1. Introduction The A3 Salerno-Reggio Calabria motorway is the principal junction between the Centre- North part and the South part of Italy. It runs parallel to the Tyrrhenian railway line, between Naples and Villa San Giovanni. The main objective of the A3 modernization project is to restore this infrastructure, realized in the 70es, to full functionality and security level according to modern design standards and regulations. General Contractor Pizzarotti & C. S.p.A. won the contract to construct the section 4b running from Altilia (Cosenza province) to Falerna (Catanzaro province). This 18.2 km section required the build of dual carriageway, carrying two 3.75 m-wide lanes and a 3 mwide emergency lane per carriageway. Nearly 30% of the project runs on viaducts and in tunnels. Four bored tunnels (three twin tubes and one single tube) were foreseen; the second major one is Monaco twin-bore tunnel (497 m for the North tube, / 729.75 m for the South tube). The excavated cross section was about 160sq.m. Design adjustments have been provided during the twin-bore Monaco Tunnel construction through the systematic application of the observational method. This method (“learn as you go”) allowed the validation of the design choices and the further minimization of the uncertainty level and associated risks, as they inevitably present, due to the increased level of knowledge obtained during construction. 2. Construction process and site design solutions During Monaco tunnel construction, significant geological, hydrogeological and geotechnical differences were found especially in the area of North portal and in the first 190 m of the bored tunnel proceeding in South direction, with respect to the Approved Final Design (AFD) model. Completely weathered quartz and filladic schists and clays, water-satured, directly overlapping the granites bed were found in this zone. Finally, exceptional rainfalls occurred during wintertime, with unforeseen consequences on progress works in Northern portal. Anomalous and abnormal displacements happened on pile-walls structure during Northern portal excavation works. This situation forced to a general update of the design reference model, in order to provide the most adequate Site Design (SD) solutions. Numerical back-analyses were performed and immediate appropriate countermeasures were taken in order to control the increase of the displacements on pilewalls. Additional piles were executed in front of the structure to reinforce pile-walls footings, additional horizontal deep drainages sets were provided in order to reduce the water loads, and temporary portal canopies were installed over the piles, top-heading level. As a result, the displacements kept below the alert threshold values. Therefore, SD focused on individuating the potential hazard for bored-tunnel excavation as a result of the updated geotechnical reference conditions, obtaining these results: – High extrusions and convergences in the tunnel and deformations on the contour; – High plasticization of the rock-mass around the tunnel; – Negative interaction with the overlaying slope; – Negative interaction with the Northbore (to be excavated after the Southbore completion); – Loss of organisation of working cycles, and increasing of execution time; – Reduced level of construction safety for site workers. As a solution, SD proposed mitigation measures to be implemented in order to reduce the potential risks associated to the identified hazards: – Ahead of tunnel face: increased tunnel face consolidation and additional round/footings consolidation, using fibreglass dowels injected by expansive mortar, finalized to ensure adequate stiffness to the excavation core (extrusions control) and to the excavation boundary (convergence control) prior to tunnel advance. – Behind of tunnel face: additional immediate closure of the cavity using a temporary pre-invert steel arch in order to minimize tunnel convergences and settlements; closure of the tunnel section with the installation of final concrete invert arch very closed to tunnel face, at a maximum distance of one excavation diameter. – Inside the pillar between the two bores: additional reinforcement of the pillar from the already excavated South-bore using fibreglass dowels injected by expansive mortar, before the North-bore excavation. A very detailed geotechnical monitoring system, consisting of: – Continuous geological face mapping before tunnel excavation, each tunnel round advance; – Systematic horizontal drilling probes, executed each 30 m inside the rock-mass ahead of tunnel face, with registration of drilling parameter; – Systematic monitoring (inclinometers + optical targets, load cells, piezometers) on the portal works and on the tunnel (strains measurements ahead of the face by extrusometers, radial convergence measures by optical targets placed along the tunnel perimeter). Was introduced throughout the construction period with the aim of controlling the residual risks. New threshold values for the construction period were established. In order to verify the design hypotheses during the construction phase, an action plan was set out according to threshold values. The presence of the tunnel design engineer on site was enhanced. Numerical back-analyses were carried out in order to verify the reliability of the updated geotechnical model and to balance the amount of the interventions to be installed. Ground-structure interaction was recorded for both ordinary and exceptional conditions (such as the exceptional rainfall events occurred during wintertime); the tunnel works behaviour and the design interventions effectiveness were successfully tested in real scale. 3. Conclusions The application of observational method during the excavation works of portal structures and during the underground excavations of the twin-bored Monaco tunnel, in A3 Salerno-Reggio Calabria motorway, have allowed the updating and upgrading of the Approved Final Design (AFD) predictions into the Site Design (SD) solutions. “Observational method” principles and criteria provided calibration and adjustment of the design solutions, based on the improved level of knowledge obtained during construction activities; thus, it was very effective in very variable geological, hydrogeological and geotechnical context encountered during realization of the whole Monaco Tunnel, with particular reference to the Northern stretch. Efficient monitoring system was applied in order to control the tunnel behaviour ahead of the face (extensometer; horizontal probe drillings) and after tunnel excavation (optical targets); thus, numerical back-analyses were carried out in order to verify the reliability of the geotechnical model. The consequent fitting and balance of the most adequate support interventions were possible with reference to the real tunnelling behaviour verification. Among others things, numerical analyses allowed to individuate in the North side the mandatory pillar reinforcement in very weathered schists and phyllites tunnel stretch in order to ensure the stability of the ground-structures system, and to be installed before the second bore advance. Therefore, the continuous close collaboration between Designer Team, Contractor, Subcontractor and Works Supervisor Team during construction follow-up was effective in ensuring the completion of the works and the minimization of potential risks, in compliance with technical and safety requirements. Systematic application of observational method guaranteed regular working cycles and avoided further advance and time disruptions.