Stress evolution around a TBM tunnel in swelling clay shale over four years after excavation

Abstract

The old Belchen tunnel tubes in the Swiss Jura Mountains were excavated with drill-and-blast in swellable sedimentary rocks, i.e., anhydrite-rich marls (Gipskeuper) and Opalinus Clay shale (OPA). Already during construction in the 1960s both rock formations caused substantial damage to the tunnel support through high swelling pressures and heave, and in later years the tubes had to be refurbished again. Important maintenance and repair prompted the construction of a new, third Belchen tunnel tube (2016–2021) with a tunnel-boring machine (TBM).In this study we present in-situ datasets acquired to investigate the stress evolution and controlling mechanisms over more than four years at a monitoring section located in a strongly faulted OPA section of the new Belchen tunnel tube. The main datasets include time series of total radial pressure, radial strain, rock water content, rock and concrete temperatures, as well as details of the geological structures obtained from analyses of borehole logs and three-dimensional photogrammetric excavation face models. Finally, a series of idealised numerical simulations explore the impact of measured temperature variations on the measured total pressures, which confirm a strong temperature effect on radial pressures related to the setting of concrete and seasonal climatic variations.We find that in our monitoring section radial pressures on the tunnel support are very heterogeneous, i.e., they range between 0.5 MPa and 1.5 MPa, and still gently increasing 4 years after excavation. The measured pressures are 2–5 times greater than measured in the old Belchen tunnel tubes and similar in magnitude to swelling pressures obtained in laboratory tests. EDZ permeability measurements, water content evolution, and radial strain data from the tunnel invert suggest that swelling processes contribute to the long term radial pressure build-up. Thermo-elastic deformation and swelling might be superimposed by local reactivation of tectonic faults and gap grout cracking at crack-initiation stress levels.