Preliminary Regional Engineering Geological assessment and Rock Mass Strength parameterization for Landslide Hazard: A case study in southwestern Puerto Rico

Rock mass strength is a controlling factor on many types of landslide failures and thus is paramount for landslide hazard estimation. A primary objective of any geotechnical characterization is to determine the engineering characteristics of the rock mass, which are used as inputs to numerical modeling of slope stability with estimates of uncertainty. Typical geotechnical investigations include borehole logging and laboratory and in-situ testing, which provide detailed exploitation of the rock mass quality at a site-specific scale. However, up scaling these traditional geotechnical approaches for regional assessments has limitations in cost and feasibility. Recently, new approaches have been published that extract regional patterns in near-surface mechanical properties and quantify rock mass quality in shallow depths based on field investigations and in-situ geotechnical investigations. We expand on this endeavor by carrying out a preliminary engineering geological assessment of the relationship between rock mass strength and landslide occurrence in Puerto Rico. Here, we utilize several recent landslide-triggering events (e.g. Hurricane Maria, Tropical Storm Isaias, and the 2020 Mw6.4 EQ), along with field and remote sensing investigations to develop a holistic approach to estimating geotechnical characteristics at a regional scale as they pertain to landsliding. The study area has undergone tectonic processes that have resulted in prominent relief and fractured rock masses, with slope variation from 10o along the coastal zone to about 38o in the central part of the island. We collected 22 field surveys in diverse geological materials to evaluate near-surface mechanical characteristics across these lithology and topographic gradients by pairing 1D shear wave velocity (Vs) surveys and engineering geological assessments based on tools such as Schmidt hammer and the Geological Strength Index (GSI) classification system. High resolution LiDAR-derived topography was used to correlate topographic derivatives with the spatial pattern of geological formations and their respective shear strength. These findings contribute to local land-use planning and preparedness enhancement against landslide phenomena, as well as serve as a template for future regional assessments.