| With the rapid economic development and large-scale urban construction, environmental geological problems are prominent day by day, and become the focus of public concern. Land subsidence, as one of the environmental geological problems, becomes a key focus because of its persistent, large-scale, big destructive and difficult to control. Therefore, the system summarizes the characteristics of land subsidence, in-depth analysis the mechanism and causes of land subsidence and quantitative evaluation various factors'influence to land subsidence is significant for developing reasonable measures to prevent and control land subsidence and promoting sustainable economic development.Land subsidence started in the 1960s and developed rapidly caused by groundwater overpumping in XX city. After the land subsidence room was established which monitored the head and settlement of every layer in 1984, the groundwater mining and land subsidence obtained the effective monitor and control. Although XX city has given the high attention and positive research to land subsidence, the previous work are limited to the monitor of groundwater and land subsidence and the characteristics, mechanism and affluent factor lacks system's analysis and research.Base on research of natural geological conditions, regional geology, engineering and hydrogeological condition, this paper combines with land subsidence history of the study area, analyses the characteristic and mechanism of land subsidence, constructs the model of groundwater pumping-land subsidence and evaluation the land subsidence by numerical simulation technology. The following conclusion can be obtained:1. The average thickness of Quaternary in XX plain,formed on Cretaceous synclinal basin, experienced the continental alluvial and several times of marine transgression, is about 100 meters, increasing from southwest to northeast. According to the hydraulic properties of formations, the Quaternary includes two aquifers:the first aquifer and the second aquifer. The top depth of the first aquifer is about 50 meter and thickness is 13 meters. The top depth of the second aquifer is about 75 meter and thickness is 10 meters. While according to the mechanical properties of formations, the Quaternary includes six aquifers:the first soft soil group, the fist hard soil layer, the second soft-hard soil layer, the first aquifer, the fourth hard soil layer and the second aquifer.2. According to the collected borehole data and geological cross section's information and based on the TIN method of GMS software, this paper construct there dimension geology model. From this model, we can obtain:the upper of first aquifer covers all XX plain and the lower of first aquifer and the second aquifer miss in the southwest. The two aquifers'lithologies exist zoning whose direction is southwest-northeast and northwest-southeast respectively.3.The overall trend of groundwater funnel area in XX city decreases from 1983 to 2008, the centers of which are located in HF textile factory northern section of JD, WX textile factory southern section of JD and RF textile factory western of city. The funnel of land subsidence and water level are basically the same before 2000, which reflects that groundwater extraction mainly causes the land subsidence. But after 2000 the funnel center for groundwater and land subsidence has shifted and some local land subsidence appears in southeast and southwest, which reflects construction causes the land subsidence.4.The subsidence in center of funnel reached 258.5mm(relative 1984.3) at the end of 2008. The land subsidence of the first soft soil group, the second soft-hard soil layer and the first aquifer accounts for 90 percent in total, include 67 percent,12 percent and 11 percent respectively.5. Deformation characteristics of soil layers are different and there are three deformation forms: elastic, plastic and elastoplastic. The second aquifer and the first hard soil are elastic. The fourth hard soil layer, the first aquifer and the second soft-hard soil layer are elastoplastic. While the first soft soil group is plastic. Deformation characteristics of elastoplastic layers in XX city didn't show no-linear characteristics considered pre-consolidation pressure which is used by many scholars, while showed some another characteristics. Another characteristic for layers is that deformation shows some stages. The fourth hard soil layer, the first aquifer and the second soft-hard soil layer are divided by two stages:①elastoplastic before 1995②elastic after 1995. The first soft soil group is also divided by two stages:①slow deformation before 1997②fast deformation after 1997. The ability of whole deformation is:the first soft soil group>the second soft-hard soil layer>the first hard soil layer>the fourth hard soil layer> the first aquifer>the second aquifer.6. The internal reason for land subsidence is soil's property, while the external is mainly groundwater pumping and static and dynamic load on the surface. The first soft soil group doesn't finish consolidation and is easy to compress. Groundwater pumping mainly affects the subsidence of soil which position is in the middle and deep (include the first hard soil layer, the second soft-hard soil layer, the first aquifer, the fourth hard soil layer and the second aquifer). While the static and dynamic load on the surface affects the subsidence of the first soft soil group. The subsidence in the middle and bottom of the first soft soil group is affected by the two factor. 7. Based the dynamic monitor data of groundwater and land subsidence, this paper has done some analytical research about the inverse method of aquitard's parameters. The analytical solution of groundwater level and land subsidence in the aquitard is obtain, which provides a new method to get the aquitard's parameters. According to the theory, the pressure conductivity coefficient for the fourth hard soil layer, the second soft-hard soil layer and the first hard soil layer can be calculated, 10 m2/month,3 m2/month and 400m2/month respectively.8.Based the above conclusion, the groundwater pumping coupled land subsidence model in the main area is constructed and the water level and land subsidence is calculated by owe program code. To some extent the result reflects the previous knowledge and provides information to establish an accurate and reliable model. |
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