| Carbonate strata are widely distributed in China,with an exposed surface area of910,000 km~2.However,the development of karst has spatiotemporal heterogeneity,it is difficult to fully judge the strength of karst development.The construction of high-speed railway was mainly based on the scheme of avoiding soluble rock in karst areas,while in southwest of China and Western Hubei,soluble rock is continuously exposed in a large area,leading to the inapplicability of conventional methods of avoiding.Western Hubei,where the E-Yu section of Zheng-Wan High-speed Railway runs through,locates at the junction of the second and third steps in Chinese topography.The terrain is undulating,the karst types are abundant,and the spatial distribution is complex.Therefore,it is difficult to accurately describe the karst development degree through the field karst geological survey.The engineering specifications in Western Hubei are harsh,karst disasters and accidents were frequent in the built karst tunnel,and karst water utilization rate in the region is high.How to avoid karst disasters,ensure construction safety,and reduce the impact on the ecological environment in the region has caused great trouble to the line selection of Zheng-Wan Railway.The karst problem has become the primary problem restricting the railway construction in the region,which restricting the traffic an-d economic exchanges between the southwest center of China.The conventional op-timization schemes of karst geological route selection are mostly qualitative analysis of karst development degree or comparison and selection based on existing routes,which lacks of quantitative analysis and mathematical model generalization.Therefore,it is of great significance and value to analyze the controlled mechanism of karst development in the study area and establish a more scientific and objective multi-level and multi-index karst geological line selection model based on the unique hydrogeological conditions in Western Hubei.Based on the geological and karst hydrogeological conditions the field survey data,this paper starts from the controlled mechanism of karst development:Study and summary the influencing factors of karst development in the study area.Consider the independence and correlation of each index and determine the influence weight of ea-ch index by the analytic hierarchy process(AHP).Determine the karst development index of each region in the study area by fuzzy comprehensive evaluation method,an-d design the ideal line position by geographic information system technology(GIS).Through the above research,the following understandings are obtained:(1)The exposed carbonate strata area accounts for 72.78%of the total area of the study area,and Triassic soluble rock accounting for 47.01%of the total exposed area of the carbonate strata in the study area.Therefore,the large-scale continuous development of karst and the accompanying karst water problem are the main reasons for the layout of high-speed railway in the study area.According to the exposure of the carbonate strata,the soluble rock strata can be divided into:continuous soluble rock strata,carbonate intercalated non-soluble rock strata,and mutual soluble rock strata.Study area has experienced the geological movement of multi-next multi-cycle,with different structural patterns,crisscross faults and mutual cutting.There are many types of folds,including overturned folds and complex folds.(2)Faults mainly control karst water drainage in the study area,and fold structure mainly controls karst spring outcropping in the study area.Karst pipes are gradually formed in the main fissures under the dissoluteion of groundwater,so that karst water in the unit can be discharged centrally.Synclines catchment capacity is significant,many karst spring and underground rivers expose along the syncline axis scatteredly.The main discharge modes in the study area can be divided into three types:the centralized discharge area of pipeline flow in the middle-low mountain valley area on the west side of the study area,the centralized discharge type of network fractures in the platform edge trough valley in the middle of the study area,and the dispersed dis-charge type of pipeline flow in the hills-low mountains depression area.The karst water in the study area is dominated by HCO3-Ca Type and HCO3-Ca·Mg Type,which are typical groundwater types in karst areas.The pH value is between 7.2 and 8.1 which is weakly alkaline.SIc value is between-0.350 and0.583,and karst water has moderate to strong erosion.(3)Combining with the karst geological background and the karst controlled mechanism in the study area,the indexes affecting the development of karst in the study area are determined as follows:soluble rock index,geological structure index,topographic and geomorphic index and hydrochemical condition index.The membership images of each index to different karst development degree has been determined by analytic hierarchy process(AHP)and fuzzy mathematics theory.Karst development index in the study area was evaluated by weighted average method through GIS.(4)Karst development in the study area is controlled by structure and topography and landform:the karst development index is relatively higher in the influence zone of fold flanks and faults,so it should be avoided as far as possible when choosing routes.The karst development index increased gradually from watershed to drainage boundary.The karst development index near the boundary between soluble rock and non-soluble rock is generally higher,because there is a concentrated discharge of groundwater at the boundary between soluble rock and non-soluble rock,resulting in local karst development.When choosing the route,it is not easy to judge that the engineering condition near the non-soluble rock is better.(5)After evaluating the karst development index in the study area,the optimal cost path function of geographic information system has been utilized to generate the optimal high-speed railway line position across the soluble rock.The line position was guaranteed to meet the design specification of high-speed railway at the same time,and the karst development index of line position is as low as possible and the distance is as short as possible. |
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