Measurement and interpretation of disc cutting forces in mechanized tunneling - A contribution to the understanding of rock failure mechanisms and the advancement of TBM performance prediction

Martin Entacher

Research output: ThesisDoctoral Thesis

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Hard rock tunnel boring machines (TBM) are equipped with disc cutters as their main excavation tools. Each cutter is nominally loaded with about 250 kN. The knowledge of cutting forces is restricted to the measurement of global thrust from which individual forces are estimated. However, it is known that actual cutting forces vary greatly with peaks that are a multiple of the nominal load. Their knowledge would open a wide range of possibilities to improve TBM tunnelling. They include gaining a deeper understanding of rock breakage, improving TBM performance prediction models, establishing new ways of geological documentation of the tunnel face as well as improving cutter and cutterhead design with respect to peak forces and load collectives. Consequently, the development of a new cutter force measurement method is presented. In contrast to previous approaches, sensors are placed in the cutter saddle as opposed to the cutter itself. Thereby, they are independent from frequently occurring disc cutter changes and thus suitable for a lasting in situ use. After extensive simulations and laboratory tests, the method was implemented on a TBM at Koralm tunnel. Results are presented with respect to the geology of the tunnel face. It is shown that anisotropic features and fractured areas can be identified. A foundation for the interpretation of cutting forces was built by means of full and small scale laboratory rock cutting tests. The analyses range from macroscopic spatial observation of crack propagation and microscopic investigations in the direct vicinity of the cutter tip to correlations of sound emissions, cutting forces and failure events. Such in-depth analyses were made possible by the development of a new small scale cutting test rig that allows for high-precision investigations in a controlled laboratory environment. The interpretation of results highlights common misperceptions in previous studies regarding the meaning of peak forces and subsequent force drops as well as crack propagation. Measurement and interpretation of cutting forces is inherently linked to TBM performance prediction. While full scale cutting tests are considered to be the best laboratory experiment in this regard, they require large samples which are hardly available before a tunnel is built. Furthermore, it was proven that common prediction models based on classical strength parameters such as uniaxial compressive strength are not capable of predicting cutting forces in certain rock types. Thus, it was shown that the newly developed small scale cutting test delivers superior input parameters for a new TBM performance prediction model. With the developed in situ measurement method and laboratory experiments at hand, tools for major improvements of TBM tunnelling were provided. A profound understanding of tool-rock interaction might even in future enable fully automated TBM control with cutting forces as the main input parameter.
Translated title of the contributionMessung und Interpretation von Diskenschneidkräften im maschinellen Tunnelbau - Ein Beitrag zum Verständnis von Gesteinsbruchmechanismen und zur Weiterentwicklung von TBM Leistungsprognosemodellen
Original languageEnglish
  • Thuro, Kurosch, Assessor B (external), External person
  • Galler, Robert, Assessor A (internal)
Publication statusPublished - 2013

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