2.4.3 The Arrival Section of the Shield Machine is...
1）Shield positioning and compound measurement of the position of the entrance at the end of the shield
When the shield is pushed to the construction range，the position of the shield and the survey control points of the shield tunnel should be accurately measured，and the relationship between the actual tunnel center axis and the tunnel design center axis should be clarified.At the same time，the tunnel door of the shield receiving well should be checked and measured，and the penetration attitude and driving deviation correction plan of the shield should be determined.Two points should be paid attention to when considering the penetrating attitude of shield：one is the deviation between the central axis of shield tunneling and the design axis of tunnel，and the other is the deviation of the position of receiving portal.These factors should be adjusted on the basis of tunnel design center axis，and the correction of deviation should be completed step by step.
In order to ensure the accuracy of arrival，the penetration survey is carried out 150 m before the tunnel penetration.The penetration survey includes the ground control network joint survey（plane and elevation）and the receiving well hole gate measurement（plane and elevation）.The accuracy of height penetration survey is≤±25 mm，and the measurement error of traverse penetration is≤±20 mm.
（1）Ground control network joint survey.The GPS static positioning technology is used to measure all the control points of the receiving and starting control network.The instruments，methods and specifications used in the repeated survey are consistent with the timing of the establishment of the ground control network.According to the results of joint measurement，the coordinates of control points are checked to ensure that the control points are consistent with each other.
（2）Receiving well gate determination.By using the total station，a certain number of measuring points are measured on the reserved steel ring of the tunnel gate，and according to the results of the measurement，the actual central coordinates of the entrance door and the distance from each point to the center are calculated.According to the calculation results，it is judged whether it is necessary to adjust the digging axis to ensure the smooth arrival of the shield.
2）Shield machine arriving at excavation operation
Take the shield tunneling operation of Nanjing Yangtze River Tunnel as an example.
The driving speed of shield arrival section is relatively stable，the disturbance to soil is reduced，and the axis control of shield tunnel is strengthened.The amount of grouting and grouting pressure at the same step should be adjusted according to the propulsion speed and ballast discharge.Because the shield tail brush has been dug in such long distance，the synchronous grouting pressure should be controlled to prevent the leakage of slurry at the end of the shield.
After the irrigation is completed，the shield continues to push forward for 2 m，and stops at a position of 4 m from the freezer wall.It begins to pull out the frozen pipe and pull all the frozen pipes located in the tunnel propulsion range from the top of the tunnel 0.3 m，and then resume the freezing.After the shield passes through the frozen section，all the frozen pipes are pulled out and filled with yellow sand.After the frozen tube is pulled out in place，the shield is pushed smoothly to the door.The propulsion pressure is gradually reduced before shield penetration，until the incision pressure is 0，and the intake and exhaust valve of the system is closed after the cutter head breaks the wall.
When the shield cutter head breaks the freezer wall and enters the working well，the incision pressure disappears completely，which is consistent with the liquid level in the well.at this time，the flow rate of the inlet and outlet slurry is controlled within 100m3/h，and the flow rate in and out of the working well is kept unchanged（the liquid level in the working well remains 2.5-3.0 m below the top surface of the shield cutter head during the receiving process）.After the shield machine is completely entered the receiving well by sealing grouting at the entrance of the tunnel，the debris in the well is removed，and the gap between the segment and the excavated surface（embedded steel ring）is filled with C20 jet coagulation soil，during which the precipitation has been running all the time.While the shield is disassembled，the temporary sealing tunnel door is used in time，and then the end ring segment is strengthened by grouting through the grouting hole reserved on the embedded steel ring.When the shield is dismantled，the permanent cast-in-place concrete tunnel door is applied under the condition of precipitation.
3）Shield arrival heading control
In addition to the purpose of correcting deviation according to the scheme of heading correction，special attention should be paid to the excavation control of the last 10 m section.Before the arrival of the shield，it is necessary to strengthen the observation and settlement monitoring of the entrance of the tunnel，and communicate with the main driver of the shield in time in order to control the excavation，such as the reinforcement of the strata，and so as to strengthen the observation of the working conditions and the monitoring of the settlement of the entrance after entering the reinforcement body.
According to the geological conditions of the entry section，the reasonable excavation parameters are determined and the written submission is made.The general requirements are：low speed，small thrust，reasonable slurry pressure and timely and full backfill grouting.In the assembly of the last 20 ring segments，the segments should be connected into a whole with a longitudinal pull rod in time to avoid loosening between the segments when the thrust is very small or there is no thrust.
Because the shield well enclosure structure adopts reinforced concrete at the shield gate，the shield must be broken before it arrives.When the shield is excavated in the last ring，it should be excavated with low speed，small thrust，small torque，reasonable slurry pressure，and through the interval tunnel after the synchronous grouting and condensation injected in the previous ring.
4）Key points and measures of shield arrival construction technology
（1）When the shield enters the arrival section，the workers shall specify the realtime mileage of the shield and the distance from the cutter head to the tunnel portal，and carry out the construction according to the determined construction technical scheme.
（2）Before the arrival of the shield，it should be checked to confirm whether the condition of the end formation meets the reinforcement requirements.
（3）Increasing the frequency of surface subsidence monitoring and feedback the monitoring results in time to guide the construction.
（4）Re-survey of conductors and elevations shall be carried out every 50 m before arriving at 200 m，and the clearance of the penetration point shall be measured to determine the position accurately.
（5）In the arrival section of shield machine，combined with the through position and referring to the design line，each ring must be carried out in strict accordance with the excavation plan.
（6）The mix proportion of grouting materials in the first six rings should be adjusted，and if necessary，special water stop materials can be injected into the outer part of the shield shell by drilling holes in the tail shell of the shield，so as to prevent the formation collapse caused by water gushing and mud gushing.
（7）In order to prevent the contact relaxation of the annular seam of the segment caused by the insufficient reaction force of the cutter head，the segment is tightened longitudinally along the tunnel with channel steel in the last 10 ring segments of the shield arriving section.
（8）Several ring segments installed after shield penetration must ensure that the grouting is full and dense，and prevent the segment from sinking and missetting.
（9）In the construction of shield arrival section，it is necessary to ensure the normal operation of slurry system and the amount of dry sand to be machined.the data of soil and sand quantity are positive，and the cut water pressure is reduced to the minimum.
（10）Closely observe the change of parameters such as the deformation of the tunnel portal and the torque of the cutter head.In case of any abnormality，the propulsion shall be stopped immediately and corresponding countermeasures shall be taken.
（11）when the shield incision is close to the shield working well wall protection 2-3 m，the viscosity and specific gravity index of slurry can be gradually reduced under the condition of determining the stability of the soil at the mouth of the tunnel，until it is replaced by clean water.
（12）When the cutter head excavates the shield working well to protect the wall，it should stop pushing immediately，replace the mud in the cut mud cabin with clean water，and clean the mud water pipeline at the same time.