In the complex environment of open-pit mining, the design and construction of haul ramps represent a critical intersection between operational efficiency and geotechnical safety. A pit ramp is not merely a road; it is a structural element of the mine wall that must support the immense weight of loaded haul trucks while maintaining the integrity of the overall slope (Golestanifar, 2018). Given that haulage costs often account for a significant portion of a mine’s operating expenses, the geometry and stability of these ramps are paramount (Sullivan, n.d.).
Before breaking ground, a comprehensive suite of geotechnical investigations is essential to ensure that the ramp can withstand static and dynamic loads, environmental changes, and the inherent variability of the rock mass.
Characterization of rock mass and physical properties
The fundamental building block of any geotechnical investigation for a pit ramp is the detailed characterization of the rock through which the ramp will be excavated. Studies highlight that the construction of slope angles and benches is highly dependent on the physical and mechanical properties of the rocks (MDPI, 2025).
Key parameters to investigate:
- Lithological mapping: identifying the different rock types (e.g., gneiss, schist, marble) and their spatial distribution is critical for predicting how the ground will behave under stress (Holley et al., n.d.).
- Intact rock strength: laboratory testing to determine the Unconfined Compressive Strength (UCS) of the rock. This informs the bearing capacity of the ramp surface and the stability of the ramp-edge berms.
- Discontinuity mapping: investigating joints, faults, and bedding planes. The orientation and spacing of these structures determine the likelihood of wedge or planar failures that could take out entire sections of a ramp (Sullivan, n.d.).
In-Situ stress and seismicity analysis
A pit ramp must be designed to withstand not only the vertical gravity loads but also the horizontal in-situ stresses present in the rock mass. Geotechnical investigations must establish the ratio between maximum stress and vertical stress (Holley et al., n.d.).
Furthermore, seismic risk assessment is a non-negotiable step. Investigations should calculate the expected maximum acceleration for the site to ensure that ramp gradients and berm widths can survive a seismic event without catastrophic failure. For instance, sites in seismically active zones may require a design that accounts for a 10% probability of exceedance in 50 years (Holley et al., n.d.).
Hydrological and hydrogeological assessment
Water is perhaps the greatest enemy of ramp stability. Geotechnical investigations must pinpoint the groundwater table and understand how it interacts with the ramp structure.
- Pore pressure monitoring: High pore water pressure reduces the effective stress in the rock mass, which can trigger slope failures (Golestanifar, 2018).
- Infiltration and advective heat transfer: modern research suggests that advective heat transfer associated with precipitation and infiltration can be used to infer percolation amounts in unsaturated soils, which is critical for understanding how water moves through the ramp foundation (Lu & LeCain, 2003).
- Drainage requirements: investigations must determine if the ramp requires a “drained condition” design. A drained slope often allows for a steeper inter-ramp angle compared to an undrained one, significantly improving mine economics (Golestanifar, 2018).
Geometric and operational design constraints
The geotechnical investigation must inform the geometric layout of the ramp. This includes determining the “Inter-ramp Angle” (the angle between the toe of a slope where a ramp passes and the toe of the bench above it) and the “Overall Slope Angle” (Nancel-Penard et al., 2019). mandatory geometric considerations are:
- Road width: standard practice often dictates a width 3.5 times that of the widest haul truck for two-lane traffic, though automation may allow for narrower designs if geotechnical conditions permit (Montana Tech, 2021).
- Gradient: a 10% gradient is common, but this must be validated against the rolling resistance of the local rock and the mechanical limits of the fleet (Montana Tech, 2021).
- Safety berms: federal regulations often require berms to be at least mid-axle height of the largest equipment (e-CFR, 2021, as cited in Montana Tech, 2021).
Pavement and wearing course evaluation
For in-pit ramps, the “pavement” is often unpaved granular material. Geotechnical investigations must assess the suitability of on-site materials for use as a wearing course. This involves testing for:
- Rolling resistance: high resistance increases fuel costs and slows production (UBC, n.d.).
- Trafficability: the ability of the material to maintain structural integrity under high tire pressures, especially when wet.
- Dust and erosion potential: identifying materials that minimize dust and resist erosion from heavy runoff.
Conclusion
The safety and profitability of an open-pit mine are inextricably linked to the quality of its haul ramps. By conducting rigorous geotechnical investigations—covering rock mass characterization, in-situ stress, hydrogeology, and material performance—engineers can design ramps that minimize the stripping ratio while maximizing safety. As mining moves toward deeper pits and autonomous fleets, these investigations will only become more critical in managing the thin line between an optimized slope and a costly failure.
References
Golestanifar, M. (2018). Governing risk elements through open pit slope optimization. Journal of the Southern African Institute of Mining and Metallurgy, 118(1), 47–55. https://doi.org/10.17159/2411-9717/2018/v118n1a6
Lu, N., & LeCain, G. D. (2003). Percolation Induced Heat Transfer in Deep Unsaturated Zones. Journal of Geotechnical and Geoenvironmental Engineering, 129(11), 1040–1053. https://doi.org/10.1061/(asce)1090-0241(2003)129:11(1040
MDPI. (2025). Influence of Blasting Approaches in In-Pit Haul Road Construction on Emission Levels and Resource Management. Applied Sciences, 15(22), 12310. https://doi.org/10.3390/app152212310
Montana Tech. (2021). Adapting Open Pit Mine Design Fundamentals to Leverage the Advantages of Autonomous Haulage Systems. Digital Commons @ Montana Tech.
Nancel-Penard, P. et al. (2019). Value-Optimal design of ramps in open pit mining. Archives of Mining Science.
Sullivan, T. D. (n.d.). Pit Slope Design and Risk – A View of the current state of the art. SAIMM.
Holley, K., Skayman, P., & Zhiwei, H. (n.d.). Geotechnical Design for Open Pits at Tanjianshan, China. SAIMM.
