CASE STUDY
Assessing Slope Stability Along US-191 at Recapture Reservoir
Ground Engineering Awards 2026 Finalist in the Technical Excellence Category
PROJECT IMPACT
Gained clarity on the extent of instability — InSAR determined that a slow-moving landslide was active only on the west side of the highway, while the east side remained stable throughout the monitoring period.
Provided long-term movement history, using Sentinel-1 SAR data from December 2019 to May 2025, providing more than five years of retrospective and ongoing ground movement information.
Quantified displacement rates, with Geofem’s analysis showing typical downslope movement of approximately 6 mm/year at selected locations, increasing to around 18 mm/year during reservoir drawdown periods.
Improved understanding of movement mechanisms. By comparing InSAR displacement data with reservoir water levels, Geofem identified a likely relationship between reservoir level and downslope movement.
Better targeting of investigation and remediation. The InSAR results helped define the spatial extent of significant movement, supporting more focused geotechnical investigation, monitoring and stabilisation planning.
Cost-effective infrastructure risk management — Geofem’s satellite monitoring provided broad-area, repeatable displacement measurements without the need for site visits or local instrumentation.
Project overview
The Utah Department of Transportation (UDOT) commissioned an assessment of slope movements affecting approximately 150 metres of the US Route 191 — a north-south highway in the Western United States — adjacent to Recapture Reservoir and around three miles north of Blanding, Utah.
The highway experienced damage associated with a slow-moving landslide for more than 25 years, including pavement damage, sheared culverts, and disrupted surface drainage.
The project aimed to better understand the ongoing slope instability affecting the highway infrastructure and assess the relationship between changes in reservoir water levels and ground movement.
"Geofem’s analysis provided insight into the behaviour of the slope over time, helping UDOT better understand the factors influencing movement and the potential implications for highway infrastructure."
The challenge
US-191 runs alongside the eastern shore of Recapture Reservoir through an area affected by a chronic, slow-moving landslide.
The instability caused persistent damage to approximately 150 metres of highway, requiring repeated pavement repairs — at times as frequently as weekly for several consecutive months.
Previous mitigation by UDOT included regrading the approximately 15-metre-high slope between the highway and reservoir, removing material from the upper slope and placing material near the toe as a berm.
However, this did not resolve the ongoing movement.
The site’s complex geology, comprising interbedded sandstone, conglomerate and swelling-prone smectitic mudstone, presented an additional challenge to understanding the landslide mechanism.
Changes in reservoir water levels were also suspected to influence slope behaviour by changing toe loading, groundwater pressures, and the mechanical behaviour of soil and rock masses.
"The key challenge was therefore to move beyond simply detecting ground movement and understand why the slope was moving, how its behaviour related to reservoir levels, slope geometry and local geology, and what this could mean for the stability of the highway infrastructure."
Landslide-induced pavement cracking soon after resurfacing US-191.
The Geofem approach
Geofem used Sentinel-1 C-band SAR imagery to estimate line-of-sight displacement velocities and time-series ground movement between December 2019 and May 2025.
"The analysis used a Small Baseline Subset approach, which improves long-term deformation monitoring by selecting image pairs with small temporal and spatial baselines to reduce decorrelation and phase noise."
The InSAR displacement results were then matched with site evidence, including the extent of pavement resurfacing, asphalt cracking patterns and sheared culvert locations, to validate the spatial movement patterns detected by satellite.
In parallel, InSAR displacement time series were compared with reservoir level data to identify whether slope movements accelerated, slowed or reversed during different reservoir conditions.
To understand the full extent of the instability, Geofem produced a velocity map and rasterised displacement map to define the active movement zone and determine whether significant deformation extended east of the highway.
Seasonal surface soil moisture data was also derived from radar satellite data, allowing soil moisture patterns to be compared with the InSAR displacement data and independently sourced rainfall and reservoir level data.
Finally, the displacement behaviour was interpreted in terms of slope mechanics, reservoir drawdown, toe support, pore pressure dissipation, shrink/swell behaviour and the role of underlying mudstone.
"This integrated approach turned satellite observations into practical engineering evidence to guide future investigation, monitoring and slope stabilisation strategy."
InSAR displacement output showing extent and movement rate of landslide
Results
The InSAR analysis provided a clear picture of the active slope movement affecting US-191 and revealed a strong relationship between movement patterns and changes in reservoir levels. The findings helped identify the extent and rate of deformation, while providing valuable insight into the likely geological and hydrological controls influencing slope behaviour.
The InSAR analysis identified a clearly active movement zone west of US-191, adjacent to Recapture Reservoir.
The east side of the highway remained stable during the monitoring period, indicating a lower susceptibility of the highway to a deep-seated landslide beneath it.
The displacement zone matched the north-south extent of newer asphalt visible in optical satellite imagery, providing an independent visual check against historical maintenance activity.
Points on or near the highway moved downslope at approximately 6 mm/year during much of the analysis period.
Movement rates increased to around 18 mm/year during periods following reservoir drawdowns from full or near-full capacity.
Slight upslope movements were observed when the reservoir reached full capacity, suggesting that water loading at the toe and swelling of the underlying mudstone may temporarily reduce or reverse downslope movement.
The most rapid downslope movement periods occurred from approximately December 2019 to July 2020 and November 2023 to August 2024, periods that coincided with the reservoir level dropping from full capacity.
Satellite-derived surface soil moisture correlated with rainfall and snowmelt but not with InSAR displacement data or reservoir level, suggesting that deeper subsurface mudstone behaviour governed slope movements.
The results indicated that reservoir level fluctuations, local geology, and delayed pore pressure response are likely key controls on slope movement.
The study demonstrated how InSAR can support highway asset management by identifying active geohazard zones, quantifying rates of movement, and improving understanding of likely movement mechanisms.
Selected InSAR points showing variable movement rates of the landslide.
Geofem Insights
When reservoir levels drop rapidly, the supporting pressure of the water is removed from the slope while pore water remains trapped within the soil. The retained increase in pore-water pressures and reduced shear strength leaves the slope more susceptible to failure. This is called the ‘rapid drawdown effect’. Highly plastic clays add further complexity by swelling on wetting and shrinking and cracking on drying. Slope movements and failures then become even more unpredictable.
"Geofem conducted a study to assess the magnitude and extent of ground movement at a challenging landslide site along one of our state highways."
Their analysis not only confirmed our initial interpretations but also provided valuable additional insight that helped further define and justify the extent of the slide mass. The work was delivered quickly and with a high level of professionalism! We really appreciated the time the team took to clearly present their methodology and findings, ensuring we fully understood the results and their implications. Communication was seamless and their support felt consistently close, responsive, and engaged throughout the remote monitoring project.
Ari Menitove, PE
Geological Engineer
Utah Department of Transportation
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