Ijmuiden breakwaters: selection of renovation options and scale model investigations for a new top layer of concrete blocks on the impermeable core

摘要

The Ijmuiden breakwaters need significant and increasing maintenance every year. Especially the top layer of concrete cubes suffers severe damage. These cubes, placed directly on a 2 m thick watertight stone asphalt layer, are not stable.Now, 50 years after construction, also the old cubes and asphalt are seriously degenerated. Just after construction it became clear that the original stone asphalt top layer was not stable due to high water pressures in the core, propagating through the open toe construction below the asphalt. The concrete cubes, placed on top, should give contra weight. It is clear that renovation is necessary in the near future. The Civil Engineering Division of the Dutch Ministry of Transport, Public Works and Water Management has completed a study on the possibilities of renovation of the breakwaters. Many different solutions have been compared: from complete reconstruction to the creation of openings in the impermeable asphalt layer. The most feasible solution seems to be the replacement of the concrete blocks by heavier blocks, eventually in combination with a fill layer of rock on the impermeable asphalt. It is expected that a fill layer increases the stability of the new blocks and enables accurate placing of the blocks despite the unequal surface of the asphalt layer. To investigate the influence of the impermeable asphalt layer on the stability of the new concrete blocks, different model tests have been executed with one or two layers of blocks, with or without a fill layer. Also the placing method is varied and the thickness of the fill layer has been changed. The paper presents the comparison and selection of different renovation options and the results of exploratory model tests compared with the actual design formulas.