Characteristics of groundwater and land subsidence evolution before and after the South-to-North Water Diversion Project in Beijing, China

Land subsidence caused by over-exploitation of groundwater has become one of the important factors restricting the sustainable development of the society and economy of Beijing. In December 2014, the middle route of the South-to-North Water Diversion Project was officially opened, and more than 1 billion m of water was delivered to Beijing every year. As a result, the pattern of water supply in Beijing has changed, and conditions have been created to reduce the exploitation of groundwater, conserve groundwater, and control land subsidence in Beijing. This paper uses a variety of monitoring data to analyze the changes in groundwater and land subsidence in the Beijing Plain before and after the South-to-North Water Diversion Project. The deformation characteristics of soil layers with different lithologies and depths under different groundwater level variation patterns are studied. The elastic and inelastic skeletal-specific storage rates of the soil layers at different deformation stages are calculated. The reasons for the large residual deformation and hysteresis deformation of the cohesive soil layer are discussed. The results show that: ® From 2015 to 2020, the groundwater level of the first to fourth aquifer groups in most areas of the plain gradually increased, and the land subsidence gradually slowed down. (2) The second and third compression layer groups in the plain are the main contributing layers to the subsidence. Except for Pinggezhuang and Yufa station, the subsidence proportion of the third compression layer group in the other stations gradually increased. The main subsidence layer is gradually transferred to the deep formation. (3) In the north and east of the plain, the groundwater levels corresponding to the second and third compression layer groups changed from falling to rising. During the drop stage of the groundwater level, the soil layer exhibits plastic and creep deformation. During the rise stage of groundwater level, the soil layer exhibits plastic deformation and elastic deformation for part of the time, which is viscoelastic plastic. The soil layer exhibits the characteristics of viscoelastic plastic. In the southern part of the plain, the groundwater levels continued to decline, and the deformation of the soil layer was always plastic and creep deformation. The sand layer mainly exhibits elastic deformation. For different stages of soil deformation, the change in elastic and inelastic skeletal-specific storage rates is not constant. As the groundwater level decreases, the soil skeletal specific storage rate shows a decreasing trend. (5) The reasons for the large residual deformation and deformation hysteresis of the cohesive soil layer mainly include two aspects. Firstly, the inelastic skeletal specific storage rate is greater than the elastic skeletal specific storage rate. Secondly, the weak permeability of the cohesive soil layer. © 2024 Geological Society of China. All rights reserved.