Designing challenges engineers to develop both economically attractive and safe designs. This might seem a paradox, but with the application of the observational method, wherein safety is checked by real-time monitoring, economic design and safety are united. The observational method is recently successfully applied in the Netherlands during the excavation works of the double layered tunnel across densely populated central Maastricht. The uncertainties in the limestone conditions and possible karst holes, combined with a length 2.5 km of tunnel, made the observational method very applicable. This paper outlines the application of state of the art real-time monitoring techniques as vital part of the observational methods' success. The individual components of the monitoring network will be discussed with an emphasis on reliability and availability of data. Actual data is presented and attention is paid to the essential relation of the monitoring team with the geotechnical engineers and excavation crew.