Features of Geological and Geotechnical Risk Sources in Public Works Projects

Abstract

Most accidents in public works projects, such as the road collapse during the construction of the Fukuoka City Subway Nanakuma Line, are caused by uncertainties in geological and geotechnical features. The influence that “the geological and geotechnical uncertainty” has on a project is called a “geological and geotechnical risk”. It is necessary to grasp the sources of a geological and geotechnical risk in advance to manage the risk appropriately. However, there are only a few examples that have considered risk sources from this perspective. This report is an analysis of the documents of “the re-evaluation of the public works projects.” First, the authors classified the sources of increase in construction cost and allocated an amount of money for each source. Furthermore, the stage at which the cost increased was divided into either design or construction. When the sources were assumed beforehand, they were divided into estrangement of geological “distribution” or “property.” As a result, the increase in cost because of geological and geotechnical sources was approximately 2 trillion yen. The source that required the most money was “the geological feature that was worse than the assumption.” The analysis revealed that “the estrangement of the geological and geotechnical distribution” appears for a design stage, and “the estrangement of the geological and geotechnical property” appears for a construction stage. Because geological and geotechnical properties present certain uncertainties, it is believed that the width of the property estrangement is larger than that of the distribution estrangement.