| The geological conditions in the karst terrain are complex and the groundwater is active.A series of karst-related disasters,such as water inrush,mud inrush,karst caves instability,and surface collapse,might be encountered during tunnel construction in karst terrain.However,the karst-related disasters generally have the characteristics of concealment,suddenness and mass-generating.Once happen,these disasters often cause huge economic losses and casualties.How to predict or evaluate the karst disasters accurately and thus to take corresponding measures to prevent the occurrence of the related disasters,has become an urgent problem to be solved for tunnel construction in karst areas.Therefore,the attentions have been payed on the four aspects,including karst water inrush,surface collapse,impact of hidden caves on tunnel structure and prediction of karst in this dissertation.The assessment systems for evaluating risk of water inrush in tunnels,assessment systems for evaluating risk of surface collapse caused by metro tunnel construction,and the synthetically dynamic system for karst prediction were established in this dissertation based on studying the occurrence mechanism of these disasters.In addition,safety distance between tunnel structure and the hidden karst caves was also studed.The main works and achievements of this dissertation are listed as follows:(1)Based on the analysis of the occurrence mechanism of water inrush in tunnls in karst terrain and the related statistical data,the factors controlling water inrush were selected to establish the evaluation indices system for water inrush in tunnels in karst terrain.All of the evaluation indices were quantitatively graded according to the four risk levels of water inrush.Parts of the evaluation indices were graded based on their measured values,while the rest were graded based on their ratings.The comprehensive weighting method was used to determine the weights for evaluation indices.The subjective weights were determined by applying the Fuzzy Analytical Hierarchy Process,while the objective weights were assigned through correlation analysis.The comprehensive weighting method considers both the subjective understanding of human to things and the objective relations between things,which can significantly reduce the error caused by only using one kind weighting method.The comprehensive weighting method makes the assessment results more accurate.(2)Risk assessment systems for water inrush in tunnels in karst terrain were established based on the extension mathematical theory,attribute mathematical theory and fuzzy mathematical theory,respectively.These three risk assessment systems were applied to evaluate the water inrush risk for one chainage section in a tunnelling project.The assessment results of the three assessment systems were not only in agreement with each other but also consistent with the practical situations of water inrush in site.This fact verified the applicability and rationality of the risk assessment systems established based on the extension theory,attribute theory and the fuzzy mathematical theory.(3)During establishing the risk system based on the extension theory,the linear nondimensionalization method was applied to process the quantitative grading intervals and measured values of the evaluation indices.The nondimensionalization process of evaluation indices could be regarded as the process of constructing membership functions for these evaluation indices,and made it possible that the evaluation indices with different units can be compared with each other.As the extension assessment results were consistent with the assessment results of the other two systems and the practical situations of water inrush,the nondimensionalization process applied in the extension assessment system was proved to be reasonable.(4)Based on analysis of the occurrence mechanism of surface collapse during construction of metro tunnel in karst terrain and the related statistical data,the representative indices were selected to establish an evaluation index system for assessing risk of surface collapse caused by metro tunnel construction in karst tarrain.In this index system,the evaluation indices were quantitatively graded and the weights for evaluation indices were assigned by using the comprehensive weighting method.The extension mathematical theory,attribute mathematical theory and fuzzy mathematical theory were applied to establish the risk assessment systems,respectively.The three risk assessment systems were applied to evaluate risk of the surface collapse a metro region in karst area.The assessment results of the three risk assessment systems were not only highly consistent with each other with a consistency ratio of 91.19%,but also matched the collapse conditions and collapse history of this region.The application results that the three established risk assessment systems of karst surface collapse have high applicability and rationality.(5)Based on the data of karst caves arranged from the existing literatures and the tunnelling projects within the Guiyang-Guangzhou railway,the distribution forms of the width and height of karst caves were fitted by using MATLAB.The fitting results indicated that the width and height of karst caves obeyed Rayleigh distribution.Considering the practical situations and the operability of numerical simulations,the shape of karst caves was equivalent to ellipse.The Response Surface method was applied to design the numerical simulation plan on excavation of tunnels under the influence of hidden karst caves.Based on the changing characters of the parameters,the parameters whose changing have distinct impact on the safety of tunnel structure were selected as the variables,including elasticity modulus,cohesive force and friction angle of the surrounding rock,and the width and height of karst cave,while the rest parameters were regarded as the constant.The displacement mutation of the key locations of tunnel structure was used as the sign of tunnel structure losing its stability.The safety distances between karst cave and tunnel structure were obtained from three categories of numerical simulations,in which the hidden karst cave was located above the tunnel,below the tunnel and in side of the tunnel,respectively.And the east-square method was used to fit the response surface functions of the safety distance between tunnel and hidden karst cave in regard to the five variables.(6)A dynamically comprehensive system was established for karst prediction for tunnelling.This dynamically comprehensive karst prediction system was composed of two parts,the assessment of underground karst development state before tunnel construction and the prediction of karst features in ahead of the tunnel face during tunnel construction.The extension theory was applied to establish a system for assessing underground karst development state.The assessment results of underground karst development state were used as the initial prediction of karst before tunnel construction,and the results were also used to guide the plan designing of kart prediction during tunnel construction.The appropriate investigation techniques were selected and combined to predict the karst features in ahead of the tunnel face during tunnel construction aiming to the different karst development state level.And the karst prediction results were used to revise and update the assessed level of karst development state.The dynamically comprehensive karst prediction system was applied in a tunnel project,and the karst prediction results were highly consistent with the karst states recorded in site,which indicated that the established dynamically comprehensive karst prediction system had good applicability and high reliability. |
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