The lack of raw natural resources has forced the European Union to initiate the search for innovative solutions within the raw materials sector. One of the most popular paradigms that has evolved from this initiative is the so-called circular economy. The main focal point of the circular economy is the recycling and reuse of the existing resources. At the same time, sanitary landfills, as the final step in the waste management hierarchy, are perceived as obsolete and should be abandoned. As a consequence, scientific research on the mechanical properties of municipal solid waste material (MSW) has decreased significantly. However, recent landfill landslides in Zaldibar, Spain (2020) and Zagreb, Croatia (2023) reminded us that landfills are still widely present. Therefore, this paper presents experimental results on shear strength properties obtained on mechanically and biologically treated MSW using a triaxial device. The results presented can be seen as a valuable tool in preventing similar tragic events across the world.