A large underground cavern design at a shallow depth requires reliable estimation of in-situ stress conditions. Geological conditions, topography, and tectonic activity influence the magnitude of the in-situ stresses. The objective of this paper is to predict and evaluate the in-situ stress state before the excavation of two pilot tunnels in a railway access cavern. The access cavern, named Mellanplan, is a part of the Korsvägen section at the West Link Project in Gothenburg, Sweden. The first phase of the construction sequence at Mellanplan involved excavation of two pilot tunnels in the top heading, where a rock pillar remained in between the pilots. Before excavating the rock pillar, SINTEF Community performed secondary stress measurements from the roof of the two pilots, utilising the 2D doorstopper method. The results from these stress measurements are assessed and an interpretation of the stress field at Mellanplan is carried out. The concepts from the final rock stress model (FRSM) suggested by Stephansson and Zang (2012) are applied in the back-calculation of the initial stress state. A 3D numerical program, RS3, is applied for parameter stress analyses, where various stress inputs are evaluated. The results from stress analyses are validated with the induced stresses obtained from doorstopper measurements. The final rock stress model demonstrates the stress field at Mellanplan as σH > σv > σh. The findings in this study reveal that tectonic stress and residual stress have greater contribution towards the major horizontal stress component. Furthermore, σv and σh. are suggested as gravity induced stresses. Due to the complexity of stresses at shallow depths, there is a possibility of geological structures reducing the already low magnitude of σh.
KEYWORDS
Back-calculation; Estimation of in-situ stresses, Rock cavern; 3D modelling, Scandinavian geology
