"Ongoing challenges in Engineering Geology for tunnelling in difficult ground"

By: Paul Marinos

Abstract

The growth of infrastructure needs has increased demands for the excavation of tunnels in poor ground or varying geological conditions. This development includes site investigation techniques, analytical design method (notably numerical), risk analysis, techniques of construction, and monitoring.  The assessment of ground for design has to be based on a sound understanding of the regional geological rules and the establishment of a geological model where data and conditions are translated into an engineering geology description. Examples of geological models and cases from both mountain and urban tunnels under complex or difficult geological conditions are presented. These include the base tunnels in the Alps in a variety of conditions at great depth, tunnels through heavily folded formations with shear zones and cataclasites, Metro works in heterogeneous and weak ground and the selection of the appropriate TBM, and the project for the Gibraltar strait tunnel.

Tunnel design requires knowledge on the quality of the material in which the tunnel will be constructed. Engineering design requires numbers and the lecture explores and discusses methods that can be used by Engineering Geologists to assess the geological factors that have an impact on the design. Since the attempt of Terzaghi (1946) to describe the characteristics of rock masses, a number of rock mass classifications have been developed and play an important role in tunnel design, providing input data on strength and deformation properties of the ground for numerical models. Together with the rock mass properties, the in situ stresses field has to be estimated or measured and this is one of the most difficult tasks.

Although the role of engineering geology has been extended into the area of defining the design parameters, the idealization process, in the form of numerical analysis, should be driven by sound geologic reasoning together with the engineering logic.

The understanding of real behaviour is indeed absolutely necessary before any calculation is attempted. Thus, the engineering geological “I.D” of the geomaterial and the stress environment define this ground behaviour such as:
• Brittle failure of strong massive rock under high stress level.
• Gravitational falling or sliding of blocks or wedges defined by intersecting structural features or
“chimney” type failure, or ravelling in disintegrated and heavily broken and loose masses.
• Formation of a “plastic” zone by shear failure under high stress relative to the strength of the rock mass with deformation problems or even squeezing.
• Swelling, in case of appropriate mineralogical constitution. Comments on the methods for design for each of those cases are discussed and the presentation concludes with a discussion on excavation methods in conventional tunneling construction with special attention to support devices in order to deal with squeezing ground. Examples from a number of tunnels from around the world illustrate the design and construction procedures are discussed.