AGS 2021 Young Geotechnical Professionals’ Night

Slope stability analyses are routinely performed to assess the safety of geotechnical designs. Stability in two dimensions is commonly assessed using the limit equilibrium method (LEM) which provides a ‘true’ Factor of Safety (FoS) based on the ratio of destabilising to stabilising forces. In three dimensions,

the application of LEM to complex 3D geometries causes some challenges. The most common alternative to LEM is the finite element method or finite difference method (FEM/FDM), which is well established for 3D analysis. However, FoS cannot be calculated directly using the FEM/FDM and needs

to be inferred using other means such as the strength reduction factor (SRF) technique.

This paper investigates the use of LEM and FEM/FDM for a large-scale open pit mine. It discusses the challenges found when applying these methods such as modelling complex 3D geometries and interpretation of SRF results. The application of the strength reduction technique is also investigated

with an assessment of how the most reliable FoS equivalent (SRF) in FEM/FDM was derived. Results from the analysis of the open pit mine are used to highlight the challenges faced in the analysis. Key findings included the need to carry out sensitivity analyses on mesh density and convergence criteria to obtain more reliable estimates of SRF, especially when LEM has difficulties. This placed additional reliance on SRF technique, increasing the need to be able to understand SRF analysis so it can be more reliably applied.

The presence of joints and fractures can dramatically affect the mechanical behaviour of a rock mass by providing planes of weakness across which frictional sliding can occur. Structures designed in or on a fractured rock mass are often subjected to dynamic stresses induced by moving loads (e.g., heavy haul train), seismic events, or mining activities (e.g., rock blasting). Thus, an accurate account of the behaviour of a discontinuous rock mass requires a realistic model for the tribological behaviour of discontinuities to evaluate the time-dependent shear behaviour of rock joints subjected to repeated loading for long-term stability assessments of jointed rock slopes, tunnels, and foundations. Therefore, a systematic dynamic triaxial series of test on replicated rough rock joint was carried out, and results clearly highlight the strength attenuation as a function of joint degradation with respect to the number of loading cycles, and a reduction in shear stiffness and enhanced ductility is also observed. A novel semi-empirical mathematical model to evaluate the equivalent dynamic shear strength (EDSS) of rock joint is proposed and validated with experimental results. EDSS is a useful parameter to provide a better assessment of joint stability and its shear strength when subjected to dynamic loading incidences (both natural and man-made) with the inevitable result of joint degradation over time. Therefore, in a real-life design perspective, using a reduced shear strength for design based on EDSS can be considered to be more realistic at a given time.

The classification of sandstones and shales in the Sydney region most often relies on the system developed by Pells et al (1978) and revised twice, first in 1998 and most recently in 2019. The system offers a classification method for the rock with five classes based on various thresholds of fracture spacing, seam content, and compressive strength with guidance on allowable ranges of geotechnical design parameters for each class. However, the successful application of the system to investigation data requires the manual, repetitive and time-consuming task of reviewing logs, core photos, test data and down hole imaging (where available). Furthermore, different engineers or geologists, regardless of experience, may produce differing interpretation. This variation is due to the sometimes subjective nature of boundary placement and reference length being examined.

As part of a large project, a significant amount of data will need to be reviewed, interpreted and classified quickly and accurately while ensuring consistency across the dataset. The author, working alongside the engineering geologist, has been tasked with designing a process that automates the classification of the sandstones and shales by interpreting digital data (.ags) with coding in Python. A Python script was developed which processes hundreds of boreholes in seconds, reviewing and classifying the data by each of the classification parameters set out in (Pells, et al., 2019) using logical rules that won’t vary by user. The final output includes an excel spreadsheet (which can be imported into software like gINT) and a series of plots which are easily reviewed by a human user where the results can be tweaked and manually manipulated as deemed necessary. Based on the current project dataset, the script has reduced the time the geologist spends on interpretation and classification by half.

This rock pillar assessment formed a part of the integrated station design of a metro station. The rock pillars located between running tunnels and 28 m deep station entrance shaft in Sydney CBD area were numerically assessed for pillar stability and excavation deformation. The shaft excavation has been mainly formed within Hawkesbury sandstone. The ground model including overall stratigraphy, in-situ stress condition, and rock/joint parameters was developed according to available borehole information and available tunnel mapping. Numerical modelling software packages including PLAXIS3D and 3DEC were used to develop a global model (continuum model) and a local model (discontinuum model), respectively. The global model captured the rock mass behaviours using Mohr-Coulomb constitutive model and considered the effects of staged construction for the entire site. The local model simulated interaction between rock mass at block scale and joints by utilising a deterministic Discrete Fracture Network (DFN) model informed from mapping data. The effects of as-built rock bolts were also incorporated in the local model. The outcomes of the assessment provided a basis for advice on pillar stability, excavation methodology and mitigation and monitoring strategies to complete the entrance shaft excavation.