Methodology for Cavity Detection under Multi-level Buildings in the Karstic Island of Boracay, Philippines

Author: Jose Carlo Eric L. Santos, MSCE, John Michael B. Gargullo, MSCE, Karen Joy R. Leobrera, MSCE, Joanne Marie T. Parafina

Presenter: (For presentation)

Most islands in the tropics with beautiful white sandy beaches are karstic in nature. One such island is Boracay, in the Philippines, with its stunning white sand beaches and world-class resorts. In the last two decades, the development of the inland areas of Boracay has been progressing at a more hurried pace in order to provide tourists and visitors the needed facilities and amenities. Most basic of these is the construction of multi-level hotels and exclusive villas, resting on cliffs and terrain offering fantastic views of the turquoise sea and golden sunset. However, since the topography of Boracay was formed through the dissolution of soluble rocks such as limestone, the island is characterized by underground drainage systems littered with sinkholes and cavities. This dissolved bedrock is often covered by residual soils thereby concealing the distinctive karst features which occur at subsurface levels only. This poses a challenge to geotechnical and foundation engineers especially when determining the stability of the underlying bedrock to support multi-level buildings. This paper focuses on the methodology used to map out the extent of subsurface cavities through the use of geophysical means, specifically the geo-electrical resistivity method. It discusses how the investigation plan for the underlying ground formation was formulated and how the locations and extent of cavities were determined. Moreover, it compares the results of the geophysical investigation with the actual cavities encountered during the foundation excavation stage. Lastly, it tackles the various technical and non-technical considerations which had to be factored in the final design in order to make the building’s foundation system safe and economical.


6th International Conference on Geotechnical and Geophysical Site Characterization – Budapest, Hungary (2021)

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