| The Loess Plateau is the cradle of Chinese civilization and an important part of the Yellow River Basin,but it is also one of the areas with fragile geological environment,severe shortage of land resources and most frequent geological disasters in China.In recent years,in order to expand urban development space,the Loess Plateau has carried out a large-scale “mountain excavation and city construction”(MECC)project,creating a large amount of new flat land for urban expansion by “cutting mountains,filling ditches,creating land and building cities”.The large-scale MECC project of Yan’an New District(YND)in Shaanxi Province is the largest geotechnical project in the collapsible loess hilly gully area,which has significantly damaged the original longterm geological equilibrium environment and caused new man-made geological disasters and environmental problems,such as significant ground subsidence,potential landslides and environmental damage,which have seriously affected the local urban development and normal operation of the project.Therefore,a systematic and scientific study on the disasters and environmental problems caused by the large-scale MECC project in YND can provide a theoretical reference for the planning and construction of the new district and similar projects and the scientific prevention and control of disasters,which is of great significance to the urbanization development of the loess hilly gully area.In this paper,the social effects,disaster effects,environmental effects and scientific prevention and control modes of the MECC project were systematically studied by using multidisciplinary intersection and combination of multiple methods.Firstly,the necessity and suitability of the MECC project in YND were discussed through the dynamic evaluation of the bearing capacity of resources and environment and the evaluation of the suitability of the construction land under different urban development modes.Secondly,the development characteristics,spatial and temporal patterns and development trends of disasters(ground subsidence,ground uplift and potential landslides)related to the MECC project were revealed based on air-skyground multi-dimensional observation.Then,the spatial and temporal dynamic response of ecological environment factors in the land creation area was revealed by remote sensing analysis,and the impact of the construction of the MECC project on the ecological and geological environment was comprehensively evaluated.Finally,the positive and negative effects of the MECC project in YND and the human-land relationship were discussed,and a scientific disaster prevention and control system for the whole life cycle of the MECC project was constructed based on the concept of NBS(nature-based solutions)and resilience.The main conclusions are as follows:(1)The resources and environment of Yan’an City were already in a serious overload before the MECC project,and the MECC project can provide additional land resources for the urban development of Yan’an without the restriction of the original topography and landscape,and relieve the pressure of urban resources and environment bearing.The old city of Yan’an has a typical “linear” urban spatial pattern,with intensive development of geological hazards and increasing contradiction between supply and demand of land resources.Before the MECC project,with urbanization and rapid population growth,the shortage of land resources was further aggravated,making the old city of Yan’an seriously overloaded in terms of resources and environment.The development model of MECC effectively avoided the influence of the original geological disasters and topography,greatly increased the proportion of available land resources(about 56.3%),eased the pressure of urban resources and environment bearing,and was an effective way to realize the expansion of suitable land for largescale urban construction in the hilly and ravine area of Loess Plateau.The MECC project in YND started in April 2012 and completed its earthworks in June 2018,creating a total of about 22.3 km~2 of flat land with a maximum cut-and-fill thickness of110 m.(2)Significant post-construction settlement was detected in YND after the MECC project,and its spatial distribution and spatio-temporal pattern were mainly controlled by the extent,intensity and timing of the filling project.The spatial pattern and magnitude of significant ground settlement up to 121 mm/a in the YND were revealed by time-series In SAR,which was mainly controlled by the extent and thickness of the filling works.In terms of time-series evolution,the long-term ground settlement in the YND showed an exponential decay trend,and the newer and thicker the loess filling foundation was,the greater the ground settlement rate and the smaller the decay trend were within the same observation period,and the longer the time required for the settlement of the filling foundation to stabilize;in terms of spatial evolution,the ground settlement area in the YND gradually shifts to the newly completed fill area,and the area of the settlement area decreases by about 32.9% from May 2016 to October 2019.The comparative analysis of the prediction results of the solidification theory and the regression parameter model showed that the hyperbolic model was most suitable for the prediction and evaluation of ground settlement in YND,and its prediction results showed that most areas of Qiaoergou in YND still had a settlement potential of 10-100 mm after the In SAR monitoring conducted in this paper,and would continue to settle for about 9.3 years(until 2029).(3)The large-scale mountain excavation and gully filling in YND broke the original stress equilibrium environment and caused significant ground uplift deformation.Spatially,the ground uplift is mainly distributed in the excavation area closely related to the urban construction,with the total area covered and the maximum uplift rate reaching 5.94 km~2 and 24.8 mm/a,respectively;temporally,the ground uplift develops faster in the early post-construction period,and tends to slow down and stabilize with time.The mass unloading(load release)caused by large-scale mountain excavation,which is significantly positively correlated with ground uplift,indicates that the scope and magnitude of load release control the spatial pattern and magnitude of ground uplift,while construction loads also inhibit the development of ground uplift.The rapid release of elastic strain energy of the original strata caused by large-scale mountain excavation and the rebound deformation resulting from stress field readjustment are the main intrinsic mechanisms of ground uplift in YND.(4)A new method for early identification and stability assessment of potential landslides combining optical remote sensing,time-series In SAR and field survey was constructed based on the universal spatio-temporal deformation law before landslide failure,and 18 potential landslides were identified in YND for the first time based on this method.The difference deformation map derived from Sentinel-1 SBAS-In SAR results clearly reveals the active boundary of potential landslides,and its combination with optical imagery can accurately identify the extent of potential landslides.The field survey and UAV mapping verified the reliability of the potential landslide identification results.An exponential model was applied to simulate the temporal deformation of In SAR measurement points within the potential landslide,and the results showed that most(about 82%)of the measurement points within the potential landslide showed primary or secondary creep stages,but there were still several potential landslides that showed significantly accelerated tertiary creep.The high correlation between the spatio-temporal deformation pattern of the potential landslide and environmental factors revealed the important controlling roles of fill thickness and rainfall on the activity of the potential landslide.(5)The ecological and geological environment of YND showed a general trend of first destruction and then gradual restoration with the construction of MECC project.By extracting the NDVI time series data for 13 years before and after the construction of MECC project,the response of the vegetation environment to the construction of MECC project is revealed,i.e.,the natural balance before the construction of MECC project,significant destruction during the construction and slow recovery after the construction.A new method for evaluating the recovery rate of vegetation considering climate effects was proposed,indicating that it may take another 12 years(until 2033)for the vegetation environment of YND to recover to the pre-construction level.The evaluation of RUSLE model showed that the MECC project of YND has reduced the“potential energy difference” of the original hilly and ravine landscape,curbed the occurrence of soil erosion and improved the original living environment.The comprehensive evaluation of ecological and geological environment quality showed that the MECC project has significantly damaged the ecological and geological environment during the construction period,but gradually restored the ecological and geological environment after the construction;in addition,the MECC project has effectively shared and relieved the urban bearing pressure of the old city of Yan’an and improved the original poor ecological and geological environment of the old city.(6)Based on the concept of NBS and resilience,a comprehensive disaster prevention and control system and model for the whole life cycle of the MECC project have been constructed.Although the MECC project of YND caused large-scale geological disasters and ecological environment damage,a new natural balance environment was gradually formed after the project.The MECC project not only solved the problem of land resources that restricted urban development in hilly and ravine areas,but also gradually alleviated the conflict between human and land.NBS and resilience theory advocate the development law and solution based on nature,conforming to nature and using nature,which can be used to enhance the harmonious relationship between human and land;based on this,the disaster avoidance mode of conforming to nature in the planning stage,the mitigation mode of protecting nature in the design stage,the disaster control mode of using nature in the construction stage and the monitoring and control response mode of restoring nature in the operation stage were proposed to serve the disaster prevention and control in the whole process of engineering planning,design,construction and operation,so as to realize the harmonious coexistence and coordinated development of human and land. |
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