# A methodology for multi-hazard interaction assessment of abandoned mines

Marwan Al Heib, Christian Franck

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Marwan Al Heib, Christian Franck. A methodology for multi-hazard interaction assessment of abandoned mines. Journal of Industrial Safety Engineering, 2024, 1 (2), pp.100018. 10.1016/j.jinse.2024.100018. ineris-04780770

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# A methodology for multi-hazard interaction assessment of abandoned mines

M. Al Heib*, C. Franck

Ineris, Institute national de l'environnement industriel et des risques, France

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## ARTICLE INFO

**Keywords:**

Hazard

Risk

Multi-hazard

Interaction

Mine

Post-mining

Abandoned

Assessment

## ABSTRACT

The number of abandoned mines is continually increasing around the world. This situation demands that the public safety of surface structures and infrastructure situated above and around abandoned mines be ensured. Abandoned mine sites pose serious risks to people who are unaware of the site's hazards. As a result, post-mining risk assessment and management remain crucial for mining authorities, policymakers, and planners. In the post-mining period, several hazards (ground movement, flooding, gas emission, pollution, etc.) are likely to affect the surface areas above closed mining sites. The effect of closed mines can lead to potentially damaging changes in surface or/and underground water flows, as well as the development of surface instabilities that can pose a grave risk for people, structures and/or infrastructure.

This paper presents a methodology for analysing the interactions between hazards around abandoned mines. The methodology was developed based on the assessment of the different hazards that may occur at the mine site, and on the interaction between post-mining hazards and other risks (natural and mining-induced). The methodology provides tools, such as an interaction matrix, and an interaction diagram, that can be used to identify the type and the level of interaction. A multi-hazard index that assesses the level of interaction at a mining site is also proposed, which can be integrated in a hazard mapping tool such as GIS (Geographic Information system) and DSS (Decision Support System). The multi-hazard assessment methodology was applied to a coal mine to demonstrate the complexity but also the utility of such a risk assessment analysis for the risk management of abandoned mines.

## 1. Introduction

Mines may, as a rule, be closed when their mineral resources are exhausted [29,47] but mines may also be closed for reasons of safety and security. In the United States, more than 557,000 abandoned mines have been registered, with 38,991 posing significant risks to both human health and the environment, according to [15]. Fig. 1 shows the density of inactive mines across Europe. There is no real count of the number of abandoned mines in Europe. The UNEXMIN's (European project) inventory listed ~8100 flooded mines in 24 European countries (2018). In Australia, the number of inactive and abandoned mines is between 50,000 and 80,000 (Unger et al., 2012). In Canada, there is no national inventory of orphaned and abandoned mine sites. Some 10, 000 orphaned and abandoned mine sites have been identified across Canada, many of which are hazardous and lack long-term closure plans [19].

Abandoned mines are areas where mining activities were brought to halt as a result of incomplete mining closure or improper reclamation.

There are many reasons for this, ranging from inadequate compliance with regulations of the environmental protection agency by operators, or even the total lack of compliance. Nowadays, most mines (coal, iron, and others) in Europe have been closed as a result of economic, environmental, and social issues [25,50]. Policy changes, which occur from time to time, are also a factor governing mine closure, particularly when governments change.

### 1.1. *Environmental and human impact of the post-mining activities*

Former mining sites constitute a serious industrial legacy, and they have social and environmental impacts [9,40,42]. The reclamation and restoration of an abandoned mining site is generally based on advance risk assessment studies [10,24,30]. Additionally, post-mining management is a significant concern for the nations and regions with a rich mining history, such as Europe, Australia, South Africa, and Canada.

Some hazards related to mining activities can sometimes occur immediately after cessation of mining operations, sometimes much later

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: marwan.alheib@ineris.fr (M. Al Heib).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Map of Europe (left) showing closed and operating mines (http://en.gtk.fi/informationservices/databases/fodd/)), the map displays the location of waste rick from mining activities, with the map of France(right) showing the abandoned mines in France.</figcaption></figure>

(even years later - Belle and Donnelly [10]). Post-mining hazards can sometimes pose serious risks for people or infrastructure such as roads, railways, water distribution networks, etc. Berest et al., [1,4,41]. The assessment of the different hazards must consider their interaction, the influence of one hazard on others. Thus, land use planning and specific planning for adequate rehabilitation of former mining sites require better tools to understand the multiplicity of hazards and their constraints.

Managing multiple hazards is a real challenge for communities. Indeed, risk management is a dynamic and iterative process that must take environmental, social, and economic factors into consideration [48].

This paper develops a systematic hazard interaction assessment applicable to the post-mining environment context. Additionally, it proposes tools (an interaction matrix, and an interaction organogram) to evaluate the interaction between different mining hazards related to post-mining. Natural hazards identified in former mining areas are also considered. Finally, this paper explores the multi-hazard methodology as applied to a closed coal mine in France.

## 2. State of the art of multi-hazard risk assessment

Interest in multirisk assessment has surged over the past few decades, both globally and within Europe. This heightened interest is particularly notable in the realm of applications and initiatives aimed at evaluating risks coming from a diverse array of natural and anthropogenic hazards [16]. It therefore becomes necessary to adopt a global perspective when assessing risks. Many authors used multi-hazard risk assessment for different categories of hazards Delmonco et al. [20,23], etc. Mavrommatis et al. [35] carried out a multi-hazard analysis as a result of climate change and provided operational recommendations for managing the interaction between the mining industry and natural hazards related to climate change. Ming et al. [37] studied a multi-hazard risk assessment framework for compound flooding. Gustafsson et al. [21] introduced a methodology aimed at establishing a comprehensive national framework for understanding the interaction of natural hazards, and they delineated various types of interactions between 20 distinct natural hazards. Among these interactions, they identified 39 cascading events, 56 instances of what they term "disposition alteration," 3 scenarios of additional hazard potential, and 17 cases of coincident triggering.

### 2.1. Definitions

A "interacted hazard" refers to a hazard whose occurrence depends on the occurrence of another hazard. In certain cases, the occurrence of one hazard reveals the existence of a second hazard. "Independent

hazards" refers to two hazards that occur independently of each other. The term "multi-hazard" refers to the existence and the occurrence of several hazards in the same territory. Delmonaco et al. [8] define multi-hazard analyses as the "implementation of methodologies and approaches aimed at assessing and mapping the potential occurrence of different types of natural hazards in a given area". The European Commission [14] defines multi-hazard as the probability of occurrence of different hazards, either occurring at the same time or shortly following each other, not as random coincidences, but because they are dependent on each other or because they are caused by the same triggering events, such as rainfalls and earthquakes. Multi-hazard assessment can be defined as a method or approach that analyses more than one hazard in the target area as well as the interrelationships between these hazards, including their simultaneous or cumulative occurrence and their potential interactions [6].

### 2.2. Advantages and limitations of multi-hazard analysis

Multi-hazard risk assessment has as its objective the identification of suitable mitigation methods and the deployment of a monitoring system [22]. Multi-hazard risk assessment may provide a comprehensive analysis of the impact of multiple hazards and can thus be used for decision making Garcia-Aristizabal et al. [17]. In fact, multi hazard/risk management decreases the cost and increases the effectiveness of interventions. In addition, multi-hazard management increases the effectiveness of prevention. For sites vulnerable to multiple hazards, the development of multi-hazard risk analyses emerged as a response to the limitations of single-hazard assessments. Unlike single-hazard analyses, these comprehensive approaches recognise the complex interplay between different hazards. They involve assessing each hazard as a component capable of interacting with others, thereby providing a more thorough understanding of the overall risk landscape. The integrative nature of multi-hazard analysis, which considers several hazards and associated vulnerabilities, provides a more comprehensive appraisal of situations where multiple hazards coexist and often interact on the same territory/site. Multi-hazard assessment tools can support decision-makers and provide them with information on mitigation measures (Komedantova et al.). However, multi-hazard risk assessment at local and regional scales remains a significant challenge due to the lack of data, causal factors, and interactions between different types of hazards [49]. Understanding the multi-hazard landscape and their interactions, as well as their simultaneous effects on mining sites, is essential.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Different tools for assessing potential interactions: (a) interaction between hazards (H), (b) type of interaction, (c) interaction matrix, (d) interaction diagram. FHn denotes factors related to the hazard.</figcaption></figure>

## 2.3. Multi-hazards assessment tools

The following tools - hazard matrices, [20,53], Kappes et al., fault trees [34], 2012, [13], multi-criteria analysis [36,38], negotiated choice, the implementation of a multi-scale GIS (Geographical Information System) and statistical modelling of vulnerability including temporal variability [33,46] - can all be used separately or in combination to assess the hazard interaction. Furthermore, back-analysis of case studies and expert input are generally used to assess the interaction between hazards [6]. The interaction matrix presents the interrelation between n hazards (H1, to Hn), where the source hazard (H1) can trigger several hazards (H, 1 to n). For instance, Fig. 2 presents an interaction matrix for five hazards (note that the interaction matrix is not symmetric). The different levels of hazard interaction can be identified and presented using colour codes. The green colour means the interaction level is very low and limited, the orange colour means that interaction level is moderate, and the red colour denotes a significant interaction and has consequences that need to be mentioned in the risk assessment. Sigtryggsdottir and al. [45] used the interrelation matrix to identify interactions between geohazards that may affect hydropower projects, and to optimise the monitoring system. The fault tree tool helps to identify the interaction between the triggering event (hazard) and the different potential events, based on the site conditions and site factors.

A full multi-hazard assessment should consider all the possible hazard sources and identify all possible interaction scenarios, including cascading effects (Gracia et al.); the cascading effect refers to a hazard which has an impact on several hazards. The interaction between hazards should consider both temporal and spatial scales. The spatial scale refers to the area where the hazard has an impact, and the temporal scale refers to the time scale during which the single hazard acts on the natural environment, according to Gill and Malamud [20]. They observed that natural hazards influence a range of spatial areas, from fractions of square kilometres to hundreds of

millions of square kilometres (a global scale). The timescale of the interaction ranges from seconds to millennia. De Ruiter et al. [7] presented and discussed several catastrophic events that show the different spatial and temporal extents covered by consecutive disasters, with times ranging from days to months or even years. Furthermore, they mentioned that the risk assessment of a single hazard is not optimal for assessing several regional hazards.

Liu et al. [32] proposed a framework for multi-hazard assessment that considers the possible hazard interactions. They suggest a flow chart that guides the user in deciding whether a multi-hazard and risk analysis is warranted. One can distinguish between one-hazard assessments and multi-hazard assessments; the different levels of analysis reflect the type of potential interactions: independent, triggering, and cascading. The third level of the methodology is a detailed quantitative multi-risk analysis. (Fig. 3)

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Hazard interaction assessment of multi-hazards based on the typology of the potential interaction ([32], modified by the authors).</figcaption></figure>
The assessment of hazard interactions relies on categorizing hazard intensity (high, average, low) according to the specific conditions of the study area. In the case where the study area is vulnerable to multiple hazards, the potential for each of three types of interactions should be considered:

* Coupled or combined dependent hazards. The studied area has several hazards with common triggering factors. Their consequences could be cumulative. In this case, one hazard can trigger one or more hazards; there is a direct causal link between one and several hazards occurring consecutively in the studied area.

* Sequential dependent hazards. The hazard modifies the conditions of one or more hazards. When one hazard occurs, the conditions for a second hazard may be met, the area becomes vulnerable, or the probability of occurrence of the second hazard is increased. The second hazard is entirely or partially dependent on the first one.

* Dependent hazards. The occurrence of a first hazard triggers, amplifies, or modifies the second. This is a chain reaction of several hazards. The dependent hazards can lead to a domino or cascade effect. Domino effects extend the diffusion of consequences in space and time beyond the scale of individual hazards.

### 3. Multi-hazard assessment in a post-mining context

In a post-mining context, the objective of multi-hazard assessment is to improve the management of abandoned and recently closed mines [48]. In such a context, the hazard assessment studies have focused on the detailed examination of a single hazard [39,44,52]. However, closed mining areas are generally not affected by a single hazard, and two or more can act at the same time or consecutively [2]. In such cases, assessing a single hazard can be intractable (and ill-advised) when multiple hazards must be considered. But in a post-mining context, the need to adopt a multi-hazard approach is not always evident: the available data for the different individual hazards may pertain to different spatial scales complicating the generation of comparisons, rankings, and aggregations. Different specialised entities and experts need to collaborate. In such cases, the use of a decision support tool DSS, [43,26] can be proposed to the decision maker and local and regional authorities.

The advantages of multi-hazard analyses for closed mines are: 1) a better assessment of the intensity of the hazards around closed mines, particularly for scenarios related to their interactions. 2) Better consideration of the vulnerabilities of a territory exposed to several hazards. 3) A global and integrated view of the risk [18], which leads to better preservation of the general interests identified around closed mines. 4) The opportunity to boost the capacity for resilience and the sustainability of the territories.

#### 3.1. Methodology of multi-hazard assessment for the post-mining area

The multi-hazard assessment methodology consists of three main steps. The first step describes the three significant hazard families: mining, natural, and technological. For the site under consideration, the single hazards should be identified based on the characteristic of the hazards and the related external factors. For instance: the sinkhole hazard depends on the depth, the dimensions of the underground cavity, the strength of the upper layer. External factors influencing the sinkhole hazard include flooding, traffic, ageing, etc.

The second step of the methodology assesses the potential interaction, based on the common factors shared by the hazards and on conditions that may trigger occurrence of the hazards. This step includes the description of the possible interactions according to their character (trigger or aggravating), their category (technical or regulatory), and their typology (dependent or independent).

The third step determines the level of interactions between hazards. The level of the interaction is based on both the intensity of the single hazards and the level of the interaction itself. The following questions should be answered by the experts in charge of studies to identify the

potential interactions between the different hazards:

* Interaction conditions: are there specific conditions to be fulfilled? What are these conditions? How is their likelihood evaluated? Is the interaction systematic?

* Intensity: to what extent should a specific source phenomenon modify the intensity of the target phenomenon? What are the parameters that determine the intensity of the target phenomenon?

* Probability of occurrence: which parameters should modify the probability of occurrence of the target phenomenon?

* Temporality: will the source and target phenomena coincide, or is there a buffer time between their occurrences? What are the parameters influencing the buffer time?

#### 3.1.1. Mining hazards description (M)

Various mining hazards can occur in closed mining areas [12,3,28,5]. They are gathered into 6 groups: ground movements; combustion and fire; hydrological and hydrogeological disturbances of mining origin; gas emissions; endogenous radioactivity of the environment; and environmental pollution (which may have an impact on water, soil and air). Table 1 provides a list of hazardous phenomena, the scientific disciplines required for their analysis, and their respective consequences. In total, we listed 18 mining hazards, and additional mining hazards can be added depending on the local mining conditions.

The combustion (COM) hazard is mainly linked to the heating of land on mine dumps, and in coal and lignite mines. Hydrological and hydrogeological disturbances/Flooding (FLO) refer to hydrological and hydrogeological disturbances that may be detrimental to land or subsoil use. In gas hazards (GAZ), mine gas may be directed toward the surface through natural and induced drains. Pollution of water, air and soil (POL) can potentially pollute groundwater and water sources. One of the causes of post-mining pollution and nuisance is the interaction between mining operations and water flows, which can lead to soil, surface water, and groundwater contamination.

#### 3.1.2. Description of natural hazards (N)

Table 1 summarises the main natural hazards that can be identified in closed mine sites (Djzanne et al.). They are mapped at all territorial scales. At the scale of the territory, such as around closed mines, natural hazards evolve as a consequence of anthropogenic factors and the impact of climate change.

Ground movements refer to any more or less sudden movement of the soil or the subsoil or of rocks destabilised under the effect of natural stresses (snowmelt, abnormal rainfall, seismic shocks, erosion at the foot of the slope, and others) or anthropogenic effects (earthworks, vibration, deforestation, mining and quarrying, and others).

#### 3.1.3. Description of technological hazards (T)

Technological hazards related to industrial activities can occur in the post-mining areas. The industrial activities take place before and/or after cessation of mining activities. A technological hazard is a hazard arising from industrial activities, and may include industrial pollution, nuclear radiation, toxic wastes, dam failures, transportation accidents, factory explosions, fires, and chemical spills. Technological hazards also may be triggered by a natural hazard or other incident or event. Table 1 lists 17 technological hazards, additional technological hazards can be added depending on the specific characteristics of a site.

### 3.2. Mining hazards interactions

Different types of interactions can be identified in the industrial and mining area [27]. Based on the feedback analysis, the following interaction cases have been identified for a mining hazard acting as a trigger for another mining/natural/technology hazard(s): ground movements can increase the probability of the flooding hazard; toxic mine gases can cause health and environmental consequences; flooding of a salt mine
<caption>**Table 1**</caption>

Summary of the mining, natural and technological hazards used in this multi-hazard analysis [11].

<table><thead><tr><th colspan="2">Mining hazards (18)</th><th colspan="2">Natural hazards (16)</th><th colspan="2">Technological hazards (16)</th></tr></thead><tbody><tr><td>Subsidence</td><td>SUB</td><td>Subsidence</td><td>SUB</td><td>Gas explosion</td><td>Exp</td></tr><tr><td>Crevasse</td><td>CRE</td><td>Localised collapse (sinkhole)</td><td>SIN</td><td>Slick fire - liquid</td><td>FEN</td></tr><tr><td>Localised collapse (sinkhole)</td><td>SIN</td><td>Dissolution of natural cavity) (gypsum, chalk, salt)</td><td>DIS</td><td>Flare fire - gas or liquid</td><td>FET</td></tr><tr><td>Massive mine collapse</td><td>MMC</td><td>Settlement, clay shrinkage, consolidation</td><td>SET</td><td>Solid fire - combustible solid</td><td>FES</td></tr><tr><td>Settlement</td><td>SET</td><td>Deep landslide</td><td>DLS</td><td>Boil over - heavy hydrocarbon</td><td>BLO</td></tr><tr><td>Deep landslide</td><td>DLS</td><td>Shallow landslide</td><td>SLS</td><td>Flammable liquefied gas<br>Exposition of boiling liquid</td><td>BLV</td></tr><tr><td>Shallow landslide</td><td>SLS</td><td>Erosion</td><td>ERO</td><td>Vaporisation of liquid jet</td><td>RPL</td></tr><tr><td>Erosion</td><td>ERO</td><td>Mudflow</td><td>MUF</td><td>Gaseous product release</td><td>RPG</td></tr><tr><td>Mudflow</td><td>MUF</td><td>Rocky landslide</td><td>RLS</td><td>Release of a liquified gas</td><td>RGL</td></tr><tr><td>Rocky landslide</td><td>RLS</td><td>Rock or block fall</td><td>RFA</td><td>Fire with toxic product</td><td>IPT</td></tr><tr><td>Rock or block fall</td><td>RFA</td><td>Avalanche</td><td>AVA</td><td>Release with nuclear substance</td><td>RSR</td></tr><tr><td>Heating or combustion</td><td>COM</td><td>Natural seismicity</td><td>NSI</td><td>Discharge of water bodies</td><td>RME</td></tr><tr><td>Mine gas</td><td>GAZ</td><td>Wildfire - Forest fire</td><td>FFI</td><td>Land movement</td><td>MVT</td></tr><tr><td>Modification of ground water regime</td><td>MWR</td><td>Flooding of lowland</td><td>FLO</td><td>Pneumatic energy release</td><td>EBC</td></tr><tr><td>Modification of river regime</td><td>MOR</td><td>Flooding due to runoff</td><td></td><td>Combustion of gas</td><td>VCE</td></tr><tr><td>Flooding low topographic</td><td>FLT</td><td>Flooding due to rising of groundwater</td><td></td><td>Explosion of solids</td><td>ENA</td></tr><tr><td>Flash flooding surface</td><td>FLS</td><td></td><td></td><td></td><td></td></tr><tr><td>Induced seismicity</td><td>INS</td><td></td><td></td><td></td><td></td></tr></tbody></table>

can induce the collapse of the mine with consequent ground movement [51]; mine flooding can trigger or worsen the sinkhole hazard.

Natural hazards can trigger a mining hazard and/or an aggravating hazard. For instance, a drought hazard may be related to hydrological and hydrogeological disturbances in mining reservoirs and may thus modify the gas flow to the surface. Earthquakes are much rarer but can lead to the collapse of shallow mines operated by abandoned rooms and pillars [31]. The slope instability of an open pit mine can be triggered by an earthquake whose epicentre is at a distance equal to or less than 100 km and with a magnitude greater than 6.0 [2].

Technological hazards as well can trigger and/or aggravate mining hazards. Here are some examples of where specific technological hazards can interact with the mining hazards. The failure of specific structures such as dams and sewage or drinking water networks can directly trigger the flooding of closed mines and cause widespread or localised collapses. The rupture of extensive bridges/works can cause surface movements of land on the flanks/fronts of open-pit mines, further leading to underground disorders. Intensive agriculture - such as large cereal farms - lead to soil erosion and ultimately to mechanical instability of the soil, resulting in a flow, or a landslide in underground mining works.

### 3.2.1. Multi-hazard matrix and mine hazards interaction organogram

In order to build an interaction matrix for mining hazards, the level of the interaction should be assessed based on the predisposition factors and the intensity level of the individual hazards. Experts need to work together to suggest interaction levels. The interaction level between hazards is estimated from the factors determining their intensity on the one hand and their probability of occurrence on the other hand. For phenomena that are not repetitive, the probability of occurrence is replaced by the predisposition of the site to occurrence of the phenomenon. Thus, the interactions between the identified hazards were hierarchised into three levels, as outlined in the methodological guide for developing Mining Risk Prevention Plans: high, medium/moderate, and low. To build the interaction matrix between the mining hazards, we selected 13 post-mining hazards whereby the hazards running down the leftmost grey column trigger the same set of hazards shown running along the top grey row. The matrix has 169 cases with 107 zero interactions between hazards, 28 low, 14 medium and 20 high interactions. Table 2 shows the interaction matrix where the first column corresponds to the source phenomenon and the first row corresponds to the target phenomenon. Thanks to the interaction matrix, one can recognise the role and the interaction of the flooding hazard (FLO) with

the other mining hazards. The interaction of the flooding hazard (FLO) with the sinkhole hazard (SIN) is high level. Flooding is the hazard that can trigger 9 post-mining hazards, with 6 of them at a high interaction level. On the other hand, the gas hazard has no impact on the other mining hazards. The interaction matrix is also asymmetric, which indicates that the interaction between hazards is not bidirectionally equal.

We also present the interaction organogram for the flooding mining hazard with different hazards shown in Table 1. To construct the interaction organogram, the experts used their knowledge, judgement and analysis of the case studies to identify the direction and the level of the interaction for each hazard pair. Thanks to this analysis we can identify the potential scenario and the several optional cascading interactions. Fig. 4 presents the interaction organogram for the flooding hazard (mining, natural and technology). It shows that the flooding hazard can interact with 11 mining hazards (gray), 8 natural hazards (green) and 3 technology hazards (pink). Additionally, the organogram provides information about the type of the interaction, and whether it is a single, double (bidirectional), and cascade interaction.

The two tools shown here highlight the potential interaction between hazards. The number and the specific hazard interaction scenario depends on the single hazards identified at the mining site.

### 3.2.2. Multi-hazard index calculation

The main purpose of multi-hazard index is to gather in one index the different hazard-related intensities for an area under study. Unlike the qualitative approach presented by the diagram and the interaction matrix, it is possible to calculate parameters quantifying the interaction. This calculation of the multi-hazard index is based on a qualitative evaluation of interactions between hazards. To calculate the multi hazard index for a mining site, four steps are required:

- Substitute the qualitative hazard level by number; that means the initial level of hazard will be replaced as following: low = 1, medium = 2 and high = 3, (Table 3).

- Substitute the interaction level by number; that means the interaction level between two hazards replaced as following: low = 1, medium = 2 and high = 3. The choice of values for hazards and interaction levels is arbitrary but must be identical in order to compare different zones of a mining site or several mining sites (Table 3).

- Calculate the adjusted hazard value (according to the following equation:
<table><caption>**Table 2**

Tentative view of interaction matrix of different hazards related to mine closure.</caption><thead><tr><th></th><th>FLOD</th><th>MMC</th><th>SUB</th><th>SET</th><th>CRE</th><th>GAS</th><th>MUF</th><th>SIN</th><th>DLS</th><th>SLS</th><th>ERO</th><th>ISI</th><th>COM</th></tr></thead><tbody><tr><th>FLOD</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>MMC</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>SUB</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>SET</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>CRE</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>GAS</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>MUF</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>SIN</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>DLS</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>SLS</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>ERO</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>INS</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>COM</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<table><thead><tr><th>Specific mining hazard (12)</th><th>Codes</th></tr></thead><tbody><tr><td>Flooding</td><td>FLOD</td></tr><tr><td>Subsidence</td><td>SUB</td></tr><tr><td>Crevice</td><td>CRE</td></tr><tr><td>Sinkhole</td><td>SIN</td></tr><tr><td>Massive mine collapse</td><td>MMC</td></tr><tr><td>Settlement</td><td>SET</td></tr><tr><td>Deep landslide</td><td>DLS</td></tr><tr><td>Shallow landslide</td><td>SLS</td></tr><tr><td>Erosion</td><td>ERO</td></tr><tr><td>Mudflow</td><td>MUF</td></tr><tr><td>Mining gas</td><td>GAS</td></tr><tr><td>Induced seismicity</td><td>INS</td></tr><tr><td>Combustion</td><td>COM</td></tr></tbody></table>

<table><thead><tr><th colspan="2">Interaction level</th></tr></thead><tbody><tr><td>No interaction</td><td>82</td></tr><tr><td>Low</td><td>28</td></tr><tr><td>Medium</td><td>14</td></tr><tr><td>High</td><td>20</td></tr></tbody></table>

$$H_{adj-i} = H_{ini-i} * \sum_{1}^{3} L_k * N_i \quad (1)$$

$H_{ini-i}$: initial hazard level; $L_k$: hazard interaction level, with k varying from 1 to 3; $N_i$: the number of different interactions (with the other hazards).

For instance, with the values given above, if $N_i$ equal 1, then the value of the adjusted hazard level ($H_{adj-i}$) can vary between 0 and 9 (Table 3). The value zero is for the case of absence of interaction between the two hazards (e.g. independent hazards). For instance, if the level of the initial hazard ($H_{ini-i}$) is low (1) and the level of interaction ($L_k=1$) with the second hazard j is low, the adjusted hazard level ($H_{adj-i}$) is equal to 1 (low) = 1*1. If on the other hand, the level of the initial hazard ($H_{ini-i}$) is high (3) and the level of interaction with the second hazard j is high (($L_k=3$), the adjusted hazard ($H_{adj-i}$)) is then equal to 9 (high) = 3*3.

For a hazard $H_{ini-i}$, corresponding to high level ($H_{ini-i}$ = 3, red), at the highest interaction level, the $L_k = L_3 = 3$ and maximally $N_i = N = 12$. $N_i$ is one less than 13 because each hazard is not expected to interact with itself. So, $L_3 \times N = 3 \times 12 = 36$. Finally, then the maximally ($H_{adj-i}$) = $3 \times 36 = 108$.

Finally, the last step of the multi-hazard index (MH) is the calculation of the value resulting from the interactions between all hazards. The multi-hazard index (MH) is defined according to the following equation:

$$MH = \sum_{1}^{n} H_{adj-i} \quad (2)$$

With n being the number of hazards identified on the site studied. $H_{adj-i}$: adjusted hazard level of hazard i ($H_i$)

$$MH = \sum_{1}^{n} H_{adj-i} = \sum_{1}^{n} \left( H_{ini-i} * \sum_{1}^{3} L_k * N_i \right) \quad (3)$$

To illustrate the calculation of the multi-hazard index, the following example for two hazards (n=2) is presented:

- the level of the first hazard is medium ($H_{ini-1}$ = 2) and the level of the second hazard is ($H_{ini-2}$ = 1).

- the respective level of interaction between these two hazards is assessed as high (3).

- therefore, the adjusted levels of the hazards 1 and 2 can be calculated as follows:

$$H_{adj-i} = 2 \times 3 = 6 \text{ and } H_{adj-j} = 1 \times 3 = 3.$$

- the multi-hazard index for the two hazards can be calculated as $MH(n=2) = 6 + 3 = 9$.

This approach is applicable to regions where multiple hazards converge, thus allowing the identification of target areas characterised by a significant number of hazard interactions, while taking into account the severity of each expected phenomenon. In this way, it is possible to prioritize sectors for overall hazard management. The multi-hazard index can be mapped using the GIS techniques or other mapping tools.

## 4. Application of the methodology to a case study

The case study concerns a sector of a closed underground coal mine in the south of France (Fig. 1). Two different mining methods were used: long-wall and rooms and pillars. Additionally, there is a mining dump holding waste coal materials. The mine was characterised by induced seismicity attributed to the geological composition, which featured very resistant and competent strata and faults. Hazards assessment studies were carried out after the shutdown of the mining activities, as recommended by the mining code in France. The study highlighted the presence of 6 post-mining hazards: ground movements
<figure><img src="image_7.png" /><figcaption>Fig. 4. Interaction diagram showing the interaction of the flooding hazard with the mining (gray), natural (green), and technology (pink) hazards.</figcaption></figure>

(subsidence, sinkhole and landslide), combustion (self-heating) and induced seismicity. Furthermore, the area is exposed to 5 natural hazards: ground movements (sinkhole, settlement), wildfire, flooding, and natural seismicity. Table 3 presents the different identified hazards (mining and natural hazards) and their intensity level (initial levels before considering the potential interaction).

To apply the multi-hazard methodology developed in this paper, two approaches were adopted: the qualitative approach based on the construction of the interaction matrix and the quantitative approach

based on the calculation of the multi-hazard index (MH). The interaction matrix was constructed based on the assessment of each natural and post-mining hazard factor, incorporating collected data and analysing the interplay between mining-related and natural hazards (refer to Table 4). One can see from the interaction matrix's asymmetry that natural hazards, such as sinkholes and flooding, significantly influence post-mining hazards in the studied area (the natural and mining settlement have no impact on the other post-mining hazards). In fact, the occurrence of events such as flooding and natural seismicity can led to

<table><caption>**Table 3**
Qualitative and quantitative correspondence of the hazard level.</caption><thead><tr><th colspan="2">Initial hazard level</th><th colspan="2">level of interaction</th></tr><tr><th>Qualitative</th><th>Quantitative</th><th>Qualitative</th><th>Quantitative</th></tr></thead><tbody><tr><td>Low</td><td>1</td><td>Low</td><td>1</td></tr><tr><td>Medium</td><td>2</td><td>Medium</td><td>2</td></tr><tr><td>High</td><td>3</td><td>High</td><td>3</td></tr></tbody></table>
<table><caption>**Table 4**

Hazard level adjustment based on the initial hazard level and the interaction level between two hazards.</caption><thead><tr><th rowspan="3"></th><th rowspan="3"></th><th colspan="3">Adjusted hazard level</th></tr><tr><th colspan="3">Level of interaction</th></tr><tr><th>Low</th><th>Medium</th><th>High</th></tr></thead><tbody><tr><th rowspan="3">Initial level of hazard</th><th>Low</th><td>1</td><td>2</td><td>3</td></tr><tr><th>Medium</th><td>2</td><td>4</td><td>6</td></tr><tr><th>High</th><td>3</td><td>6</td><td>9</td></tr></tbody></table>

<table><caption>**Table 5**

Coal mine -intensity level (low = green, moderate = orange, severe and very severe = red) of the 6 mining hazards and 5 natural hazards. The number, 1, 2 and 3 correspond respectively to quantitative intensity level of the mining hazards.</caption><thead><tr><th colspan="2">Hazard</th><th>Low</th><th>Medium</th><th>Severe</th></tr></thead><tbody><tr><td rowspan="6">Mine hazards (6)</td><td>SIN</td><td>1</td><td>2</td><td></td></tr><tr><td>SUB</td><td>1</td><td>2</td><td></td></tr><tr><td>LSG</td><td>1</td><td></td><td></td></tr><tr><td>SET</td><td>1</td><td></td><td></td></tr><tr><td>COM</td><td>1</td><td>2</td><td></td></tr><tr><td>INS</td><td>1</td><td>2</td><td></td></tr><tr><td rowspan="5">Natural hazards (5)</td><td>SIN</td><td></td><td></td><td>3</td></tr><tr><td>SET</td><td></td><td>2</td><td>3</td></tr><tr><td>NSI</td><td></td><td>2</td><td></td></tr><tr><td>FLOD</td><td></td><td></td><td></td></tr><tr><td>FFI</td><td></td><td></td><td>3</td></tr></tbody></table><table><thead><tr><th colspan="2">Hazard</th><th>Code</th></tr></thead><tbody><tr><td colspan="2">Sinkhole</td><td>SIN</td></tr><tr><td colspan="2">Subsidence</td><td>SUB</td></tr><tr><td colspan="2">Deep landslide</td><td>LSG</td></tr><tr><td colspan="2">Settlement</td><td>SET</td></tr><tr><td colspan="2">Combustion</td><td>COM</td></tr><tr><td colspan="2">Induced seismicity</td><td>INS</td></tr><tr><td colspan="2">Sinkhole</td><td>SIN</td></tr><tr><td colspan="2">Settlement</td><td>SET</td></tr><tr><td colspan="2">Natural seismicity</td><td>NSI</td></tr><tr><td colspan="2">Flooding</td><td>FLOD</td></tr><tr><td colspan="2">Wildfire</td><td>FFI</td></tr></tbody></table>

catastrophic scenarios, significantly increasing the occurrence of post-mining ground movement hazards such as sinkholes, subsidence, and landslides. Further analysis will reveal additional post-mining scenarios. Based on the interaction matrix (Table 5), the total number of the interactions between mining and natural hazards is 35: low = 17, medium = 4 and high = 14. The majority (11) of high-level interactions concern natural hazards.

The effect of interactions can increase the initial level of the hazard by at least one (cf. Table 4). Therefore, four of the six mining hazards could be raised to the high hazard level. This assessment should be discussed and carefully analysed because it has significant social and economic impact on risk management (cost, life, industry, etc.).

The second part of the application of the methodology is the calculation of the multi-hazard index for first for the mine as a whole, and then for three sectors of the mine. We concentrated only on the interaction between mining hazards for the sake of simplification. The site is divided into three sectors S1, S2, and S3, and the first sector S1 has only one post-mining hazard (a sinkhole), while sectors S2 and S3 have six post-mining hazards with different levels (see Table 6).

The calculation proceeds according to the four steps presented in Section 3. The first step is the determination of the value of the adjusted hazard level as a function of the number and the level of interaction. For this case study, we identified 13 interactions between the mining hazards (see Table 6). Table 7 presents the initial hazard

level and value for the mine, and the number of interactions for the mining hazards. The second step is the adjustment of the initial hazard level, where the new level and the value of the adjusted mining hazard was calculated for two initial hazard levels: low and medium. The initial hazard level is updated: low becomes medium and medium becomes high. Table 7 presents the results where the adjusted hazard value runs from 0 to 12. This is very useful for recognising the significance of the interaction, and for classifying the hazards for management purposes.

The same steps were carried out for the three sectors of the mine. In addition, the multi-hazard index based using relation (3) was calculated. One can notice that sector one is characterised by a single hazard, and the initial level of this (sinkhole) hazard is unchanged. The value of the multi-hazard index for sectors two and three (S2 and S3), are respectively equal to 28 and 37. Both sectors have the same number of mining hazards (6), but the hazard levels are different. (Table 8)

## 5. Conclusions and recommendations

The paper presents an analysis of multi-hazard interactions and their assessment for closed and abandoned mines, emphasizing the economic and social benefits of multi-hazard risk assessment. A comprehensive methodology was developed, yielding qualitative (interaction matrix) and quantitative (multi-hazard index) tools, preceded by the compilation of potential post-mining, natural, and technological hazards. The qualitative assessment focuses on understanding the key characteristics of each hazard type, guided
<caption>**Table 6**</caption>

Multi-hazard interaction matrix and assessment of the level of the interaction This is given by colour: red = high, yellow = medium, green = low, and by the number of interactions.

<table><thead><tr><th rowspan="2">Source hazards</th><th rowspan="2"></th><th colspan="6">Mining hazards (Target)</th></tr><tr><th>SIN</th><th>SUB</th><th>LSG</th><th>SET</th><th>COM</th><th>INS</th></tr></thead><tbody><tr><td rowspan="5">Mine hazards (6)</td><td>SIN</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SUB</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>LSG</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SET</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>COM</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="5">Natural hazards (5)</td><td>INS</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SIN</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SET</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NSI</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FLOD</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>FFI</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<table><thead><tr><th>Hazard</th><th>Code</th></tr></thead><tbody><tr><td>Sinkhole</td><td>SIN</td></tr><tr><td>Subsidence</td><td>SUB</td></tr><tr><td>Deep landslide</td><td>LSG</td></tr><tr><td>Settlement</td><td>SET</td></tr><tr><td>Combustion</td><td>COM</td></tr><tr><td>Induced seismicity</td><td>INS</td></tr><tr><td>Sinkhole</td><td>SIN</td></tr><tr><td>Settlement</td><td>SET</td></tr><tr><td>Natural seismicity</td><td>NSI</td></tr><tr><td>Flooding</td><td>FLOD</td></tr><tr><td>Wildfire</td><td>FFI</td></tr></tbody></table>

<table><caption>**Table 7**</caption><thead><tr><th colspan="2">Interaction level</th></tr></thead><tbody><tr><td>No interaction</td><td>31</td></tr><tr><td>Low</td><td>17</td></tr><tr><td>Medium</td><td>4</td></tr><tr><td>High</td><td>14</td></tr></tbody></table>

Assessment of the interaction level and adjustment of the post-mining hazard.

<table><thead><tr><th rowspan="2">Mine hazards</th><th colspan="3">Initial hazard level</th><th colspan="3">Level and number of interactions</th><th colspan="4">Adjusted hazard</th></tr><tr><th>Low</th><th>Medium</th><th>Low</th><th>Medium</th><th>High</th><th>Medium</th><th>High</th><th>Medium</th><th>High</th></tr></thead><tbody><tr><td>SIN</td><td></td><td></td><td>3</td><td>0</td><td>0</td><td></td><td></td><td>3</td><td>6</td></tr><tr><td>SUB</td><td></td><td></td><td>1</td><td>1</td><td>1</td><td></td><td></td><td>6</td><td>12</td></tr><tr><td>LSG</td><td></td><td></td><td>0</td><td>0</td><td>1</td><td></td><td></td><td>3</td><td>0</td></tr><tr><td>SET</td><td></td><td></td><td>0</td><td>0</td><td>0</td><td></td><td></td><td>0</td><td>0</td></tr><tr><td>COM</td><td></td><td></td><td>2</td><td>2</td><td>0</td><td></td><td></td><td>3</td><td>6</td></tr><tr><td>INS</td><td></td><td></td><td>2</td><td>0</td><td>1</td><td></td><td></td><td>0</td><td>10</td></tr></tbody></table>
<table><caption>**Table 8**
Assessment of the interaction level, adjustment of the post-mining hazard, and calculation of the multi-hazard index (MH) for the three sectors S1, S2, and S3 of the mine.</caption><thead><tr><th rowspan="2">Mine hazard</th><th colspan="3">Initial hazard level</th><th colspan="3">Level and number of interactions</th><th colspan="3">Adjusted hazard level</th><th colspan="3">Adjusted hazard value</th></tr><tr><th>S1</th><th>S2</th><th>S3</th><th>Low</th><th>Medium</th><th>High</th><th>S1</th><th>S2</th><th>S3</th><th>S1</th><th>S2</th><th>S3</th></tr></thead><tbody><tr><td>SIN</td><td></td><td></td><td></td><td>3</td><td>0</td><td>0</td><td></td><td></td><td></td><td>3</td><td>3</td><td>6</td></tr><tr><td>SUB</td><td></td><td></td><td></td><td>1</td><td>1</td><td>1</td><td></td><td></td><td></td><td>6</td><td>12</td><td></td></tr><tr><td>LSG</td><td></td><td></td><td></td><td>0</td><td>0</td><td>0</td><td></td><td></td><td></td><td>3</td><td>0</td><td></td></tr><tr><td>SET</td><td></td><td></td><td></td><td>0</td><td>0</td><td>0</td><td></td><td></td><td></td><td>0</td><td>0</td><td></td></tr><tr><td>COM</td><td></td><td></td><td></td><td>2</td><td>2</td><td>0</td><td></td><td></td><td></td><td>3</td><td>6</td><td></td></tr><tr><td>INS</td><td></td><td></td><td></td><td>2</td><td>0</td><td>1</td><td></td><td></td><td></td><td>0</td><td>10</td><td></td></tr><tr><td colspan="12">Multi-hazard index (MHI)</td></tr><tr><td colspan="12">3 32 41</td></tr></tbody></table>

by expert judgment and real case studies. Completion of the qualitative assessment paves the way for the quantitative approach, which involves determining the number and severity of interactions and adjusting hazard levels accordingly. This leads to the creation of a multi-hazard index, applicable to single or multiple mining sites, facilitating the mapping of multi-hazard intensity interaction. The assessment aims to analyse interactions between hazards, including possible combinations and cascading effects using these two complementary tools. Overall, the results underscore the importance of considering hazard interactions, drawing on the analysis of individual hazards, expert input, and real-world case studies.

The multi-hazard assessment methodology was applied to a coal mine exposed to post-mining and natural hazards. The area is divided into three sectors having a different number and intensity of post-mining hazards. The multi-hazard assessment showed that it is advisable to upgrade the level of the post-mining hazard by at least one level (low to medium and medium to high) in order to be able to consider the hazard interactions. In addition, the calculation of the multi-hazard index allows the three sectors to be compared; the first sector shows little reason for carrying out a multi-hazard risk assessment, in contrast to the second and the third sectors.

Multi-hazard assessment provides a real advantage for mining regions. Policymakers and stakeholders should therefore bring together experts capable of assessing the interaction of hazards. The potential consequences of merely assessing each single hazard separately can lead to an increase in the costs of mitigating hazards, and in specific cases to a catastrophic scenario with severe social and economic consequences. The multi-hazard methodology devised for post-mining sites, along with the knowledge generated, should be further refined within the framework of research projects dedicated to the mining and post-mining sector. The expectation is that the multi-hazard analysis and multi-risk assessment would become regarded as the primary tools to be used when conducting case studies across Europe and beyond. Another indicator of success is therefore the uptake of these tools within wider projects, networks, and dialogues.

### CRediT authorship contribution statement

M. Al Heib: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation, Conceptualization. C. Franck: Writing – review & editing, Methodology.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors are thankful to Ineris experts from Ineris and thanks the

European Union's Research Fund for Coal and Steel (RFCS) for funding the European project "POMHAZ" grant agreement No 101057326.

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