Training Webinars on Weak Rock Characterisation

The Large Open Pit (LOP) project is an international research and technology transfer project focused on the engineering of large slopes associated with open pit mines. It is an industry-sponsored and funded project that was initiated in 2005. Among the initiatives mandated by the LOP sponsors was the development of a series of guidelines on the state-of-practice related to the design and stability of large slopes associated with open pit mines. The dissemination of the knowledge that has been created through LOP Guidelines (see here) and supporting research initiatives, and the education and training of the professionals who work in open pit mining, are also mandates of the LOP. To facilitate this dissemination, and education and training, the LOP Sponsors have agreed to allocate a separate budget for Education and Training under LOP III.

In collaborating with WSP Mining, the following training webinars on Guidelines for Weak Rock Characterization have been developed in English, Spanish, Portuguese, and French. If you have any questions about the content, feel free to reach out to the LOP team (here) or any of the WSP contributors listed in the series below.

• Introduction to Weak Rock: This training module provides a foundational overview of weak rock and its importance in geotechnical engineering and open pit slope design. Weak rock is defined as material with strength between 1 and 25 MPa (R1–R2), occupying a transitional zone between soil and strong rock and often exhibiting soil-like behaviour. The module explains how weak rock is influenced by factors such as weathering, alteration, mineralogy, porosity, and water content, which significantly affect its strength and stability. It introduces key international classification systems (ISRM and ISO) and highlights the need for subdividing weak rock categories due to variability in behaviour. Common types of weak rock, including sedimentary rocks, saprolites, and hydrothermally altered materials, are presented along with their formation processes and engineering challenges. The module also outlines key difficulties in drilling, sampling, and testing weak rock, as well as the importance of considering groundwater effects and degradation over time. Watch here.

• Core Logging, Sampling, Photography: This training module provides practical guidance for collecting reliable geotechnical data in weak rock to support engineering design. It emphasizes the need for immediate, rig-side logging to preserve in-situ conditions before core degrades due to drying, swelling, or handling. Weak rock requires a combined soil–rock approach, considering factors such as water content, weathering, alteration, and discontinuities. The module highlights best practices for sampling, stressing the importance of capturing weaker intervals and avoiding bias toward stronger material. It also outlines methods to preserve sample integrity through careful handling, wrapping, and rapid packaging to maintain natural moisture conditions. Additional guidance is provided on drilling considerations, core recovery, and proper communication with drilling teams to improve sample quality. Core photography and boxing techniques are covered to ensure accurate documentation and quality control of data collected in the field. Watch here.

• Field Testing Methods:This training module introduces practical methods used to assess the strength of weak rock in the field for geotechnical and mining applications. It explains how weak rock behaves differently from strong rock and highlights the limitations of simple manual classification methods. The module focuses on three key index tests: point load, Equotip hardness, and needle penetration testing. Each method is described in terms of application, advantages, and limitations, with emphasis on data reliability and the need for site-specific correlations. Special attention is given to factors such as water content, sample condition, and variability of weak materials. The training highlights that no single test is sufficient and encourages combining field methods with laboratory data. Watch here.

• Downhole Geophysics Surveys:This training webinar introduces downhole geophysical survey methods used to characterize weak rock in geotechnical and mining applications. Weak rock, defined by low strength (1–25 MPa), often presents challenges such as poor core recovery and limited laboratory testing data. This module explains how downhole geophysics helps overcome these challenges by providing continuous, in‑situ measurements along the borehole. The webinar focuses on the practical application of key logging techniques, including optical and acoustic televiewers, full waveform sonic logging, and gamma‑gamma density measurements. These methods allow engineers to identify fractures, structural orientations, and zones of weak or altered rock that may not be visible or preserved in core samples. The training highlights how these tools complement traditional geotechnical investigations to improve accuracy and reliability. A key theme of the module is reducing uncertainty in weak rock environments. By filling gaps caused by incomplete sampling, downhole geophysics supports better estimation of rock strength, rock quality (RQD), and in‑situ conditions. The webinar also discusses interpretation techniques and emphasizes the importance of correlating geophysical data with laboratory results for site-specific calibration. Operational considerations are also addressed, including borehole stability, tool selection, and data quality requirements. These factors are critical to ensuring successful surveys and reliable outputs in challenging weak rock conditions. Watch here.

• UCS and TCS Testing: This webinar presents practical guidelines for uniaxial (UCS) and triaxial (TCS) compressive strength testing of weak rocks (R1–R2) used in open pit slope design. It explains how weak rocks differ from strong rocks in terms of porosity, compressibility, and sensitivity to water content, and why these factors significantly influence strength testing results. A key focus is on triaxial testing, which is essential for developing reliable strength envelopes because weak rock behaviour is highly dependent on pore pressure and confinement. The webinar outlines proper testing procedures, including saturation, consolidation, and shearing stages, and highlights the importance of using effective stress for accurate strength interpretation. Special emphasis is placed on sample preservation, stressing that maintaining in-situ moisture content is critical. Drying or re-saturating samples can alter structure and lead to incorrect strength estimates, potentially resulting in unsafe designs. The session also provides guidance on selecting appropriate confining pressures that match real slope conditions and discusses alternative testing methods, such as direct shear, when samples are limited. Watch here.

• Direct Shear Testing: This presentation outlines guidelines for direct shear testing of weak rock (R1–R2) for open pit slope design. It emphasizes the importance of collecting representative samples, as weak rock is difficult to recover and prone to bias toward stronger material. The document describes testing methods (ASTM and ISRM), equipment, and procedures, including specimen preparation, size requirements, and maintaining natural moisture content.  Normal loads for testing are selected based on expected slope failure conditions, typically ranging from 50–700 kPa. Single-stage tests are preferred for weak rocks to minimize specimen damage, while multi-stage tests may be used when samples are limited.  Results are interpreted using key plots (e.g., shear stress vs. normal stress) to determine strength parameters. Both linear (Mohr–Coulomb) and nonlinear envelopes are used to model shear strength. Careful result validation and reporting are essential for reliable slope stability design. Watch here.