Strength, deformability and permeability of kakiritic rocks from the Gotthard base tunnel

Abstract

The 57-km long, new Gotthard Base Tunnel (GBT) crosses over a 1.1-km long stretch containing kakirites, i.e. weak rocks which, depending on the degree of tectonisation, may exhibit severe squeezing behaviour upon tunnel excavation. This tunnel stretch was already identified as a key problem in the early planning phase, necessitating intensive geological exploration and laboratory investigations over a 14-year period, including research and development in experimental techniques. The laboratory investigations for the GBT provided not only a considerable amount of data about the geotechnical parameters of the kakirites, but also valuable information about their basic behaviour and the adequacy of various testing techniques for this type of rock. Specifically, by using specially designed testing equipment and procedures, it was shown that, (1) conventional rock mechanics triaxial tests cannot provide reliable results for kakirites; (2) the kakirites obey the principle of effective stress in Terzaghi's classical form and consequently it is indispensable to measure the pore pressure during triaxial testing, analogously to the consolidated drained (CD) and consolidated undrained (CU) tests in soil mechanics; (3) the effects of a possible loss of water and desaturation during sampling, transport, storage and preparation of specimens can be successfully mitigated by subjecting the specimens to flow-through before the actual triaxial testing; and, (4) CD tests on partially saturated specimens that are obtained by means of flow-through can provide reliable and reproducible results. This saves a significant amount of time, allows a greater number of tests to be performed, and allows results to be obtained more quickly, which is particularly important for decision-making during tunnel advance.

Keywords

• Squeezing
• Pore pressure
• Triaxial testing