We present a method for risk assessment of groundwater drawdown induced land subsidence when planning for subsurface infrastructure. Since groundwater drawdown and related subsidence can occur at large distances from the points of inflow, the large spatial extent often implies heterogeneous geological conditions that cannot be described in complete detail. This calls for estimation of uncertainties in all components of the cause-effect chain with probabilistic methods. In this study, we couple four probabilistic methods into a comprehensive model for economic risk quantification: a geostatistical soil-stratification model, an inverse calibrated groundwater model, an elasto-plastic subsidence model, and a model describing the resulting damages and costs on individual buildings and constructions. Groundwater head measurements, hydraulic tests, statistical analyses of stratification and soil properties and an inventory of buildings are inputs to the models. In the coupled method, different design alternatives for risk reduction measures are evaluated. Integration of probabilities and damage costs result in an economic risk estimate for each alternative. Compared with the risk for a reference alternative, the best prior alternative is identified as the alternative with the highest expected net benefit. The results include spatial probabilistic risk estimates for each alternative where areas with significant risk are distinguished from low-risk areas. The efficiency and usefulness of this modelling approach as a tool for communication to stakeholders, decision support for prioritization of risk reducing measures, and identification of the need for further investigations and monitoring are demonstrated with a case study of a planned railway tunnel in Varberg, Sweden.
Design of excavations and permanent underground structures requires accurate predictions of e.g. deformations and earth pressures for both the short-and long-term. As excavation depths increase and/or the proximity to adjacent infrastructure decreases, there is a need to improve and develop existing design methods and validate numerical models. The first part of this paper revisits the measurement data from a previous excavation, the Göta Tunnel in Gothenburg, Sweden, in order to benchmark a contemporary constitutive soil model, called Creep-SCLAY1S. This study looks into time series including e.g. final dewatering of the excavation, followed by the development of pore water pressures, earth pressures and deformations over time (until 2 years after excavation). The model predictions are in general in good agreement with the measurement data up to final dewatering. However, installation effects due to drilling are believed to have caused continued deformations which are difficult to capture in the numerical model. Part two of the paper presents details of a recently instrumented excavation in soft clay in Central Gothenburg. The measurement data comprises e.g. pore water pressures, deformations as well as vertical and horizontal earth pressures at three locations under the permanent structure. Continued long-term measurements are planned and the existing and future data are believed to provide valuable insights on the development of the stress state and earth pressures under permanent structures in soft clay.
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