## 1 Introduction  
In the current evolving energy context, characterized by an increasing of variable renewable energy (VRE) in the electricity mix, the development of flexible energy storage systems (FESSs), such as pumped storage hydropower (PSH) or compressed air energy storage (CAES) plants are required. PSH is the most mature technology to provide ancillary services to the electrical grid [1]. PSH systems accounted for 150 GW worldwide in 2016 (40 GW in the European Union) [2] and the capacity could be 325 GW in 2030 [3]. Underground pumped storage hydropower (UPSH) is an alternative to store large amounts of electrical energy with low environmental impacts [4; 5]. Other energy storage systems such as Li-ion batteries are more efficient, but more expensive to install [6; 7]. Geomechanical studies of the underground infrastructure are required to assess the technical feasibility of subsurface energy storage plants. [PERSON] et al. [8] carried out a stability analysis of the underground infrastructure of UPSH plants in closed mines. The stability of the powerhouse cavern and the effect of air pressure on the excavations (tunnels and air shafts) during the operation time were analyzed. [PERSON] and [PERSON] [9] studied the behavior of a limestone mine as UPSH plant, at 671 m depth with a volume of 9.6 million m\({}^{3}\). [PERSON] et al. [10] presented the opportunities for large-scale energy storage in geological formations in mainland Portugal. UPSH, CAES and gas storage systems (hydrogen and methane) were analyzed. [PERSON] et al. [11] analyzed compressed air storage caverns in rock salt considering thermo-mechanical cyclic loading. The study concludes that the stability is affected by the operating pressure (10 MPa) and the increased creep rate accelerates the volume convergence. [PERSON] et al. [12] applied empirical analysis and numerical methods to provide a support design for an underground water-sealed oil storage cavern. [PERSON] et al. [13] developed a numerical model to analyze the stability of a small-spacing two-well salt cavern used as gas storage. [PERSON] et al. [14] investigated the thermodynamic and geomechanical performance of CAES systems in concrete-lined rock caverns. [PERSON] et al. [15] developed an equation for prediction of displacement at the key point on the high sidewalls of powerhouse caverns, considering four basic factors (the rock deformation modulus, the overburden depth, the height of the powerhouse and the lateral pressure coefficient of the initial stress). [PERSON] [16] suggested the idea to use a closed underground mine as a lower reservoir and develop an UPSH plant in 1960. At the end of the 1960s, Swedish engineers proposed the exploitation of a surface upper reservoir and the construction of a new lower water reservoir in an underground rock cavern [17]. [PERSON] suggested an optimistic future for the development of UPSH plants [18]. The Mount Hope project, located in northern New Jersey (USA) was proposed in 1975 [19]. It intended to use the facilities of a closed iron mine as a lower water reservoir, but it was never developed. In 1978, an UPSH project was presented with a lower reservoir formed by a network of \(15\times 25\) m elliptical tunnels, at a depth of 1000 m [20]. During the 1980s, a project to install an UPSH plant was proposed in the Netherlands [21], but the project was finally not developed due to the poor quality of the rock mass. [PERSON] assessed the possibility of constructing UPSH plants in the Bukit Timah granite of Singapore [4]. Coal mining structures in closed underground coal mines in the Asturian Central Coal Basin (ACCB), NW Spain, have been proposed as a lower water reservoir of UPSH plants [22; 23]. Recently, several studies have been also carried out in Germany to assess the possibilities to develop UPSH plants on closed underground coal mines in the Harz and Ruhr regions [24; 25; 26]. [PERSON] et al. [27] and [PERSON] et al. [28] proposed a closed slate mine in Belgium as a lower reservoir of an UPSH plant.