北京平原区现阶段主要沉降层位与土层变形特征

超量开采地下水引发的地面沉降已成为北京平原区最主要的地质灾害之一.精准识别现阶段地面沉降主要贡献层位,查明不同水位变化模式下土层变形特征,对实现地面沉降精准防控,建立合适的地下水-地面沉降模型具有重要意义.本文根据北京市7个地面沉降监测站内分层标和水位近十几年观测资料,对不同深度土层沉降变化特征和主要沉降层位进行了精准...

MAIN SUBSIDENCE LAYERS AND DEFORMATION CHARACTERISTICS IN BEIJING PLAIN AT PRESENT

Land subsidence caused by over-exploitation of groundwater has become one of the most important geological disasters in the Beijing Plain. The important tasks are to accurately identify the main contributing layers of land subsidence and to analyze the characteristics of soil deformation under different water level change modes. They are of great significance for establishing a suitable groundwater-land subsidence model and achieving precise prevention and control of land subsidence. This paper uses the extensometer and corresponding groundwater level observation data at land subsidence monitoring stations in Beijing for the past ten years. It accurately identifies the main deformation layers and reveals the deformation characteristics at different depths of soil layers. It then analyzes the deformation characteristics of different compression layer groups and sand layers under different water level change modes. It discusses the reasons for the large residual deformation and the deformation lag of the clayey soil layers. The results show the following findings. (1)The main subsidence layers are the second compression layer group(middle-deep strata) and the third compression layer group(deep strata) in the Beijing land subsidence area. The average subsidence ratio is 31.01% and 60.73%. The proportion of subsidence is gradually increased. (2)The amount of soil deformation at different depths and its proportion in the total subsidence are not only closely related to variation of groundwater level, but also related to the lithology and thickness of the soil layer. When the thickness of the compressible soil layer is large, even if the groundwater level drops small, it can produce a large amount of deformation. (3)The deformation characteristics of different lithological soil layers under different water level change modes can be summarized into 5 categories. The sand layer is mainly characterized by elastic deformation. The cohesive soil layers of different depths have elastic, plastic and creep deformation. The soil layers have obvious characteristics of viscoelastic-plastic deformation. (4)The groundwater level in the plain has changed from falling to rising in 2017. The deformation characteristics of the soil layers are quite different. The first compression layer group is transformed from elastoplastic deformation to elastic deformation. When the second and third compression layer groups are mainly cohesive soil, the soil layer always exhibits plastic and creep deformation. If it is mainly sand layer, it can show the plastic and creep deformation before 2017, and the plasticity, creep and elastic deformation after 2017, with obvious viscoelastic-plastic features. (5)The large residual deformation and deformation lagging of the cohesive soil layer are mainly caused by two factors. First, the inelastic water storage rate is greater than the elastic water storage rate. Second, the excess pore water pressure in the cohesive soil layer dissipates slowly, and the soil layer exhibits delayed water release, which results in delayed soil deformation.