Rock Mechanics Assessment Techniques

Fracture and Strain Measurement in Rock Mechanics When rocks deform under stress, they accumulate strain—changes in length, shape, or volume. But once fracture occurs, the testing environment changes. New surfaces form, the load path shifts, and the local strain field can be significantly altered. If the edge of a rock breaks during testing, it can influence the accuracy of nearby strain measurements. Stress concentrations near the fracture can affect strain readings, and in some cases, gages close to the break may no longer reflect the intended measurement direction or magnitude. This leads to an important consideration in sensor selection and placement. A Tee Rosette, made up of two independent strain gage grids at 0° and 90°, is a versatile tool for evaluating planar strain states—especially when principal strain directions are known and stable. It’s commonly used in uniaxial or biaxial loading conditions and can help characterize directional strain fields in heterogeneous materials like rock. However, in fractured specimens or where the strain field evolves rapidly, assumptions made at the start of the test may no longer hold. The gage orientation may no longer align with principal directions, and readings must be interpreted in the context of evolving boundary conditions. Using independent grids allows for flexibility in data analysis and can improve spatial sensitivity in two directions. For full 3D strain state evaluation, additional sensors or out-of-plane measurement methods are required. Key points: 🔹 Fracture alters the stress field and can compromise strain data near the break 🔹 Edge breaks can reduce measurement reliability if not accounted for 🔹 Tee Rosettes are effective for in-plane strain measurement when strain directions are known 🔹 Careful interpretation is needed when fractures develop during the test 🔹 In rock mechanics, strain measurement is about understanding what those numbers mean in a material that may not behave predictably.

DFN-Based Vent Raise Assessments: Worst Case Assumptions or Data Driven Reality Designing a vent raise or a large ore pass is never just about excavation—it’s about understanding and managing the geotechnical risks that come with it. Traditionally, kinematic assessments have been the go-to method for evaluating potential wedge failures along the walls or face of a raise. These conventional methods typically focus on identifying the largest possible wedge that could form, without consideration of the location of the intersecting structures and the feasibility of the wedge forming. While this is useful for conservative designs, it often falls short when justifying the largely unsupported configurations commonly used in many raises. This is where Discrete Fracture Network (DFN) modelling provides a contrasting approach. Unlike traditional methods, DFN-based methods don’t just look for the “worst-case” wedge. Instead, they provide a probabilistic and spatially realistic picture of what’s actually likely to occur. By leveraging structural data from pilot boreholes, DFN models can: ✅ Reproduce the anticipated fracture network along the entire raise length ✅ Identify the distribution and size of potentially unstable wedges ✅ Predict where kinematic overbreak is most likely to occur The result? A much clearer, defensible, and data-driven assessment of raise stability that supports better design decisions and risk communication. In today’s risk-based mining environment, this approach can be critical. Whether applied stochastically or deterministically, DFN modelling provides a robust framework for evaluating and managing geotechnical uncertainty, helping operators move beyond “rule of thumb” and into evidence-based design. Curious about how DFN modelling can improve your next raise or ore pass project? Let’s connect—I’d be happy to share more. #GeotechnicalEngineering #VentRaiseEngineering #MiningEngineering #StructuralGeology #DFN #RockMechanics #MineDesign #InnovationInMining WSP Mining & Metals

Hello everyone , Today we have an interesting topic to discuss, In underground mining, geotechnical testing is essential to assess ground conditions, ensure stability, and design appropriate support systems. The common tests and methods used include: 1. Rock Quality Designation (RQD): • Measures the degree of jointing or fracturing in a rock mass, indicating the quality of the rock. High RQD suggests strong, competent rock. 2. Point Load Test: • A quick test to estimate the uniaxial compressive strength (UCS) of rock by applying a load to a rock sample until it fails. 3. Uniaxial Compressive Strength (UCS) Test: • Measures the strength of rock or soil under uniaxial compression. It provides critical information for rock stability and load-bearing capacity. 4. Triaxial Compressive Strength Test: • Similar to the UCS test but considers confining pressures, simulating underground conditions more accurately. It provides insights into how rock behaves under stress from multiple directions. 5. Direct Shear Test: • Assesses the shear strength of rock or soil, which is vital for understanding sliding or failure potential along joints or bedding planes. 6. Brazilian Tensile Strength Test: • Determines tensile strength by applying force to a rock disc, offering insights into its resistance to tensile forces, which are common in mine openings. 7. Slake Durability Test: • Tests the durability of rocks, especially important for rocks prone to weathering or breaking down when exposed to water, like shale or claystone. 8. Permeability Test: • Assesses the ability of rock or soil to allow fluids to pass through, which is essential for understanding groundwater flow and pressure in mine workings. 9. Laboratory Density and Porosity Testing: • Measures density and porosity of rocks, providing insights into rock mass behavior, such as how much water it can absorb or store. 10. Acoustic Emission Monitoring: • Tracks micro-cracks in rock to predict failure or stress build-up, helpful in understanding rockburst risk in deep underground mines. 11. Borehole Geophysical Logging: • Uses techniques like resistivity, seismic, and gamma-ray logging in boreholes to assess in-situ rock properties without disturbing the formation. 12. Geomechanical Mapping and Structural Analysis: • Involves detailed mapping of joints, faults, and fractures to understand the rock mass’s structure and orientation, providing data for ground support design. 13. Seismic Tomography and Microseismic Monitoring: • Detects and monitors seismic events to assess stress changes in the rock mass, which helps in predicting rockbursts or structural failures. These tests help geotechnical engineers make data-driven decisions about excavation methods, support systems, and long-term stability in underground mining operations.

The Geological Strength Index (GSI) is used for more fractured rock because it helps assess the mechanical behavior of rock masses, particularly those with significant fracturing, jointing, and other discontinuities. In fractured rock masses, the presence of joints, faults, and cracks can significantly reduce the overall strength and stability of the rock compared to intact rock. Key reasons for using GSI in fractured rock: 👉Accounts for Discontinuities: GSI incorporates both the degree of fracturing (or jointing) and the surface condition of these fractures. More fractured rock masses tend to behave differently compared to intact rocks, so using GSI helps estimate the reduced strength due to these fractures. 👉Realistic Strength Estimation: Unlike intact rock, fractured rock masses do not follow simple mechanical strength formulas. GSI is a semi-empirical tool that provides more realistic estimates of rock mass strength by considering the extent of fractures and how these fractures interact with each other under stress. 👉Input for Other Models: The GSI value is used in various geotechnical models (e.g., Hoek-Brown failure criterion) to predict the behavior of rock masses under stress. In fractured rock, these predictions are critical for determining how the rock will respond to excavation, loading, or tunneling. 👉Classifies Different Rock Conditions: GSI allows for a broad classification of rock masses from intact to highly fractured or weathered rock. In fractured rock masses, the index helps engineers select appropriate methods for support and design in tunnels, slopes, and foundations. 👉In summary, GSI is specifically useful for fractured rock because it provides a structured way to assess and quantify the impact of fracturing on the rock's mechanical properties, which is crucial for accurate engineering analysis and design.