## 3 Results of Literature Review and Economic Assessment  
### Novel Use of Existing LRC Cavern for UIPHES  
#### 3.1.1 History and Characteristics of LRC Caverns  
Research and development on LRC cavern started in Sweden in the 1990's for the storage of natural gas under shallow depth and under high pressure till 28 MPa (including some tests at 52 MPa) [14]. The main distinctive features of LRC are the following: - the pressure of the gas is opposed by the surrounding rock mass, - a liner of steel or polymer is taking care of absolute tightness, - a low ratio of cavern diameter to the thickness of rock-cover protects the cavern against the risk of ground lift-up thanks to the built-up of a near-cone induced by the coefficient of friction of the rock material, as illustrated by two of the three models of Figure 1. [PERSON] et al. have extensively simulated from Sweden trials the conditions of this lift-up and performed sensitivity analyses on 20 variable factors [17]. From their work, Figure 2 shows an assessment of the safety factor against lift-up depending on normalized storage pressure and on normalized storage depth. [PERSON] et al. studied several design issues of LRCs in 2015, for their possible application to CAES, including ground uplift, buckling of liner, cycling and fatigue and concrete plug [18]. To evaluate the integrity of cavern sides, in 2016, [PERSON] et al. performed a study of fracture patterns of rock mass around a pressurized storage cavern based on the element-free Galerkin (EFG) method with a cohesive crack model. Quite logically they mention that a value higher than 1.2 is preferred for the local stress coefficient K0, equal to the pressure horizontally borne by the rock mass divided by the vertical pressure induced by gravity [19]. Fortunately, for shallow grounds below 200 m depth, the condition of medium-high K0 (i.e., value of from 0.8-1.5) is met for a high share of rock masses on Earth. The Swedish program, accomplished with the partnership of the French company Gaz de France (nowadays named ENGIE) started in1988 by a pilot test of three shaft-shaped caverns of 4.4 m diameter and of 15 m height at a shallow depth of only 50 m in the Grangesberg granite mountain. Tested liners were steel, thinner stainless steel and polymer sheets. Later, a larger evaluation and commercial operation program was realized after 1999 at the Skallen mountain, using four large shaft-shaped caverns 36-meters in diameter, 50 m in height, 40,000 m\({}^{3}\) volume, at a depth of 115 m, with a large cupola and a curved form bottom [14]. One illustration is shown on Figure 3. The host rock in the area is crystalline gneiss of good quality, intersected by amphibite dikes of poor quality. After a test period of one and a half years, the facility has been in commercial operation as a part of the Swedish gas grid since early 2004 [20]. Despite tensile fracturing occurring in the unreinforced concrete layer placed between rock mass and liner, these caverns remain operational until this moment. Figure 1: Three models for action of ground against lift-up due to gravity. W is weight, alpha is the angle of friction of the rock mass, usually 30–45 degrees [19]. One LRC cavern was developed at Hokkaido (Japan) in 1990 using a decommissioned coalmine at a 450 m depth. Its function was the storage of compressed air for a pilot CAES at 8 MPa. This LRC takes the shape of a 57 m long tunnel, 6 m diameter. The liner is composed of butyl synthetic rubber inside a thin shell made of prefabricated concrete blocks, using more synthetic rubber at the liaison of blocks [15]. Figure 3: Vertical plane section of the Skallen gas storage plant, showing one of the 4 caverns [19]. Figure 2: Computerized value of the safety factor against lift-up related to normalized storage pressure (dimensionless value of gas pressure in cavern divided by rock weight pressure at cavern ceiling) and related to normalized storage depth (dimensionless value of cavern ceiling depth divided by cavern inner radius) [17].