Raise Boring Machine

Overview

A raise boring machine (RBM) is a specialized vertical drilling rig that excavates raises (ore passes, ventilation shafts) from lower working levels upward through staged drilling and reaming. The machine drills a small pilot hole from the bottom level, then pulls a reaming head upward through the pilot to enlarge it to final diameter in a single pass. Raise borers dominate modern mining because they eliminate dangerous hand-mucking in raises and deliver finished shafts with minimal fragmentation and ground disturbance.

Traditional raise sinking via drop-blast (creating a pile of broken ore below that falls to lower level) is slow, dangerous, and leaves poor collar conditions. Raise borers complete shafts in weeks rather than months and produce vertical openings suitable for direct installation of ore passes, conveyor systems, and ventilation fans.

Two-Stage Drilling Process

Stage 1: Pilot Hole (bottom-up): The Rotary Head and Drive grips the Kelly Drive Rod (top drill section with square key interface) and rotates at 30–80 rpm. The Pilot Hole Bit, typically a 200–400 mm diameter core sampler or chisel bit, is fed downward by the Feed Thrust Cylinder at 500–3000 kN thrust. The pilot hole advances at 2–10 m/hour depending on rock strength and drill string compliance.

Drill Pipe Extension Sections are added as the pilot hole deepens. Drill string torque is applied via a Torque Reaction Arm and proportional Torque Control Cylinder, typically 500–2000 kNm for 3 m diameter raises in hard rock. The drill string is suspended from the Spindle Shaft on tapered-roller Spindle Bearing Assembly, which carry both thrust and rotary loads.

Once the pilot hole reaches the upper level (detected by sudden torque drop or breakthrough indication), the rig transitions to reaming.

Stage 2: Reaming (top-down): The rig is repositioned above the raise collar. A Reaming Head Assembly (expandable or fixed) is pushed down through the pilot hole. The Reaming Tool Shell contains carbide inserts arranged in spiral rows at increasing diameters. As the head moves upward through the hole, Reaming Shell Expansion Mechanism hydraulic pressure (or mechanical linkage) gradually opens the shell, bringing outer cutters into contact with the rock.

The reaming head advances upward at 1–5 m/hour, continuously enlarging the hole diameter. Feed force (thrust) is reduced compared to pilot drilling, typically 300–1000 kN, because the reaming cutters work in relief and require less force-to-fracture ratio. Torque increases due to larger cutting diameter, reaching 1500–2000 kNm.

Operational Constraints

Pilot Hole Straightness: A 10° deviation in a 200 m raise creates 35 m of lateral offset at the upper end, unacceptable. Drill-string stiffness is maximized through optimal rod wall-thickness and minimize unsupported span. Modern rigs employ Linear Guide Rails on the mast that keep the Spindle Traveling Carriage vertical to within 1°.

Thread Connections: Drill rod connections (typically pin-and-box or API threads) must be torque-controlled and inspected regularly. Over-torque causes thread stripping; under-torque allows slipping. Typical thread connection torque is 50–150 kNm depending on rod diameter. A single failed connection mid-drill requires pulling 300+ rods (2–3 days lost time).

Rock Quality and Hole Stability: Hole collapse in weak ground (poor RQD, clay zones) can trap the drill string. Probe drilling (small test holes ahead) or pilot-hole camera inspection identifies problem zones for contingency support (resin anchors, casing).

Reaming Tool Selection

Two main reaming approaches:

1. Fixed (non-expanding) reaming head: All cutters are active throughout the upward pass. Simpler, no hydraulic complexity, but requires precise pilot-hole centering. Used in hard, stable rock.

2. Expanding (hydraulic or mechanical) reaming head Reaming Shell Expansion Mechanism: Outer cutters are closed during descent and opened progressively during ascent. More forgiving of pilot-hole drift, but adds complexity and requires controlled reaming-shell opening pressure (20–40 bar differential).

Most raises >2 m diameter use expanding heads for safety and flexibility.

Typical Timeline and Costs

A 300 m diameter × 3 m raise:

Cost per meter: $400–$700/m, depending on rock strength and raise diameter.

Maintenance and Wear

Drill Pipe:

• Thread connections inspected every 500 m drilled; worn threads replaced
• Pipe body wear from bit eccentric motion; 10–20 mm erosion typical over project life
• Replacement cost: $500–$1000 per rod (many rods = major expense)

Spindle Bearings Spindle Bearing Assembly:

• Tapered roller bearings support 1–2 million cycles per project
• Preload adjusted annually; typical bearing life 5–10 years
• Bearing replacement: $20k–$50k for spindle-down work

Reaming Inserts Reaming Carbide Cutters:

• Tungsten-carbide insert wear 5–10 mm per raise; replaced every 2–3 raises
• Cost: $15k–$40k per tool set

Hydraulic System:

• Filters changed every 500 operating hours
• Fluid analysis sampled monthly to detect contamination
• Seal replacement during fluid change cycles

Safety Considerations

• Drill-string failure: catastrophic if chuck release occurs during rotation; safety pins and redundant holding mechanisms required
• Torque reaction: reaction arm swing hazard if control pressure lost; emergency pressure relief prevents uncontrolled motion
• High-pressure hoses: 280 bar hydraulic lines subject to failure; annual non-destructive test and hose age replacement
• Confined space: pilot and reaming holes are entries for inspection; respiratory protection and rescue equipment mandatory

Advanced Features in Modern RBMs

• Torque limiting: proportional hydraulic torque control within ±50 kNm to prevent string breakage
• Automated rod handling: powered rod rack and chasing gear reduce manual labor and thread-wear risk
• Laser/gyro navigation: navigation-grade inertial measurement or gyroscope feedback detects drift in real-time; warning alerts for >5° deviation
• Pressure-monitoring telemetry: wireless data logging of feed, torque, and rotary pressures for performance trending and failure prediction