The evolution characteristics and mechanism of the land subsidence in typical areas of the North China Plain

The North China Plain (NCP) is one of the biggest groundwater systems in the world, and land subsidence occurs commonly. Due to diverse sedimentary environments, different geological conditions, and intensive groundwater exploitation, the mechanism of land subsidence is complex. With the help of earth observing technique and traditional monitoring tools, the disaster process of land subsidence was monitored, and the developing history and current situation were analyzed. Combined with strainstress diagrams and soil mechanics tests, the difference features of land subsidence and the mechanism of hysteretic deformation were analyzed. Some conclusions have been reached:(1) The spatial distribution of land subsidence is significantly distinct. The land subsidence areas are mainly located in the areas of the Quaternary sedimentary depressions, having characteristics of west-east sub-zone and north-south subsection. The spatial development of land subsidence is partially controlled by NE-and NW-trending structures. The land subsidence in the Cangxian uplift is serious in that the tectogenesis is relatively inactive in Quaternary and the loose sediments are very thick. Also, there exist paleochannels of three layers with the same strike as the structures, whose groundwater resource is abundant and has been intensively exploited. (2) The development of subsidence is correlated with the local groundwater exploitation history, and the major contribution layers to the land subsidence have varied with the change of exploited layers. The contribution of the strata below 100m has increased in the Beijing plain. The land subsidence of the Tianjin plain mainly comes from the compression of the strata below 300 m. (3) The drought is an important and indirect factor leading to subsidence by reducing natural recharge and leading to the increase of groundwater exploitation for emergency water supply. Besides, in the groundwater exploitation, much attention should be paid to the effect of the heavy weight of densely constructed buildings, dewatering of foundation pits, and the exploitation of deep geothermal water.(4) The hysteresis of land subsidence is obvious, and the time of hysteresis can last for twenty-five years. Besides the consolidation, the creep is another important reason for the hysteresis. The creep is obvious for the Pleistocene strata, which can reach 28.3% of the total deformation for the Late Pleistocene strata in the Cangxian uplift. (5)The deformation characteristics of the soil layer bear strong relationship to the physical characteristics and change pattern of groundwater levels. The strata at different depths have distinct deformation characteristics such as elastic, visco-elastic, and visco-elastic-plastic deformations. Shallow aquifer groups indicate typical elastic deformation.