This paper presents the geotechnical characterization of a 400 m long, 40 m deep permanent excavation for the Mall Plaza project in Manizales, Colombia. The site, located in steep terrain within a densely populated area, is intersected by a major regional fault and its ~100 m wide damage zone. Subsurface materials include graphitic schists, with a diorite intrusion within the fault zone, overlain by volcanic ash from Ruiz Volcano and colluvial deposits from Quaternary mass movements. The region, with 2000 mm/year rainfall and nearby active faults capable of M7 earthquakes, poses high geotechnical risk. Initial conventional borings proved insufficient, prompting a detailed investigation with high-quality core drilling (100–130 mm) and geophysical methods. Field conditions were continuously monitored and the geotechnical model refined during construction. The case reflects the complexity of fault damage zones in the Colombian Andes, where landslides are frequent and site characterization is challenging. Lessons learned are applicable to similar settings.
