Mechanical excavation of tunnels using Roadheader is getting widespread and popular due to their capability of excavating precisely soft to medium strength rock without weakening the surrounding rock. Roadheader is flexible, very mobile, safe, and environmentally friendly machinery. It can excavate smoothly various sizes, shapes, and types of tunnel openings.
The more widespread use of the mechanical excavation systems is a trend set by increasing pressure on the mining and civil construction industries to move away from the conventional drill and blast methods to improve productivity and reduce costs. The additional benefits of mechanical mining include significantly improved safety, reduced ground support requirements and fewer personnel. These advantages coupled with recent enhancements in machine performance and reliability has resulted in mechanical miners taking a larger share of the rock excavation market.
Roadheaders are the most widely used underground partial-face excavation machines for soft to medium strength rocks, particularly for sedimentary rocks. They are used for both development and production in soft rock mining industry (i.e. main haulage drifts, roadways, cross-cuts, etc.) particularly in coal, industrial minerals and evaporitic rocks. In civil construction, they find extensive use for excavation of tunnels (railway, roadway, sewer, diversion tunnels, etc.) in soft ground conditions, as well as for enlargement and rehabilitation of various underground structures. Their ability to excavate almost any profile opening also makes them very attractive to those mining and civil construction projects where various opening sizes and profiles need to be constructed.
The roadheader is a hybrid mechanical excavator consisting of a boom mounted cutting head, a loading machine usually involving a conveyor and a crawler travelling track to move the whole machine forward into the rock face. The cutting head can be a general purpose rotating drum mounted in line or perpendicular to the boom or can be particular function heads such as jack hammer like spikes, compression fracture micro wheel heads like those on larger tunnel boring machines like a gigantic chain saw for dicing up rock or simple jaw like buckets of traditional excavators. Given the variable geological conditions encountered on most tunnel projects, the roadheaders are now being built in such a way that the same machine could be used as a drill rig, be equipped with different cutterhead styles, operated with various cutting speeds or be equipped with a hydraulic breaker and alternatively a bucket (shovel). Roadheaders were and continuously are being made more versatile.
In general, Road-headers can be divided into two types; milling (axial) type, where the cutter head rotating around the boom axis, and ripping (transverse) type, where the head rotating perpendicular to the boom axis. Road-headers are also available in size ranging from 30 ton up to 120 ton, cutting motor power ranges from 80 kw up to 500 kw and a maximum torque up to 2.5 times the running torque. Moreover, Road-headers are available in single, double, telescopic, and articulated boom.
Roadheaders were first developed for mechanical excavation of coal in the early 1950s. Nowadays, it plays a major role not only in mining industry but in tunneling excavation projects as well due to continual performance increases brought about by new technological developments and design improvements. Design of Roadheader has been improved since its first development, and these improvements include increased machine weight, size and cutterhead power, improved design of boom, muck pick up and loading system, more efficient cutterhead design, metallurgical developments in cutting bits, advances in hydraulic and electrical systems, and more widespread use of automation and remote control features. All these improvements lead to long life hi-tech machinery with high cutting capabilities.
Machine weights have reached up to 120 tons providing more stable and stiffer (less vibration, less maintenance) platforms from which higher thrust forces can be generated for attacking harder rock formations. . The cutterhead power has increased significantly, approaching 500 kW to allow for higher torque capacities. Modern machines have the ability to cut cross-sections over 100m2 from a stationary point. Computer aided cutterhead lacing design has developed to a stage to enable the design of optimal bit layout to achieve the maximum efficiency in the rock and geologic conditions to be encountered. The cutting bits have evolved from simple chisel to robust conical bits. The muck collection and transport systems have also undergone major improvements, increasing attainable production rates. The loading apron can now be manufactured as an extendible piece providing for more mobility and flexibility. The machines can be equipped with rock bolting and automatic dust suppression equipment to enhance the safety of personnel working at the heading. They can also be fitted with laser-guided alignment control systems, computer profile controlling and remote control systems allowing for reduced operator sensitivity coupled with increased efficiency and productivity.
Mobility, flexibility and the selective mining capability constitute some of the most important application advantages of roadheaders leading to cost effective operations. Mobility means easy relocation from one face to another to meet the daily development and production requirements of a mine. Flexibility allows for quick changes in operational conditions such as different opening profiles (horse-shoe, rectangular, etc.), cross-sectional sizes, gradients (up to 20, sometimes 30 degrees), and the turning radius (can make an almost 90 degree turn). Selectivity refers to the ability to excavate different parts of a mixed face where the ore can be mined separately to reduce dilution and to minimize waste handling, both contributing to improved productivity. Since roadheaders are partial-face machines, the face is accessible, and therefore, cutters can be inspected and changed easily, and the roof support can be installed very close to the face. In addition to these, high production rates in favorable ground conditions, improved safety, reduced ground support and ventilation requirements, all resulting in reduced excavation costs are the other important advantages of roadheaders.
The hard rock cutting ability of roadheaders is the most important limiting factor affecting their applications. This is mostly due to the high wear experienced by drag bits in hard, abrasive rocks. If the rock is very abrasive, or the pick consumption rate is more than 1-pick/m3, then roadheader excavation usually becomes uneconomical due to frequent bit changes coupled with increased machine vibrations and maintenance costs. A significant amount of effort has been placed over the years on increasing the ability of roadheaders to cut hard rock. Most of these efforts have focused on structural changes in the machines, such as increased weight, stiffer frames and more cutterhead power. Extensive field trials of these machines showed that the cutting tool is still the weakest point in hard rock excavation. Mini-Disc Cutters could increase the ability of the roadheaders for hard rock excavation while providing for lesser cutter change and maintenance stoppages. This new cutting technology holds great promise for application on roadheaders to extend their capability into economical excavation of hard rocks. In addition, using the mini-disc cutters, a drum miner concept has been developed for application to hard rock mine development.