**Empirical Analysis and Geomechanical Modelling of an Underground Water Reservoir for Hydroelectric Power Plants**

**Javier Menéndez 1,\* , Falko Schmidt <sup>2</sup> , Heinz Konietzky <sup>3</sup> , Antonio Bernardo Sánchez <sup>4</sup> and Jorge Loredo <sup>5</sup>**


Received: 28 July 2020; Accepted: 20 August 2020; Published: 24 August 2020

**Abstract:** The European Union policy of encouraging renewable energy sources and a sustainable and safe low-carbon economy requires flexible energy storage systems (FESSs), such as pumped-storage hydropower (PSH) systems. Energy storage systems are the key to facilitate a high penetration of the renewable energy sources in the electrical grids. Disused mining structures in closed underground coal mines in NW Spain have been selected as a case study to analyze the construction of underground pumped-storage hydropower (UPSH) plants. Mine water, depth and subsurface space in closured coal mines may be used for the construction of FESSs with reduced environmental impacts. This paper analyzes the stability of a network of tunnels used as a lower water reservoir at 450 m depth in sandstone and shale formations. Empirical methods based on rock mass classification systems are employed to preliminarily design the support systems and to determinate the rock mass properties. In addition, 3D numerical modelling has been conducted in order to verify the stability of the underground excavations. The deformations and thickness of the excavation damage zones (EDZs) around the excavations have been evaluated in the simulations without considering a support system and considering systematic grouted rock bolts and a layer of reinforced shotcrete as support system. The results obtained show that the excavation of the network of tunnels is technically feasible with the support system that has been designed.

**Keywords:** mining structures; underground reservoir; empirical analysis; numerical modelling; energy storage; hydropower plants
