Understanding Key Block Failure
The stability of key blocks is crucial for assessing geotechnical risk, including failure volume and runout distance. Their Factor of Safety (FoS) is calculated using limit equilibrium methods, and blocks with FoS below one are systematically removed from the slope to estimate maximum slope unravelling. This data helps predict runout distances and identify high-risk areas for proactive monitoring or hazard control. However, to apply this method reliably, the structural inputs must be accurate.
Mapping Unreachable Slopes Safely
Highly structured rock masses require accurate definition of geological features, including faults, joints, and shears, particularly when failure mechanisms are structurally controlled. In both research applications, traditional mapping techniques were not viable due to steep slopes and hazardous field conditions. A contact-free, photogrammetric approach using ShapeMetriX was essential to obtain reliable data to use for advanced analysis in RocSlope3.
Application 1: Wedge Failure in a Steep Inter-Ramp Slope
Following the completion of open pit mining and the start of underground operations, a 140 m high wedge failure occurred along a steep inter-ramp slope (IRA = 63°). The failure coincided with seasonal snowmelt in Spring 2023, suggesting that the transient pore pressures in the slope triggered the failure. Due to safety concerns, direct access to the failure zone was not possible, but aerial photogrammetry made it accessible from above.
Mapping the Slope With ShapeMetriX
Using drone flyovers, high-resolution overlapping images were processed into a 3D model of the slope and led to reproducible, statistically admissible measurements of geological structures using ShapeMetriX by 3GSM. From the 3D model, geologists mapped several dominant structural features, including two high-angle faults (72°/252° and 64°/357°) and a low-angle basal shear (20°/345°). These formed the boundaries of a clearly defined wedge block.
Modelling With RocSlope3: Validating Failure Mechanisms
The mapped structures were then exported to RocSlope3, where limit equilibrium-based 3D block failure analysis was performed under both dry and elevated pore pressure conditions. The combination of ShapeMetriX’s high-resolution structural data with RocSlope3’s 3D block stability analysis tools provided a clear, volumetric understanding of wedge geometry, failure potential, and associated risk.
Results: Failure Volume and Sensitivity to Pore Pressure
Initial shear strength parameters, based on back-analyses of similar faults at the site, were set as follows:
- All structures: c′ = 5 kPa, ϕ′ = 25°.
- Basal shear plane (adjusted): c′ = 0 kPa, ϕ′ = 20°.
Under dry conditions, the model yielded a Factor of Safety (FOS) of 1.1. However, applying 20% saturation to simulate snowmelt infiltration reduced the FOS below 1.0, confirming a likely failure scenario. The predicted failure volume was 85,520 m³.
Observed Behavior: Confirming the Value of High-Resolution Mapping
While no major slope unravelling was predicted after removing the key block in the initial model focusing on the dominant wedge defined by persistent structures, field observations following the failure confirmed several smaller block detachments along nearby benches. To replicate this domino effect, higher-resolution photogrammetry and incorporation of a higher number of less persistent geological structures into the 3D model are required.
Application 2: Stability Assessment of a Highly Structured Slope
In a second example from the same region, ShapeMetriX was used first to map a 45° inter-ramp slope with a high density of discontinuities. The goal was to evaluate potential slope hazards in a similarly structured rock mass and identify slope optimization opportunities in future pushbacks.
The model captured 4,540 mapped geological structures, which were analyzed for block formation potential. The mapped data was then exported to RocSlope3, where 19,571 valid blocks intersecting the slope were automatically identified and analyzed.
Model Stability and Risk Screening
A uniform set of shear strength parameters was applied to all mapped structures:
- c′ = 5 kPa, ϕ′ = 30°.
The 3D block model produced in RocSlope3 from ShapeMetriX data indicated a highly stable slope condition, with over 99% of blocks exhibiting a Factor of Safety greater than 1.0. While a small number of blocks fell below this threshold, the predicted unravelling volume was minimal. The model provided a volumetric basis to assess the scale of potential unravelling if any key blocks were to be removed in future designs.
Why Contact-Free Mapping Matters
This case study illustrates how drone photogrammetry and 3D block theory work in tandem to support efficient, reliable, and safe slope analysis. 3GSM’s photogrammetric tools enabled data collection that would have been physically impossible to achieve by hand. Without that structural input and the seamless integration with RocSlope3, key block analysis could not have been conducted to the same level of accuracy.
In geotechnically hazardous environments, the ability to gather critical structural data without setting foot on the slope isn’t just a time-saver — it’s a technical necessity.
Try ShapeMetriX and RocSlope3 for Your Next Project
Whether you’re assessing wedge failures, mapping inaccessible slopes, or building a structural geologic model for stability analysis, ShapeMetriX delivers the resolution and reliability you need. When paired with RocSlope3, it enables an integrated workflow from structural mapping to 3D block stability analysis, bridging field data and geotechnical design. Start a free trial of ShapeMetriX and RocSlope3 today.