As underground mining operations continue to move deeper, geological conditions are becoming increasingly challenging. Weak rock formations, fractured zones, fault structures, high in-situ stress, and groundwater problems create significant risks for tunnel stability and construction safety.

For underground mine reinforcement, both small-diameter pipe grouting systems (small pipes) and self-drilling anchor bolt systems (SDA systems) are widely used for ground improvement and pre-support. Although both technologies can enhance surrounding rock stability through grouting, their structural design, installation methods, and performance characteristics are fundamentally different.

This article provides a comprehensive comparison between self-drilling anchor bolts and small-diameter pipes, focusing on their structures, underground mining applications, construction processes, and advantages of self-drilling technology.

1. Structural Comparison of Self-Drilling Anchor Bolts and Small-Diameter Pipes

1.1 Structure of Self-Drilling Anchor Bolts

A self-drilling anchor bolt is an integrated ground support system combining:

• Drilling
• Grouting
• Anchoring

into a single construction process.

Unlike conventional anchoring systems that require a separate drilling rod and casing, the hollow anchor bar itself functions as the drilling rod during installation and remains permanently embedded as part of the support structure.

A typical self-drilling anchor bolt system consists of:

Hollow Anchor Bar

The hollow bar is manufactured from high-strength alloy steel with a continuous external thread and a central hollow channel.

Its main functions include:

Transmitting drilling torque;

Serving as a grout delivery channel;

Providing long-term reinforcement capacity.

Common sizes include:

• R25. R28. R32. R38. R51;
• T30. T40. T52. T76 and larger diameter systems.

Different diameters can be selected according to rock conditions, anchoring depth, and load requirements.

Drill Bit

The drill bit is installed at the front end of the hollow bar and directly cuts through the rock formation.

Depending on geological conditions, different types can be selected:

• Cross carbide bits;
• Button bits;
• Spherical alloy bits;
• Mud bits.

The drill bit selection directly affects drilling efficiency and hole stability.

Coupler, Plate and Nut

For deeper reinforcement requirements, multiple hollow bars can be connected using couplers.

The bearing plate and nut transfer the load from the surrounding rock to the anchor system, improving overall stability.

Grouting System

After or during drilling, grout is injected through the hollow center of the bar.

The grout fills fractures and voids around the anchor, creating a composite structure:

Steel Bar + Grout + Surrounding Rock

which improves the load-bearing capacity of the rock mass.

1.2 Structure of Small-Diameter Pipes

Small-diameter pipe grouting systems are commonly used for temporary reinforcement and pre-support in underground construction.

A typical small pipe system consists of:

Steel Pipe Body

The pipe is usually manufactured from steel tubes with:

Diameter typically ranging from 2550 mm;

Length commonly between 36 m.

The pipe itself does not have drilling capability and requires a separate drilling process before installation.

Grouting Holes

Small holes are distributed along the pipe surface, allowing grout to penetrate into surrounding soil or fractured rock.

The purpose is to:

Fill cracks;

Improve ground strength;

Create a reinforced zone around the excavation.

Installation Structure

The installation process generally includes:

Drilling a hole;

Inserting the pipe;

Sealing the hole;

Injecting grout.

This traditional approach has been widely applied in tunneling and underground engineering.

2. Application Differences in Underground Mining

Applications in Underground Mines

Fractured Rock Zones

Underground mines frequently encounter:

• Fault zones;
• Broken rock masses;
• Weathered formations;
• Loose surrounding rock.

During small pipe installation, borehole collapse may occur before pipe insertion, reducing installation reliability.

Self-drilling anchor bolts overcome this limitation because the hollow bar remains inside the borehole during drilling, providing continuous support and improving construction success rates.

Advance Support for Tunnel Excavation

Small pipes are commonly used for:

Face reinforcement;

Temporary pre-support;

Local grouting reinforcement.

Self-drilling anchor bolts can be used for:

Advanced reinforcement;

Self-drilling pipe roof systems;

Deep grouting reinforcement.

They are particularly suitable for fast excavation environments where construction speed and safety are critical.

Deep Underground Mining

As mining depth increases, surrounding rock faces:

Higher stress;

Greater deformation;

More complex instability mechanisms.

Self-drilling anchor bolts provide stronger reinforcement through the combination of high-strength steel and grout bonding, improving:

Tensile resistance;

Shear resistance;

Rock mass integrity.

3. Construction Process Comparison

3.1 Small-Diameter Pipe Installation Process

Traditional small pipe construction generally includes:

Step 1: Drilling

A separate drilling machine creates the borehole according to the designed angle and depth.

Step 2: Pipe Installation

The prefabricated steel pipe is inserted into the drilled hole.

Step 3: Hole Sealing

The opening is sealed to prevent grout leakage.

Step 4: Grouting

Cement grout or chemical grout is injected into the surrounding formation.

Step 5: Grout Curing

The reinforced zone develops after grout hardening.

Limitations

Although the method is mature, it has several challenges:

More construction steps;

Longer installation time;

Difficulty in unstable formations;

Potential borehole collapse.

3.2 Self-Drilling Anchor Bolt Installation Process

The self-drilling anchor bolt installation process is simplified:

Step 1: Install Drill Bit

The drill bit is connected to the front end of the hollow bar.

Step 2: Drilling

The hollow bar directly penetrates the rock while transmitting drilling torque.

Step 3: Simultaneous Grouting

After drilling, grout is injected through the hollow channel.

Step 4: Install Plate and Nut

The anchor system is completed.

Key Advantages

The entire process integrates:

• Drilling;
• Installation;
• Grouting;
• Anchoring.

This significantly reduces operation steps and improves construction efficiency.

4. Advantages of Self-Drilling Anchor Bolts in Underground Mining

4.1 Superior Performance in Difficult Ground Conditions

Self-drilling anchor bolts are particularly effective in:

• Highly fractured rock;
• Weak formations;
• Collapsible ground;
• Water-bearing zones.

The embedded hollow bar helps maintain borehole stability during installation.

4.2 Faster Construction Speed

Compared with conventional small pipe systems, self-drilling technology eliminates:

• Drill rod removal;
• Separate pipe installation;
• Additional casing operations.

This makes it highly suitable for rapid underground excavation.

4.3 Higher Load-Bearing Performance

After grouting, the self-drilling anchor forms a composite reinforcement system.

The interaction between:

• Steel bar;
• Grout body;
• Rock mass

provides improved structural stability and long-term support performance.

4.4 Lower Overall Project Cost

Although the initial material cost may be higher than small pipes, the total project cost can be reduced through:

• Reduced labor requirements;
• Fewer construction procedures;
• Higher installation success rate;
• Faster excavation progress.

4.5 Better Compatibility with Modern Mining Equipment

Self-drilling anchor bolts can be installed using:

• Hydraulic drilling rigs;
• Bolter machines;
• Underground mining equipment.
• This supports more efficient and mechanized mining operations.

5. Self-Drilling Anchor Bolt as an Alternative Solution

With the development of deeper and more complex underground mining projects, self-drilling anchor bolts are becoming an effective alternative to traditional small pipe reinforcement methods.

5.1 Alternative for Fractured Rock Pre-Support

Traditional method:

Drilling Pipe Installation Grouting

Self-drilling solution:

Drilling + Grouting + Anchoring in One Operation

Suitable for:

• Fault zones;
• Broken rock formations;
• High-risk excavation areas.

5.2 Alternative to Some Pipe Roof Applications

For short and medium-length advance support requirements, self-drilling anchor systems can replace certain small pipe roof applications.

Benefits include:

• Lower space requirements;
• Faster installation;
• Higher reinforcement capacity.

5.3 Alternative for Permanent Reinforcement

In areas requiring:

• Deep anchoring;
• High load capacity;
• Long-term stability,

self-drilling anchor bolts combined with pressure grouting can provide a reliable permanent support solution.

Conclusion

Self-drilling anchor bolts and small-diameter pipes both play important roles in underground mining reinforcement. However, their technical approaches are different.

Small-diameter pipes mainly provide grouting-based ground improvement, while self-drilling anchor bolts provide integrated drilling, grouting, and structural anchoring.

With increasing demands for safer, faster, and more efficient underground mining, self-drilling anchor bolt technology is becoming an important solution for complex geological conditions, especially in fractured rock masses, deep mines, and high-risk excavation environments.

Integrated installation, strong adaptability, and high load-bearing performance make self-drilling anchor bolts a promising alternative for next-generation underground mine support systems.

Categories

Mining

Keywords

Self-Drilling Anchor Bolts, Small-Diameter Pipes, Underground Mining Support