with the depth as a whole,and within the depth range of the three layers,it increases with the depth,and the pore structure is gradually complex.（3）Due to the particularity of the soil at 40 cm depth,the compressibility of the soil layer is studied.Under the conditions of 0.5%salt content and 90%compactness,a group of remolded soil samples with different water content gradients（15%,16.9%,19%,21%）were prepared for compression test.The results showed that:（1）The void ratio of each soil sample was the same in the initial state.When the load increased to200 k Pa,the compression curve began to show significant indigenous differences,and the compressibility increased with the increase of water content.（2）When the load reaches 400 k Pa,the difference of soil samples under different moisture contents is the largest;（3）when the load reaches 3200 k Pa,the compressibility of each soil sample is similar as a whole,but there are still some differences due to different moisture content.The compressibility indexes show the following rules:（1）With the increase of water content,the compressibility coefficient a1-2 increases linearly;（2）the compression modulusσk showed a linear decreasing trend;（3）The rebound index Cvfirst decreases and then increases,and the lowest point is the optimal moisture content of 16.9%;（4）Due to the increase of water content,the connection mode between soil changes,resulting in the abrupt change of structural yield stress with 16.9%（Wop）water content as the boundary point.When the water content is less than Wop,the structural yield stress is larger,and when the water content is greater than Wop,it is significantly smaller.Under the condition of 0.5% salt content and 16.9%water content,the remolded soil samples with different compaction gradient（C80,C85,C90,C95）were prepared for compression test.The results show that:（1）the void ratio of the soil samples with different compaction degrees varies greatly under the initial state,and the void ratio decreases with the increase of compaction degree under the same load.（2）In the load range of 100 k Pa～400 k Pa,the e-log P compression curve shows an inflection point in turn,and the void ratio difference between the compaction degrees decreases.（3）When the load reaches 3200 k Pa,the compression state of the soil is basically the same.The compression modulus and structural yield stress increase linearly with the increase of compactness,while the rebound index decreases linearly,and the compression coefficient decreases with the increase of compactness.（4）The mercury intrusion test was carried out on remolded soil samples under different water content,compaction degree and consolidation load,and it was found that the pore distribution curves showed three peaks.The pore volume with pore diameter less than 0.4μm is hardly affected by the above three factors.The change of pores is mainly that the medium,large and ultra-large pore volumes larger than 0.4μm are compressed and transformed into each other.Under all levels of load,with the increase of water content,the total pore volume decreases,the medium pore content increases,and the large pore content decreases.When the load reaches 200 k Pa,the pore size classification results take the optimal moisture content as the mutation point.The macropore content of the soil samples with less than this moisture content is significantly higher than that of the soil samples with more than this moisture content,while the change rule of the medium pore content is opposite.Under the load of 3200k Pa,the contents of micro,small and super large pores in soil are basically the same.The increase of water content increases the content of medium pores and decreases the content of large pores.The pore cumulative curves of soil samples with different compaction degrees are quite different.Under each consolidation pressure,with the increase of compaction degree,the peak volume of pores larger than 4μm decreases,and the peak interval of pore diameter moves to the direction of small pore diameter,resulting in the decrease of total pore volume.With the increase of consolidation pressure,the pore curves with different compactness tend to be concentrated.When the load reaches 800 k Pa,the difference between soil samples with different compactness decreases significantly,the linear of cumulative pore curves is basically the same,and the pore distribution curves are also similar.When the load reaches 3200 k Pa,the pore volume cumulative curve and pore volume distribution curve between the compacted soil samples are similar,and the content of each pore size is basically the same,indicating that the structure of the soil tends to be homogeneous. |
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