<p>The closure process of upstream-raised tailings’ dams represents an engineering challenge in terms of material variability, operational complexity, and design. The determination of the current conditions of tailings in the dam and understanding their geotechnical behavior are crucial steps for decommissioning. Accordingly, this paper evaluates the geotechnical response of iron ore tailings disposed of at a planned-to-be-decommissioned dam through laboratory tests, field tests, a fully instrumented pilot excavation, and numerical analysis. Triaxial tests were used to determine the tailings’ critical state line (CSL), while CPTu results were used to investigate the tailings' state at the field. Then, a finite element model was used to perform a simulation of the pilot excavation conducted. For the first time, a fully instrumented excavation on tailings was monitored and analyzed to enhance the knowledge of tailings behavior and promote safe decommissioning designs. The combined analysis of laboratory and field tests presented good agreement with the void ratio obtained from in-situ sampling. The numerical model accurately captures the overall response as verified by the instrumentation. Besides, the excavation was found to mobilize a large portion of tailings despite its limited size. The results reinforce the need for careful investigation and design of the decommissioning work as full-scale operations may mobilize a large volume of tailings, augmenting the risks involved.</p>