Geotechnical Investigation of the April 2022 South Wall Collapse at the Kuelap Fortress

Kuelap is a major archaeological complex constructed by the Chachapoyas, a pre-Columbian culture that inhabited the northeastern Andes. Its principal feature, the Kuelap Fortress, measures approximately 584 by 110 m, and is built at an elevation of 3,000 m atop the Barreta mountain. Over 1,500 years old, the fortress is a significant heritage site for Peru and the world. It was built by carefully stacking limestone slabs with soil-based mortar, forming a substantial structural fill enclosed by a perimeter stone wall façade averaging 20 m in height. This construction enabled the formation of flat, gently sloping platforms on challenging mountainous terrain suitable for habitation. Archaeological records indicate some structural issues since the late 1890s, including wall bulging, fissures, and localized failures. This paper examines a major failure event that occurred on April 10, 2022, along a section of the south wall. The failure involved several stages beginning with a wedge-like geometry and progressed into a more complex and larger final failure. No significant rainfall or seismic activity preceded the event. A multi-institutional, interdisciplinary team led by PUCP conducted a comprehensive post-failure investigation, including field reconnaissance, surveying, laboratory testing, and analytical studies. Given the wedge-shaped geometry of the initial failure surface, three-dimensional limit equilibrium analyses were performed. This paper presents the principal findings of the geotechnical investigation and discusses the likely contributing factors to the failure.