A Study On Hydrogeological Characteristics Of Clay Layer With Rich Lime Concretion In Xuzhou Area

The clay layer of Quaternary Epipleistocene in Xuzhou area generally contains lime concretion(also known as shajiang),which is named ?3-4 clay layer by local engineering academic circles.The lime concretion results in good permeability of the clay,which seriously affects the rail transit construction in Xuzhou city.Therefore,it is necessary to ascertain the spatial distribution and hydraulic characteristics of the clay layer with rich lime concretion in Xuzhou area,and establish groundwater flow and land subsidence models.A comprehensive and objective understanding of the clay layer must be conducted by the qualitative and quantitative analysis.The investigated area in this paper includes five districts in Xuzhou:Gulou,Quanshan,Tongshan,Yunlong and Jiawang,totally covering an area of approximately 880km2.According to the 4,748 borehole data reference,the paper adopts Kriging interpolation method to form the contour maps of the buried depth and thickness of the clay rich in lime concretion in the study area.Combining the topography and geomorphology characteristics,the distribution of lime concretion can be summarized as follow:the buried depth of lime concretion in the study area is normally from 0 to 16m;the buried depth is relatively shallow in the vicinity of hills and piedmonts.It shows a pattern that the depth and thickness grow from hills to plain area.The buried depth is deeper along the old floodplain of the Yellow River,which is due to the depositing of thick silt in history.In order to study the hydraulic characteristics of lime concretion clay,a layered pumping test was carried out on the?3-4 clay layer of Erhuanbeilu Station on Line 2 and the hydraulic conductivities was conducted by using the AquiferTest software.Integrating the pumping tests of other stations,it was found that the permeability of the clay is related to the content of lime concretion in it.The higher the content of lime concretion is,the greater the permeability is.The water level of the bedrock observation hole decreased greatly during the pumping period,indicating that there was a hydraulic connection between the?3-4 clay layer of the Erhuanbeilu Station and the underlying bedrock.When water was pumped in the ?3-4 clay layer,bedrock fissure water supplied the layer.On the basis of above study,the regional water flow model of the study area was established,identified and verified using the Modflow module in the GMS software and the observation of the water level.Next,the local flow model of Erhuanbeilu Station was nested in the regional flow model.Combined with the observation data of the water level at the Erhuanbeilu Station,more reasonable parameters of the local site were obtained after further adjusted and fitted.The subsidence model was built on the basis of the flow model of Erhuanbeilu Station,and the Sub Module in the GMS software was used for simulation calculation.The purpose of constructing a numerical model of flow and settlement is to predict the water level and subsidence under a specific pumping scheme.This paper takes Erhuanbeilu Station as an example,predicts the foundation pit rainfall using the verified nesting model of Erhuanbeilu Station.When the 15 wells in the pit were pumped at a rate of 110m3/d,the maximum drawdown in the pit was 16m in continuous 12-day pumping,and the drawdown was 1.9m within a range of 600m in the foundation pit.The maximum influence radius of group well pumping was about 700m.The paper also calculated that the maximum subsidence value in the foundation pit during pumping is about 10mm,and the subsidence value in the range of 300m in the foundation pit exceeds 5mm,and the subsidence value is only 2.5mm within 500m from the center of the foundation pit.The land subsidence caused by rainfall in the foundation pit under this forecast scheme is within the scope of monitoring and warning,and will not cause environmental geological disasters.As for other stations of urban rail transit construction,the modeling method of Erhuanbeilu Station is helpful when forecasting water level and subsidence according to the needs of the actual project.