Cathodic protection has been applied to the immersed zone and buried zone for the port steel structures in Japan. However, the protective current flowed in the buried zone is not clearly understood because the difficulty of the measurement. Therefore, a constant value is used in the design protective current density at every depth of the steel. In this study, to improve the accuracy of the design protective current density, the cathodic protection characteristics of the buried zone was investigated by monitoring the current density and the potential of the steel of the buried zone
in a single pipe pile of very long Haneda D runway contact taxiway. Conclusions are as follows:
(1) In the seabed soil, as the depth from the seabed was deeper, the period until the 　　　potential of steel reach the protective potential was longer. However, the effect is
　fully obtained at the every depth of pile by a cathodic protection.
(2) The current density flowed into the seabed soil was larger than the design current
　density in the vicinity of the seafloor at initial. Then, it tended to decrease because of
　environmental improvement of the steel surface by the occurrence of the electro
　-coating. On the other hand, at the location which was deeper than -10m from the
　seabed, it was very small current density compared to the designed current density.
(3) Regarding a cathodic protection mechanism in the seabed soil, the cathodic
　protection was considered to be achieved, even if the supplying protective current
　was very small in the seabed, due to various effects such as the decrease of dissolved
　oxygen concentration, the restraint of anode reaction and the decrease of the
　equilibrium potential by the increase of pH.
(4) The supply of protective current was affected by the soil resistivity based on the
　ground survey. In addition, we proposed a design method of cathodic protection
　considering the soil resistivity, based on the potential and current density distribution
　analysis using the finite element method.