Roadheaders are versatile machines with a long history that offer many advantages when constructing tunnels, mines or underground facilities. Their use dates back to the first half of the 20th century – predating the TBM – and they have been used across the globe.
To get a better sense of what to consider when buying a roadheader, we talked to Wilhelm Papst, Sales Support Manager for Mechanical Cutting and Construction for Sandvik Construction. He is based in Zeltweg, Austria, where Sandvik Construction rock cutting equipment and roadheaders are built. He has nearly 40 years of experience in roadheader manufacturing.
What are some of the advantages of roadheaders?
With a roadheader you can make 90 degree cross cuts and maneuver around curves with small radii, which you cannot do with a TBM. So, it is well suited for constructing cross passages and adits for ventilation equipment, etc. You can cut the exact profile that you require – a horseshoe, for example – whereas with a TBM the excavation can only be circular, which means that in some cases you may be over-excavating.
Another advantage of the roadheader is that it provides a safe operation because you don’t loosen the surrounding rock, whereas with blasting there is a potential that a piece of rock could come loose. Also a roadheader allows a much more precise tunnel profile. Roadheaders provide precision within 5 cm, whereas drill-and-blast is on the order of 30 to 35 cm. The more precise the profile is, the more savings are achieved because less shotcrete is required to fill the voids. The precision is made possible by electronic navigation using an external theodolite. A surveyor marks a set point, and then the machine works off that set point.
How do rock properties affect the decision to use roadheaders?
The use of roadheaders is limited by the properties of the material you are cutting. With the latest technology of Sandvik Construction roadheaders, we can economically cut all rock with compressive strengths up to 130-140 MPa. Above this limit it is still possible to cut the rock, but the cutter pick consumption begins to increase to where it is not as economically feasible. But there are some projects that only allow the use of roadheaders because the use of drill-and-blast is restricted. This is the case sometimes in urban areas or below buildings – wherever there may be the potential for damage due to blasting shock waves. In those cases, the roadheader is the best option.
On the other end, in areas where the rock may be less than 100 MPa, mining with a roadheader may be even cheaper than drill-and-blast. The advantage of a roadheader application is that it is a continuous mining operation. You cut the material, it falls down at the face, it is picked up by the loading mechanism, transported from the machine and hauled away. Drill-and-blast is a discontinuous operation. You have to drill, load the explosives, set the charge, wait for the fumes to dissipate and then you have to load and transport the material in that sequence. We have had tunnels in the past with advance rates of over 20 m per day.
When considering a roadheader, you need to consider the abrassivity (Cerchar index) of the ground. In rock with a high content, for example, pick consumption is higher. But again there are instances where there are no other options. Our clients, for example, have completed projects in New York City (East Side Access) and in Japan (Toyota plant) where blasting was not permissible and there was no other feasible way to excavate.
How does the size of the tunnel impact the use of roadheaders?
There is actually no limit on how big the tunnel is, but rather how small. The machine has to have space on both sides and above the machine to operate. You need a minimum of about 45 cm on the sides and 30 cm above the highest point of the machine. In large diameters when you using a roadheader, you cut three sections on the top heading, and then three cuts on the bench – for a total of six cuts – and you are able to create large-diameter spans.
The smallest roadheader we have has a loading width of 3.5 m, which can be customized smaller if the customer requires. Our most common roadheader used for tunneling – the MT 720 – has a 4.56-m wide loading table. For tunneling projects, the two big models MT 720 and MT520 represent about 90 percent of what we build. The smaller roadheaders we sell are used for hydro projects or small sewer or water tunnels, whereas the most road and rail tunnels will use the bigger machines.
What advances have been made in roadheader technology recently?
We now have options for different for conveyors – conveyors behind the machine that can be moved up and down, left or right, so that trucks can be set. They can also be used with a bridge belt conveyor suspended on the roof. There are also many electronic options in addition to the guidance system. We offer an automatic cutting system that allows the machine to automatically cut the face – you only need to advance the machine manually. This avoids overloading the cutter motor. We have connectivity to the tunnel’s communication network – which either links wirelessly or via LAN. We have report-generating software using data from sensors of the machine. The machine is fully PLC-controlled with a screen in the cabin that shows operating data. We have different versions of the cabin that include air conditioning and pressurized cabins that prevent dust from entering the cabin. Roadheaders can also be equipped with de-dusting systems, generators and ventilation.
What are the next steps in roadheader development?
We are continuing to see advances in hard rock cutting technology including cutting with slow revolution of cutterhead, which allows for better torque. We are also seeing continuous improvement in the quality of the cutting tools, including tungsten picks that provide longer life. These advances, along with the improving electronics and communications, allow roadheaders to get better and better every year.