Study On Construction Safety Risks Control Technology For Tunneling Adjacent To Existing Buildings(Structures) Within Shield Launching/Arriving Section

With the development of urban rail transit,crossing the existing underneath metro lines is such a highly risky project in the construction of metro tunnel that we have to face.Immediately after being launched at Shangmeiling Station of Line 9 of Shenzhen Metro,the shield machine has to cross the existing underneath Line 4 of Shenzhen Metro,and just before arriving at Hongling Station of Line 9 of Shenzhen Metro,the shield machine has to cross the complex geological conditions of the existing underneath Line 3 of Shenzhen Metro.All of these examples represent this project has multi-level risk features in the shield machine launching/arriving section,crossing the existing underneath metro lines and in the complex geological conditions.These risk sources are all serious hazards for shield tunneling in the urban rail transit projects.Under the superposition of multi-level risk sources,how to obtain the key factors of construction safety risk control measures and then adopt the targeted risk control measures,which is of great significance for the safe and successful execution of the project.At the same time,it also brings valuable reference for the design and construction of rapid development of metro project under similar conditions within China.This paper mainly carried out the following works:(1)In Shenzhen Metro Line 9 project,the shield machine crosses the existing underneath Line 4 of Shenzhen Metro in water-rich ground after being launched at Shangmeiling Station of Line 9;also,the shield machine crosses the existing underneath Line 3 of Shenzhen Metro in heterogeneous ground in the arriving section at Yuhongling Station of Line 9.With this project as an example,the multi-level risk factors of Earth Pressure Balance(EPB)type of shield machine are identified and analyzed with the consideration of water-rich ground,heterogeneous ground,adjacent to the launching/arriving shafts and crossing the existing underneath metro tunnel,in the meantime,the concept of risk control in shield tunneling with multi-level risk sources is proposed and a risk source checklist is developed.(2)Based on the multi-level risk source control theory and by means of written investigation by extensive experts of different disciplines and different levels,and using the analytic network process in shield engineering risk assessment and based on the expert investigation results,risk assessment model is established,by which the order of the weighing of different levels of risk control technologies are revealed under the circumstance of multi-level risk factors.The research indicates that top 3 nos.of 2^ndlevel risk indicators are:no effective filling in the tunnel shield ring gap,poor reinforcement quality in the break-in and break-out part and adverse effects in water-rich area.After that,the expert scoring method is used to evaluate the level of risk sources,which proves that the weight ranking obtained by the analytic network process is suitable for construction safety risk control of crossing existing underneath structures project.Steel sleeve sealing technology and shell annular space grouting technology are the key control technologies to reduce and control the above three top risks.(3)Through the study of the mechanism of steel sleeve function,the influence of preset pressure parameters,influence extent and adaptability of the steel sleeve sealing technology in shield machine launching/arriving section,the effects of settlement,ground stress release rate and ground stress state caused by the steel sleeve with the preset pressure parameters had been obtained.It showed that the preset pressure of steel sleeve can significantly affect the horizontal displacement of tunnel initial face.The horizontal displacement of tunnel face could be reduced by increasing the preset pressure of steel sleeve.In the influence zone of preset pressure of steel sleeve,increasing the preset pressure can reduce the ground settlement in the launch section due to the sensitivity of soil strength parameter;the increasing of preset pressure can reduce significantly the ground deformation when shield machine passing through the ground treatment zone.At the same time,this paper puts forward the influence extent and scope of the steel sleeve preset pressure,as well as prove feasibility of using steel sleeve sealing technology in the launch section of shield machine.(4)The research on the impact of the key parameter of annual clay-shock grouting technology and its parameter optimization and effect of risk control for the clay-shock technology had been carried out.It is found the grouting pressure of clay-shock significantly affected the ground settlement at the third stage,the maximum ground settlement could be reduced by 70%if the annulus grouting pressure increased from 125k Pa to 170k Pa.The grouting pressure of clay-shock has direct influence on the excavated ground,such as stress release ratio of excavation ground,stress state of surrounding ground,failure mode and failure extent of the ground,etc.It is suggested that the quality control of the annulus grouting should be mainly based on the grouting pressure and the clay-shock pressure should be measured during the construction process.In order to precisely control the ground settlement caused by the shield passing in the third stage,several measures should be taken,such as,increasing annulus grouting pressure in the arch part,optimizing the distribution mode of grouting,and supplementing the grouting reinforcement in the the existing metro tunnel,which leads to reduction of impact and influence of shield machine on existing metro tunnel and minimize the construction safety risk at the same time.