Excavation TBM cutters in operation The quality and lifetime of the excavation tools have a noticeable effect on the tunneling performance and the economic efficiency of mechanized tunneling. The high material quality of the tools ensures the productive removal of soil or rock on the one hand,...
Excavation TBM cutters in operation
The quality and lifetime of the excavation tools have a noticeable effect on the tunneling performance and the economic efficiency of mechanized tunneling. The high material quality of the tools ensures the productive removal of soil or rock on the one hand, and low downtimes due to high durability on the other. The excavation tools are installed on the cutter head – in soft ground called cutting wheel. With the rotational movement of the cutter head and the thrust of the hydraulic jacking stations, the excavation tools loosen the rock and / or the soil at the tunnel face. Depending on the geology, the cutter head is equipped with TBM cutting knives and / or disc cutters (discs) or rippers. To ensure that a project remains within the calculated budget, also with respect to tooling, the excavation tools must perform as forecast even with extremely high rock strengths, abrasion and water pressures.
The nature of the ground conditions determines the choice and positioning of the excavation tools for the TBM cutter head or the cutting wheel. Only if the tools are perfectly matched to the geology, the TBM can achieve high tunneling performances. However, the respective ground conditions challenge the reliability of the tools in different places. In soft soils, precautions must be taken to stop the disc cutters from getting clogged. During excavation in hard rock, such as granite, the TBM disc cutters must pass the absolute hardness test.
The correct steel mixture and an exact work process are of crucial importance in the materials composition and production of the disc cutters. The high-alloy tool steel of the cutting ring must not only be very hard to resist wear, the material toughness is also an important factor for its lifetime. Excessively brittle steel would lead to early material spalling at the cutting ring during rock bursts, which happen occasionally. Excavation tools up to 8 inches for smaller excavation diameters are also made of high-alloy tool steel.
In addition to the geology, limited space at the cutter head and the maximum possible load on the disc cutter bearings are also crucial factors when choosing the appropriate tool size. A wide range of cutting knives, buckets, rippers and other tools complement the spectrum for operations in loose soils. With respect to these types of tools, the durability and material composition are also key factors in optimizing time and costs. The durability of the excavation tool edges and/or teeth is improved enormously by soldering carbide metal – tungsten carbide. Depending on the geology, cutting knives, buckets and rippers are given a welded hard facing on the tool body to protect it against abrasive soil paste.
The geology determines the design of the TBM cutter head
The choice of different types of disc cutters and their spacing already plays a decisive role in the structural planning phase during cutter head design, to ensure optimum excavation of the rock. In addition to saving time, safety also counts: tools in wear-intensive positions are easily accessible and the correspondingly prepared back-loading system allows for the tool to be replaced from behind protected by the cutter head.
The tools' geometry is strongly influenced by the geology and the cutting wheel design.