Self-Drilling Pipe Umbrella vs. Advance Small-Diameter Pipes in Tunnel Support

Time:2025-11-15From:sinorock View:

In tunnel engineering, mine roadways, and various underground space development projects, ensuring the stability of the excavation face is a top priority for construction safety. To deal with weak and fractured ground, engineers often adopt pre-support technologies. Among them, the self-drilling pipe umbrella and advance small-diameter pipes are two widely used but often confused methods. Although they share the same goal—reinforcing the ground before excavation—their working mechanisms, construction efficiency, and applicable conditions differ significantly. This article systematically explains the differences between the two from aspects such as definitions, construction principles, applicable conditions, advantages and disadvantages, and real engineering comparisons, helping engineering professionals make scientific selections.

1. What Are Advance Small-Diameter Pipes?

Advance Small-Diameter Pipes

Advance small-diameter pipes are a traditional and economical pre-support method. They typically use small-diameter steel pipes (such as Φ42 mm or Φ48 mm), with the front end shaped into a tapered tip for easier penetration.

Working Principle and Construction Process

Drilling: A drilling rig creates holes at a designed angle in the arch area outside the excavation contour.
Installation: Prefabricated advance small-diameter pipes are inserted into the drilled holes.
Grouting: Cement grout or chemical grout is injected into surrounding soil or rock through grout holes on the pipe. After hardening, the grout forms a supporting arch.

Key Characteristics

“Drill first, install later”: This is the core feature, with separate procedures.
Applicable Ground: Mainly used in soils or fractured rock with some self-stability.
Advantages: Low cost, mature technology, simple construction.
Disadvantages: In very loose or water-rich formations, hole formation is difficult, collapse is common, and the pipe cannot be installed properly, resulting in low efficiency.

2. What Is a Self-Drilling Pipe Umbrella?

Self-Drilling Pipe Umbrella

The self-drilling pipe umbrella is a major upgrade to traditional advance small-diameter pipe technology. It integrates the functions of drill rod, pipe, and grout channel. Its core component is the hollow anchor rod (self-drilling anchor rod), which has full-length threads and a drill bit.

Working Principle and Construction Process

Integrated Drilling and Installation: The self-drilling anchor rod with its drill bit is directly connected to the drill rig. It is rotated and drilled to the designed depth, and the rod itself remains in the ground as the permanent support structure.
Grouting: After drilling, grout is injected through the central hole of the hollow rod. The grout flows out through the bit and fills the gap between the rod and the borehole wall, forming a dense grout envelope.

Key Characteristics

“Drilling and installation at the same time.”
Applicable Ground: Designed specifically for running sand, silt, cobble layers, and other highly unstable, easily collapsing formations.
Advantages: High success rate, fast construction, good support performance, and improved corrosion resistance due to grout encapsulation.
Disadvantages: Higher material cost and greater technical requirements.

3. Case Study

metro tunnel project

In a metro tunnel project, the original design used advance small-diameter pipes. The construction section mainly passed through Quaternary cobble layers and Tertiary highly weathered conglomerate. The cobble layer had large particle size, high content, and good cementation. When the tunnel excavation entered the full-face conglomerate layer, the traditional advance small-diameter pipes could not penetrate the hard rock effectively, causing grouting difficulties and preventing the formation of a continuous reinforcement arch. Meanwhile, multiple continuous water inflows appeared at the excavation face, severely affecting construction safety and progress.

3.1 Solution

After comparison of multiple options and field trials, self-drilling hollow anchor rods were adopted to replace the original design. Specifically, R25 (3 m) and R32 (5 m) self-drilling anchor rods were used together with a cement–water glass two-component grout system. A uniform and dense reinforcement layer with a thickness of 1.5 m was successfully constructed within a 140° range of the tunnel vault, achieving dual goals of ground reinforcement and water control.

3.2 Performance Comparison

Significant Improvement in Reinforcement

Original advance small-diameter pipe grouting: maximum reinforcement thickness 1.0 m
Self-drilling anchor system: reinforcement thickness 1.5 m
Reinforcement range increased by 50%, improving bearing capacity

Water Inflow Significantly Improved

With advance small-diameter pipes: multiple leakage points
With self-drilling anchors: excavation face remained dry, water inflow completely controlled

Notable Settlement Control
Advance small-diameter pipes:

Daily maximum settlement: 2 mm
Stabilized cumulative settlement: 8–9 mm
Maximum settlement: 12 mm

Self-drilling anchors:

Daily settlement controlled within 1 mm
Stabilized cumulative settlement: 6–7 mm
Maximum settlement: 9 mm
Settlement control improved by about 40%, significantly enhancing construction safety.

3.3 Case Insights

This case fully demonstrates the adaptability and reliability of self-drilling anchor technology in complex geological conditions. Its integrated drilling–support–grouting process effectively solves the technical bottleneck of traditional advance small-diameter pipes and provides valuable experience for similar tunnel projects.

4. How to Make the Right Choice?

Self-Drilling Pipe Umbrella

Choosing between a self-drilling pipe umbrella and advance small-diameter pipes is not a simple cost comparison but a comprehensive decision based on geology and project requirements.

Choose Advance Small-Diameter Pipes When:

Ground conditions are relatively stable and collapse is unlikely
Project budget is tight
Construction schedule is flexible

Choose Self-Drilling Pipe Umbrella When:

Facing cobble, running sand, high water table, or easily collapsing formations
High safety and schedule requirements leave no room for collapse risks
A more durable and higher-quality support structure is needed

5. Conclusion

Both methods have their advantages: the self-drilling pipe umbrella excels in efficient construction and strong support, while advance small-diameter pipes offer cost-effectiveness and flexibility. In real projects, decisions should be based on geological conditions, schedule requirements, and budget. Understanding their fundamental differences is crucial for engineers to design safe, economical, and efficient tunnel support solutions.

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