In order to strengthen the shield tunneling control and reduce the vertical movement of the tunnel and the change of the internal force of the structure, the following principles should be followed in the online shield construction of the tunnel support trolley:
The calculation value of earth pressure in the balance state of earth pressure is strictly used as the earth pressure setting value of shield tunneling construction, and the theoretical excavated quantity is strictly used as the control value of shield spoil;
The control principle of the vertical direction (elevation) of the shield machine: strictly control the vertical, horizontal and inclination deviation of the shield machine, reasonably control the stroke difference of the propulsion jack, and avoid large-scale axis rectification action. As the clear distance between the upper and lower tunnels is very small, in order to reduce the disturbance of shield construction on the surrounding soil and reduce the impact on the construction of the lower tunnel, large deviation correction is prohibited, and the rectification principle is "frequent correction, less correction".
The synchronous grouting is timely and sufficient, and the secondary grouting is timely carried out through the grouting hole behind the shield machine;
Check the cutting tools in the reinforcement area before the overlapping tunnel section to ensure the integrity of the cutting tools.
1、 Strictly control the main driving parameters
In the process of shield driving, according to the geology, overburden thickness, ground construction conditions and combined with the monitoring results of surface subsidence, the earth bin pressure is adjusted to reduce the pressure fluctuation. In order to create good conditions for segment assembly, it is necessary to control the deviation, minimize the disturbance to the soil, strengthen the monitoring of excavated quantity, and prevent over excavation and under excavation. The synchronous grouting amount should be adjusted in time according to the advancing speed, mucking amount and surface monitoring data, and the deviation between the construction axis and the design axis and the stratum deformation should be controlled within the allowable range.
1. Selected pressure value of soil bin
The earth pressure calculation value under the earth pressure balance state is strictly used as the earth pressure setting value of shield tunneling construction, and the earth pressure of each section is calculated according to the stratum and groundwater depth. Only a small amount of adjustment can be made in the construction under the premise of the approval of the project engineer. At the same time, the displacement and pressure of soil can be adjusted by monitoring the depth of soil.
2. Control of slag discharge
Considering the actual loose coefficient of soil layer, the mucking amount of each ring is controlled according to the monitoring data and experience.
The theoretical excavation amount shall be strictly taken as the control value of the shield spoil, and the deviation of the excavated amount per ring shall not exceed 1m3. In order to avoid the statistical error of quantity, the soil box should be cleaned as much as possible in the construction. Once the phenomenon of over excavation occurs, it must be fed back truthfully. In the subsequent grouting construction, excessive grouting will be targeted.
3. Propulsion and propulsion speed
If the excavation speed is too slow, it will disturb the soil greatly and is not conducive to the control of slag discharge. If the excavation speed is too fast, it is not conducive to the stability of the working face, and it is easy to cause the unstable change of soil bin pressure. The selection of driving speed and thrust is for the purpose of maintaining the earth bin pressure, and the shield tunneling speed is determined to be 20 ~ 40mm / min according to the overlapping section; during the excavation process, the excavation speed and thrust shall be adjusted timely and reasonably according to the actual situation.
2、 Shield attitude control
Due to the influence of stratum, tunnel curve, slope change and operation, the shield can not advance completely according to the designed tunnel axis, but will produce certain deviation. When the deviation exceeds a certain limit, it will cause the tunnel lining to invade the limit, reduce the clearance at the tail of the shield, worsen the local stress of the segment, increase the ground loss and increase the surface settlement. Therefore, effective technical measures must be taken to control the driving direction and correct the excavation deviation in time.
In view of the fact that the soil mass where the shield tunneling is located in the overlapped tunnel has been disturbed in advance, and the distance between the upper and lower tunnels is small, it is necessary to strictly control the vertical deviation of shield tunneling, reasonably select the tunneling trend, and reduce the risk of "falling head". In the process of tunneling, the shield must keep advancing smoothly, properly improve the penetration, reduce the deviation correction and reduce the disturbance to the front soil.
3、 Grouting behind synchronous wall
The ground loss caused by shield construction, the reconsolidation of remolded soil disturbed or sheared around the shield tunnel and the infiltration of groundwater are the important reasons for surface settlement. In particular, in order to reduce and prevent the settlement of the overlapping section, in the process of shield tunneling, sufficient slurry material should be injected at the back of the segment which has been released from the shield tail to fill the annular construction space of the shield tail and support the surrounding rock mass of the segment. The condensed slurry will be used as the first waterproof barrier of the shield construction tunnel to enhance the waterproof capacity of the tunnel, and provide early stability for the segment and make the pipe The integration of sheet and surrounding rock mass is conducive to the control of shield tunneling direction and the final stability of shield tunnel. If necessary, double slurry should be injected to further accelerate the consolidation time of segment filler, stabilize the formation as soon as possible, and reduce disturbance and subsidence.