Characterizing the Topographic Changes and Land Subsidence Associated with the Mountain Excavation and City Construction on the Chinese Loess Plateau

Abstract

A mega project, Mountain Excavation and City Construction (MECC), was launched in the hilly and gully region of the Chinese Loess Plateau in 2012, in order to address the shortage of available land and create new flat land for urban construction. However, large-scale land creation and urban expansion significantly alters the local geological environment, leading to severe ground deformation. This study investigated the topographic changes, ground deformation, and their interactions due to the MECC project in the Yan’an New District (YND). First, new surface elevations were generated using ZiYuan-3 (ZY-3) stereo images acquired after the construction in order to map the local topographic changes and the fill thickness associated with the MECC project. Then, the interferometric synthetic aperture radar (InSAR) time series and 32 Sentinel-1A images were used to assess the spatial patterns of the ground deformation in the YND during the postconstruction period (2017–2018). By combining the InSAR-derived results and topographic change features, the relationship between the ground deformation and large-scale land creation was further analyzed. The results indicated that the MECC project in the YND has created over 22 km2 of flat land, including 10.8 km2 of filled area, with a maximum fill thickness of ~110 m. Significant uneven ground deformation was detected in the land-creation area, with a maximum subsidence rate of approximately 121 mm/year, which was consistent with the field survey. The strong correlation between the observed subsidence patterns and the land creation project suggested that this recorded uneven subsidence was primarily related to the spatial distribution of the filling works, along with the changes in the thickness and geotechnical properties of the filled loess; moreover, rapid urbanization, such as road construction, can accelerate the subsidence process. These findings can guide improvements in urban planning and the mitigation of geohazards in regions experiencing large-scale land construction.

Observation of Surface Displacement Associated with Rapid Urbanization and Land Creation in Lanzhou, Loess Plateau of China with Sentinel-1 SAR Imagery

“…For Regions D and E, some newly significant displacement areas were discovered during the period from December 2019 to March 2021, particularly in Region D. We can see from Figure 11c,d that the severe decorrelation signals occurred in the new displacement areas of Region D, indicating that the lands in these areas were created between December 2017 and December 2019. Besides, the displacement rate maps illustrate that non-uniform displacement exists in the MECC areas; this may be correlated with the thickness of the filling loess [4] and will be discussed in detail in Section 5.3. Compared with Figures 10 and 11, we found that most of MECC areas completed before 2005 were basically stable, and the main urban of Lanzhou was also stable.…”

“…The time-variable pseudo displacement signals can be generated in the case when the DEM errors are not exactly corrected [24]. However, a completed processing procedure for the DEM errors is not obtained in those previous studies [2,4,5,23].…”

“…As a consequence, a mountain excavation and city construction (MECC) project has become one of the major ways to solve the contradiction between more and more people and less and less land in the process of rapid urbanization. In cities such as Yan'an [4], Lanzhou [5], and Lanzhou New District [2], northwest China, tens of square kilometers of urban land have been created by large-scale MECC projects. The MECC projects can largely contribute to the development of local economy and urban construction.…”

“…The MECC projects can largely contribute to the development of local economy and urban construction. However, they can have a major impact on the local geological environment when the project intensity largely exceeds the geological bearing capacity, such as geohazards, soil erosion, as well as water and air pollution [3,4], among which the major concern is severe surface displacement. Severe surface displacement poses a significant threat to the construction of urban areas and may bring the risk of damaging the urban infrastructures, such as buildings, pipelines, and roads.…”

“…However, those recent InSAR studies on surface displacement monitoring associated with MECC projects were mainly focused on the one-dimensional line-of-sight (LOS) direction or vertical direction (directly projected by assuming that no horizontal displacement occurred); this is satisfactory for detecting the spatial distribution of the active displacement areas, but it is insufficient for investigating the spatial displacement patterns of the created land in details. Furthermore, the ongoing MECC projects can lead to dramatic changes in local terrain [4], thus resulting in large digital elevation model (DEM) errors in SAR data processing. The time-variable pseudo displacement signals can be generated in the case when the DEM errors are not exactly corrected [24].…”