# The Future of Abandoned Shallow Mines as a Function of Precipitation Under Changing Climate
## 3 Extraction Methods and Potential Geohazards  
The main extraction method used in underground shallow mines is the room and pillar method (Figure 2). This method is widely used in shallow mining operations (up to a few hundred meters below ground level). The principle of the method is to extract ore while leaving pillars in place to support the land above the deposit. Depending on the quality of the material, the thickness of the extraction, and the depth of the mining area, the dimension of the pillars left in place is more or less important. The height of the pillars can reach several tens of meters. The space cleared between the pillars is called the \"room\", hence, the name of the method. Figure 1: Density of the underground cavities (chalk, gypsum, limestone), certain zones present more than 20 cavities per town. When mining ends, this is referred to as \"mining by abandoned rooms and pillars\". If during or after mining, the rooms are backfilled, or when the pillars collapse, the land above the extraction zone presents a lower risk of collapse compared to when the rooms are abandoned, unless the extracted ore dissolves in water. The objective of mining is primarily to increase production and optimise operation, by enhancing the operating rate while maintaining \"acceptable\" safety and stability conditions in the short term. However, these conditions are not sustainable in the long term given the degradation processes of underground mines; consequently, all underground mines are destined to collapse, to varying degrees, over the long term. These collapses can be localised (such as sinkholes) or generalised, with varying degrees of impact on the surface. Increasing the safety factor, defined as the ratio of the strength to the pillar load, more than two is not enough to avoid the mine's collapse [5]. Ref. [6] presented 17 examples of massive collapses that occurred in various mining activities around the world, including salt, coal and limestone mining. The surface area of these collapses ranges from 0.1 hectares to approximately 300 hectares. These events have caused injuries and destroyed houses, such as the collapse in Clamart (Hauts-de-Seine) in 1961, which resulted in 21 deaths and 50 injuries. In Europe, several countries have experienced ground movements linked to underground cavities, for example, Belgium, Great Britain, Germany, the Netherlands, Italy, Spain, Poland, etc. In France, several dramatic collapses have occurred, for example, the Clamart collapse (1961) causing 21 deaths and 36 injuries, the Chateau-Landon collapse causing 7 deaths (1910), and the Hautil collapse (1991). While the loss of life is relatively low compared to other geohazards, such as massive landslides, the consequences for infrastructure can be significant. The stability of an underground mine depends on the pillar stress and the strength of the extracted material. The average vertical stress of the pillar (\(\sigma_{p}\)) can be calculated using the tributary (Figure 3), considering the depth of the mine (\(H\)) and the extraction ratio (\(t\)). For complex configurations and abandoned mines, the average vertical stress pillar is a function of several parameters, as expressed by the following equation:  
\[\sigma_{p}=f(\gamma,\tau,F,H,G,T) \tag{1}\]  
where  
\(\gamma\): the density of the overburden. \(\tau\): the extraction ratio, which depends on the geometry of the rooms and the pillars. \(F\): the fault impact on the stress distribution. \(G\): the configuration impact (slope, valley, etc.) on the stress distribution. \(T\): the time and ageing impact. Figure 2: Room and pillar method for shallow underground mines—example from Paris basin, limestone room and pillar mine. Water influences the behaviour of faults and discontinuities, as well as the short- and long-term behaviour of rock mass. It can reduce the strength and the effective stresses.