Physical, Mineralogical, and Geochemical Signatures of Slip Surfaces in Landslide-prone Tropical Soils: Añasco, Puerto Rico

The high frequency of hazardous rainfall-induced landslides in Puerto Rico necessitates thorough comprehension of the causes and mechanisms of mass wasting. We focus on the soil properties and structures that facilitate sliding through geochemical and mineralogical analysis of soil samples, with the goals of identifying potential failure planes related to expandable argillic horizons, buried soils, and/or relict parent material. This study determines the mineralogical and geochemical components of 16 soil samples from ~1.5 m soil profiles at three sites adjacent to past landslides in Añasco, Puerto Rico, using thermogravimetric analysis (TGA), X-ray fluorescence (XRF), and X-ray diffraction (XRD). Two sites were much more deeply weathered compared to the third, containing only non-expandable 1:1 clays (kaolinite) as opposed to 2:1 clays (e.g. smectite). Potential failure planes were identified by diagnostic increases with depth in clay content, occurring at ~80-110 cm in one highly-weathered soil as well as a less-weathered soil, as well as at ~140-160 cm in another highly-weathered soil. Strengthened understanding of soil conditions and properties in landslide-prone mountainous tropical regions – especially clay-rich horizons that become mechanically weak when water-logged – contributes to landslide hazard mitigation efforts in Puerto Rico, with broader applications in other comparable regions.