Drilling Jumbos

A drilling jumbo is a mobile rig with one or more hydraulic booms, each carrying a rock drill on a feed, used to drill the blast holes at the face of a tunnel or mine drift. It is the key machine of the drill and blast method: the jumbo drills a precise pattern of holes, the holes are charged with explosives and blasted, the rock is mucked out and the cycle starts again. Jumbos are also called face drilling rigs, and similar carriers are used for rock bolting and for drilling holes for ground reinforcement.

Drill and blast remains the most flexible way to excavate tunnels in hard rock. It works in almost any rock, for any cross-section and for short or long tunnels, and the equipment can be moved from one project to the next. The jumbo decides how quickly and accurately the face is drilled, which in turn affects the quality of the blast, the amount of overbreak and the overall advance per round.

Brief History of Drilling Jumbos

Mechanical rock drills appeared in the nineteenth century, when compressed air drills were used on large Alpine railway tunnels. For a long time these drills were handheld or mounted on simple bars and columns, and several drills were often fixed to a wheeled or rail-mounted platform to work on the face at the same time. These large multi-drill platforms are generally seen as the origin of the name jumbo.

The biggest change came with hydraulic rock drills, introduced in the early 1970s. They were more efficient, less noisy and produced no oil mist compared with pneumatic drills, and they allowed faster penetration. At the same time, hydraulic booms with parallel holding and rubber-tyred diesel carriers replaced the older rail-mounted rigs in many projects. From the 1980s and 1990s computer-controlled jumbos appeared, able to position booms according to a drilling plan loaded into the rig. Current machines combine precise positioning, data logging and, increasingly, battery-electric carriers.

How a Drilling Jumbo Works

Carrier. A rubber-tyred, articulated chassis with a diesel engine for tramming, or a battery-electric drive. At the face, the hydraulic pumps are usually driven by electric motors fed by a cable from the tunnel power supply.
Booms. One, two, three or more hydraulic booms that can swing, lift, extend and rotate. Parallel holding keeps the feed at a set angle as the boom moves, which helps drill parallel holes.
Feed. A long beam along which the rock drill slides. Feed length determines the length of the round, commonly several metres.
Rock drill. A hydraulic hammer that delivers rapid blows to the drill steel while rotating it. Flushing water carries the cuttings out of the hole and cools the bit.
Drill string. Drill steel, couplings and a carbide button bit. Some rigs carry rod handling systems to add rods for long holes.
Operator platform or cab. The operator controls the booms from a cab or platform, often with a basket on a separate boom for charging and scaling.
Control system. On modern rigs, the drilling plan is loaded into the rig, sensors report boom and feed positions, and the system guides or automatically positions each hole.

Types of Drilling Jumbos

Single Boom Jumbos

Compact rigs for small drifts, narrow vein mining and small tunnels, typically in cross-sections of a few square metres to about 30 square metres.

Twin Boom Jumbos

The most common configuration in mining and medium tunnels. Two booms drill at the same time, giving a good balance of speed, cost and manoeuvrability.

Three Boom and Multi-Boom Jumbos

Rigs for large road and rail tunnels with sections of 100 square metres and more. The booms work in parallel to drill the full face quickly, and many rigs have an extra boom for a service platform.

Rail-Mounted and Gantry Jumbos

Rigs mounted on rails or on gantries for long tunnels or very large sections, such as some hydropower tunnels and caverns.

Low-Profile Jumbos

Rigs with reduced height for low mine openings.

Rock Bolting and Cable Bolting Rigs

Closely related carriers equipped for drilling and installing rock bolts or cable bolts, usually in a single mechanised process.

Size Classes

Class	Typical tunnel cross-section	Typical use
Small	about 6 to 30 m²	Narrow vein mining, small drifts, utility tunnels
Medium	about 20 to 100 m²	Mine development, rail and water tunnels
Large	about 60 to 200 m² and more	Road tunnels, large rail tunnels, caverns

Figures are indicative and overlap. Boom configuration and reach vary by manufacturer and model.

Attachments and Options

Beyond the basic booms and rock drills, jumbos can be specified with a range of options that change what they can do at the face. Service platforms or baskets on an extra boom give access for charging explosives, scaling, installing mesh and fitting ventilation ducts. Rod handling systems allow longer holes for probe drilling and grouting ahead of the face. Cable reels feed electric power to the rig, and water booster pumps keep flushing pressure up when the supply is weak. Other common options include bolting and meshing units, measuring instruments for hole depth and angle, profile scanners, cameras, air conditioning for hot mines and fire suppression systems. On larger rigs, boom and feed geometry can be chosen to suit tunnels with cross passages, curves or wide caverns.

Main Manufacturers

Epiroc, formerly part of Atlas Copco, with its Boomer range of face drilling rigs.
Sandvik, with a wide range of tunnelling and mining jumbos and in-house rock drills.
Komatsu, active in underground hard rock drilling.
Resemin, a Peruvian manufacturer known for compact rigs for narrow vein mining.
Furukawa, the Japanese manufacturer of rock drills and tunnelling jumbos.

Main Uses of Drilling Jumbos

Drilling blast holes in road, rail and water tunnels.
Mine development in drifts, ramps and crosscuts.
Production drilling in some underground mining methods.
Drilling for rock bolts, grout injection and probe holes ahead of the face.
Caverns for hydropower, storage and underground stations.
Scaling and enlarging existing tunnels.

Buying and Operating Tips

Match booms to the section. The rig must reach the whole face and still fit in the smallest opening, including curves, passing bays and ventilation ducts.
Invest in drilling accuracy. Accurate, parallel holes give cleaner blasts, less overbreak and smoother walls. Computerised positioning quickly pays back on long tunnels.
Keep the drill string in good condition. Bit grinding, correct feed pressure and good flushing water are essential for penetration rate and steel life.
Plan power and water. Jumbos need a reliable electrical supply and clean flushing water at the face.
Train the operators. Even with automation, operator skill still makes a large difference to cycle times and blast quality.

Safety

The face after a blast is one of the most hazardous places in a tunnel or mine. Loose rock must be scaled and ground support installed before anyone works beneath the new roof. Modern jumbos have protective cabs with falling object protection, which keep the operator further from the face. Other hazards include drilling into misfired explosives from a previous round, noise, dust, contact with moving booms and high-pressure hydraulics. Good lighting, water sprays, hearing protection and a clear procedure for checking the face for misfires are essential.

Trends and Technology

Face drilling rigs are becoming increasingly automated. Many rigs can now drill a full round with little input from the operator, following a drilling plan and logging each hole’s position, depth and drilling parameters. This data is used to refine blast design and to map rock conditions ahead of the face. Battery-electric carriers, which use the battery for tramming and the electric mains supply for drilling, are replacing diesel engines in many mines. Remote operation and scanning of the tunnel profile after each blast are also spreading.

Key Terms

Drill and blast: the tunnelling method based on drilling, charging and blasting the face.
Round: one cycle of drilling and blasting, and the length of tunnel it advances.
Feed: the beam on which the rock drill travels.
Parallel holding: automatic control that keeps the feed at a constant angle as the boom moves.
Cut: the central group of holes that creates the initial opening in each blast.
Overbreak: rock removed beyond the design profile.

Frequently Asked Questions

Why is it called a jumbo? The name comes from the large multi-drill platforms used in early mechanised tunnelling, which were large compared with handheld drills.

How many holes does a tunnel face need? It depends on the size of the section, the rock and the explosives used. Small drifts may need a few dozen holes, while large road tunnels may need well over a hundred.

Can a drilling jumbo install rock bolts? Jumbos can drill bolt holes, and some carriers are fitted for bolting, but dedicated bolting rigs are usually used for mechanised bolt installation.

Are drilling jumbos electric? Drilling is usually powered by electricity from the tunnel supply. Tramming has traditionally used a diesel engine, but battery-electric carriers are now widely offered.

How long is a typical drill and blast round? Round length depends on the rock, the section and the feed length of the jumbo. Rounds of several metres are common in tunnelling, and shorter rounds are used in poor ground where support must follow closely.