| Shenzhen is the first urbanized city in China and the main area of karst distribution in China.At present,the area of karst detected accounts for about 10.8%of the urban area,and most of them are covered karst.How to accurately and economically determine the characteristics of urban underground karst development and distribution is a major problem in urban karst exploration.It is also an important basis for regional karst research.In the early stage,more than 20 karst collapses occurred,which were distributed in the thinner areas of the city,resulting in casualties of human and animal,collapse of residential houses,halt of Engineering projects,and huge losses.Shenzhen City,as one of the four major central cities in the planning of Dawan District of Guangdong,Hong Kong and Macao,has a fast development speed and a large economic total,but its land resources are limited and its population density is high.Once the geological disaster of karst collapse occurs,it will cause heavy casualties and property losses.The study of the spatial distribution of urbankarst areas,and further research on the risk assessment of ground collapse in karst area not only has important practical significance,also has the important academic value.This research takes the area of 30km~2 in a district of Shenzhen as the research area,a multidisciplinary and comprehensive approaches been used to study the detection,development distribution and collapse risk of urban karst.In this study,the geological environment background of karst development has been analyzed comprehensively,and the comprehensive exploration method suitable for urban karst has been adopted.The development characteristics,spatial distribution rules and influencing factors of karst development are revealed.Based on the analysis of the influencing factors of karst collapse,the mechanism of karst collapse has been further discussed.Based on the particularity of urban environment,the urban planning map has been applied to risk assessment.Combined with the basic geological conditions of karst collapse,artificial conditionsand the vulnerability conditions of disaster bearing body,the risk assessment system and model of the study area have been constructed by using a variety of assessment methods,and the risk grade assessment of the collapse disaster of the study area has been realized.A total of the following results have been achieved:(1)A comprehensive study of the background of regional geological environment has been carried.The soluble rocks in the study area are marble and limestone of the lower Carboniferous subgroup.The soluble rocks are mainly distributed in the Quaternary alluvial and residual slopes,and a small amount of soluble rocks are under the sandstone and granite of the Carboniferous water measuring group.The main types of groundwater are karst fissure water and loose rock pore water.The water content of karst water is moderate,and the water content of loose rock pore water is poor to medium.The geological structure of the regional is complex,and there are many faults round the study area.The Bi Ling fault,Tang hang fault,Pingshan fault and the Xianshui Lake fault have the greatest influence on the regional karst development.(2)The comprehensive karst detection method for urban environment has been adoped.On the basis of previous scientific research achievements,following the principle of key investigation in key areas and aiming at exploring karst caves and soil caves,we have been triedfour geophysical methods,including high density electrical method,geological radar,elastic wave CT and transient electromagnetic method,and drilling methods,to detect key areas.A total of 48 lines of high density electrical method and geological radar profiling have been completed.Fourteen surface surveying lines,three pairs of elastic wave CT imaging,two transient electromagnetic surveying lines and 43 geological boreholes have been collected.Combined with the collected borehole data,through repeated comparative tests,it is considered that the high density resistivity method is more suitable for urban karst exploration in the study area.At the same time,the abundant drilling data are used to verify and supplement the geophysical exploration results.The location,scale,shape and filling of karst caves in the study area are proved.It is a fast,economical,non-destructive and effective means to reflect the information of urban underground karst.(3)Study on karst development characteristics,spatial distribution.The karst in the study area is mainly the covered karst,which distributed in the whole study area,the development pattern of the karst is the cave,soil hole,karst grooves,which at the level of weak to moderate development on the whole.The most of caverns are semi-filled and fully filled caverns,the filling material is composed of gravel cohesive soil,silty clay,silty soil,breccia and karst debris.From west to East,the filling of karst cave presents a process of gravel clay clay,sandy clay,fine sand and medium coarse sand.The cavity with no filling and part of half a pack of karst cave in the study area are at the stage of development.The cave profile in the study area show elliptical or irregular shape.The overall distribution of karst caves shows great heterogeneity.The development of karst caves is mainly small or medium-sized,mainly distributed in the east and west of the study area.Large karst caves are mainly distributed in the east,and only a small number of large and extra large karst caves are distributed in the middle.The multi-layer karst caves are mainly distributed in the east side of the Pengxi mining area,Niujiaolong area and Salt Water lake area,with obvious regional characteristics.The distribution law of karst elevation is higher in the West and lower in the East,which is consistent with the change of regional topography.The vertical distribution of karst shows that the development gradually weakens with the increase of depth,and generally develops in the range of 15-40m,and the development depth of multi-layer karst cave is shallow.With the increase of burial depth,the proportion of small and medium-sized karst caves decreases gradually,while the proportion of large to oversize karst caves increases gradually.The depth of karst development reflected in the plane is shallow in the west,the deepest in the middle,and the second in the east.The karst development in the study area is mainly influenced by hydrological conditions,topography,geological structure and lithologic conditions.Among them,geological structure plays a leading role,and there are many fault structures in the area.Under the action of abundant surface water system and groundwater supply,it not only provides good conditions for the development of underground karst,but also controls the direction of karst development.(4)The origin and mechanism of karst ground collapse are studied.The analysis shows that the karst collapse in the study area is mainly affected by karst development,lithology,and caprock conditions,and there are natural and human factors such as atmospheric precipitation,fluctuation of groundwater level and human pumping groundwater.The collapse mechanism of the study area affected by surface water infiltration,groundwater drop is studied.The process of rainfall induced collapse in the collapse point of Salt Water lake is simulated and verified.In karst areas with thin overburden,under the influence of rainfall or water accumulation,when the upper soil reaches saturation or has a certain depth of rainfall,the soil mass increases seriously,and karst ground collapse may occur.(5)Establishment of risk assessment system for urban karst ground collapse.Based on the analytic hierarchy process,sensitive factor analysis,expert decision-making and other methods,this paper mainly considers the influence of three basic factors on the collapse,i.e.the degree of karst development,the amplitude of groundwater level variation and the depth of karst development.It mainly considers that the urban karst collapse is affected by the human activity intensity,and the urban planning map is included in the evaluation calculation,and the risk assessment model of karst ground collapse is established.According to the mathematical model of risk assessment,the risk value of karst collapse in the study area is carried out.The study area is divided into four risk levels:high,medium,low and risk-free.Among them,the high risk area of karst collapse is 2.53km~2;the medium risk area is 6.5km~2 and the low risk area is 4.74km~2.The risk-free area is the non carbonate distribution area in the study area,with an area of about 15.44km~2. |
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