Stability Analysis Of Loess Landslide In Loess Tableland Edge Of Heifangtai Irrigation Area

Loess landslides, as a kind of geological disasters, be similar to floods, hurricanes and othernatural disasters. It has brought a huge loss to people's lives and property and the nationaleconomic construction, such as buried villages and farmlands, destroyed plants, interruptedtraffic, blocked rivers, and caused casualties, etc. In recent years, landslides of the Loess Plateautriggered by agricultural irrigation is attracting the attention of the public, the media andengineering geological community. Therefore, it has important theoretical significance andpractical value to study the formation conditions, formation mechanism and stability ofirrigation-induced loess landslides, and to reveal the landslides how to nurture, how to developuntil the landslides die out.In this paper, we focus on the landslides of the Heifangtai loess platform in YongjingCounty, Gansu Province. First of all, through summarized the research status ofirrigation-induced loess landslides and collected large number of documents, the geologicalenvironmental background and the characteristics of the landslide hazard are analyzed. Secondly,researching the groundwater migration law in the irrigation process by computer numericalsimulation based on indoor and outdoor test data. And the relationship between the slope stabilityand irrigation infiltration process was analyzed. The process of four movement stages of loesslandslide was simulated using PFC2Dsoftware. The main conclusions as follow:1)35landslides have been found in the study area, mainly loess landslides andloess-bedrock landslide. Large numbers of landslide are loess landslides and primarily are theunstable medium-sized shallow landslides. Small numbers of landslide are loess-bedrocklandslides and usually are the more stable large-scale deep landslides. Six kinds ofcross-sectional shape types and two kinds of longitudinal shape types are proposed. Thedevelopment characteristics of landslide shape are described and summarized from length, height,width, slope, the main slip direction and so on. The boundary feature of loess landslides andloess-bedrock landslides are summarized from the development features of landslide trailingedge and landslide front edge. Four kinds of deformation process and two kinds of movement patterns are proposed.2) Groundwater recharge and discharge in loess formation were analyzed, and on this basis,the hydro-geological conceptual model and mathematical model were established.Three-dimensional groundwater unsaturated flow in irrigation area was simulated by FEFLOWsoftware. The analysis of results shows that: with the different annual irrigation amount, thedistribution of groundwater was unlike. However, the overall trend is that the groundwater levelwas increased with the increasing annual irrigation amount; groundwater flow was from west toeast; water was excreted in the eastern platform margin and on both sides. The saturated zonethickness of center layer of the loess platform is gradually increasing with the increasing annualirrigation amount, and progressively becomes thinner thickness from the center to thesurrounding. Under different irrigating schemes, the change rates of groundwater level wereunlike in certainly platform locations. In the first five years of irrigation, groundwater levelraised the fastest, and groundwater seepage was tended to a steady flow with increasingirrigation time. The change rates of groundwater were not only with the annual irrigation amount,but also with the location in loess platform. According to the data of the monitoring points, therelationship curve was obtained between the annual irrigation amount and the change rates ofgroundwater level.3) Base on the mechanism of landslide formation and the research of groundwater migrationlaw in study area, unsaturated shear strength and seepage force of the general limit equilibriummethod was proposed. Then it was researched from eight aspects, and the computing formulas ofthe stability factor were derived in detail. Unsaturated shear strength and seepage force of thegeneral limit equilibrium method was programmed by the MATLAB language, and a slopestability assessment procedure was accomplished.4) Take a typical slope of the Heifangtai loess platform in study area as an example. UsingPUSG program the slope stability was calculated with a series of the designed calculationscheme, for instance, taking into account different conditions (under different in the amount ofirrigation), taking into account unlike evaluation methods (traditional limit equilibrium method,unsaturated shear strength and seepage force of the general limit equilibrium method). Contrastthe results obtained by the two methods and concluded that with the increase of annual irrigationamount, slope stability factor showed a decreasing trend, but the impact of groundwater on theresult is different. The calculated results of the stability factors based on the traditional limitequilibrium method show that with the increase of annual irrigation amount, stability factor decreased from1.045to1.003, the slope stability factor greater than1and less than1.1,indicating that the slope is in an unstable state and in creep-tensile stage, and the groundwaterwas a little impact on slope stability calculation. The calculated results of the stability factorsbased on the unsaturated shear strength and seepage force of general limit equilibrium methodshow that with the increase of annual irrigation amount, stability factor decreased from1.283to0.796. When the annual irrigation amount is greater than500×104m3, the slope stability factorsare less than1, indicating that the slope is in an unstable state. Therefore, the500×104m3is alimit value, which we called the limit value of the annual irrigation amount of slope stability.When the annual irrigation amount is less than the limit value, the slope will be in the stable state.In other words, when the annual irrigation amount is larger than the limit value, the slope will bein an unstable state. The groundwater was a relatively large impact on slope stability calculation.5) Combined with the characteristics of loess slides's deformation and movement inresearch area, using PFC2Dsoftware to simulate the process of four movement stages of loesslandslide, such as, soften-erosion stage, peristalsis-tensile stage, sliding-failure stage,stable-compacted stage, and the simulation results are analyzed. It is a reproduction for the entireprocess of loess landslides from the deformation, damage to the demise.