Geotechnical risk assessment

Geotechnical risk assessment / risk management is used at all stages of design, construction, operation, and closure of mines, dams, slopes, excavations, landslide management, geohazards and is a relatively mature field with numerous books and other references.

Boundary Bay sunset panorama

Boundary Bay dyke discharge — Gord did his own risk assessment when buying his house below sealevel in Ladner near Vancouver.

 

Whitman risk diagram 010003j_2

Whitman (1984) f-N Diagram is a seminal contribution to geotechnical risk assessment

The chemical and nuclear industries have been leaders in risk analysis and risk management  for decades. Whitman’s 1984 paper, “Evaluating calculated risk in geotechnical engineering” in the ASCE Journal of Geotechnical Engineering is a seminal work in the field and a good introduction. It uses f-N (frequency / number of deaths ) diagrams for presenting information on societal risks and plots the annual risk of failure (probability) against consequences and maps acceptable risk curves.

 

 

 

 

 

 


Here are some highlights from my own practice

I usually start with a Failure Modes Analysis (FMA) as a transparent and compact method to screen risks. This approach was developed by FERC (the US Federal Energy Regulatory Commission) for hydropower dam safety. It is used to categorize indentified potential failure modes. A strength of this approach is that it focusses on failure modes, it only looks at consequences obliquely (which allows a much simpler approach, with less debate (which is saved for the most important failure modes later).  The team assessing the failure modes works through each one, and assigns it to one of four categories:

“Category I – Highlighted Potential Failure Modes – Those potential failure modes of greatest significance considering need for awareness, potential for occurrence, magnitude of consequence and likelihood of adverse response (physical possibility is evident, fundamental flaw or weakness is identified and conditions and events leading to failure seemed reasonable and credible) are highlighted.

Category II – Potential Failure Modes Considered but not Highlighted – These are judged to be of lesser significance and likelihood. Note that even though these potential failure modes are considered less significant than Category I they are all also described and included with reasons for and against the occurrence of the potential failure mode. The reason for the lesser significance is noted and summarized in the documentation report or notes.

Category III – More Information or Analyses are needed in order to classify these potential failure modes to some degree lacked information to allow a confident judgment of significance and thus a dam safety investigative action or analyses can be recommended. Because action is required before resolution the need for this action may also be highlighted.

Category IV – Potential Failure Mode Ruled Out Potential failure modes may be ruled out because the physical possibility does not exist, information came to light which eliminated the concern that had generated the development of the potential failure mode, or the potential failure mode is clearly so remote a possibility as to be non-credible or not reasonable to postulate.”

Often there is quite a lot of engineering analysis required up front to categorize the risks. Failure modes that classify as Category III need further analysis (and sometimes site investigation to be able to reclassify). Put in simple terms, the big stones go into Category I — these are “highlighted” and need careful attention and often quite a lot more work. They tend to feed into a Failure Modes and Effects Analysis (FMEA) discussed below. Category II are noted and need good engineering, but are more a part of routine design and operation. Category III need more work. And Category IV are written down and discarded as not really possible.

When I was first introduced to this process in 2004, I was really skeptical — where was the power in this? Why not just go to an FMEA? But quickly I came to realize that this approach really highlights the critical risks quickly and efficienctly, THEN, one can bring in a higher power tool and further understand the probability and consequnces and remedial measures.

One this first project where I was involved in the FMA, the mine assembled a multiparty group to evaluate the failure modes (with representatives of the mine, local stakeholders and government, and consultants) to jointly evaluate the failure modes. The mine agreed in advance to remediate all of the Category I failure modes. This was a very powerful approach and highly recommended.


Much of my work is landform design and mine reclamation / mine closure planning. As part of my 2002 thesis research, I assembled a list of 142 potential failure modes for reclaimed landscapes and use this as a starting place for an FMA analysis.

 

Post reclamation landslide on waste rock dump at a Rocky Mountain coalmine in 1996

Post reclamation landslide on waste rock dump at a Rocky Mountain coalmine in 1996

Landform design / mine reclamation potential failure modes (McKenna 2002)

Physical: Animal-induced erosion, Avulsion and flooding; Bank erosion; Blockage: Beaver dam; Blockage: Ice jam; Blockage: Log jam; Breaching; Breakwater damage; Catastrophic outflow; Channel deposition; Creep (and solifluction); Debris flows; Deep-seated landslides; Desiccation; Drought; Earthquake / tectonism; Explosion; Fan deposition; Flowslide / liquefaction; Fluctuating water levels; Glacial advance; Gullies, headward erosion of ; Ice mounds; Ice push; Iceberg scour; Icing; Infilling of water body; Infiltration; Isostatic rebound; Lake overturning / mixing; Meteorite; Migrating dunes; Overtopping; Permafrost creation; Permafrost destruction; Piping; Raindrop erosion; Road construction; Road / trail deterioration; Sea level change; Sheet erosion; Shoreline erosion; Silting up of wetlands/lakes; Slipoff failures; Slumps; Snow avalanche; Spalling; Spring sapping; Stream avulsion; Stream flooding; Subsidence /consolidation; Tsunami, surge, sieche, waves; Tunnel collapse; Undermining; Vandalism or sabotage; Volcanism; War; Wind erosion and deposition;

Biological: Blight / disease; Blowdown / tornado; Browsing; Burial by sand; Climate change; Cultivation; Dusting of leaves; Failure to set up nutrient cycle; Fertilizing; Fire: Forest; Fire: Grass; Fire: Muskeg / peat; Fire: Topsoil; Grazing; Groundwater contaminating plants; Groundwater contaminating animals; Ice push; Ice storm; Irrigation; Logging; Loss of diversity; Micro-climate change; Non-native invasion offsite; Non-native invasion onsite; Not self-sustaining; Nutrient accumulation; Over-fishing; Over-hunting; Over-grazing; Over-trapping; Pseudo-climax; Salt accumulation / salt pan; Salt fluctuations in water; Vegetation undercut by gullies; Unwanted succession; Use of herbicides; Use of pesticides; Water table: Fluctuating; Water table: Too high; Water table: Too low

Chemical: Acid rock drainage (ARD); Cementation; Coal fire; Decementation; Gas evolution; Leachates; Piping in dispersive clays; Radioactivity; Spontaneous combustion

Human health and safety: Barrels and debris; Bioaccumulation / food chain; Boreholes / wells; Breach / flooding; Cliffs; Deteriorating road; Dust; Dynamite / blasting caps; Fences; Fog; Gas; Groundwater contamination; Highwall ravelling / falls; Ingestion; Old bridges; Old buildings; Old equipment; Pits and water bodies; Power lines / substations; Quicksand; Radioactivity; Rapids / falls; Shafts / portals; Silicosis; Sinkholes; Slides / flowslides; Soft shoreline on lakes; Thin ice; Untrafficability; Uneven ground; Wildlife or insect attack

Failure to perform intended function: Act as aquifer; Aesthetics; Affect water temperatures; Avoid sterilizing ore; Barrier: Site; Barrier: Sound; Contain / isolate fluids; Contain / isolate solids; Corridor: Industrial; Corridor: Road; Corridor: Wildlife; Corridor: Transportation; Corridor: Wildlife; End land use; Firewall; Fluids : Isolate; Foundation soils; Groundwater barrier; Littoral zone; Meet corporate objectives; Meet design or code; Regulate water discharge; Regulatory: Continue to meet future regulatory approval; Regulatory: Receive regulatory approval; Support infrastructure; Support vegetation; Trafficable; Trap sediments; Water: Attenuate; Water: Gather; Water: Release; Water: Store; Water: Transmit; Water: Cleaning/ treatment; Waves: Attenuate / block.

 

The next step is a full Failure Modes and Effects Analysis (FMEA). The following is typical of a table used for geotechnical risk analyses

Typical format for Geotechnical FMEA Table

Typical format for Geotechnical FMEA Table

 

Often an event tree is needed to work out the probabilities.

Most industrial companies and mines have their own risk rating system and risk acceptance table. Here is a simplified one.

Typical risk matrix

Typical risk matrix (simplified)

 

The consequence is usually chosen as the most severe of:

  • lives lost / injuries
  • cost (including lost production, litigation)
  • environmental damage
  • reputational risk

And in the last decade, usually the reputational risk is the most severe for major catastrophic events such as a dam breach.

Critically, the risk acceptance criteria for a mine (which is often based implicitly on expected value, with an implicit cost-benefit relationship for remedial measures) is recognized as different than for stakeholders or others (who may have lower risk tolerance). More recently, numerous papers have been published regarding societal acceptance for low-probability high consequence events (say for example, an extreme earthquake causing a dam failure in an urban setting). Certain risks may be rated as intolerable in the extreme case, or as in the ALARP region (where mitigations are put in place to reduce the risk to as low as reasonably practicable). Different groups will have different risk tolerances.

 

Frank Slide, Alberta Canada

Frank Slide, Alberta Canada