Abstract: The land subsidence in Beijing is divided into north and south areas, and the southern area mainly includes Lixian-Yufa subsidence zone. The subsidence rate in this area has accelerated in recent years and reached 50-75 mm/a during 2017-2019, showing new subsidence characteristics. Based on the subsidence and water level observation data of 7-year time series from 2011 to 2017 at Yufa Subsidence Monitoring Station, the only subsidence observation station in the south subsidence area. The deformation characteristics of soil layers, especially clay layers, are systematically analyzed. Then the elastic and inelastic specific storage, vertical hydraulic conductivities and deformation lag time of the clay layers are estimated by the stress-strain diagram method and the type curve method. The results show that there are both elastic deformation and plastic deformation in each soil layer, and the plastic deformation is the main deformation with obvious deformation hysteresis. However, the deformation rate of each soil layer presents different characteristics. The results of the stress-strain diagram method show that the inelastic specific storage of the first and second clay layers is between 1.49×10^-5 and 2.10×10^-5, the elastic specific storage is between 3.93×10^-5 and 8.38×10^-5, the inelastic specific storage is 2-4 times of the elastic specific storage, the vertical hydraulic conductivity is between 6.20×10 ^-6~4.39×10^-5 m/d, and the soil deformation lag time is 2.1-12.8 years. The specific storage of the shallow layers in the study area estimated by the type curve method is about 4.8×10^-4, and that of the deep layer is between 1.08×10^-4 and 2.05×10^-4. The specific storage of shallow layer is 2-4 times of that of deep layer, and the vertical hydraulic conductivities are 1-2 orders of magnitude larger than that of the deep layers.

Key words: land subsidence, clay layer, deformation characteristics, hydrogeological parameter, stress-strain diagram method;
type curve method