**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. Menendez 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. Uddin and Asce [9] studied the behavior of a limestone mine as UPSH plant, at 671 m depth with a volume of 9.6 million m<sup>3</sup> . Carneiro 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. Khaledi 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. Liu et al. [12] applied empirical analysis and numerical methods to provide a support design for an underground water-sealed oil storage cavern. Chen et al. [13] developed a numerical model to analyze the stability of a small-spacing two-well salt cavern used as gas storage. Rutqvist et al. [14] investigated the thermodynamic and geomechanical performance of CAES systems in concrete-lined rock caverns. Zhu 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). Harza [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]. Sorensen 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 × 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. Wong 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 [24–26]. Pujades et al. [27] and Kitsikoudis et al. [28] proposed a closed slate mine in Belgium as a lower reservoir of an UPSH plant. The slate mine consists of nine large caverns with an available volume of 550,000 m<sup>3</sup> .

This paper analyzes the stability of a network of tunnels as a lower water reservoir at 450 m depth in sandstone and shale rock formations in closed mines. The rock mass was characterized according to the rock mass quality (Q-System) and the rock mass properties were estimated. In addition, 3D numerical analysis using FLAC 3D have been performed to verify the stability of the excavations. The maximum thickness of the excavation damage zone (EDZ) and the vertical and horizontal displacements when the support system is applied have been compared to the unsupported case in central and transversal tunnels. The axial force, bending moment and shear force in the shotcrete layer have also analyzed. The results obtained show that the excavation of the network of tunnels is technically feasible with the support system that has been designed.
