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3D slope stability analyses of the April 2022 failure at the south wall of the Kuelap Fortress
Thesis

3D slope stability analyses of the April 2022 failure at the south wall of the Kuelap Fortress

Sebastian Gabriel Aucca
Master of Science (M.S.), Drexel University
Sep 2024
DOI:
https://doi.org/10.17918/00010763
pdf
Aucca_Sebastian_202413.58 MB
PDF Embargoed Access, Embargo ends: 30 Nov 2026

Abstract

Geotechnology Peru--Kuelap Site Material degradation Seepage Slopes (Physical geography)--Stability Wedge-shaped failure surface Civil Engineering Geology
This research focuses on the collapse of the south wall of Kuelap Fortress, which occurred on April 10, 2022. The fortress is over 1500 years old and it has long experienced structural issues including cracks and movements along of some portions of the wall and a major failure at Access 01. The site features a massive limestone perimeter wall filled with limestone slabs and fine -grained soil-based mortar. Following the collapse, a multi-institutional investigation, led by the Pontificia Universidad Católica del Perú (PUCP), was launched, forming the foundation of this study. The research integrates the findings of the PUCP-led investigation with 3D slope stability analyses to determine the collapse's causes. Factors such as pore water pressure, material degradation, and the impact of drainage systems installed between 2000 and 2005 were evaluated. Three 3D slope stability software tools--Slide3, SWedge, and Slope3D--were tested to model the failure. After validation, Slope3D was selected for detailed analysis. The study aimed to model the failure by defining the slope geometry before and after the event, estimating engineering parameters for the materials involved, and assessing pore pressure conditions. Since direct access to the site was limited, the pre- and post-failure geometries were reconstructed using photos and point cloud data. The failure was characterized as a wedge-shaped slip surface, 12 meters high and 5.42 meters deep. The material properties of the fortress’s structural fill were estimated using rock mechanics methods to account for its anisotropic masonry-like structure. Due to limited rainfall and pore pressure data, estimates were made using a range of pore pressure ratios. A total of 26 slope stability analyses were conducted, including comparisons between 2D and 3D models, back-analyses, and potential role of a buried 4-inch PVC pipe. The comparison revealed that 2D models underestimated stability, highlighting the importance of 3D modeling for capturing the complex geometry. In dry conditions, the 3D model yielded a Factor of Safety (FS) above 1.3, consistent with the fortress’s long-term stability. In wet conditions (with r_u = 0.1), the FS dropped to above 1.1, underscoring the significant impact of pore water pressures. Back-analyses models showed that for the wedge failure to occur requires high pore pressure conditions (r_u > 0.2) combined with low shear strength of the structural fill. Notably, the required high pore pressures could not be explained by a high antecedent rainfall, nor unusually high recent rainfall activity that exceeded historical records. Additional contributing factors may include moisture buildup in the structural fill due to impeded drainage due to presence of modern mortar with low permeability in the perimeter facing. Or a long-term shear strength degradation of the structural fill possibly be due to long-term weathering or exposure to seismic activity, such as a 7.5 magnitude earthquake in November 2021. The study also examined the potential role of a buried 4-inch PVC collection drainage pipe installed about 20 years ago. This pipe, designed to channel surface water, was found to reduce the FS by 5 and 8% with respect to the base cases under dry and wet conditions, respectively. The FS could decrease further if the pipe had leaks, thus increasing pore pressures. In conclusion, the failure of the south wall was likely the result of multiple interacting factors, including increased moisture content and pore pressures, and shear strength degradation of the structural fill. Additionally, the possible weakness caused by the PVC pipe installation in the early 2000s. The study recommends further geotechnical testing, installation of pore pressure monitoring devices, and an onsite rainfall gauge to better understand the relationship between rainfall events and pore pressure generation.

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