January, 2022 - WAARTSY

How a Geotechnical engineer confirms the stability of a slope through Numerical simulation

Area identification

As per the requirement of the ABP target and approved mine scheme, areas are to be decided by the mine planning engineer where mining excavation will be done in near future. It can be in bench-scale, inter-ramp scale or large scale. That identified area is to be approved by a mine planning engineer.

2. Rock/ rockmass/ properties and collection of other inputs

After identifying the area, all rock properties data from already been tested from the laboratory to be gathered. The field observational data, geometrical data and all geological data i.e compressive strength, tensile strength, shear strength, cohesion, poison’s ratio, young modulus and other rock/ site-specific constants, which are needed for numerical modelling need to be gathered.

3. Structural modelling

All structural data i.e. joint properties, fault/fold/dyke attributes and shear zone related data to be collected as an input for numerical modelling. Hydrogeological data is also to be recorded. RMR, mRMR, RQD, GSI and other geotechnical data are also required for numerical simulation.

4. Pit design/ dump design

Either the mine planning engineer will provide the pit design in Dxf format or the Geotech engineer will design pits in planning software. These pit/dump design geometries will be the main input for the numerical simulation.

5. Numerical modelling and FOS determination

Considering all the geological, geometrical and other mining parameters, numerical modelling shall be done in a number of iterations to determine the Factor of safety in each concerned section. For numerical modelling, any readily available method (BEM/FEM/DEM) in either 2D or 3D can be used.

6. Analysis of the numerical model result

Geotech engineer will analyze all the results of numerical simulation determined by the modelling software. Those shall be discussed with the mine manager, production engineers and mine management for their acceptance. Displacement analysis, damage analysis and stress analysis shall also be analyzed by geotechnical engineers ad planning engineers.

7. Aligning the design with govt/ statutory guidelines

Not only the acceptance of mine management but also the designs will be aligned with all the statutory guidelines (i.e DGMS, IBM, MoEF, PCB etc)

8. Re-interpretation and redesign of pit and dump

If the numerical simulation result is OK and accepted by the mine management then those designs shall be released for fiel implementation but those are not OK, then again reiteration shall be done considering different input parameters.

9. Finalization of pit design/ dump design

If all the output results seem OK, then those designs shall be released to the production dept for field implementation.

RESOURCES REQUIREMENT:

A trained Geotechnical engineer (Mining engg/ geologist or Civil engg)
Trained mining engineer in mine planning software
High configuration desktop/laptop system (at least 32GB RAM)
Mine planning software (SURPAC/ Data mine/ Vulcan or similar software)
Numerical modelling software (FLAC/MAP/Phase/UDEC/3DEC/Plaxis/Galena/Slide or similar software)

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.

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

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.