## 3 Micro-Settlement Control Techniques
### Grouting Pressure Maintenance Technique Around the Pipe During the Pipe-Roof Jacking Process  
To control surface settlement caused by the construction of individual pipes in the pipe roof, the grouting pressure must be carefully regulated to achieve saturation grouting pressure. Proper regulation prevents excessive settlement due to insufficient low pressure and surface heaving due to excessive pressure. The relationship between grouting pressure and surface settlement, as derived from Equation (20), is illustrated in Figure 12.  
Figure 13 illustrates the relationship between pipe spacing and surface settlement under different grouting pressures. As the pipe spacing increases, the impact of subsequent pipe installations on the first pipe gradually decreases. When the pipe spacing exceeds \(\lambda\)i = 15.996 m, the influence of subsequent pipe installations outside the settlement trough on the first pipe becomes negligible. Meanwhile, an increase in grouting pressure alters the surface settlement behavior. Specifically, the initial settlement decreases, followed by surface heaving, with the heaving magnitude gradually increasing. The minimum surface settlement is observed at a grouting pressure of 0.28 MPa, indicating that this pressure represents a reasonable grouting pressure. Therefore, selecting an appropriate grouting pressure ensures that the surface settlement remains within a relatively small range.  
The pipe-roof jacking must possess functions such as portal pressure maintenance, lubrication and pressure maintenance around the pipe, and synchronous grouting. To achieve  
Figure 11: Schematic diagram of the pipe roof construction sequence.  
Figure 12: Relationship between grouting pressure and surface settlement.  
this, a device for maintaining portal pressure and a system for lubrication and pressure maintenance around the pipe were developed, as illustrated in Figure 14. During soil excavation using a micro shield machine, the cutter head's diameter is larger than that of the steel pipe, thus creating a gap that significantly contributes to settlement during the pipe jacking process. To address this issue, grout is uniformly injected at a constant pressure to stabilize the borehole walls and prevent collapse and settlement. Additionally, it serves as a lubricant to reduce resistance. To maintain pressure and reduce resistance around the pipe, a lubrication slurry pipeline system is installed within the pipe, along with a nozzle system evenly distributed around the pipe wall. These components together form an ultra-long slurry jacket, which reduces resistance on the outer surface of the steel pipes in the pipe roof.  
A longitudinal main grouting pipeline, which runs the entire length of the steel pipe within the pipe roof, is installed internally and connected to circumferential branch pipelines via three-way valves. To prevent abrupt pressure loss in the lubrication chamber due to cracks or voids in the soil, compartmentalized partition rings are installed at regular intervals along the steel pipe, as depicted in Figure 15.  
Figure 14: External wall pressure maintenance and resistance reduction system.  
Figure 13: Relationship between pipe roof spacing and surface settlement under different grouting pressures.