| Sanming City,located in the central of Fujian Province,is the main area of Fujian Eco-province construction Strategy,where the regularly planned Sanming Ecological Industry and Trade Zones promotes the spatial integration and cross-regional development of Sanming urban City,Shaxian and Yongan.As large number of projects distributed in karst areas,the research on karst collapse is particularly important during the construction process of industrial and trade zones.Since the 1970 s,karst collapses have been often reported within the region,which resulted in property loss and casualty,as well as seriously hindering the local economic construction and ecological environmental protection.Therefore,it's of great significance to investigate and analyze the characteristics of karst collapse,further then to study the distribution and formation conditions of it,for urban planning,construction,site selection,prevention and control of karst collapse,and protection of geo-ecological environment.This thesis conduct systematically and thoroughly studies on the karst development characteristics,karst collapse and relevant environmental issues within the karst area in Sanming Industrial and Trade Area,based on detailed survey of surface karst geology,karst hydrogeology and karst collapse,large number of borehole data,limestone dissolution test,underground karst cave physical detection test,and physical and mechanical index of overburden.The results of this thesis are listed as follows.Limestones in the study area can be identified as Chuanshan Formation microcrystalline limestone and Qixia Formation chert-bearing limestone according to lithology and stratigraphy features,while being divided into exposed,covered and buried limestone areas according to their burial conditions.It's generally characterized by subtropical karst landform of karst hills and depressions combination,which can be classified into three kinds: the karst solitary peak landform represented by Wanshouyan in Sanming,the karst residual hill-depression combination dominated by Dahu Basin of Yongan,and the major fault basins and underground karst landform combination mainly in Sanming Basin.The karst caves underground in Dahu Basin of Yongan developeddiscontinuously in horizon,which are distributed in limestone area level of 30-10 m,-10-25 m,-70-100 m,with mainly size of 0.5-3.0m.the The dominated form is karst pipeline,and the number of karst caves decreases rapidly,with the increase of burial depth.According to the karst development degree,the Dahu Basin of Yongan is divided into three zones: strongly developed karst area in Zengtian-Xiahe,mediumly developed karst area in Linggan-Hufeng and weakly developed karst area in Yixi-Puling.The karst geomorphology development in Dahu Basin is in the stage of topographic maturity.The karst caves underground in Sanming Basin can be roughly divided into two parts: the upper karst caves developed in the 197.5-103.3 m section with a diameter of3-5 m and a maximum of 20 m,and the lower karst caves in the 109.0-84.5 m section with a diameter of 1-3 m and a maximum of 13 m,with little filling or no filling.Sanming Basin is divided into strongly developed karst areas,considering the karst development degree.The influence factors in karst development of the area can be summarized as follows:1)The karst development is influenced by composition,texture and micro-fissure situation of the lithologic mass.2)The geological tectonism is so closely related to karst that not only controls the direction of karst development,but also affects the scale and size of it,proved by higher karst rate on the fault zone.3)The groundwater hydrological network is the dominant factor of karst development certified by obviously stronger karst development degree in runoff and drainage areas of surface water and groundwater.4)The neotectonic movements can change the cyclic alternating conditions of groundwater,thereupon then change the karst zones in limestone and the spatial location and direction of karst development correspondingly.It is the first time to use microtremor to detect underground karst cave full of water and demonstrate its suitability.It is deemed to be feasible for carrying out large-scale exploration by comprehensive geophysical methods of high-density resistivity and microtremor in this area,using part of borehole data as calibrate to contrast the results.It's available to guide engineering investigation in Karst area,to provide physical basis for drilling hole layout,to reduce drilling engineering quantity and shorten exploration period.The limestone dissolution rate of the area is not constant,which shows a fluctuation law consistent with the change of groundwater temperature,rising with theascending groundwater temperature.There are additionly many factors affecting the dissolution rate of limestone,including the limestone texture,the fissures situation and the velocity of groundwater.The dissolution rate in micro-fissures developed limestone is relatively high.Based on the catastrophe series method,the susceptibility assessment shows that the central area of Dahu Town,such as Kengyuan Village and Hufeng Village,belongs to the most susceptible collapse region.The margin of the Dahu Basin such as north of Kengyuan Village and south of Luojiaping belongs to the susceptible area.The other buried karst areas belong to more stable or stable area.As for Sanming Basin,except the Chenda Town area classified into the most susceptible collapse region,the other buried karst areas belong to more stable or stable areas of karst collapse.This is the first time to use the catastrophe series method to quantitatively evaluate the susceptibility of karst collapse in the study area.The karst hydrological system is more complex than other hydrological systems,thus the drainage of groundwater is prone to trigger karst collapse geological hazards in covered karst area.The ecological protection of groundwater in karst areas should be managed by the whole runoff process from the recharge area to the discharge area,especially strengthen the management for the recharge area,by strictly forbidding the construction of landfills and industrial zones with high pollution.Karst collapse within the research area mostly concentrated in depressions of Dahu Basin and Sanming Basin,and can be divided into natural collapse and artificial collapse in terms of genesis.There are four modes of karst collapse mechanism :(1)corrosion-gravity collapse mode;(2)suffosion collapse mode;(3)soil rheology-differential pressure field-caprock failure and load-increasing effect collapse mode;(4)overloading collapse mode.The research is based on the analysis of sediment dating and development process in stratiform karst caves of Wanshouyan and Linyin Stone forest,comparing with stepped karst caves in Qixian cave.It shows the area has undergone at least four uplifting phases and three intermittent periods since the late Middle Pleistocene.The overall activity intensity is weak,showing neotectonic movement characteristics with intermittent and differential rising.The impact of karst collapse on urban construction include four aspects as follows:the impact on urban planning,the impact on embankment engineering,the impact ongeological relics protection and the impact on mine ecological environment restoration.The train of thought and corresponding measures for karst collapse prevention should refer to the basic conditions,the triggering conditions and the causes of collapse. |
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