Namikata underground LPG storage site, one of the world's largest hydraulic containment type underground LPG storage site is composed by a dual-purpose cavern for butane/propane storage and two connected propane caverns to provide 450,000 tons stockpile capacity. The Namikata site has passed the acceptance test and started the operation in March, 2013.

Since the air-tightness of hydraulic containment type underground LPG storage cavern is ensured by the stable groundwater filed from cavern excavation throughout the operation phase, therefore, in order to maintain the groundwater filed and prevent the extension of unsaturated zone during cavern excavation, the authors have designed water curtain tunnel and horizontal/vertical water curtain boreholes with 10m spacing to enclose the storage caverns, these boreholes are perpendicularly intersected with the primary fractures orientation for better water curtain efficiency.

For the evaluation on hydraulic behaviors and water curtain efficiency during cavern excavation, the authors have integrated the derived permeability in investigation and water curtain boreholes to establish the 3D groundwater simulation model and upgraded it with progressive excavation for more accurate estimation. The estimated pore-pressure and seepage were compared with measured values in cavern excavation for evaluation. For the low water curtain efficiency location, the authors designed additional water curtain boreholes to ensure the sufficient pore pressure in the cavern vicinity. On the other hand, grouting works were applied on the evaluated high permeable zone as countermeasure. After the construction of storage caverns were completed, the water curtain tunnel and water curtain boreholes were submerged and compressed to recover the decreased groundwater field in cavern excavation. Then, the access tunnel was also submerged and the storage caverns were examined the air-tightness designated by High Pressure Gas Safety Act. As the results, the pore-pressure and groundwater field were maintained as estimation throughout the cavern compression till the air-tightness test, consequently, the water curtain system has been verified its efficiency.

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