1450 1)“Two-Step” Grouting Process When lost circulation or borehole collapse occurs during drilling, forward segmented grouting is implemented to protect and reinforce the borehole wall. Then, a perforated casing is installed, and segmented reinforcement grouting is conducted using slurry stoppers within the casing to form a long-term waterblocking and reinforcement curtain, as shown in Figure 4. Figure 4 – Sectional grouting with perforated pipe schematic diagram 2)Downhole Segmented High-Pressure Grouting Technology Based on the characteristics of different strata, differentiated segmented treatment strategies are implemented. The treatment section length is set to 30–80 m for fractured strata, 20–30 m for broken strata, and 5–10 m for extremely broken strata. One-way perforated casings and specialized high-pressure slurry stoppers with a pressure-bearing capacity of 35 MPa are developed to achieve high-pressure segmented control, ensuring repeatable and reliable sealing at any position within horizontal boreholes. 3)Highly Adaptive Grouting Materials A series of grouting slurries is developed to accommodate complex geological conditions. Modified cement-based materials are suitable for permeation reinforcement in fractured strata. High-solid-content and high-fluidity cement-based materials are applied for splitting and compaction in argillized zones. Clay– cement slurry is utilized for long-distance fracture water plugging, while chemical slurry is employed for permeation grouting in porous and fine sand layers. 4. APPLICATION PRACTICE 4.1 Case Ⅰ: Xianglushan Tunnel Treatment Project of the Central Yunnan Water Diversion Project The No. 5 Adit of this tunnel traverses the Shihuiyao Fault Zone, with a total length of 1245.9 m and a maximum overburden depth of 552 m. The surrounding rock is highly fractured, the external water pressure ranges from 1.00 to 2.11 MPa, and the maximum mud and water inrush volume during the early stage reached 1000 m³. Following in-tunnel grouting, the monthly advance rate was limited to 3.3–4.7 m. For the remaining 552 m unexcavated section, surface directional drilling and grouting technology were applied for the first time to conduct water-blocking and reinforcement treatment of the tuff stratum, with six kilometer-scale high-precision boreholes completed and a deviation rate of ≤ 1.9‰ (Figure 5). The maximum grouting pressure reached 15.06 MPa, and the total grouting volume amounted to 92612.70 m³. After treatment, the stratum water permeability coefficient was reduced to below 2 Lu, the self-stability of the surrounding rock was upgraded to Class Ⅲ, and the working face remained dry without water inrush. Slurry diffusion traces were clearly observed, the fracture sealing rate exceeded 90%, and the water-stop curtain range exceeded 8 m (Figure 6). Therefore, the monthly advance rate of the drill-and-blast method increased to 60 m, representing an approximately 15-fold improvement in construction efficiency.
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