The Relationship Between Land Subsidence And Groundwater Exploitation In Typical Areas Of The North China Plain

The North China Plain is one of the most serious land subsidence areas in China.According to the topography and hydrogeological characteristics,the North China Plain can be divided into three parts: piedmont,central area and coastal plain area,among which the coastal plain area is the most serious land subsidence area.Cangzhou is located in the coastal plain area.According to the comparison between the average exploitation intensity of deep groundwater and the land subsidence rate for many years in Cangzhou area,the land subsidence is directly related to the exploitation of deep groundwater.Based on the comprehensive consideration of soil mechanics parameters and regional groundwater flow field,this paper explores the controlling factors of land subsidence in typical sections of the North China Plain,and studies the response relationship between settlement of different aquifers and groundwater exploitation through numerical simulation.There is a relationship between soil consolidation and land subsidence.According to layerwise mark data,the contribution of deep soil settlement is dominant in Cangzhou.According to the high pressure consolidation test,the strata of Cangzhou are normally consolidated or underconsolidated soil within 0-150 m,and overconsolidated soil below 150 m.The degree of consolidation increases with the increase of depth,that is,the soil layer with a large contribution of subsidence has a high consolidation state.The formation compression is mainly related to the compression characteristics of cohesive soil.The basic physical test and high-pressure consolidation test on the cohesive soil samples from Cangzhou SNDC-1 borehole show that the local cohesive soil has the characteristics of large non-uniformity coefficient,poor grain grading,and greater compression coefficient in shallow layer than in deep layer,indicating that shallow cohesive soil is easier to compress than deep layer.The clay content,water content and porosity of cohesive soil decrease with the increase of depth,indicating that the basic physical indexes are also indicative of the consolidation state of cohesive soil.By comparing the stratigraphic characteristics in Shijiazhuang,Hengshui and Cangzhou,with the regional from the piedmont plain area to the coastal plain area,the particle size of the clay soil changes from coarse to fine,and the percentage of clay particles increases.And Cangzhou cohesive soil stratum thickness is greater than the Hengshui area,shows that big thickness of cohesive soil is the main internal cause of land subsidence caused by Cangzhou.Under the same depth condition,the clay content of Cangzhou area is generally higher than that of Hengshui area,and the sand content is lower than that of Hengshui area.Within 200-400 m of the main mining section,the natural porosity ratio in Cangzhou area is higher than that in Hengshui area.These indexes can better represent the characteristics that the deep cohesive soil in the coastal plain area is easier to compress than that in other areas.Based on the coupled model of soil and water in Shijiazhuang-Hengshui-Cangzhou section,the reasons of groundwater level sinking funnel and land subsidence in deep aquifer in Cangzhou region were simulated and analyzed considering the difference of mechanical properties of soil in piedmont plain,central plain and coastal plain.Aquifer IV will produce significant contribution to compression even if it is not directly exploited.When the shallow groundwater is overexploited,the upflow effect will also cause significant increase in the settlement of aquifer III.These understandings can provide theoretical basis for the subsequent control of land subsidence.