| The Loess Plateau is full of ravines and broken terrain,and the space suitable for urban development is extremely limited,which seriously restricts the local social development.To solve the shortage of land resources in the process of urban development,a large area of land was built for urban construction in the loess area through the major loess project of"cutting mountains,filling ditches and creating land".This is the largest geotechnical project in the loess region,and it is also rare in the history of urban construction in the world.Those projects not only greatly changed the original loess geomorphic,but also changed the original loess structure,affecting the characteristics of loess permeability and infiltration mechanism,leading to the redistribution of water.Water is the key factor to induce loess landslide,land subsidence and other geological disasters,it is urgent to study the permeability characteristics and infiltration mechanism of loess in major engineering areas,to provide scientific basis for the prevention and control of major engineering geological disasters of loess.Based on the research status of permeability of loess at home and abroad,it was selected the typical loess tableland,aiming at the loess permeability changes and water infiltration process under the influence of major engineering,and comprehensively uses the methods of field investigation,laboratory experiment,in situ test,theoretical analysis,model building,etc.,to elucidate the non-Darcy permeability characteristics of loess,to reveal the microscopic mechanism of the change of loess permeability characteristics under the loess project,to clarify the dynamic water infiltration and migration process in loess,to crack the influence mechanism of engineering interface effect on water infiltration and migration,to propose the optimal control measures of water in the loess engineering area,and a whole-chain loess seepage research system of"theoretical innovation-phenomenon analysis-mechanism cracking-engineering prevention and control"was constructed.The main conclusions are as follows:（1）The correlation between hydraulic gradient,sample uniformity and saturated hydraulic conductivity was established,and the test method of loess saturated hydraulic conductivity in laboratory was improved.The traditional form of Darcy’s law is not suitable for describing water flow processes in loess under low hydraulic gradient,since there is a threshold hydraulic gradient（i0）and the relationship between the hydraulic gradient（i）and seepage velocity（v）is highly non-linear.The existence of bound water is the main reason for the change in the i–v curves.The formula fully reflects the existence of i0 and the non-linear variation of the i–v curve in non-Darcy seepage processes and is of practical value because it can be used to simulate both Darcy and non-Darcy seepage process in loess.Therefore,the accurate permeability coefficient of loess can be obtained by Darcy’s law only when the hydraulic gradient range is strictly controlled in the laboratory permeability experiment.Sample uniformity test found that different sample preparation methods would lead to differences in sample uniformity.Based on the mechanical response of soil under impact load in the process of ramming,the traditional sample preparation method was improved to obtain permeable soil samples with good uniformity.At the same time,the saturated permeability test results of soil samples with different homogeneity show that there is a negative correlation between the sample uniformity and the saturated permeability coefficient under the same dry density condition,that is,the better the sample uniformity,the greater the saturated permeability coefficient.Therefore,it is necessary to fully test the homogeneity of the sample before conducting the permeability experiment to obtain the accurate permeability coefficient.Furthermore,Kozeny-Carman equation was introduced to construct the functional relationship between saturated permeability coefficient and sample uniformity,and the saturated permeability coefficient could be used as an index to reflect the sample uniformity.（2）The difference of homogeneity and permeability between undisturbed loess and remolded loess before and after land reconstruction project was compared and analyzed.The influence law of loess compaction on pore size distribution was clarified,and the microscopic mechanism of loess permeability change was revealed.The results of large-scale in-situ engineering simulation show that there are significant differences in the homogeneity and permeability anisotropy between undisturbed loess and remolded loess.The dry density of undisturbed loess has little spatial variation and is homogeneous on the whole.The spatial variation range of the dry density of remolded loess is obviously greater than that of undisturbed loess,and the vertical dry density is the most obvious change,which is heterogeneous on the whole.Similar to the results of the uniformity test,the remolded loess had obvious interlayer interface due to the effect of lamination tamping,and the dry density in each layer decreased with depth.The top of the remolded loess was overcompacted,and the bottom was undercompacted,showing significant stratification characteristics.The vertical permeability coefficient（Kv）of undisturbed loess is greater than that of horizontal permeability（Kh）,and the permeability is anisotropic in the range of 2.56～5.1Kv/Kh.However,there was no significant difference between Kv and Kh in remolded loess,and the range of Kv/Kh was between 0.68 and 1.32,and the permeability was basically isotropic.The main reason for the difference of permeability between undisturbed loess and remolded loess is the redistribution of pore size under the action of tamping.The results of mercury injection test show that the effective porosity of undisturbed loess is greater than that of remolded loess on the whole,and the pore（0.1～3μm）and mesopore（3～100μm）are the most obvious changes in the compaction process,which are the main space of pore changes and determine the permeability coefficient of loess.（3）The seepage pattern of water infiltration and redistribution in undisturbed loess were revealed,and the dynamic infiltration and migration processes of water in different patterns were described quantitatively.In situ infiltration test results show that there are three patterns in undisturbed loess:uniform infiltration,preferential infiltration and macroporous dominant infiltration,among which uniform infiltration and preferential infiltration are common,and macroporous dominant infiltration has randomness and contingency.During the infiltration period,the uniform infiltration was semi-elliptical in vertical.Preferential infiltration first expanded from fingerlike to vertical flow,and gradually changed to semi-ellipse.The vertical migration rate of uniform and preferential infiltration is greater than the lateral migration rate.During the redistribution period,the water in the uniform infiltration area migrated vertically and laterally,and water consumption led to the narrowing of the range of uniform infiltration.The preferential infiltration was still a semi-elliptical vertical and lateral flow,and there was little difference between the vertical migration rate and lateral migration rate(7.55×10-6 cm/s～1.38×10-5 cm/s),and all were lower than the migration rate in the infiltration period(1.73×10-4 cm/s～2.83×10-4cm/s).In the infiltration period,all the infiltration water was transformed into the soil water increase within the monitoring range of ERT.In the initial period of infiltration,priority infiltration was dominant（75.4%）and uniform infiltration was supplementary（22.9%）,and then gradually changed into uniform infiltration（57%）and priority infiltration was supplementary（40.5%）.During the redistribution period,under the influence of multiple heavy precipitation,the net precipitation supplement changed into soil water increase within the monitoring range of ERT,and about 33.73%of water migrated to the deeper yellow soil in a stable state.In the initial stage of redistribution,water migration was dominated by uniform infiltration（58.3%）,supplemented by priority infiltration（41.7%）,and then gradually changed to be dominated by priority infiltration（60%）and supplemented by uniform infiltration（40%）.（4）It is elucidated the law of water infiltration and migration under the influence of major loess projects,cracked the mechanism of interface effect produced by engineering construction on water infiltration and migration,and proposed the water optimization control measures of engineering construction in loess region.The results of large-scale in-situ engineering simulation show that there are significant differences in the infiltration law between the undisturbed loess and the remolded loess,which mainly depend on the difference of permeability,the effect of excavation and filling interface and the effect of the interface between the remolded loess soil layers.Under the influence of osmotic anisotropy,interlayer capillary water isolation and interlayer water retention,water in the remade loess mainly migrated laterally,and mainly migrated from the digging-fill boundary to the undisturbed loess.The digging-fill boundary is the dominant channel for water infiltration.On this basis,the SEEP/W seepage analysis model was established.The results showed that78.2%of the water flowed from the excavation and filling interface to the undisturbed loess,and only 21.8%of the water was discharged from the blind ditch at the bottom.Under the condition of transient rainfall infiltration,almost no water was discharged from the bottom blind groove.Therefore,the water regulation function of the blind ditch at the bottom of the fill project is limited.The drainage capacity of the blind ditch under different engineering measures is simulated and analyzed.The method of combining excavation and filling interface tamping,blind ditch slope increasing and infiltration hole is proposed to control the water in the fill engineering. |
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