Evaluating ground/underground risk

The risk of ground movement falls under accidental risk, which shares a common terminology with technological risk. Nevertheless, the natural phenomena at play give this type of risk its own distinctive characteristics, which influence the methods of evaluation: an incomplete knowledge of the context in which the phenomena occur, the diversity of events, a probability of occurrence that is difficult to estimate, a high intensity that cannot be reduced using risk management measures, etc.
Risks of natural origin were the first to be the subject of risk prevention plans (PPRs) at the territorial level, as enacted by the so-called “Barnier” decree of February 2, 1995. A PPR regulates the use of soils by identifying the natural risks, and how to avoid them, within a given area. The types of risks accounted for in these plans include floods, ground movement, avalanches, forest fires, earthquakes, volcanic eruptions, storms and cyclones (for the French Overseas Territories).

“Post-mining” risk

These risk prevention plans, later known as natural risk prevention plans (PPRNs), served as a model for the mining risk prevention plans (PPRMs) mandated by the “Post-Mining” decree of March 30, 1999. While the only mining still being done in France today is the extraction of salt, bauxite and kaolin, old closed mines can still represent risks and nuisance.
The purpose of a mining risk prevention plan is to ensure the safety of people and goods by defining soil usage guidelines in areas vulnerable to mining-related risks.
Ground movement (subsidence, collapses and landslides), along with floods and surface emissions of dangerous gases, are the three types of natural risk identified when evaluating “post-mining” risks. Other mining risks include soil and subterranean water pollution and ionizing radiation.

> Typology of ground movement risks

Evaluating ground movement risks

At the core of a ground movement risk evaluation is a process of mapping the area in question for natural hazards.

• The crucial first step, after the scope of the study has been set, is the cataloguing and processing of data. Unlike an industrial system, which is well known to its operator, the context in which the natural hazard may produce disturbances is only partially or completely unknown.  Studying context requires collecting a large amount of technical data (meteorological, morphological, geological, hydraulic, hydrological, etc.), from archives, and analysing geographical information (populations, habitats, etc.). The data collected is used to create a map that describes observed or historic phenomena.
• The first step in evaluation is to determine a baseline of natural hazards in the given zone. The characteristics of ground movement are highly variable: each event is unique and does not reoccur under the same conditions.
• Thus, it is necessary to determine a baseline of natural hazards to serve as a simplified representation of the wide diversity of possible characteristics. This baseline is usually either the most severe potential event likely to occur on a centennial basis (at least once per century), or the most severe historic event to have occurred in an equivalent configuration.
• The second step is to define natural hazard zones. This is done by identifying geologically homogenous sectors in which ground movement may occur. Analyses of existing data as well as ground surveys are used to complete this stage.
• The third step is to estimate the probability of an occurrence: most ground movement events are not repeated and so it is almost impossible to quantify estimated frequency. Estimation is thus based on the site’s susceptibility to the event, which is determined by analysing the site’s configuration, potential triggers or aggravating factors.
• The fourth step is to quantify the intensity of the natural hazard. Given the diverse nature of ground movement, it is difficult to quantify its likely physical characteristics. They are often qualified based on the event’s damage potential (impact on stakes), the scale and cost of protective measures that should be implemented in case of occurrence, and surveillance data which are used to assess the progress of movements.
• The map of natural hazards produced during the evaluation is then combined with a map of stakes in the territory, which helps organize urban planning in the zone under study.