Although there is still a sustained trend to model versatility, as demonstrated by Sandvik’s latest MT520 model, there is also a wide range of models available for economical tackling of ground from coal and soft rock (Sandvik MR200 to MR600 series) to the MH600 Series hard-rock machine (formerly the AHM 105 IC). In the middle are those models used most often for tunnelling from the MT300 (former ATM 75) to the MT700 Series (former ATM 105 C) and including the MT520.

New power
Part of the versatility of the MT520 is as a result of its modular design. At 115t it is comparatively light for a cutter motor power of 315kW. The key to its cutting power and stability is the speed of the cutter, and layout of major components rather than the weight of the machine.

Further versatility in dealing with different types of rock can be introduced by changing the set-up of the cutterhead itself, especially the gearing. There are four types available for Sandvik’s MT720/MT620 roadheader with its 300kW cutter drive. For the hardest rocks (110-180MPa), the possible net cutting rate (CR) using the R400-TC87 cutterhead is slim and, obviously, low at around 10-18 solid cubic metres per net cutting hour. At the other end is the R400-TC38 that can excavate up to at least 160 solid cubic metres per net cutting hour if the material is no harder than 60MPa. As an example, the typical use range for the Bilbao Metro in solid limestone has been up to 100MPa and cutting rate up to 30 m3/net cutting hour using the R400-TC72 cutterhead.

Sections
Although roadheaders are relatively mobile, with a tramming speed of up to 15m/min, and can therefore work efficiently in excavating junctions and non-standard profiles, for main tunnel drives it is more efficient to use a roadheader on the same longitudinal alignment. Therefore the cutter boom should be able to reach the full design section of a tunnel, unless two roadheaders work side-by-side for larger sections. Compared to larger TBMs and drill jumbos, roadheaders may therefore be limited in section, although the larger models do have sufficient reach for most standard transport tunnel sections.

Using the Sandvik range as an example again, the new MT520 can reach up to 8m cutting height. The cutting profile of the MT720 that can cover from one position is up to 8.3m wide and 6.1m high, or 48m2 face area. Using the extended telescopic boom of the cutterhead (with a sump-in force of up to 880kN and 650-mm stroke), this can be increased to 9.1m wide by 6.6m high (61m2 face area), see Figure 1.

For larger sections and potentially unstable open faces, sequential excavation methods can further utilise the mobility and versatility of roadheaders by working on several ‘partial’ faces in sequence according to the excavation design. In this method roadheaders operate similarly to conventional hydraulic excavators but usually for harder rock.

Mobility is promoted by the availability of diesel power packs rather than having to handle high-voltage cable feeds for the electro-hydraulic drives. In the case of Sandvik MT720 the diesel drive is a Cat model C-9 rated at 224kW at 1800 rev/min. The engine is certified to the latest EPA Tier IIIA and European Community Stage IIIA emission standards.

Attachments
An alternative to roadheaders for occasional requirements is to employ cutterhead attachments for standard hydraulic excavators. Although not suitable for the demands of full-section excavation, they can find tunnelling applications for removing moderately hard rock bands from softer matrix in the SEM (using the same standard hydraulic excavator carrier), accurate profiling and scaling in harder rock, and trenching for drainage.

Accuracy
Another important factor in the choice of roadheaders as the main means of excavation is accuracy. This is now a matter of intense competition between the main excavation methods. In common with drilling for blasting, roadheaders now have guidance systems available that not only maintain the excavation in the right alignment but ensure that the design profile is reached as close as possible. Such systems are a comparatively new development for roadheaders since early attempts were hampered by vibration and carrier stability issues in relation to the survey reference points.

Successful roadheader guidance and profile monitoring systems have now been developed by VMT, Geodata and others. Using a console screen displaying the preprogrammed design profile, the roadheader operator can guide the cutterhead in its optimum excavation sequence up to the profile limit, using continuous calculation of the cutterhead position.

As well as the operator profile display, the Sandvik MT720 carries sensors for the cutter boom vertical position, an elongation sensor for the boom telescopic movement, and a two-axis inclinometer for pitch and roll. Using an on-board data processor, the values from the sensors can be used for the continuous calculation of the cutterhead position. As a survey reference, a motorized theodolite is linked visually to two reflective prisms and by radio link to the roadheader. The theodolite uses another prism on the tunnel structure as a backsight for directional reference.

With good profile control there can be savings in support materials and costs, with fewer rockbolting requirements to secure the rock and minimal concrete for both primary and permanent support according to the tunnel design.

Josef Pichler of Sandvik claims that the accurate profile achievable with roadheaders results in less disturbance of the surrounding rock mass, and so less loss of friction in the rock parting planes.

As an example of possible cost savings put forward by Pichler, with a 8m by 6m section of 40m2 area and 17m circumference, a typical 200mm radial additional overprofile for support needs will result in 50-100mm using a roadheader but 250-300mm using drill and blast. The additional overcut will require around 3.4m3 of concrete per linear metre of tunnel. With a 6m advance per day, over 26 working days a month, this will require an additional (26x6x4)m3 of concrete. At a cost of EUR 140 (USD 199) per cubic metre this can result in an additional cost of EUR 74,300 (USD 105,620) per month.

Vibrations
Fans of roadheaders over drill and blast point mainly to the lower vibration levels claimed for their operation compared to blasting in urban situations. Tests of roadheader action in dolomitic limestone in Sorrento, Italy, used sensors in the tunnel to measure peak particle velocity at distances of four, seven, 12, 17 and 27m from the cutting face and at 0.5m from the wall. The peak particle velocity ranged from 2.5- 3.5mm/s at the closest sensor to the face, down to 0.5-1.0mm/s, 27m away.

The transmission of cutting vibration through the body of a roadheader and into the ground is reduced by a disc support ring of the main turret that helps absorb the vibration. There will still be some vibration transmitted from the cutterhead through the rock, but this will be alleviated by a sustained cutting action helped by careful design of the cutterhead pick pattern.

There may be other situations where, for operational requirements or the proximity of neighbours, blasting is entirely impractical, unless the use of blasting mats and many other precautions is acceptable. For example, in the refurbishment of the Frejus rail tunnel, Mont Cenis, France, a Sandvik MT620 roadheader has been employed to increase the sectional capacity of the tunnel – behind barriers – whilst the existing railway continued in operation.

Major projects
Among major projects in which roadheaders are being used successfully, together with other methods, are the Brisbane Airport Link, Australia, by the Thiess-John Holland JV, and the East Side Access project in New York by the Dragados-Judlau JV. Rock compressive strengths range from as little as 1.10MPa in Brisbane Tuff, itself a very variable strata, up to 99.80MPa maximum in the Aspley Formation. This has resulted in net CR from 7-160 solid cu m per net cutting hour.

The MT520 and MT720 have been used successfully in commencing the Line Three of the Bilbao Metro (T&TI Oct 2010). The MT720 weighs 135t compared to the MT520’s 115t, and has a 300-kW cutter motor within 533kW total drive power. The MT720 has also been used, with two units in parallel, for driving a highway tunnel in Dortmund, Germany.


An Aker Wirth roadheader at the Caldecott Tunnel site in Callifornia Figure 1, Comparison of profiles achievable with the Sandvik MT520 roadheader using inline – softer rock – and transverse cutterheads Roadheaders can work in close proximity with other systems, as here in the Mont Cenis rail tunnel, Frejus, France, which remained in operation A transverse cutterhead on a Sandvik MT720 roadheader, similar to that used on the Bilboa Metro in Line Three in Spain Construction of the Bilboa Metro Line Three employs this new Sandvik MT520 roadheader, plus the MT720