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Landslides Risk Assessment

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Mining Risk Assessment

Natural and manmade slope portfolios require landslide risk assessment in order to ensure sustainable management. Slopes present hazardous geomorphological processes such as landslides which can occur discretely (some deformation or cracking followed by a large movement) or continuously (deformations ranging between a few millimeters and several centimeters per years with occasional accelerations followed by a return to “average velocity”).

How Do We Manage Hazards or Risks?

Any slope, anywhere in the world, has a probability of occurrence of first failure. Continuously sliding phenomena all have a probability of sudden acceleration (multiple failures over a long period of time).

Technically, reliability is the chance of not having a failure, so it is the numeric complement to the probability of failure.

Enhancing a single slope’s or multiple slopes’ reliability means reducing their probability of failure by mitigation. However, managing portfolios of slopes, like for any other hazards portfolio, must make sense from both the economic and safety point of view.

Since various failures within a portfolio will generally cause different consequences (to users, public, environment, traffic, infrastructure, etc.), enhancing the reliability alone will be fallacious from the economic, safety and social point of view.

To optimize the reliability enhancement approach, one has to combine the probability of failure and failure consequences, i.e. look at the risks that each slope can generate. An efficient reliability enhancement effort should be seen as an “enterprise risk management” action, even if the “enterprise” is a government.

Operational, Tactical and Strategic Risk Management Planning Requires a Well-defined Glossary

Below we summarize some of the technical terms applied to landslide risk assessment.

Failure

Landslide failures are generally characterized by rather high frequencies and extremely variable consequences, hence highly variable risks.

We will note that “failure” should be carefully defined, as it can occur either suddenly, like the one-time “fragile” failure of a dam, or slowly, in the form of a continuous movement with discrete bursts of acceleration, paroxysms (continuous slide of large alpine landslides).

The latest edition of the risk and crises glossary proposed in this website will be strictly used through this paper to ensure clarity.

Reliability

The probability of failure evaluation can follow many different approaches depending on the level of knowledge available. That includes geotechnical data, climate data, design construction and maintenance, etc.

Frequency and Probability

These terms require clear definitions:

  • Frequency (fq) is the average number of events (within a population) during a given unit of time (failures per year in the Andes, for example).
  • Probability (pf) is the chance that a failure may occur next year (for example 5% for a specific slope).

When the frequency is small, the number that represents it looks very similar to the number that represents a probability. Thus, many people confuse one for the other. However, for fq>0.1 the values of p start differing significantly.

Figure 1 shows the fq=f(pf) or pf= f( fq) graph.

landslide risk assessment
Figure. 1 Frequency vs. Probability. Vertical axis probabilities (limited by definition to 1), horizontal axis corresponding value of 1/frequency. Frequency expressed in events per year.

Mathematical rules allow to evaluate fq=f(p) or p= f( fq) for any value of fq or p given a few conditions.

Consequences

Consequences C are generally multidimensional, because lives, infrastructures, the environment, cultural assets, etc. are generally at stake.

Any risk assessment that only looks at a partial list of dimensions is biased and censored.

Risk

Risk is the combination of the probability of failure (a number between 0 and 1) and the multidimensional consequences of that failure. Thus, risk has the dimensions of the consequences. Annual probabilities are oftentimes used, however; when looking at a facility with a certain expected lifespan, we can look at the probability over that lifespan, which is more understandable by the public simply because the numbers are more familiar.

Risk Tolerance

Risks (from any hazard) can be tolerable or intolerable from an economic, social or ethical point of view. Intolerable risks can be manageable or unmanageable.

Manageable means that the risk can become tolerable within the realm of credibility by allotting sustainable mitigative funds. Unmanageable means the risk remains intolerable, no matter the mitigative funds allotment.

It is obvious that this type of repeatable and rational definition requires means other than the classic Factor of Safety against sliding and arbitrary classifications normally performed based on intuition or experience.

Once risks (i.e. pf,C) are evaluated for an entire hazard portfolio, then a graph as the one in Figure 2 can be drawn. The orange curve is the risk tolerance (societal or corporate) threshold. The case, locale, culture, and society are among the driving parameters to its definition.

landslide risk assessment
Figure. 2 a pf-C graph with various risks scenarios and risk tolerance threshold. Three classes of risks are visible. Blue=tolerable, yellow=intolerable but manageable, red=intolerable and unmanageable.

In Figure 2 we can see the three different classes of risk, namely the blue, yellow and red classes.

The blue shape represents tolerable risks. Therefore, by definition, in a risk management approach, one can put them aside until there are no more yellow and red risks. They are generally operational. If they have high frequencies and low consequences, they affect routine maintenance and operations.

The yellow risks are intolerable but manageable. Mitigation can reduce their probabilities (brought below tolerance in the realm of credibility).

The last class, the red risks, are unmanageable. Mitigation cannot make them tolerable within the realm of credibility unless we change the system.

Landslides Risk Assessment

Risks that are amenable to sustainable and economic mitigation below tolerance by hazard probability reduction are tactical risks. Intolerable and unmanageable risks are strategic risks. They require system alterations to reduce consequences and bring the risk under tolerance.

A strategic shift would mean, for example, to permanently evacuate a region. The system would be altered. As another example, buttressing a dam to reduce its breach probability is a tactical mitigation, whereas changing the dam location, breaching it to end its service life, or not building it in a certain area represents strategic mitigation.

Tactical risks are under management responsibility, whereas strategic risks, might require upper management or governments to shift their objectives, incentives, planning, etc.