Field observations of landslides and related materials following Hurricane Maria, Puerto Rico

During September 2017, Hurricane Maria caused widespread landsliding throughout mountainous regions of Puerto Rico, with more than 71,000 landslides being subsequently identified from aerial imagery (Hughes et al., 2019). Most landslides apparently mobilized as debris flows and occurred within soil (unconsolidated material overlying saprolite and bedrock) and saprolite overlying less-weathered rock (e.g., Bessette-Kirton et al., 2019a). To better understand the characteristics of Maria-triggered landslides, debris flows, and materials in which landslides occurred, we performed reconnaissance-level studies of 118 landslides, 46 soil exposures generally within landslide scars, 24 saprolite exposures, and 37 rock exposures. Results from these studies are provided herein. Landslides studied were mostly selected from aerial imagery collected between 9-15 October 2017 (Quantum Spatial, Inc., 2017), with emphasis placed on four study areas where landslides were particularly numerous, and which were previously studied (Bessette-Kirton et al., 2019b) using aerial imagery and lidar data. However, we also selected landslides for field study that were spread across the mountainous parts of the main island of Puerto Rico, and which occurred in geological formations in which Maria-induced landslides were numerous. Finally, we selected landslides for field study that were relatively easily accessible from roads, although we attempted to mostly evaluate landslides that did not have clear association with roadway construction or drainage. However, a previous study found that landslides in Puerto Rico are approximately five times more likely near roads compared to away from roads (Larsen and Parks, 1997). Field studies were performed sporadically between June 2018 and March 2022.
 
Data provided with this release are in the form of a file geodatabase developed using ESRI ArcGIS and which comprises four point feature classes; Soil_Descriptions, Saprolite_Descriptions, Rock_Descriptions, and Source_Areas, with the latter describing locations from which landslides initiated. Measurements of landslide location and dimensions were made in the field using tape measures, laser rangefinders, hand levels, clinometers, pocket transits, geological compasses, and positioning systems on mobile devices (that is, cellular telephones, tablets) that utilize GPS techniques and distance from cellular antennas. Unconfined compressive strength and undrained shear strength under field conditions were approximated for some materials using hand penetrometers and hand vane-shear devices, respectively, with values provided in Consistency/Compactness (unconfined compressive strength) and Notes (undrained shear strength) fields. Soil and saprolite colors were visually estimated using Munsell color charts. Data were collected in the field on mobile devices running ESRI ArcGIS Collector software and subsequently compiled using ArcGIS desktop software. Photographs were collected of many (but not all) features, and these are provided also in the geodatabase. The following paragraphs describe the attributes of each of the four point feature classes. Additional information is provided in the accompanying metadata file.
 
Soil_Descriptions
OBJECTID – Unique number assigned by software to identify each point observation.
 
Shape – Type of feature, in all cases “point.”
 
Soil Type – Visually and manually estimated primary soil type modified by soil types present in lesser amounts, with soil types generally of “clay,” “silt,” “sand,” and “gravel.” “Clay” consists of particles 0.002 mm diameter and finer, “silt” consists of particles 0.002 mm - 0.075 mm diameter, “sand” consists of particles 0.075 mm - 4.75 mm diameter, and “gravel” consists of particles 4.75 mm - 75 mm diameter. Secondary soil type may modify primary type (for example, “silty clay”) if more than 30% of the secondary type is present. Non-primary soil type content also may be described as “trace” if 1% - 10% is present, “little” if 10% - 20% is present, “some” if 20% - 35% is present, and “and” if 35% - 50% is present.
 
Munsell Color – Visually estimated from hand specimens at field moisture content using Munsell color charts.
 
Field Moisture – Increasing wetness from “dry” (powders or doesn’t change color when warmed) to “moist” (can be rolled into a thread 1/8” in diameter or changes color when warmed) to “wet” (soft and sticky or holds a molded shape) to “saturated” (freely drains water).
 
Plasticity – Estimated from specimens at field moisture content or higher, with “non-plastic,” “slightly plastic,” “moderately plastic,” “plastic,” and “highly plastic” representing the ease with which soil could be rolled into 1/8-inch-diameter threads, with ease increasing toward “highly plastic” and decreasing toward “non-plastic.”
 
Field Dry Strength – Generally estimated from specimens at field moisture content or lower and ranging from “none” or “very little” (crumbles when handled) to “slight” (can be broken easily) to “medium” (can be broken with some effort) to “high” (can be broken with great effort but not powdered) to “very high” (cannot be broken or powdered using finger pressure).
 
Cementation – Estimated generally from silty, sandy, and gravelly field specimens as “none,” “weak” (breaks with finger pressure), “moderate” (cannot be broken with fingers but can be with hammer), or “strong” (breaks with effort using hammer).
 
Structure – Description of any structure identified, such as fracturing, bedding, slickensides, crumb, etc.
 
Consistency/Compactness – Estimated from specimens at field moisture content, with some estimates provided quantitatively from hand penetrometer measurements in kg/cm2. For clay, values increase from “very soft” (squeezes between fingers when hand is closed; 0-0.25 kg/cm2) to “soft” (easily molded by fingers; 0.25-0.5 kg/cm2) to “medium stiff” (molded by strong finger pressure; 0.5-1.0 kg/cm2) to “stiff” (dented by strong finger pressure; 1.0-2.0 kg/cm2) to “very stiff” (slightly dented by strong finger pressure; 2.0-4.0 kg/cm2) to “hard” (slightly dented by pencil point; >4.0 kg/cm2). For silt and sand, values increase from “very loose” (squeezes between fingers when hand is closed; 0-0.25 kg/cm2) to “loose” (molded by fingers; 0.25-0.75 kg/cm2) to “medium dense” (dented by strong finger pressure; 0.75-2.0 kg/cm2) to “dense” (slightly dented by strong finger pressure; 2.0-4.0 kg/cm2) to “very dense” (slightly dented by pencil point; >4.0 kg/cm2).
 
Particle Description – General descriptions of the particle size distribution and of particles themselves.
 
Notes – Any notes that the recorder thought were pertinent.
 
GlobalID – Unique identifier assigned by software during geodatabase formation.
 
Saprolite_Descriptions
OBJECTID – Unique number assigned by software to identify each point observation.
 
Shape – Type of feature, in all cases “point.”
 
Soil Type – Visually and manually estimated primary soil type modified by soil types present in lesser amounts, with soil types generally of “clay,” “silt,” “sand,” and “gravel.” “Clay” consists of particles 0.002 mm diameter and finer, “silt” consists of particles 0.002 mm - 0.075 mm diameter, “sand” consists of particles 0.075 mm - 4.75 mm diameter, and “gravel” consists of particles 4.75 mm - 75 mm diameter. Secondary soil type may modify primary type (for example, “silty clay”) if more than 30% of the secondary type is present. Non-primary soil type content also may be described as “trace” if 1% - 10% is present, “little” if 10% - 20% is present, “some” if 20% - 35% is present, and “and” if 35% - 50% is present.
 
Munsell Color – Visually estimated from hand specimens at field moisture content using Munsell color charts.
 
Field Moisture – Increasing wetness from “dry” (powders or doesn’t change color when warmed) to “moist” (can be rolled into a thread 1/8” in diameter or changes color when warmed) to “wet” (soft and sticky or holds a molded shape) to “saturated” (freely drains water).
 
Plasticity – Estimated from specimens at field moisture content or higher, with “non-plastic,” “slightly plastic,” “moderately plastic,” “plastic,” and “highly plastic” representing the ease with which soil could be rolled into 1/8-inch-diameter threads, with ease increasing toward “highly plastic” and decreasing toward “non-plastic.”
 
Field Dry Strength – Estimated from hand specimens at field moisture content or lower and ranging from “plastic” (easily molded) to “friable” (crumbles with finger pressure) to “weak” (crumbles under light hammer blows) to “moderate” (withstands several firm hammer blows) to “strong” (withstands heavy, ringing hammer blows) to “very strong” (only dust produced by any hammer blow).
 
Cementation – Estimated generally from silty, sandy, and gravelly field specimens as “weak” (breaks with finger pressure), “moderate” (cannot be broken with fingers but can be with hammer), or “strong” (breaks with effort using hammer).
 
Structure – Description of any structure identified, such as fracturing, bedding, slickensides, crumb, etc.
 
Consistency/Compactness – Estimated from specimens at field moisture content, with some estimates provided quantitatively from hand penetrometer measurements in kg/cm2. For clay, values increase from “very soft” (squeezes between fingers when hand is closed; 0-0.25 kg/cm2) to “soft” (easily molded by fingers; 0.25-0.5 kg/cm2) to “medium stiff” (molded by strong finger pressure; 0.5-1.0 kg/cm2) to “stiff” (dented by strong finger pressure; 1.0-2.0 kg/cm2) to “very stiff” (slightly dented by strong finger pressure; 2.0-4.0 kg/cm2) to “hard” (slightly dented by pencil point; >4.0 kg/cm2). For silt and sand, values increase from “very loose” (squeezes between fingers when hand is closed; 0-0.25 kg/cm2) to “loose” (molded by fingers; 0.25-0.75 kg/cm2) to “medium dense” (dented by strong finger pressure; 0.75-2.0 kg/cm2) to “dense” (slightly dented by strong finger pressure; 2.0-4.0 kg/cm2) to “very dense” (slightly dented by pencil point; >4.0 kg/cm2).
 
Particle Description – General descriptions of the particle size distribution and of particles themselves.
 
Parent Rock Type – Estimated during field evaluation.
 
Bedding Spacing, Orientation - Estimated and measured during field evaluation.
 
Fracture Spacing, Orientation - Estimated and measured during field evaluation.
 
Fracture Condition – Estimated during field evaluation.
 
Hardness – Estimated from hand specimens and ranging from “soft” (moldable) to “friable” (crumbles or powders) to “low” (gouged deeply with knife or hammer) to “moderate” (scratched readily with knife or hammer) to “hard” (scratched with difficulty with knife or hammer) to “very hard” (can’t be scratched with knife or hammer).
 
Notes – Any notes that the recorder thought were pertinent.
 
GlobalID – Unique identifier assigned by software during geodatabase formation.
 
Rock_Descriptions
OBJECTID – Unique number assigned by software to identify each point observation.
 
Shape – Type of feature, in all cases “point.”
 
Rock Type – Estimated during field evaluation.
 
Bedding Spacing, Orientation - Estimated and measured during field evaluation.
 
Fracture Spacing, Orientation - Estimated and measured during field evaluation.
 
Fracture Condition – Estimated during field evaluation.
 
Hardness – Estimated from hand specimens and ranging from “soft” (moldable) to “friable” (crumbles or powders) to “low” (gouged deeply with knife or hammer) to “moderate” (scratched readily with knife or hammer) to “hard” (scratched with difficulty with knife or hammer) to “very hard” (can’t be scratched with knife or hammer).
 
Strength – Estimated from hand specimens and ranging from “plastic” (easily molded) to “friable” (crumbles with finger pressure) to “weak” (crumbles under light hammer blows) to “moderate” (withstands several firm hammer blows) to “strong” (withstands heavy, ringing hammer blows) to “very strong” (only dust produced by any hammer blow).
 
Decomposition/Mineral Alteration– Estimated during field evaluation.
 
Notes – Any notes that the recorder thought were pertinent.
 
GlobalID – Unique identifier assigned by software during geodatabase formation.
 
Source_Areas
OBJECTID – Unique number assigned by software to identify each point observation.
 
Shape – Type of feature, in all cases “point.”
 
Location – Location on landslide where point was recorded, including “flank,” “headscarp,” “toe,” and “other.”
 
Failure Mode – Failure mode of the landslide (Cruden and Varnes, 1996), including “translational,” “rotational,” and “complex.”
 
Source Evacuation – Relative amount of landslide volume that evacuated the source area during movement, including “full” and “partial.”
 
State of remaining slide mass, if any. – Increasing disaggregation and reworking observed in landslide deposit from “coherent” to “disrupted” to “fragmented” to “remolded.”
 
Landslide Complex – Whether the landslide was part of a complex (Cruden and Varnes, 1996; also noted therein as “composite”) and, if so, state of the complex, including “no,” “yes, new,” “yes, reactivated full,” and “yes, reactivated partial.”
 
Base Located Within – Material in which the landslide base was located. For landslide bases at material boundaries, both materials are listed. Observations include “rock,” “saprolite,” “soil,” “sap/rock,” “soil/rock,” and “soil/sap,” where “sap” indicates saprolite.
 
Hillslope Curvature – Overall curvature of the landslide location before the landslide occurred, including “concave,” “convex,” “planar,” and “other.”
 
Damage – Human-made structures damaged by the landslide, if any, including “house,” “road,” “none,” and “other.”
 
Vegetation – General description of vegetation in the landslide area.
 
Modified Slope – Pre-landslide slope modifications that appeared to influence landslide occurrence.
 
Land Use – General description of land use.
 
Mean Slope (degrees) – A measured estimate of the overall pre-landslide source-area slope inclination.
 
Slope Length (m) – A measured estimate of the along-slope length of the landslide source area.
 
Width (m) – A measured estimate of the mean landslide source area width.
 
Mean Depth (m) - A measured estimate of the mean landslide source area depth.
 
Maximum Depth (m) - A measured estimate of the maximum landslide source area depth.
 
Notes – Any notes that the recorder thought were pertinent.
 
Acknowledgements:
 
This work was supported in part by the Additional Supplemental Appropriations for Disaster Relief Requirements Act, 2018 (P.L. 115-123). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References:

Bessette-Kirton, E.K., Cerovski-Darriau, C., Schulz, W.H., Coe, J.A., Kean, J.W., Godt, J.W, Thomas, M.A., and Hughes, K. Stephen, 2019a, Landslides Triggered by Hurricane Maria: Assessment of an Extreme Event in Puerto Rico: GSA Today, v. 29, doi:10.1130/GSATG383A.1
 
Bessette-Kirton, E.K., Coe, J.A., Kelly, M.A., Cerovski-Darriau, C. and Schulz, W.H., 2019b, Map data from landslides triggered by Hurricane Maria in four study areas of Puerto Rico: U.S. Geological Survey data release, https://doi.org/10.5066/P9OW4SLX.
 
Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes. In: Turner, A.K., Schuster, R.L. (Eds.), Landslides, Investigation and Mitigation, Transportation Research Board Special Report 247: National Research Council, Washington, D.C., p. 36–75.
 
Hughes, K.S., Bayouth García, D., Martínez Milian, G.O., Schulz, W.H., and Baum, R.L., 2019, Map of slope-failure locations in Puerto Rico after Hurricane María: U.S. Geological Survey data release, https://doi.org/10.5066/P9BVMD74.
 
Larsen, M.C., and Parks, J.E., 1997, How wide is a road? The association of roads and mass-wasting in a forested montane environment: Earth Surface Processes and Landforms, v. 22, p. 835-848.

Quantum Spatial, Inc., 2017, FEMA PR Imagery: https://s3.amazonaws.com/fema-cap-imagery/Others/Maria (accessed October 2017).