SBT – Experiences with tunnel support using yielding elements at construction lot SBT2.1

Abstract The use of yielding elements is a highly effective method in deep rock tunnel construction for protecting the shotcrete, particularly in the early stages, from excessive stress caused by high rock loads. Installing multiple rows of yielding elements results in a ductile and flexible support. By deliberately allowing additional deformations, stresses are transferred into deeper rock regions in accordance with the NATM philosophy. Consequently, the stress‐strain relationship of the yielding elements must be adapted to the time‐dependent properties of the shotcrete in relation to the excavation rate. Experience at the SBT2.1 construction site has shown that, when using yielding elements, not only the precise location of the elements within the circumference but also their positioning within the cross‐section is key to success. It is also essential to determine the correct timing for closing the yielding elements in order to achieve a load‐bearing ring closure in a timely manner and avoid excessive loosening of the rock mass. Detailed geotechnical monitoring and continuous assessment of the observed system behaviour are crucial. This article discusses positive and negative experiences and describes measurement methods and procedures with the aim of successfully utilising yielding elements.