Research On Surface Deformation Mornitoring In Collapsible Loess Area Based On Time Series InSAR Technique

With the accelerated urbanization process,the contradiction between people and land is becoming more and more prominent in the cities on the Loess Plateau in the northwest of China.The situation of strained land forces many cities to adopt the strategy of“cutting mountains to build cities”.The construction of the new district of Yan’an is one of the largest geotechnical engineering projects in the collapsible loess region in the world,covering an area of 78 km~2.The large-scale movement of“cutting mountains and filling valleys”,the rapid urban construction process and the complex loess geological conditions have caused extensive and violent surface deformation in this region,which has brought great threats to the sustainable development of economy,the safety of people’s lives and the protection of ecological environment.Therefore,it is of great significance to study the spatial distribution and development process of surface deformation in the new district of Yan’an.Persistent Scatterer Synthetic Aperture Radar Interferometry(PS-InSAR)can obtain surface deformation on the scale of millimeters by analyzing theSAR image covering the same scene.It is widely used in the field of surface deformation monitoring.PS-InSAR is a differential interferometry technology based on high coherence points,which requires the existence of artificial attachments or exposed rock in the monitoring area for a long time.Its monitoring effect is not good for fields,forests and bare lands.For the phase calculation based on a single reference point,it is easy to cause phase mixing and unwrapping failure.Small Baseline Subset InSAR(SBAS-InSAR)makes use of interference subsets with short time-space baselines for differential processing.It can effectively improve the number and density of observation points in the low-coherence area,and obtain a higher precision for monitoring the surface deformation of bare land after earthwork.Based on PS-InSAR and SBAS-InSAR methods,this thesis monitors the surface deformation of Yan’an after earthwork in order to study the influence of large-scale engineering activities on the surface of collapsible loess area.The main research contents and achievements are as follows:(1)Taking PS-InSAR and SBAS-InSAR as examples,the basic principle of surface deformation monitoring by time series InSAR technique is introduced,and the error sources and elimination methods of InSAR method are analyzed;(2)Based on PS-InSAR and SBAS-InSAR methods,the surface deformation information of Yan’an after earthwork is calculated by using the Sentinel-1A data of 56scenes from 2015 to 2019.The difference between the surface deformation results of different methods is about 4.5 mm/a.The results show that the surface deformation in the new district of Yan’an is severe after earthwork,and the average surface deformation rate is between-70 and 30 mm/a.The maximum cumulative settlement is-300 mm in the subsidence zone between Shanghai Road and Zichang Road in the new district of Yan’an,and the maximum cumulative uplift is 100 mm in the big data industrial park to the northeast of Yan’an Learning Academy.Combined with the coherence ofSAR image and the data of 21 surface settlement monitoring points at Yan’an Airport,the theoretical and practical accuracy of InSAR deformation results are analyzed,which are 2.5 mm/a and 10 mm/a respectively;(3)The distribution of surface deformation in the new district of Yan’an has a high correlation with earthwork.Overlaying the surface deformation area with the earthwork area,it is found that more than 50%of the uplift occurs in the excavation area and more than 60%of the settlement occurs in the fill area.Based on the engineering construction and the analysis of the collapsibility of loess,the surface settlement of the fill area may be caused by various factors,such as the collapsible deformation of backfill soil under water and the compression deformation of the loess foundation under the external load and its own gravity.The surface uplift phenomenon after excavation may be caused by the release of pressure in the upper surface caused by excavation;(4)Based on Kalman filtering,the process smoothing and trend prediction analysis of 12 deformation points in the new district of Yan’an and 4 deformation points in the old urban area of Yan’an are carried out.The results show that the surface of the old urban area of Yan’an is basically stable,and the surface deformation rate of the new district of Yan’an begins to decline after the rapid deformation stage from 2015 to 2019.In addition,the overall surface deformation begins to approach a stable period.