Rock Drilling with Small and Medium Rotary Drilling Rigs: Challenges and Solutions

Abstract

Rock drilling has long been a major challenge in rotary drilling rig construction, especially for small and medium-sized rigs. In a case study involving moderately weathered limestone in Sichuan, China, a KR125A rig successfully drilled a 900mm diameter hole, penetrating 1000mm into rock and extracting intact rock cores. This demonstrates the high reliability of the KR125A and provides an effective solution to the difficulties small and medium rotary drilling rigs face in rock drilling.

1. Technical Challenges of Rock Drilling with Small and Medium Rotary Rigs

Small and medium rotary drilling rigs encounter several technical challenges in rock drilling:

1. Insufficient axial pressure – Limited downward force reduces drilling efficiency.
2. Lightweight design – Lower machine weight affects stability under rock resistance.
3. High impact from rock – Hard formations generate strong reactive forces, risking equipment damage.

The inability to achieve “leap-type rock fragmentation” efficiently is due to:

• Rock heterogeneity – Rocks are anisotropic, non-uniform, and discontinuous, with complex underground conditions.
• Machine limitations – Balancing mobility, weight, and stability is challenging for small rigs.

A breakthrough was achieved in Sichuan, where a medium-sized rig drilled a 900mm hole, penetrating 1000mm into limestone with a uniaxial compressive strength of 30MPa in just 1.2 hours. The method involved using a barrel drill with carbide picks to grind and fatigue the rock, followed by specialized tools to break and extract the core.

2. Rock Drilling Construction Process

In April 2013, a rotary drilling rig operated in Chongqing, Sichuan, drilling into moderately weathered limestone (30MPa UCS). The setup included:

• Hole diameter: 900mm
• Drilling depth: 1000mm
• Equipment: 4×9.5m locking drill pipe with a carbide-tipped barrel drill

Drilling Mechanism:

• Grinding action: The barrel drill’s rotation (self-rotation + orbital motion) abrades the rock.
• Pressure application: A combination of constant pressure (static load for steady grinding) and pulse pressure (impact load for localized fracturing) enhances cutting efficiency.
• Variable speed effect: Fluctuating rotational speeds due to uneven rock hardness create additional impact, accelerating rock breakage.
• Cooling & lubrication: Continuous water or mud injection prevents overheating and extends tool life.

3. Rock Core Extraction

After reaching the target depth, the rock core must be fractured and retrieved. Due to the rig’s limited weight and torque, a wedge-type drill bucket is used:

• A wedge is inserted between the core and the borehole wall.
• Downward force (F) generates a radial splitting force (F1), breaking the core.
• The barrel drill then extracts the fractured core.

4. Rock Drilling Mechanism Analysis

The success of rotary drilling rigs in rock penetration is attributed to:

• EU EN791 safety compliance: Ensures structural stability while maintaining mobility.
• Optimal weight distribution: Enhances pressure transmission and drilling efficiency.

Three-Stage Rock Fragmentation Process:

1. Grinding Fragmentation (Low axial pressure)
   • Contact pressure < rock strength → Friction-based cutting.
   • Produces fine particles with moderate tool wear.
2. Fatigue Fragmentation (Moderate axial pressure)
   • Repeated impacts induce micro-cracks, weakening the rock.
   • Eventually forms larger fragments.
3. Leap Fragmentation (High axial pressure)
   • Contact pressure ≥ rock strength → Sudden, large-scale breakage.

Small and medium rigs primarily utilize grinding and fatigue fragmentation, compensating for lower axial pressure while maintaining agility. Proper tooling and techniques ensure successful core extraction.

5. Key Conclusions

1. Feasibility of rock drilling – Proper rig design and tool selection enable small/medium rigs to drill into rock effectively.
2. Locking drill pipe is necessary – Essential for weight transfer, but demands high structural reliability.
3. Specialized tooling – Core extraction requires tailored drill bits, a critical research focus for future improvements.
4. Process optimization – Balancing pressure, rotation speed, and cooling (water/mud injection) is vital.
5. Weight distribution – Optimal allocation of drill pipe, tools, and mast weight ensures stability without sacrificing mobility or fuel efficiency.

This approach resolves the long-standing challenge of rock drilling with small and medium rotary rigs, offering a practical and efficient solution for hard rock conditions.