3.1.1　Technical Difficulties

3.1.1　Technical Difficulties

When shield method is used to construct shallow overburden tunnel in water area（river，lake，sea），it mainly faces the following risks and problems.

（1）Shallow overburden is easy to produce roof caving and permeable water flow.In order to reduce the slope of the line，the overlying soil in the bottom section of the water is very shallow.For example，in the north-south phase of Nanjing Metro，the minimum overlying soil thickness of the tunnel is only 0.97 m，which is very shallow overburden when the shield passes through the Qinhuai River；The diameter of the fourth Yibeihe tunnel in Germany is 12.33 m，and the thickness of the most difficult soil layer is only 7 m；The shallower overlying soil is only 5.5 m，less than half the diameter of shield machine，in Dalian Road Tunnel，Huxing East Road，Huangpu River，Shanghai.Under the condition of high head pressure and shallow overlying soil，the soil pressure balance in front of the knife plate is not easy to establish.Water sand often enters the shield machine through the opening of the cutter head and the tail of the shield from the cracks in the disturbed soil，resulting in flooding of the shield machine，resulting in machine damage accidents from time to time.

（2）Shallow overburden is easy to cause tunnel floatation.Under the water，the shield is subjected to uneven force，the attitude of the shield rises，the slope is difficult，the tunnel floats up，and the axis is difficult to control.After the assembled tunnel ring is removed from the tail of the shield，the tunnel floats because the upper ballast and self-weight cannot resist the buoyancy caused by groundwater.If the corresponding reinforcement measures are not taken，it is easy to cause local cracking and leakage of the tunnel.

（3）Shallow overburden is easy to cause sand flow and piping phenomenon.Construction in sand，sandy silt and other easily liquefied soil layer，due to shield propulsion cutter head rotating cutting extrusion disturbance，coupled with high head pressure（sometimes up to 0.3-0.5 MPa）.Liquefied sand soil seeps into the tunnel along the shield tail and tunnel joint.If measures are not taken in time，local foundation emptying，tunnel subsidence，spiral fracture and tunnel failure may occur.

Taking the initial section of Nanjing Yangtze River Tunnel as an example，due to the influence of line control，the shield initiation section（K3+600～730）belongs to ultra-shallow buried（Fig.3.1）.The thickness of the shallower overlying soil is 5.5 m，which is only 0.37D.The main crossing strata are fluidizing silty clay strata，and the construction of shield in shallow overlying soil is easy to cause the following problems.

（1）Because of the small vertical pressure，it is difficult to control the attitude of shield propulsion.

（2）Because of the thin overlying soil layer，it is difficult to control the water pressure of the incision，and the mud water is easy to pop out of the ground，and the mud splits the ground layer and destroys the slurry balance.

（3）The soil is soft and the end soil is prone to instability，water and sand gushing，formation collapse and so on.