Fatalness Assessment Of Earth Fissure Hazard In Suxichang Area Based On GA-ANN Technology

A series of environmental geology problems including land subsidence and earth fissures were caused by extensive exploitation of the groundwater in Suxichang Area. As a result,all these problems not only brought huge economic loss but also affected local social stability. In order to hold the spatiotemporal distribution characteristic and law of earth fissures in the future in study area to provide the impersonal and scientific geology gist for setting up measures about disaster prevention, proceeding the risk assessment and carrying out the plan of land utilization rationally, it is of significance to evaluate the earth fissurehazard in Suxichang Area.Recently,the research related to fatalness assessment of earth fissure hazard is limited both here and abroad, and the external literature has not been searched yet whereas the methods applied to other regions by some domestic scholars are not fitted to the practical situation in Suxichang Area. Although the previous work on estimating earth fissure hazard of Suxichang makes a number of achievements accumulated, it needs improving that whichevaluating indicator can be selected and which evaluation method can be used .Based on the knowledge and demands above, six evaluating indicators are selected under the careful analysis of the primary influence factors of earth fissure in research areaand the evaluation model of earth fissure hazard is established using GA-ANN technology.Ⅰ. Analysis of the primary influence factors of earth fissureThe occurrence of earth fissure is controlled by six factors——the discrepancies in the forms of bedrock, the aggradation structure of Quaternary period, the asymmetry of land subsidence and excessive development of ground water.The geology environment has been particularly affected by the undulating shape of palaeobasement that dominates the spread of palaeochannel and the palaeohydrodynamic condition and by which, Quaternary deposit has been influenced intensively. The exploration information of borehole and geophysical prospecting reveals that 68 percent earth fissures keep a uniform with the underlaying burial hill in trend and it shows that earth fissure is consistent with areal formation trellis. Obviously, there exist an intimate relationship between bedrock relief and the occurrence of earth fissure hazard.Quaternary lithology and structure is instable in the zone where bedrock relief varies greatly and the hydrogeology condition is different, which is mostly represented by the diversity distribution of aquifer and clay soil. In addition, the resistance to deformation of surface soil is another necessary condition for earth fissure hazard happenning. Earth fissure hazard arises easily in the place where the plasticity of surface soil is poor.The properties of aquifer and clay soil are the main controls of asymmetry land subsidence. Under the condition of groundwater recession, the aquifer thicker and the sandstone grain grosser, the degree of compaction caused by leakage from aquifer is bigger.The amount of deformation of the clay soil is determined by the thickness and mechanical property of clay soil.Over-pumping groundwater, as an important inducing factor for earth fissure hazard,can lead the consolidation deformation of soil strata within the range of drawdown of water level. As a result, the depressed area is formed. When the amount of subsidency exceeds the anti-pulls limit of soil strata, earth fissure hazard takes place.II. Selection and quantification of evaluating indicatorBased on the analysis of each influencing factor and considering the factual material in study area, this theme selects the depth of bedrock burial, the degree of bedrock relief, water level, the gradient of land subsidence, transmissivity and the thickness of clay soil to evaluate the fatalness of earth fissure in Suxichang Area.The depth of bedrock burial and the degree of bedrock relief are the quantizationindicators that express bedrock relief influencing on earth fissure hazard. The degree of bedrock relief is the difference between the depth of bedrock burial at each evaluation cell location and the maximal depth at neighborhood cell location.Transmissivity and the thickness of clay soil represent the action the aggradation structure of Quaternary period on earth fissure hazard. Suppose that the physical property of groundwater in the study area is uniform everywhere, transmissivity is the integration of porous medium lithology and thickness of aquifer and it reflects the ability aquifer compressedThe gradient of land subsidence is the quantization indicator for inhomogeneity of land subsidence. Pull apart stress is concentrated in the edge zone of subsidency where earth fissures is prone to happen and the gradient of land subsidence is dominant. Water level is the quantization indicator that groundwater overpumping may lead earth fissure hazard. The piezometric level of the second confining layer is selected which is the main developing layer in study area in this paper.III. Fatalness assessment of earth fissure hazardBP network overcomes the difficulty of establishing specific arithmetic formula in the service of fatalness assessment of earth fissure hazard and it can attain the nonlinear mapping between influencing factors and danger level of earth fissure hazard. The traditional BP algorithm adjusts network weights in the basis of error gradient descent. Owing to existing multi-part least point on the error curved surface, it makes the rate of convergence become slow and network is easier to drop into local minima point during training. Therefore , the initial weights play a key role in the process of getting network result.GA has the search ability in full scale and it is free from the restriction of spatialsearch condition. This method is able to obtain the global optimum solution according to probability, so it possesses comprehensive applicability.GA-ANN combines BP with GA to take advantage of GA to overcome the intrinsic shortcomings of BP, that optimizes the initial weights of ANN with GA.Finally, the GA-ANN model of fatalness assessment of earth fissure hazard is established which takes the six evaluating indicators as the input of network and the fatalness coefficient of earth fissure hazard as the output. The structure of network is 6-15-25-1. The result shows that GA-ANN model is more precise than BP model based on atest using these two model respectively.According to the danger classification, the research area is divided into four zones— —danger zone, sub-danger zone, sub-safe zone and safe zone. The danger zone is centred in the east of Changzhou, the middle of Wuxi and the south of Jiangyin. The safe zone is mainly distributed in the northeast of Zhangjiagang, the east of Changshu city and the north and southwest of Changzhou city. The sub-danger zone and sub-safe zone is the transition zone. The result shows that the danger zone is the zone where the bedrock relief is larger together with the depth of bedrock burial being smaller, the extent of drawdown of groundwater is serious, and the lithology and thickness of aquifer is discontinuous.