2.3.3　Broken of the Door of Tunnel

In order to ensure the stability of the retaining structure of the original well，the hole drilling is carried out in two stages in the process of opening the hole door.The first stage begins to be chiseled after the soil reinforcement inspection of the end well is passed，and the shield initiation facility is completed before going down the well；The second phase is carried out quickly after the assembly and debugging of the shield and other initial preparations are completed，as shown in Fig.2.23.

Fig.2.23　Frozen excavation surface after door breaking and complete breakage in the initial section

Before chiseling，set up a double row scaffolding，from the top to the bottom layer chisel.In the first stage，the external concrete and steel bars are chiseled from top to bottom in five levels，and the inner steel bars are reserved，so as to protect the end stratum before starting.When chiseling，the excavated steel bar is first chiseled out and cut off by oxygen welding，and then it continues to be chiseled until the steel bar on the face of the earth is exposed.The order of concrete chiseling in each layer is first middle，then left and then right.When the shield cutter head reaches 0.5≤1 m in front of the enclosure structure，the excavation is stopped，then the remaining steel bar is cut off，the remaining continuous wall is pierced，and the rest of the continuous wall is checked to determine whether there is no steel bar.

The emergency measures for the chiseling process are as follows：

（1）abnormal conditions are found，which are quickly supported by planks and steel pipes to prevent the collapse of soil outside the continuous wall，and then grouting reinforcement is carried out from outside the retaining wall as soon as possible.

（2）when the pressure of rock and soil is high，the pre-made steel mesh is welded with the steel bar of the enclosure structure quickly and then supported and stabilized by plank and steel pipe，and then grouting reinforcement is carried out on the periphery of the envelope structure，and grouting reinforcement is carried out in the tunnel door at the same time.