| In Hokkaido、 tunnel constructors often encounter rocks with toxic elements of natural origin like arsenic or lead. They measure them after the judgment criteria for toxic elements in soils by human activities as a makeshift. However、 the makeshift is sometimes thought to result in the increase of tunnel construction cost by excessive countermeasure. As a case study、 authors analyzed the relation between estimation result of arsenic and countermeasure of a tunnel constructed in hydrothermally altered area in Hokkaido. The tunnel has 1、300m in length and was opened in 2007. By the preliminary survey、 most of the tunnel ground was estimated to leachate over-the-criteria arsenic. Therefore all the rock muck from the tunnel was planned to be measured after the criteria (enclosed in double rubber sheet under filled up road). While tunnel construction、 contaminant content and leachate concentration of arsenic was checked almost every 10 meters of pilot boring cores、 and over-the-criteria arsenic was confirmed in many samples. Therefore、 the countermeasure was carried out according to the first plan. The authors examined rock types along tunnel formation and correlated them to arsenic leachate concentration. Rock of the tunnel formation consist of Ltf (Lapilli tuff)、 Vtf1 (Welded lapilli tuff)、 Vtf2 (Welded lapilli tuff)、 Alt (Alteration of sandstone、 mudstone and tuff)、 Tf1 (Tuff to lapilli tuff)、 Tf2 (Tuff to lapilli tuff). We confirmed that average arsenic leachate concentration of Tf1、 Tf2 and Ltf was under the criteria. That means if we could obtain such result previously by detailed geological estimation、 the constructor did not need to measure rock muck from Tf1、 Tf2 and Ltf (48% in total tunnel length). |
