Protection of the 400 kV OHL support foundation structure against a disaster caused by the Dunajec River meandering

摘要

The paper discusses the protection of the 400 kV OHL support foundation structure against a potentialdisaster caused by a sudden change of River’s stream direction.First part of the article refers to the meandering of Dunajec River, the second part refers to themeandering of Wisłok River.Part OneDuring the flood in June 2010, the Dunajec River meandered towards the existing support position,shifting its stream channel by several dozen meters. Formed 8-10 meters away from the supportposition was an escarpment about 6 meters high as the bank of the Dunajec River bed. The escarpmentis continuously eroded by the strong current along its outer meander. There is an extremely high riskof the escarpment being washed out by the river, which would pose an immediate threat to the safetyof the 400 kV power line whose support position may find itself in the very river current. This wouldundoubtedly result in the destruction of a section of the line with the approximate length of about1,690 meters, comprising three suspension supports, and additionally in the risk of damage to twotension supports at the ends of that section. In the course of works aimed at identifying the methods toprotect the power line against the potential breakdown that would result in considerable material losses(removal of the debris, reconstruction of the line, damages) and in power cuts during thereconstruction, a number of potential solutions were considered:a) River bank protection,b) Reconstruction of the line along the present or a modified route,c) “Reinforcement” of the support foundation.Following consultations with the Investor, option c) was chosen which guaranteed the relativelylowest cost and shortest duration of the works combined with a short shut-down period.The design involved protection of the support position by means of encasing the existing supportfoundation with a cuboid of cavity walls.The cavity walls were designed as 80 cm thick barrettes set at the depth of about 15 meters. Early inthe spring of 2011, execution of the foundation reinforcement started according to the agreed andapproved design. The whole of the works took 10 weeks. During the construction works, the line wasshut down over a period of 5 weeks only for the execution of the underground cavity walls.Part Two:When building 400 kV OHL Krosno-Tarnów (2003-2005) at the stage of designing the line it occurredthat Station no 189 is situated very close (around 30m) to the bank of the Wisłok River. The WisłokRiver is meandering in the neighbourhood of the tower foundations. The local government and theowner of the river decided that at the stage of designing, the foundation of the tower should beprotected before being destroyed by possible changes of the course of the river. The foundation wasdesigned as the group of 4 Franki piles foundations (per one leg) and additionally protected by steelwalls made of special steel braces to the 8m depth. The building permission was given and line wasbuilt.