The sixties saw huge achievements in technology. Humanity entered the space age by putting a man on the moon. The computer mouse, the computer game, handheld calculator and the first ATM were all products of the decade. For the tunnelling industry, the late 1960s were the first time roadheaders appeared on tunnelling sites – having been developed initially for coal mining. While a comparatively small player in the tunnelling market when compared with rival equipment types such as TBMs, roadheaders have been responsible for excavating some of the most challenging tunnel lengths in fiercely varying geology.

"Roadheaders were first introduced and developed for excavation of coal in Hungary in the 1950s," says Karl Mitterndorfer, president of machine manufacturer Antraquip.

"But the Hungarians couldn’t manufacture effective machines – it was a communist country then. The machine became successful when the British and the company Voest Alpine of Austria, bought by Sandvik, started building the machines in the early 1960s." By the late 1970s roadheaders were widely used and accepted on tunnelling projects.

Cutterhead Choice
The fundamental tool of any roadheader is the cutterhead. Two cutterhead types have been developed for roadheaders. The transversal-axial cutterhead rotates parallel with the boom, ripping the rock. The axial cutterhead rotates perpendicular to the boom and mills the rock.

There is much debate about when each cutterhead type should be used. Machine supplier Webster Machines argues: "Between the two types of roadheader cutter heads, the ripping and the milling heads, the ripping type is more suitable for hard rock cutting. This is due to more efficient cutting during sumping, due to more efficient cleaning of the face, and because the picks attack the tunnel face perpendicularly and thus utilise the machine’s total weight as reaction force to the pick’s cutting force."

Antraquip’s Mitterndorfer says: "If the face of the tunnel is hard to penetrate but there are some small softer openings or cracks it is definitely easier to do this with an longitudinalaxial head. If no such cracks are available and you have to go through a hard solid tunnel face, in our opinion, the transverse head is superior. Sometimes, the customer has a preference and we only recommend, but if the customer insists we will supply what they want."

On either type of head, an optimal design is necessary to match the rock cutting characteristics and maximise the production rates.

The design parameters include the spacing between picks, location of picks, their tilt angle and the skew angle.

Push for hard rock
Competition among the various excavation technologies is fierce. For a while some equipment and methods had a strong niche that only they served. But as technology has advanced the manufacturers have been pushing equipment into new territory. Roadheaders have traditionally operated in softer and friable rock, but in the past 50 years the machines have developed greatly. Roadheaders have steadily increased in weight, size and cutterhead power to tackle tougher rocks.

"The first roadheaders were very small," says Mitterndorfer. "They were 12t to 13t and had very little power. They were only able to cut rock with a strength of 5,000psi (35MPa). Now we are looking at 30,000psi (210MPa). The range went from 12t up to around 135t, which is what Sandvik is doing.

"Naturally, today everything is sturdier and the materials are better. Hydraulics are a completely different world to hydraulics 30 years ago. Electrics are completely different. You also have a computer in the machines and can run them remotely. It’s safer now, certainly. There are a lot of differences."

The heavier machines and higher cutterhead torque apply a much greater force onto the rock face. But when facing very hard rock conditions, the equipment is still playing catch up with TBM technology.

Sandvik’s Wilhelm Papst, manager for sales support and mechanical cutting, says that there are still economic limits and it is not always best to use a roadheader for tunnelling.

"The TBM has its advantages over a roadheader," he says. "This is mainly the fact that you can cut very hard material. The roadheader is limited, machines can economically cut up to 80MPa. Our hard rock miners can cut up to 120 to 130MPa, but it depends on how the rock is fractured. If the rock is fairly fractured, you can cut even harder material, but if it is solid then we say 120MPa is the economic limit."

Mitterndorfer adds: "Indeed, a roadheader is limited by the hardness of the material. In addition, a TBM is justifiable if the tunnel’s length is such that it pays to spend so much money – a TBM costs far more than a roadheader.

"If a tunnel is longer than two miles [3.2km], I normally put it aside and say it’s a TBM job. However, to use a TBM in an application with many rock faults and where the material to cut changes quickly from soft to extremely hard or vice versa, can be extremely difficult. The danger is, the TBM can get stuck and cannot go forwards or backwards. Then you have a problem. It’s a nightmare for every tunnelling engineer."

Mitterndorfer adds that this is the reason that the design engineers of the 18,660ft (5.6km) New Irvington Tunnel in California did not allow the use of a TBM. The Southland/Tutor Perini JV is using two Antraquip roadheaders for the project as well as controlled detonations to excavate the two headings.

Work has been challenging with high groundwater inflows and pressures, some squeezing ground areas and extensive pre-grouting.

Gritted teeth
The teeth of the roadheader have also developed to meet the challenges of tougher ground. According to Webster, picks are tipped with tungsten carbide and are available as radial ‘flat’ bits or rotating, conical ‘point attack’ bits. Most modern roadheaders use point attack bits. The general rule for carbide selection is: for hard-rock cutting, soft carbide is used because it does not shatter on impact. Soft rock is excavated with hard carbide, which has high wear resistance. Webster explains, "The spacing between picks must be optimised by analysing rock failure behaviour and anticipated depth of penetration. Also, cutter head layout must be balanced by controlling the placement of the cutters to create an even pick force distribution to minimise vibrations. This issue is of crucial importance since the increased spacing means less number of cutters and potentially higher vibrations. The head vibration can have adverse effects on the production rate, machine life, and maintenance. Consequently, an optimised cutter head design, achieved by well balanced pick distribution and minimised vibration, can enhance the performance of a given machine."

Pick wear is mainly influenced by the rock properties and the degree of mineral interlocking.

Major manufacturers have proprietary empirical tables and graphs for pick consumption in rocks of various unconfined compressive strengths and content of abrasive minerals. Also, tables with production rates (m3 per hour) are available for each cutter head power in rock and minerals of various UCS (MPa). If the roadheader utilisation rate as a percentage of the available face time (net cutting time) is known, relatively accurate pick cost and shift production rate estimates can be prepared.

Battle against Blasting
Roadheaders are often pitched against drill and blast technology. The systems share the ability to respond to changing ground conditions. Blasting is typically more efficient in harder rock conditions. "Depending on the material," says Antraquip’s Mitterndorfer, "if the rock is too hard for a roadheader or is just marginal for a roadheader then it might be more efficient to just drill and blast. Drill and blast technology has advanced too; in the early days, I remember 30 or 40 years ago, a tunnel that was excavated with drill and blast really looked bad – loads of over-excavation – but the technology improved substantially there."

However, the quiet of the roadheader is helping it win back ground. Sandvik’s Papst says, "Sometimes you are simply not allowed to use explosives. Especially when excavating near historical buildings or through urban areas.

"Environmental reasons may also block the use of explosives. In these cases customers may use roadheaders even if it is more expensive."

Mitterndorfer adds, "In many situations in today’s world you’re not allowed to drill and blast or it is very difficult to do it. Following 11 September 2001, the US has become very restrictive on handling any explosives. Everyone needs to be certified. If you’re in an explosive area, then there’s a danger of explosives. So you need to have your electric system approved as necessary.

"You have to also meet the local standards, which are sometimes difficult".