## 3 Micro-Settlement Control Techniques
### Groundwater Level Control Techniques  
Tunnel construction often results in groundwater loss and a subsequent drop in groundwater levels, causing soil layers to undergo over-consolidation settlement. According to Equation (31), the degree of over-consolidation settlement depends on soil properties and the height of the groundwater level drawdown. Therefore, over-consolidation settlement curves are calculated for different soil types and specific groundwater control measures are proposed as follows:  
(1) Over-consolidation settlement calculations for soil layers  
Figure 16 illustrates the relationship curve between groundwater level drawdown and surface settlement derived from Equation (31). As the groundwater level decreases, the over-consolidation settlement in soil layers gradually increases, albeit at a slower rate. A 1-m drop in groundwater level causes an increase of 0.54 mm in an over-consolidation settlement. Similarly, Figures 17 and 18 present the relationship curves between groundwater level drawdown and surface settlement for medium sand, coarse sand, gravelly sand, as well as silt sand, fine sand, and cohesive soil layers. The medium sand, coarse sand, and gravelly sand layers exhibit a faster rate of increase in over-consolidation settlement as the groundwater level decreases. For every 1-m decrease in groundwater level, the over-consolidation settlement of the soil increases by 2.15 mm. The fine sand and silt sand layers exhibit the fastest rate of increase in over-consolidation settlement as the groundwater level decreases. For every 1-m decrease in groundwater level, the over-consolidation settlement of the soil increases by 3.05 mm. The cohesive soil layers, however, are unaffected by groundwater level drawdown. In summary, a decrease in soil particle size leads to a reduction in the elastic modulus of the soil, while the rate of over-consolidation settlement increases as the groundwater level drops. Figure 16: Over-consolidation settlement in highly permeable soils. Figure 15: Installation diagram of components for compartmentalized-partition-ring-based grouting pressure maintenance and resistance reduction technique in ultra-long pipe jacking. Groundwater control measures  
Taking into account the over-consolidation settlement rate and permeability of different types of soil, develop targeted groundwater level control measures. Strictly monitor the height of groundwater level reduction during tunnel construction for gravel, crushed stone, and highly permeable strata, and if necessary, use the grouting method to control the groundwater level. For medium sand, coarse sand, and gravel sand formations, it is necessary to strictly control the lowering of groundwater level during tunnel construction. This type of formation has a permeability greater than \(10^{-3}\) and belongs to a highly permeable formation. The grouting slurry is prone to loss and the efficiency of the grouting method is low. Therefore, the effective method for controlling the groundwater level is freezing. The freezing method uses a pipe curtain to freeze and forms a closed freezing reinforcement circle with a certain thickness along the tunnel perimeter, preventing groundwater loss during tunnel construction. Strict control is required to reduce the groundwater level during tunnel construction in silty and fine sand formations. The permeability of this type of stratum is \(10^{-5}\)-\(10^{-3}\), belonging to the medium permeability stratum. Effective methods for controlling the groundwater level include grouting and freezing. The grouting method injects grout into the soil pores in front of the tunnel face by drilling holes in the surface, reducing the permeability coefficient of the soil and minimizing groundwater loss during tunnel construction. For cohesive soil layers, it is necessary to strictly control the decrease of groundwater level during tunnel construction. This type of stratum has a permeability of \(<\)\(10^{-5}\) and belongs to low-permeability strata.