Slope Stability Assessment Of Irrigated Loess Terraces

China has the largest loess covered area in the world,which accounts for about 6.63% of China's total land area.Under the influence both of the natural conditions and human activities,Not only loess landslides have been the limitations of social economy in our country,but also become the most serious geohazards in our country.The high steep slope of the Loess Plateau provides favorable conditions for the birth and occurrence of the loess landslide under the tectonic stress.With the tableland-beamMao geomorphic evolution processing,Specifically,a large number of landslides were induced by the irrigation and the geological forces inside and outside in the development of Tableland-Beam-Mao,such as the Yongjing County in Gansu Province,the south of Baoji-Changxing and The downstream of the Jinghe River.Due to the special geological structure and human activities,there ara a lot of large loess landslides developed in Heifangtai region frequently which is the typical irrigated agriculture area in loess Plateau.More than 120 landslides occurred around Heifangtai which caused 38 casualties,direct economic losses over 400 million yuan and the indirect economic losses over 1 billion yuan from 1968.Thus,more and more attention is concentrated on this area due to the severe geohazard situation.The most previous researches were carried out about loess landslide classification,the landslide mechanism,stability evaluation,saturated and unsaturated soil mechanics,the loess landslide prevention and control measures.Whilst,no three-dimensional slope stability evaluation were studied to guide the early warning and prevention of loess landslides.The first aim of this study is to provide basic conditions and model parameters for the subsequent risk control and disaster management by obtaining engineering geological conditions,geotechnical parameters and groundwater flow field information.The second aim is to extend the 2-dimensional stability evaluation methods at single point to 3-dimensinal method at regional scale to improve the practical application and provide scientific reference for landslide risk analysis and geohazard management.This is an interdisciplinary research,with related theories including engineering geology,soil mechanics,geographic information science and computer information science.Based on the comprehensive analysis of the existing research,the engineering geological conditions were obtained by field survey and mapping,which could support the subsequent engineering management and model calculation.The three-dimensional high-density electrical method was conducted to generate the parameters of rock,soil and groundwater flow combined with the borehole results,indoor and outdoor experiments.The characteristics of spatial distribution of landslide were concluded by having a field investigation.After the normalization of all collected data in ArcGIS platform and ENVI environment,the regional slope stability was calculated with the rising and falling of groundwater level.According to all above result,a new path of agricultural development was proposed,which could achieve geohazard prevention and control,protect the life and property and social stability,and improve the economy sustainably.The main conclusions are as follows:(1)There are 5 faults and two groups of joints in the northwest and northeast of Heitai,which cut through the loess layer,red clay layer,resulting in a large number of cracks inside.These tectonic fissures,joints and ground cracks provide major channel for the groundwater flowing from loess to red clay and running out from pebble layer finally.The bedrock in the loess landslides has been soften by the infiltrated groundwater,which cause the reduction of strength at the same time.Thus,the engineering measures are not suitable for controlling these landslides because of the lack of hard bedrock.(2)Using indoor and outdoor experiments to obtain the basic physical and mechanical parameters of malan loess.The results of indoor ring shear test show that the water content has a significant impact on the residual shear strength.When the water content was increasing,the shear resistance was decreasing and the cohesion was beginning to increase to a critical value and decreased rapidly close to zero.Whilst,the residual friction angle was decreasing to the critical value and increasing a little.The moisture content were 5.2% and 20% when the internal friction angle and cohesion were reached the peak respectively.(3)Under the current conditions,the results of Scoops3 D show that the area in the status of unstable,less stable,potential sliding and stable in the Heitai are 145,900m2,58,375m2,226,850m2,and 19,852,025m2 respectively,corresponding to the percentage of 0.72%,0.29%,1.12% and 97.87%.If the groundwater level is reduced by 7m,the percentages of the unstable area,the less stable area and the potential landslide area are decreased to 0.19%,0.001% and 0.73% respectively,the percentage of the stable area increased to 99.08%.If groundwater level was 7m higher than current status,and the percentage of unstable area,less stable area and potential sliding area dramatically increased to 3.89%,0.75% and 1.45%,respectively.The area of stable area is drastically reduced to 93.91%.Therefore,the rise of the groundwater level in the Heitai could decreases the regional stability,while the reduction of the groundwater level could improve the stability of the area.The Regional stability factor of is very low,at middle of the Moshigou,Jiaojia landslide trailing edge,Jiaojia landslide group,around the Luojiabanpo,There is a greater likelihood of disasters in these areas.