Observational method applied to the decision optimizing of foundation method in Kujala Interchange on silty clay subsoil

Observational method is a powerful approach to dealing with uncertainty in subsoil conditions. In the presented case study, Kujala Interchange, constructing test embankments and applying the observational method enabled to replace many initially planned pile slab foundations with ground-supported road embankments. The residual settlements of these embankments were controlled by means of preloading accompanied with monitoring. This paper demonstrates how a decision tree analysis can be employed to assess the feasibility of constructing the test embankments. The prior probability of acceptable settlements of the ground-supported embankments is estimated for a typical soil profile in Kujala area via Monte Carlo simulation. This prior probability is then updated via monitoring results and Bayes’ theorem. Lastly, the expected costs of each design alternative are derived based on their respective probabilities and the actual cost savings acquired at Kujala Interchange. The results of the decision tree analysis confirm that constructing the test embankments and minimizing the pile foundations were the optimal decision in this case study. In sum, this paper shows how the observational method can be employed to reduce the expected costs and environmental impact of foundation design characterized by significant uncertainty in subsoil conditions. However, it is concluded that besides monetary costs, one should also include non-monetary consequences such as carbon dioxide emissions.