A discussion of the cause of land subsidence in the northeast of the Xiongan New Area

In order to deeply study the main controlling factors of land subsidence in the northeast of the Xiongan New Area, the layered monitoring points of Daying Town are taken as the research object, and the causes of land subsidence are discussed based on the conventional soil test, high pressure consolidation test and layered settlement observation data combined with previous research results. The results show that the total thickness of the Quaternary loose layer in the layered monitoring points of G1 hole in Daying Town is 412 m, and the thickness of the shallow layer is 170 m. The viscous soil accounts for 66.4%−80.2%. The structure is loose and the sand-clay interlayer alternates frequently, and the water release condition is good. The shallow viscous soil has finer particles, good sortability, large porosity, and more than 0.25 liquid index, showing a soft plastic and plasticable state. The dead weight compression coefficient ranges from 0.03 to 0.43, with an average value of 0.07. Compared with the deep soil, the shallow viscous soil is of stronger compressibility. The shallow cohesive soil layer is dominated by underconsolidation, normal consolidation and underconsolidation. The average OCR of a depth of 0−90 m is 0.55, and that of 90-280 m is 0.89. Plastic deformation and permanent settlement are easy to occur. Xiongan New Area mainly exploits shallow groundwater, which is closely related to land subsidence. From December 2020 to December 2021, the monitoring results of the layered monitoring points of Daying Town show that the total settlement of the Quaternary loose layer is 61 mm. Among them, the subsidence contribution of the shallow Quaternary unconsolidated layer at a depth of 5−160 m is the largest, which is 42 mm, showing the characteristics of plastic deformation. The subsidence contribution of the deep Quaternary strata at 160−415 m is 19 mm, which shows the characteristics of viscoelastic-plastic deformation. The plastic deformation of cohesive soil layer with low consolidation and high compressibility caused by the excessive exploitation of shallow groundwater is the main cause of serious settlement.