Study On Floating Mechanism Of Shield Tunnel Under Shut Down For Pressure Maintenance Condition

Shield tunneling is one of the common construction methods of underground rail transit.This paper,taking a shield interval of Xuzhou railway traffic engineering,through research methods of literature review,theoretical derivation and numerical simulation,analysis the phenomenon and its mechanism of shield tunnel up-floating which experienced the condition of shut-down and pressure-retaining construction.Combined with the engineering practice,the results of theoretical calculation and numerical simulation are analyzed.The calculation method in the case is concluded which can be applied to forecast and disaster prevention.The engineering measures to control the tunnel rise trend are enumerated.The main conclusions of this paper are as follows:(1)Due to the heterogeneity of the internal structure of the shield machine,when the tunnel construction stops,the relatively heavy shield head will sink and the relatively light shield tail will rise.In case that the shield tunneling machine stops to maintain pressure,it is necessary to timely monitor and rectify the shield tunneling attitude,appropriately increase the thrust of the lower side jack and decrease the thrust of the upper side jack to maintain the stable attitude of the tail of the shield.(2)The attitude change of shield is closely related to the nature of soil.With the increase of the elastic modulus,cohesion,and internal friction angle of the surrounding soil,the amount of the upward and downward motion of the tail and head of the shield tunneling machine decreases,that is,the attitude of the shield tunneling machine tends to be stable.But the impact of the soil cohesion and internal friction angle on the attitude of the shield tunneling machine is relatively small.In addition,the slope of soil e-lgP curve is also one of the indicators to reflect the attitude change trend of shield.(3)During the synchronous grouting period,the amount of floating on the pipe plate is positively correlated with the grouting pressure.The amount of tube floating is negatively correlated with the elastic modulus,cohesion and internal friction angle of soil,but the cohesion and internal friction angle have little influence on the final amount of tube floating.(4)The main control measures include setting up complete monitoring system,adjusting jack thrust flexibly,formulating reasonable shield posture control scheme and adjusting measures to local conditions of grouting process are enumerated tocontrol the tunnel rise trend.Furthermore,it is suggested to monitor and adjust the starting section of shield,such as collection of shield advance and grouting parameters for the subsequent excavation which can provide direct and effective reference data.The paper has 66 figures,5 tables,and 90 references.