Mechanism Of Inrush Hazards In Tunnels Passing Through Altered Granite Zone And Method Of Construction License Assessment

Water and mud inrush events during tunnel excavation usually occur in dissoluble rock strata(limestone,dolomite etc.).However,an increasing number of inrush events have taken place in tunnels passing through granite,tuff,resulting in severe disasters and economic losses.At present,the researches and engineering practices on water and mud inrush hazards in tunnels mainly focus on soluble rocks.There is a lack of systematic studies on the mechanism and treatment measures for water and inrush hazards in granite tunnels.Therefore,with the water and mud inrush hazards in the Liangshan Tunnel and the Denghuozhai Tunnel as case studies,this paper presents the forecast techniques,treatment measures and mechanism of water and mud inrush in granite tunnels.The results can provide some references and experiences for similar projects in the future.It is extremely important to carry out safety risk management during the construction of mountain tunnels.However,existing research approaches are seriously lagging behind the engineering applications.For the ongoing excavated tunnels,there are various influence factors leading to low reliability,poor accuracy and weak practical applicability for risk assessment results.These factors include subjectivity in determining index weights and difficulty in obtaining the certain,precise and complete evaluation indexes.Because of this,the existing risk assessment techniques fail to meet the requirements of standardization,informatization and security management for tunnel constructions.Therefore,in this paper,a new risk assessment method was established by combining case-based reasoning,geological prediction and rough set theory.This novel method had been applied in engineering practice and the feasibility had also been verified.By analyzing the inrush events in two deeply-buried tunnels,this paper revealed that the inrush hazard is catastrophic,instantaneous,intermittent,highly destructive and featured by large influence range and high sand and mud content.First,the comprehensive advanced prediction method based on geological,geophysical and drilling exploration was established,by considering the unique development environment of inrush hazards.The scale and composition of the altered zone were identified.The measure of “consolidation before cleaning” was taken to deal with the inrush deposits for breach sealing and reinforcement.Energy release and stress reduction during tunnel construction and regular drainage during operation were achieved by dewatering measures,namely,“drainage supplemented by sealing,water drainage and solids remaining”.Reinforcement measures,including full-face curtain grouting,double-layer shed pipes and horizontal jet grouting pipes,were adopted to reinforce and seal the highly disturbed rock and soil in advance.The conventional and modified CD methods were employed for safe tunnel excavation.Finally,technical measures,including enhanced support stiffness,double-layer primary support and timely installation of secondary support,were taken to effectively prevent large deformation of soft rocks.With the above measures,both tunnels have passed the altered granite zone successfully.Triggered by excavation or blasting,the underground water and debris may rush into tunnel like a mudflow,which is the essence of inrush hazards in tunnels.Three conditions are indispensible for the occurrence of inrush hazards,namely,material,space and triggering conditions.This paper introduces the inrush hazards of the Denghuozhai Tunnel and the Liangshan Tunnel,and analyses the mechanism of inrush hazards from aspects of material and space conditions.Studies on the lithofacies,minerals and alteration characteristics in these two tunnels reveal that the inrush hazards in the Denghuozhai Tunnel and the Liangshan Tunnel occurred in the contact zone between geodetic granite and tuff,and the altered geodetic granite zone,respectively.The types and evolution of the altered rocks are identified which indicates an intrusion of volatile-rich magma into the wall rocks at shallow depth under tensile tectonic environment.Albitized,sericitized and clayinitized alteration thus occurred near the contact zone or the altered zone,leading to the formation of tens of meters wide,loose and deeply-buried soft mineral belts,such as sericite,kaoliniteand montorillonite,etc.These soft mineral belts provide the materials and space for inrush hazards.Meanwhile,the hydrogen and oxygen isotopes in the clay minerals have further proven that clay was the product of hydrothermal alteration,instead of weathering.It is found from the physical,mechanical and hydrological experiments that a high degree of alteration causes swelling,disintegration and argillization of rock and soil,and thus weakens physical and mechanical properties of rocks.This weaking further lowers the triggering threshold of inrush events.Therefore,when tunnels are excavated in such altered rock masses,inrush events are likely to be triggered by small disturbances.The experiences from these successful cases will be a valuable references and experiences in the design and construction of similar engineering in the future.For sample sets of practical tunnel collapse cases,based on the dependency principle of attributes,the attribute importance degrees and weights were calculated before and after attribute simplification.It was discovered that the simplification based on the dependency of attributes was excessive in risk assessment of tunnels.Both extra-reduction and low reliability of the results were due to the granularity representation of knowledge and the restriction of decision information table.There was another possible problem,i.e.,too much attribute simplification results could lead to choose impossibility and the attribute importance degrees and weights could not even be determined.Based on the situation,this paper proposed a method of determining the attribute importance degrees and weights upon the information entropy.It was a new attribute simplification method as well as the related software.This method had been applied in practical tunnel collapse cases and the results were good enough.This method could filter the risk influence factors,abandon insignificant/extra factors,calculate the objective weights of every factor and solve the problem that weights were assigned completely by subjectivity.Based on past tunnel incidents and technical regulations,the risk carrying environment and risk-caused factors were set up by level analysis method for typical risk incidents,such as collapse and inrush disasters.After the integration and conclusion of relevant information and factors,the assessment index system was built for typical risk incidents,and this laid a foundation for rough sets in case-choosing and evaluation procedure.The main risk carrying environment and geological prediction results were analyzed for to-be evaluated tunnel,then cases with similar attribute characters were chosen from the case library.After that,the attribute importance degrees,index weights and attribute simplification of similar accident cases were calculated based on rough sets theory,which optimized evaluation index selection indirectly.Finally,optimized evaluation indexes and weights were applied to fuzzy comprehensive evaluation model so that the comprehensive assessment of risk events probability was achieved for the to-be evaluated tunnels.