Underground cavern for the energy storage (oil and gas) has been constructed because it has a merit in the cost of maintenance and in the durability through the long-term service period. However, today, rational structures in the construction, the operation and also the maintenance are demanded. And in order to maintain the function and the performance that is set as the initial condition, preliminary study based on the result of examination considering the time dependent behavior of the structure is necessary. Here, the current state of the method of evaluating the cost and the performance of the underground structure are summarized and problems are pointed out based on the concept of the life cycle cost.
Now a lot of structures are constructed in the underground. However, it is said that if they have the same function and the performance, those construction cost is more expensive compared with that of constructed on the ground. In Japan, mountain method tunnels for the rail and the road have been constructed because of their restricted linear planning condition due to steep geographical features. Also a large-scale underground cavern for a pumping type power plant has been inevitably constructed from the condition demanded on its function. On the other hand, underground caverns for energy storage, such as gas and oil, have been planned and constructed because of the economic advantage. Namely the total cost during a long service period is not so high due to the long-term durability and maintenance free of the underground. For a long time, underground structures such as tunnels and caverns have been considered as eternal structures which do not need large-scale repair and reinforcement all over their service period. However that situation is changed by the accident where a part of the tunnel lining concrete fell down in piece in 1999. As a result, following reasonable facts were recognized again.
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The performance of the structure decreases with the time passage.
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The load considered in the design of the structure is to have the uncertainty, so it is possible that the load exceeds the predictions at the design stage during the long term service period and this may cause the damage or the collapse of the structure.
In other words, it became clear that the preliminary examination considering how to maintain an initial function and performance is necessary. Also the appropriate action at each stage of the design, construction and the maintenance is necessary even for the Fig.1 Cavern for energy storage underground structure that consists of concrete and rock. Then there is no merit in the underground structure? The purpose of this paper is to summarize a current problem concerning the evaluation of the safety and cost concerning the underground structure and to grope for new approach on them.
In Japan, a lot of infrastructures constructed at a period so-called high growth of economy after the 1960's begin to cause various troubles to their functions after about 50 years.
