Some big gas companies in Japan plan to develop a kind of "early warning and shut-off system" to minimize the number of suspended customers and to help early restoration of gas supply after an earthquake. Those systems, however, will be effective only if the estimate of damage is sufficiently accurate. If it is not accurate enough, the optional shut-off will cause even greater secondary disaster. The author has already proposed a method of estimating damage ratio for buried pipelines. This method is more reasonable than any other method in that this takes account of three fundamental factors in the quantitative manner, i.e., the deformability of a pipeline system, the ground conditions of the areas in which the pipeline systems are installed, and the intensity of earthquake ground motion. The very basic idea of the proposed method had been applied for the "Recommended Practice for Earthquake-Resistant Design of Medium- and Low Pressure Gas Pipelines' published by the Japan Gas Association (JGA). In this Design Practice, the earthquake-resistance of a pipeline system is evaluated on the basis of the "deformability" of the pipeline system, which is evaluated by using the "ground-block model", and in reference with the ground conditions that are classified on the basis of the nonuniformity of topographical structure. This paper discusses that the ground-block model is an almost rigorous expression of the nonuniform ground deformations. This also shows that the investigations of damage to gas pipelines, by JGA, during both the 1993 Kushiro-oki and the 1995 Hyogoken-nanbu earthquakes, proved clearly the effectiveness of the method used in the "Recommended Practice." This fact will also support the reliability of "deformability evaluation" based on the ground-block model.
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