Risk-Driven Decision Making Within the Observational Method: Case Study Based on the New International Airport of Mexico City

In geotechnical design, the Observational Method poses as an attractive solution for reducing construction costs without compromising safety, especially when dealing with a high level of uncertainty. Additionally, the benefits of the Observational Method can be elevated when it is applied in a probabilistic concept. Designing the soil improvement of the runways for the New International Airport of Mexico City (Nuevo Aeropuerto Internacional de la Ciudad de México—NAICM) holds significant risk due to the extremely soft soil, the soil-related uncertainties and the strict pavement operation requirements. Instead of opting for an over-conservative and costly design, the Observational Method was adopted in order to steer the soil improvement works according to monitored soil behaviour. The analysis presented in this paper, which is based on an example inspired by the NAICM, employs a probabilistic framework, composed of several probabilistic tools, in order to estimate the reliability of the design. Specifically, incoming monitoring (soil response) data is utilized in several reliability updating steps, giving insight into the probability of the design meeting the operational requirements. Moreover, assessing the reliability of a design allows for the quantification of risk, which can pose as a strong motivator during the decision-making process. Design decisions, such as application of mitigation measures, can be made according to the direction of risk minimization. Finally, the entire procedure of the Observational Method and the steering of the design throughout the soil improvement phase are illustrated in a decision tree. This paper draws conclusions on the benefits of incorporating probabilistic concepts in large scale projects with strong uncertainties, as well as utilizing risk as motivation for decision making, which eventually proves to be valuable for project management.