| Karst Collapse is a major type of geological disaster for cities located in karstic areas whichare promoting the process of urbanization. With these characteristics such as concealeddistribution, burstiness, group-occurring and severity, karst collapse does serious harm toproduction, construction, people’s lives and property. In Guanghua basin, the well-developedconcealed karst, overlying thin Quaternary gravel layer, un-steady clay footwall with lowcohesive force and dramatic human activities are the main factors for collapse accidents inGuangzhou city.At present, researches about karst collapse largely focus on description of features ofkarstification and discussions of disaster-causing factors on the basis of karst geologicalsurvey. Risk assessment of karst collapse contains such the following two aspects as riskprediction of karst collapse and vulnerability assessment of hazard-affected body. Riskprediction of karst collapse has been paid much attention to in the past two decades andresearch achievements have been obtained in a considerable scale, while at the same time,consensus concerning with vulnerability assessment of hazard-affected body has not beentargeted. Karst collapse mainly occurs in Northwest Guangzhou’s Huadu and Baiyun districts.Nowadays, the times and scale of karst collapse in these kartic areas are increasing asGuangzhou city expansion heading for the North, which makes it an urgent need foromnifarious investigation and researches in the aspect.Based on previous investigations and studies, the subject of risk analysis of the karstcollapse in Guangzhou Baiyun district was studied further in details, the main contents andconclusions can be listed as follows:（1）The formation condition and mechanism of karst collapse were summarized. Theoccurance of karst collapse is controlled by interaction of basic collapse requirements anddynamic factors. The collapse mechanism is influenced by different effects, multiple factorswith regard to physics and chemistry.（2）The natural geography conditions, comprehensive engineering and geologic situations,features of kartification, distribution regularity of karst collapse of Baiyun district wereinvestigated and inducted. The regional comprehensive engineering geological map, primaryprofiles of cross section and distribution display of subsidence site（group）were drawn basedon previous investigations and arrangements. These foundamental collections andpreparations are of great necessity for regional research of karst collapse.（3）The knowledge system of project risk management, methods of risk identification, analysis flow of risk estimation and common probability distribution, methods of riskevaluation were summarized. A simplified probabilistic analysis model was described andapplied in the failure risk analysis of North dam built for retaining impoundment of a miningoperation and processing plant located in karst terrain in Florida.（4）The basic risk factors resulting in regional karst collapse were indentified. Weightingfactor of risk elements affecting karst collapse regarded as a risk event was calculated byanalytical hierarchy process method and the risk factors were ordered according to weightcomputing. Eight factors which mostly influence karst collapse were choosen as the basicparameters involving in calculation of karst collapse probability. The eight factors are:overburden thickness, karst development degree, groundwater depth, bedrock lithology,overburden structure and bottom plate soil properties, groundwater exploitation intensity,underground activity intensity, ground surface construction load and vibration.（5）The grade standard basic risk factors of regional karst collapse fuzzy assessment wasformulated. The fatalness classification of regional karst collapse can be shown as the mostlikely subsidence area（Ⅰ）, the more likely subsidence area（Ⅱ）, the likely subsidence area（Ⅲ）, the lower likely subsidence area（Ⅳ）and the stable area（Ⅴ）. The probabilitycalculation model of regional karst collapse was established on the basis of Bernoulliempirical probabilistic theory. Based on the probabilistic model, the criteria of karst collapseprobability was determined: for are（aⅠ）,P (R)∈[0.95，1.00); for are（aⅡ）,P (R)∈[0.85，0.95);for area （Ⅲ）,P (R)∈[0.70，0.85); for area （Ⅳ）,P (R)∈[0.60，0.70); for area（Ⅴ）,P (R)∈[0.10，0.60). The whole Baiyun district was divided into322risk assessmentelements and fatalness of karst collapse for each element was classified according tocalculated subsidence probability. Based on the above classification and calculation, thecriteria map of karst collapse for each element was composed. The conclusions can be listedas follows:5assessment units corresponding Jiang cun, Gu lougang, Quan xi, Xi hu and Shegang successively are attributed to the most likely subsidence area;73assessment unitsattributed to the more likely subsidence area distribute in Central and Southern Jiang gaotown and Midwest part of previous Shi jing town, special attention ought to be emphasized inpractice because the dynamic factors may improve the fatalness class of subsidence in theseareas;131assessment units belonging to the likely subsidence area distribute in Ren he town,Central and Western Zhong luotan town and mideast part of previous Shi jing town, theaffected area is the largest; The total unit of the lower likely subsidence area and the stablearea is113, they mainly distribute in most part of Tai he town, east Zhong luotan town and previous east Shi jing town. From the perspective of engineering investigation and karsticground foundation reinforcement, suggestions were put forward to the engineeringconstruction in the discussed karstic terrain. |
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