Study On Critical Overburden Thickness And Stability Of Shield Tunnel Crossing Under The Lake

With the rapid development of Chinese economy and the continuous expansion of city size,the problem of urban traffic congestion is also urgent to be solved,and the subway is the best way to solve this problem undoubtedly.Due to the complexity of urban subway construction environment,sometimes inevitably need to pass through the waters,such as lakes,rivers,compared with the land tunnel,due to it’s complex geological conditions and a higher standard of construction control,lead to difficulty of underwater tunnel construction is also greatly improved,while it is also more difficult to control the tunnel stability.The shield tunnel under the lake was constructed for the first time in Hefei,and the corresponding construction experience was lacking.In addition,the groundwater weakened the stratum parameters,so the stratum collapse and tunnel water gushing were easy to occur during the construction.Therefore,it is necessary to study the stability of underwater shield tunnel construction.Based on the engineering background of shield tunnel crossing Swan Lake between Qimen Road Station and Dadongmen Station of Hefei Metro Line 3,this paper clarifies several commonly used calculation methods of critical overburden thickness at home and abroad,and clarifies their engineering applicable conditions.Based on FLAC3 D software,the three-dimensional numerical model of tunnel construction under the lake was established,and the effects of parameters such as overburden soil thickness,tunneling pressure,grouting pressure,elastic modulus of grouting material and thickness of grouting layer on the stability of shield tunnel were analyzed,it is expected that the research results can be used for reference for the construction of underwater shield tunnel in the future.The main research contents and results of this paper are as follows:(1)The critical overburden thickness of shallow buried shield tunnel under lake can be obtained by the simplified minimum burial depth calculation method and the improved minimum burial depth calculation method considering the soil friction effect.The results obtained by the simplified minimum burial depth calculation method are conservative and the construction safety factor is higher.When the tunnel is buried deep,the excavation of the tunnel will lead to the formation of soil pressure arch,so the calculation method considering the overlying soil arch effect should be adopted.For undersea tunnel excavation works,the minimum thickness of rock overburden can be preliminarily determined according to the Norwegian empirical method and the Japanese method of minimum water inflow.(2)Combined with the project of hefei metro line 3 cross swan lake,The 3D numerical simulation analysis of the construction process of shield tunnel under different soil thickness is carried out,the results show that: the maximum bottom uplift located above the center line of the double line tunnel,with the increase of thickness of overburden soil,the uplift of the lake bottom displacement decreases,when the thickness of the overlying soil reaches a critical value of 8 m,lake bottom displacement value curve tends to be stable,along with the change of thickness of overburden soil to increase the thickness of overburden soil to control has no obvious effect of increase of the tunnel.(3)The influence of tunneling pressure,grouting pressure,elastic modulus of grouting material and thickness of grouting layer on the stability of shield tunneling is studied by single factor method.The results show that the deformation trend of the tunnel segment and the sublake soil layer is the same under different parameters.The segment shows an overall upward trend,and the sublake uplift is approximately symmetrical in distribution.The maximum value is located above the center line of the double-line tunnel.The uplift deformation of the segment and the lakebed can be effectively controlled by increasing the excavation pressure,grouting pressure,elastic modulus of the grouting fluid and the thickness of the grouting layer in a certain range,but too large excavation pressure will increase the uplift displacement of the lakebed.(4)Based on the specific engineering geological conditions of Swan Lake area and the previous research results,the risk prevention measures and construction control measures for the construction of shield tunnel under Swan Lake were proposed.Numerical simulation was carried out on the shield construction process under the condition of grouting reinforcement and unreinforcement of overlaying soil,and the displacement of the lakebed and segment were analyzed.The calculation results show that the maximum uplift of the lake bottom is about 17 mm and the maximum uplift of segment is 21.1 mm under the condition of unreinforcement.After the reinforcement of soil layer,the maximum uplift value of the lake bottom is about 11.2mm,and the uplift value of the segment is 13.8 mm,indicating that the reinforcement of soil layer can effectively control the tunnel floating.