Prediction of fault zones based on geological and geotechnical observations during tunnel construction

Abstract

During the construction of the Semmering Base Tunnel, Lot SBT1.1, the drives have already encountered several fault zones in the Greywacke Zone. Because of the high overburden, the exact position of these fault zones is unknown at tunnel level; a common problem for all tunnelling projects in mountainous regions. Simple exploration drilling techniques such as percussion drillings, where only cuttings and not cores are won, do not always provide enough information to precisely specify the position of the fault zones or their nature ahead of the face. This is reason enough to examine other possibilities for the short‐term prediction of fault zones with differing characteristics ahead of the face. Usually displacement data evaluation provides the basis for a short‐term prediction of the system behaviour. However, experiences from Lot SBT1.1 show that applying this approach solely does not always yield satisfying results. A further systematic analysis of selected geological data can improve the short‐term prediction. In particular, changes of discontinuity and rock mass characteristics mapped at the tunnel face are analysed to spot significant trends indicating fault zones ahead of the face. These trends are then related to and verified by the results of displacement data evaluation. This combination of rock mass characteristics mapped at the face and state‐of‐the‐art evaluation of displacement data has helped to improve the reliability of short‐term predictions during the tunnel excavation.

”Interlocking strength” in tunnelling – an attempt at classification

“…The direct English translation "interlocking strength" was established by [8], among others. Unlike the German term Verbandsfestigkeit, "interlocking strength" is used in the existing literature, although increasingly to describe the interlocking of individual grains at mineralogical and geological scale [9][10][11].…”

Short‐term prediction of shotcrete utilization based on observations during tunnelling

“…The interlocking strength in the unsupported excavation area is understood as a qualitative sum parameter for the estimation of the interaction of the stress field near the cavity with the rock mass. The categorically qualitative evaluation of the interlocking strength is described in [7]. For the present consideration, it is assumed that the displacement level also increases with increasing loosening.…”

“…Die Verbandsfestigkeit im ungestützten Ausbruchsbereich wird als qualitativer Summenparameter zur Abschätzung der Wechselwirkung des hohlraumnahen Spannungsfelds mit dem Gebirge verstanden. Die kategorisch qualitative Bewertung der Verbandsfestigkeit ist in [7] beschrieben. Für die vorliegende Betrachtung wird davon ausgegangen, dass das Verschiebungsniveau bei zunehmender Auflockerung ebenfalls zunimmt.…”

“…Geological-geotechnical short-term predictionBased on the engineering-geological documentation of already excavated areas, the collected information has to be continuously evaluated and a geological short-term prediction ahead of the current face has to be produced. Graphs ease the determination of the trend developments and also the extrapolation of the same [5][7].…”

Tunnel Misalignment with Geostatic Principal Stress Directions in Anisotropic Rock Masses

“…Asymmetric deformations of the tunnel walls have been observed on tunnels in structured rock masses, such as phyllites and slates. For instance, asymmetric radial displacements at the tunnel wall, horizontal displacements at the crown and axial displacements at the springline are often measured on the shotcrete support of NATM tunnels in anisotropic rock masses (Schubert & Budil, 1995;Goricki et al, 2005;Schubert et al, 2005;Schubert & Moritz, 2011;Klopcic & Logar, 2014;Lenz et al, 2017). Those asymmetric deformations are commonly associated with the anisotropic properties of the rock mass and with localized heterogeneities.…”