Towards a uniform definition of rock toughness for penetration prediction in tbm tunneling

In TBM tunneling, performance prediction is a major issue since calculated excavation costs and construction time of a tunnel project are mainly based on it. Prediction is dependent on the accuracy of geological and geotechnical input parameters. Besides rock strength, toughness of the excavated rock has a significant influence on penetration and cutting efficiency as increasing toughness requires greater energy to induce complete failure. Yet, existing definitions of rock toughness are not adequate or suitable for incorporation in a performance prediction model for TBM tunneling. To develop a common definition and classification system, we used standard laboratory tests (Uniaxial Compression Test, Brazilian Tensile Test). Based on this test data we analyzed several factors that can characterize rock toughness like the ratio of compressive to tensile strength (Z-coefficient), ratio of plastic to elastoplastic strain, specific failure energy and destruction work. We expect future analysis to focus on Zcoefficient but we aim to revise the classification system of Schimazek and Knatz as results showed no good correlation. Also the ratio of plastic to elastoplastic strain is a promising tool for future research. Obviously, destruction work characterized rock toughness but the determination of this parameter depends a lot on machine stiffness and settings.