1. Recommended Rock Drill Size (Top Hammer)
For Class II surrounding rock, the rock drill power should be medium to high, typically:
✅ Rock drill impact power
18–25 kW (most commonly used range)
Suitable for hard, intact rock with good penetration efficiency
✅ Common rock drill models
HC95 / HC109
COP 1838 / COP 1840
YT28-class hydraulic equivalent (hydraulic type preferred)
These drills provide sufficient impact energy to maintain high drilling speed without excessive tool wear.
2. Drilling Parameters (Typical)
| Parameter | Recommended Value |
|---|---|
| Hole diameter | Ø38–45 mm |
| Hole depth | 3.0–5.0 m (can reach 6 m with extension rods) |
| Impact energy | 300–450 J |
| Rotation torque | 250–400 Nm |
| Flushing medium | Water or air-water |
3. Matching Drill Rod & Bit
For Class II surrounding rock:
🔹 Drill rod
T38 or T45 threaded rods
Length: 3.05 m / 3.66 m
🔹 Drill bit
Button bit (cemented carbide)
Diameter: Ø41 / Ø43 / Ø45 mm
Suitable for hard rock, long service life
4. Suitable Rock Drilling Machine Types
Based on your previous focus on tunnel equipment, the following machines are well matched for Class II rock:
Single-arm hydraulic rock drilling jumbo
(most common and economical choice)
Double-arm jumbo for higher advance rates
Mini jumbo (only if tunnel section is small, ≤3×3 m)
Recommended boom propulsion beam length:
4.5–5.9 m for standard tunnel blasting cycles
5. Why This Size Is Suitable for Class II Rock
Class II rock is hard but stable, requiring:
Sufficient impact energy to maintain penetration
Stable rotation torque to prevent bit jamming
Oversized drills (≥30 kW) increase cost and wear without significant efficiency gain
Undersized drills (<15 kW) result in slow drilling and frequent bit replacement
✅ Summary Recommendation
For drilling Class II surrounding rock, choose a rock drilling machine equipped with:
18–25 kW hydraulic rock drill
Ø38–45 mm hole diameter
T38/T45 drill rods
Single-arm jumbo as the most practical solution
If you want, you can tell me your tunnel size, drilling depth, and blast hole diameter, and I can help you select a more precise rock drill model for your project.
