2.3.1 Initial Construction of Auxiliary Engineerin...
1）Construction of initial reaction frame.
The reaction frame is a circular reinforced concrete structure，which mainly provides a solid force point for the origin of the shield machine，so that the shield Jack has enough reverse force to push the shield forward.The construction height of the reaction frame is large，so it needs to be poured in layers.The formwork is generally assembled with bamboo rubber board，the support system is full of red scaffolding，the scaffolding adopts steel pipe，and the square wood+arc I-shaped steel is used as the internal support.
The steel plate is embedded in the construction of the reaction frame，which is used to connect the steel pipe support of the steel pipe sheet（Fig.2.19）.During construction，the steel pipe piece shall be positioned first，and then the steel pipe is cut according to the measured distance between the steel tube sheet and the reinforced concrete reaction frame.One end of the steel pipe support is welded to the embedded steel plate of the reaction frame，and the other end is welded to the corresponding position of the steel pipe sheet.Steel tubes are filled with concrete of a certain strength before installation to enhance their stiffness.
2）Initial pedestal construction.
The initial pedestal is arc structure，the longitudinal two reserved grooves and the transverse one reserved grooves，and the grooves are reserved operating space for shield assembly and welding.Cast-in-place reinforced concrete structure is used in the initial pedestal and reaction frame.
（1）In order to facilitate the initiation of shield and offset the beginning of shield，the top surface of the structure is adjusted in the construction of initial pedestal.The size of the body is adjusted according to the center line of the shield design，and the adjustment of H-shaped steel is controlled according to the requirements in the construction of the height adjustment of the pedestal.
（2）In order to facilitate the welding of shield tail and middle shield during shield assembly，save the welding time of shield assembly，the reserved transverse welding operation groove structure of starting base shall be moved to a small distance of small mileage，and the length of reinforcement and position of embedded parts shall be changed accordingly.
Fig.2.19 Construction of initial reaction frame
3）Installation of long guide track
After the installation of the initial pedestal，because there is a small distance between the front end of the reference guideway and the soil in front of the initial pedestal（that is，the thickness of the wall at the end of the shield working well and the gap left in order to facilitate the installation of the temporary sealing device for the door），in order to ensure the safety and accurate initiation of the shield，two long guide tracks are installed in the hole ring and in the corresponding position of the guiding track of the original pedestal，and the installation inclination angle position is consistent with the reference guiding track，The lower part of the guideway is supported by H-shaped steel，and the lower part of H-shaped steel is fixed by steel plate and expansion bolt（Fig.2.20）.
When the shield cutter head reaches the position of the hole door，there is a 20 cm gap between the cutter head and the bottom of the hole door.In order to prevent the cutter head from planting，fill the yellow sand and compaction in this position，pour the concrete guide slot or the embedded guide rail.The diaphragm wall position portal is broken according to the forward direction profile of the shield.No overexcavation is allowed to prevent the gap between the shield shell and the diaphragm wall from causing the shield head or segment to sink.
Fig.2.20 Installation of starting base and sliding rail
4）Temporary sealing device for hole door
In order to prevent the mud from entering the shield working shaft from the annular building gap between the tunnel ring and the shield shell during the initial process of the shield，affecting the mud pressure of the shield excavation surface，the stability of the excavated surface soil mass and the construction in the shield，the sealing device with good performance must be provided at the tunnel portal before the shield starts（Fig.2.21）.According to the construction experience of shield machine in the past，the folding blade seal pressure plate has the advantages of good force，good sealing，simple operation，good stiffness，safety and reliability.