Five Things you should know about slope stability

Special Thanks to Dr Loren J Lorig

Modern-day mining requires the optimization of pit slopes to ensure that the slopes are stable and economic to mine. While several methods are available to help design and monitor the stability of the slopes, there are five major aspects that geotechnical engineers should know when they are involved in slope stability studies. The collection of appropriate data from the project site and the challenges of sampling bias, the problems of using average values in the design of excavations, the impacts of extreme natural events on ground stability, the importance of design validation, and the future trends in slope design, analysis, and monitoring for enhanced security of personnel and resources are presented here.

1. Characterization: why should we focus on the weakest materials and how does sampling bias work against us
2. Data analysis: Why it is wrong to use average values in design.
3. Design analysis: what are the impacts of extreme events (earthquakes and rainfall) on slope stability
4. Design validation: why design validation is essential
5. Future trends: how slope swill be studies in the future

1. Characterization: In this pillar- a geotechnical engineer has to determine the values of RMR, Q value, GSI, more coulomb, and Hoek- Brown parameters (detail definition and significance of each parameter has been described in a different post on this site www.waartsy.com). But every engineer should know to choose the sample rock type. The section should not be the best rock. The focus should be on the weakest parts of the rock mass. By focusing on deterministic analysis and stochastic analysis- proper values of Cohesion ( c) and Փ (angle of internal friction) only give accurate output while simulating a particular geometry otherwise the output can be anything that can lead to an unsafe design.

• Data Analysis: Well, here we can take a real-life example. Suppose one day you have loose motion and the next day after taking medicine you are suffering from constipation. So, if we take the average of your stomach problem then the first day it is ‘-100’ because of loose motion, and the next day the value is ‘+100’ because of constipation. Hence the average is ‘0”. So you are completely OK with your stomach. You neither have loose motion nor have constipation.

Exactly the same thing is applicable for rock mass also. You can not take the average. This is the problem of average value.

Rock mass variability can significantly reduce Factor of Safety (FOS) and increase Probability of Failure (POF)”. Hence our consideration within the rock mass variability is very much valuable to determine FOS and POF.

• Design Analysis: obviously every miner thinks about the perfect design to bring optimum production and optimum stability on a single page. But there are only a few engineers who think about the extreme events which can affect the design. Yes, that is the right way to analyze a design. The effects of extreme events have to be considered. Earthquake or similar big events has to be incorporated into large-scale designs. Seismic effects, heavy rainfall, dynamic mining all these things have to be incorporated.
• Design Validation: all designs are based on assumptions that must be confirmed for the design to be valid.

Example of reinforced concrete- concrete must be tested and shown to exceed required strength in order to validate the design.

For open-pit mines, we need to validate assumptions: rock quality and strength, structural setting (rock fabrics, faults, etc); water levels.

• Future Trends: Machine learning and artificial intelligence have grown quickly in the last 5-6 years and have demonstrated success in some application areas, ML and AI are effective in data-rich environments where the governing equations are unknown. In geomechanics, we are a data-poor environment and the governing equations are unknown. In this regime numerical modeling is effective.