Diagnosis system for the caisson breakwater applied FM-CW GPR

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  • ＦＭ‐ＣＷ地中レーダを応用したケーソン防波堤内部空洞診断システム
  • FM-CW地中レーダを応用したケーソン防波堤内部空洞診断システム
  • FM CW チチュウ レーダ オ オウヨウシタ ケーソン ボウハテイ ナイブ クウドウ シンダン システム

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Abstract

Breakwaters which defend ports from high waves are mainly constructed with box caissons in Japan. Caisson breakwaters made of reinforced concrete are often damaged by wave forces during a long period after the construction. The inner sand of the damaged caissons flows out. As a result, growth of the cavities in the caissons is accelerated. In order to detect the existence of the cavities, we first observe the condition of the breakwater surface and measure the elevation or the shape of the caissons. Next, we bore the breakwater from its surface and insert a waterproof camera into the holes to observe the cavities. This method takes much time and cost.<br> GPR (ground penetrating radar) is commonly applied to detect cavities in the underground or concrete constructions. However, the investigation depth of conventional impulse GPR system is shallow and they can detect the cavities only up to about 1m depth behind the concrete structures.<br> We have developed a diagnosis system which was equipped with a FM-CW (frequency modulation coutinuous wave) GPR, for investigating the existence and thickness of cavities inside caissons from the breakwater surface in a non-destructive manner. Furthermore we designed an automatic diagnosis program so that any engineer, regardless of the experience on GPR surveys, could operate the program and evaluate cavities in caissons immediately at the field site.<br> This automatic diagnosis program needs electrical properties (relative dielectric permittivity and electrical conductivity) of the concrete crown of the caissons. We measured electrical properties of concrete core samples at the same frequency range of the FM-CW GPR system.<br> Parameters needed for the automatic diagnosis program were decided by a numerical modeling of the electromagnetic wave propagation in breakwaters, to which the electrical property values obtained by the core sample measurements were applied. Diagnosis results are simply visualized and are stored in a built-in PC by the automatic diagnosis program.<br> In this paper, we introduce technical features of the diagnosis system and demonstrate exploration results at Niigata-higashi port and Kanazawa port.<br>

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