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Rock Slope Stability 

 

Summary

This review discusses rock slope stability at open pit mines. A brief review on accidents related to slope stability and stability assessment of a rock slope are mentioned here. Links to books, online courses, computer codes and software are given, as well as several consulting companies with experience in rock slope stability issues. Fluid flow in jointed rock masses is also discussed.

INTRODUCTION

My introduction to rock slope stability was Bomvu Ridge in Swaziland as I sat on a view point with Professor Jennings who was smoking his pipe and contemplating a solution to the instability of the mine's sideslopes. I was there to help understand the flow of water in the rock joints and hence predict the water pressure distribution. A quick search of the internet now tells me that early man was extracting specularite from the mine as long ago as 40,000 B.C., making it the oldest mine in the world.

My most recent encounter with rock slope stability was in New Mexico last year. If you approach Chaco Canyon from the south along the route used for over a thousand years, you see the cliffs stretch from one horizon to the other. Only directly in front of you there is a break, a gash dominated by a towering mesa. Gingerly, for the site inspires awe, you round the mesa and quietly enter the still valley. Proceed a short way up to the main building; perhaps this was the palace, or the temple, or merely the granary of the vast empire that was ruled from this site for over 400 years. Regardless of its longevity and grandeur, in the early 1900s the rock slope behind the structure failed and obliterated about a third of the building. Today the rubble of the failure makes a good viewing point.


TECHNICAL BACKGROUND

Rather than trot out the usual fundamentals of rock slope stability which is well documented in books, I encourage you to go to the fascinating site where you will find a beautifully presented story of regional-scale rock slope stability currently being investigated by the Norwegian Geotechnical Institute.

I am sure there is more to the site than I know, but go there and you can download free Chapter 9 of Practical Rock Engineering by Hoek. I did and it's a gem and as good an example as any.

In order to know more on slope stability, I would suggest to go through the papers listed in the Publication section of this review.


STABILITY ASSESSMENT OF A CRITICAL SLOPE

It is very important to conduct a stability assessment of a critical slope and takes necessary measures to avoid any accidental slope failures. Stability assessment through a probabilistic and risk analysis approach brings in light all the factors which can cause these failures. Risks are identified, remedial measures are suggested and a cost study is done covering both long-term and short term stability of the rock slope.

I, Priyadarshi Hem and my team members Pablo Urrutia, Rebeca Barja and Shujing Zhang carried out a probabilistic stability analysis using GoldSim on the case study "A slope stability problem in Hong Kong" (Hoek, 2007), to assess the short-term and long-term stability of the Sau Mau Ping slope. Four remedial measures (reduction of slope height, reduction of slope angle, reinforcement and drainage) were considered for improving the long-term stability of the slope; short-term stability was not considered an issue under the modeled conditions. Costs were estimated for each of the remedial measures. Reducing the slope angle was selected as the preferred alternative due to a technical and economic point of view.

The report on this project can be found here: Stability Assessment of a Critical Rock Slope.

Some more papers on Stability Assessment of rock slope are listed below:


SLOPE STABILITY ACCIDENTS

Indonesia Freeport highwall disaster. We don't usually hear about tragedies in surface mines, but they happen. According to NIOSH, slope stability accidents and highwall failures are among of the leading causes of fatalities at U.S. surface mining operations. Here are some slope stability fatalities that occurred around the world:

October 29, 2011: The Kentucky surface coal mine, Centertown: Two workers were killed when a pile of rock crushed their truck as they drove near a highwall.

April 17, 2007: Tri-Star Mining, Inc., Barton, Allegany County, Maryland: 2 miners got buried under 93,000 tons of rock resulting from a massive highwall collapse. Less effective company's ground control plan and inadequate examinations of highwall slope could not identify the weak highwall. Severe subsidence in the underground coal seam nearby caused the highwall to be extremely fractured.

October 9, 2003: Freeport-McMoRan Copper & Gold Inc., Indonesia: Two workers were killed, five were injured and six were missing.

November 13, 2002: Master Aggregates Toa Baja Corp., Cantera Isabela, Puerto Rico: A part of the highwall collapsed onto the bulldozer operator crushing him to death.

Mine Safety and Health Blog lists all the major slope stability accidents that happened in last decades around US mines.

Stanley Mining Services Limited has prepared a safety practice compliance procedure against highwalls which everyone shall keep in mind.


PUBLICATIONS

Slope Stability Model of the Questa Rock Pile - Phase 1

Slope Stability Model of the Questa Rock Pile - Phase 2

Analysis of Safety Aspects and Mining Practices for Effective Ground Control in Surface Mining

A US Department of Labor Report on a collapse of the highwall occurred in the Coalburg highwall miner pit at the Martin County Coal Corp.

Combat Rockfall and Slope Instability Related Accidents in Surface Mines

Assessing and Monitoring Open Pit Mine Highwalls

Survey Slope Stability Monitoring: Lessons from Venetia Diamond Mine

Rock Mass Properties for Surface Mines

Detecting Problems with Mine Slope Stability

Some more publications on stability assessment are mentioned in the section "STABILITY ASSESSMENT OF A CRITICAL SLOPE" mentioned above.


BOOKS

If you are not wedded to the use of the internet as a copious source of free information, and do not mind the expense of thick books, the following are some you may wish to have on your shelf on the topic of rock (and soil) slope stability-they are available from Amazon.com and all cost about a $100 each, new or used:

COURSES

Image from Large Open Pit Mine Slope Stability Project, CSIRO Australia. Giving and attending courses on rock slope stability is a profitable pastime. Here are some courses that might interest you:

COMPUTER CODES

Once you have the basic data about the slope and the rocks of which it is comprised, you may want to feed the data into a computer code to calculate the factor of safety and seek out the optimum slope geometry. Here are some sites that provide free software or which offer to sell relevant computer codes:

Geotechnical and Geoenvironmental Software Directory is run and maintained by Tom Spink who says that he updates the list at least four times a year. This is the most comprehensive site I found and you probably need go no further in your search for a suitable code. I did and here follows the results of my time on the internet.

I simply quote from the site's introduction which says it all rather nicely:

Rocscience software has been creating easy to use, reliable software since 1996. Our experienced team has expertise in a variety of disciplines, from geotechnical specialties through software marketing and sales. When you need us, we're ready to help - to provide information about our software functionality, free technical support or details on the status of your order. If you ever have any questions, be sure to email us at [email protected]

An amazing site that I recommend you go to is at www.ejge.com. While at this site take time out to examine the many fascinating pages, in particular the on-line geotechnical journal and the review of slope stability computer codes.

ROCKPACK III is a package of programs useful for all phases of rock slope analysis and design where stability is controlled by the orientations and characteristics of rock mass discontinuities (joints, bedding, foliations, faults, etc).

Professor Kroeger at the Southern University Illinois has free download software.

Another source of free software is the Geoengineering Website where a pretty cartoon lady will greet you a vaguely European accent.

SLOPE/W is the leading slope stability software product for computing the factor of safety of earth and rock slopes. With SLOPE/W, you can analyze both simple and complex problems for a variety of slip surface shapes, pore-water pressure conditions, soil properties, analysis methods and loading conditions.

Andreas Gunter of the German Federal Institute for Geosciences and Natural Resources describes a suite of powerful looking codes. I could not tell if these codes are free or whether you will be charged; either way take a look and if they interest you contact Gunter.

GeoStru Software is described thus on their website which came up at about 150 in my Google search: GeoStru is an enterprise, in the southernmost part of Italy, started by a group of engineers in cooperation with geologists and topographers to create and distribute software tools for environmental, structural and geotechnical engineering in geology and topology applications. See their site and products list at http://www.geostru.com/ .

HCItasca with offices across the world including the U.S., Canada, and South Africa. They have FLAC/Slope, an advanced factor-of-safety determination for rock and soil slopes in 2D.

O. Hungr Geotechnical Research has some codes he swears by.

Zostrich Geotechnical promises help with use of their proprietary codes on a case by case basis.

MORE ANALYTICAL SOFTWARE

The following is from a previous InfoMine page-a list of analytical software for rock slope analysis:

Geologic Modeling

Drill Hole Logging

The following all should have drillhole logging input/output routines available for their systems. They are, however, generally not in geotechnical format:

  • Maptek Home of VULCAN 3D geologic modeling software
  • Datamine Software Ltd. 3D geologic modeling software, stereonet plots, data log (drillhole) logs

Slope Stability

  • GEO-SLOPE International Ltd. makers of SLOPE/W, a 2D slope stability program with probabilistic capabilities
  • GEOSYTEM software (GEOSLOPE)software
  • Interstudio S.r.l.
    Circular failure analysis - Macintosh based
  • Rocscience Inc. - geomechanics software and research Formerly associated with the University of Toronto and E. Hoek, this site contains the programs DIPS (stereonet analysis), UNWEDGE (wedge failure analysis) and the finite element code PHASE2.
  • Geocomp Corp. GEOSLOPE (DOS version) for wall stability, slope indicator data reduction, rock wedge stability
  • TAGAsoft 2Dand 3D slope irregular solid slope stability analysis, seismic analysis
  • Software from EJGE A collection of soils type programs,(bearing capacity, elastic stress analysis, 2D Bishop's analysis)
  • Datasurge - GEOROK A complete suite of rock mechanics programs including stereonets, wedge analysis, blasting, etc.

Underground Excavations

  • Rocscience Inc. - geomechanics software and research Formerly associated with the University of Toronto and E. Hoek, this site contains the programs DIPS (stereonet analysis), UNWEDGE (wedge failure analysis) and the finite element code PHASE2.
  • Datasurge - GEOROK A complete suite of rock mechanics programs including stereonets, wedge analysis, blasting, etc.

Numerical Stress/Strain Analysis

Programming Resources

CONSULTANTS

The internet and Infomine's consultants listing yields these consultants who claim expertise in rock slope stability:

FLUID FLOW IN JOINTED ROCK MASSES

To establish the factor of safety of a rock slope, you need the water pressure in the rock mass. My master thesis was called Determination of the Hydraulic Potential Distribution in Jointed Rock Masses. I have a faded copy of the typed script somewhere in the attic, but who would want to read it now. There are so many excellent free publications on the internet on this topic that old documents and books and paper you have to pay for might as well not exist. On the topic of fluid flow in jointed rock masses and hence impact on rock slope stability I recommend a look at these two documents that come to your computer at no cost and no effort:

Google list three papers that are frequently quoted on the topic of fluid flow in jointed rock masses, namely

  • Equivalent Continuum Model for Coupled Stress and Fluid Flow Analysis in Jointed Rock Masses by M. Oda
  • The Elastic Coefficient of Double Porosity Models for Fluid Transport in Jointed Rocks by J. Bergman
  • Characterizing Rock Joint Geometry with Joint System Models by W. Dershowitz.

I tried accessing these papers; I needed a password for one, I needed to send money to get the next, and the third provided text of this complexity-while nevertheless summarizing the field and topic:

Phenomenological equations (with coefficients to be determined by specified experiments) for the poroelastic behavior of a dual porosity medium are formulated, and the coefficients in these linear equations are identified. The generalization from the single-porosity case increases the number of independent coefficients for volume deformation from three to six for an isotropic applied stress. The physical interpretations are based upon considerations of different temporal and spatial scales. For very short times, both matrix and fractures behave in an undrained fashion. For very long times, the double-porosity medium behaves like an equivalent single-porosity medium. At the macroscopic spatial level, the pertinent parameters (such as the total compressibility) may be determined by appropriate field tests. At an intermediate or mesoscopic scale, pertinent parameters of the rock matrix can be determined directly through laboratory measurements on core, and the compressibility can be measured for a single fracture. All six coefficients are determined from the three poroelastic matrix coefficients and the fracture compressibility from the single assumption that the solid grain modulus of the composite is approximately the same as that of the matrix for a small fracture porosity. Under this assumption, the total compressibility and three-dimensional storage coefficient of the composite are the volume averages of the matrix and fracture contributions.

I understood all this because of my Master's thesis and a training in law, but I did not read the rest of the paper. I quote it, however, to impress future readers of my learned publications.

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