| Large machinery compaction in soil dumps of the opencast coal mine is one of the most serious problems of soil degradation.The study of compacted minesoil is of great importance for the maintaining of hydrological balance and sustainability of land reclamation and ecological restoration.However,few studies have so far been conducted to evaluate the effects of large machinery compaction on soil structure and hydraulic properties.Thus,the severely compacted soil dump in the Pingshuo Opencast Coal-mine was selected as the specific study area.The soil sampling scheme was divided into two part: one part came from the compacted soil samples in dumps,the other came from the simulative compacted soil samples in the labotory.By X-ray computed tomography(CT)scanning,the 3D structure of macropores of soil samples was reconstructed.Besides,soil hydaulic properties of compacted soil were tested in the labotory.Moreover,the soil pore fractal model was used in the prediction of soil water retention curve and unsaturated hydraulic conductivity,and the prediction were proved effective.The results are as follows:(1)Through CT scanning and 3D reconstruction software,the pore structure characteristics of the soil after compaction are intuitively understood.The number of macropores decreased with the increase of compaction degree.Large pores of uncompacted soil samples in laboratory and mining areas were more sensitive to soil compaction.In the uncompacted soil samples,the connectivity of macropores was better than the compacted samples,and the poor connectivity and more isolated macropores were found in the compacted samples.(2)Macropore structural parameters such as pore number,porosity,surface area,volume were analyzed.Soil compaction has a great influence on soil macropore parameters,and the number and porosity of macropores decreased as the soil bulk density increase.In the laboratory-collected soil samples,larger pores(larger than 1000 μm)were more sensitive to compaction.In the dump-collected soil samples,macroporosity decreased with increasing bulk density.The depth of soil also caused variations of macropore parameters.Compaction also significantly affected soil macropore connectivity.With the increase of compaction degree,The macropore connectivity of laboratory soil samples was significantly reduced.Although the average Euler number of soil samples in the mining area did not show an obvious tendency,soil connectivity showed the regularity under different soil depth or different sampling points.It also showed strong regularity;the bulk density of soil samples was closely related to the number of pore throats and pore thickness,indicating that soil compaction is the main reason for the destruction of macropore connectivity.(3)The hydraulic characteristics,soil water content,saturated hydraulic conductivity and water diffusion rate of compacted soil were measured in laboratory.The results showed that the soil water content decreased significantly with the increase of matrix potential in the lower suction section,and the soil moisture content decreased with the increase of matrix potential in the high suction section.Under the same matrix potential condition,the soil water content decreased with the increase of bulk density.The results also showed that effective water content(AW),available water content(RAW)and delayed moisture content(SAW)were positively correlated with soil bulk density;field water holding capacity(FWC),wilting coefficient(WC),water content(SW)and capillary water content(CRM)were negatively correlated with soil bulk density.Soil compaction had a significant effect on the saturated hydraulic conductivity.With the increase of the bulk density,saturated hydraulic conductivity decreased.Under different compaction conditions,soil moisture diffusion rate is positively correlated with soil water content.With the increase of soil water content,the soil water diffusion rate gradually increased.With the increase in soil water head potential,the soil specific water capacity decreases gradually.The difference between the low suction segments was obvious,and the difference between the high suction segments was not obvious.(4)Based on the fractal theory of pore structure,the prediction model of soil hydraulic characteristics was established.According to the KZ-BC model,the soil water characteristic curve and the fractal Mualem model were used to predict the accuracy of soil unsaturated hydraulic conductivity.The effect of compaction on the soil water characteristic curve and the predicted value of unsaturated hydraulic conductivity were similar to the measured results in the laboratory.There were some differences between the measured values and the predicted values of the mining area,and the possible reason was the complex factors in the mining area.To get more accurate predict values,the fractal model needs to be further corrected in the prediction of field soil samples.(5)Based on the economic,social and ecological perspectives,this paper analyzed the mechanism of soil compaction mitigation in mining areas,and proposed the corresponding reclamation measures.Feasible suggestions such as the organic fertilization,mechanical operation arrangement,stability maintenance of dumping site,vegetation restoration,tillage,compaction prediction models were included in the practice of sustainable development and management in mining areas. |
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