To investigate the effect of the impurities

such as gravels and voids

in backfilled soil on the performance of HVDC common grounding electrode

a model of grounding current field is established based on the finite element analysis software COMSOL to analyze the effects of the impurities’ radius

buried depth and position on distribution of grounding electrode electric field. The results show that:（1） when the radii of the gravel and void are close to the grounding-electrode radius

the maximum electric field intensity on the surface of coke gets to the minimum;（2） when the relative buried depth of the impurities is about 30% and the impurities are in the position of a quarter of the section above the coke

the electric field intensity on the coke surface get its maximum;（3）all the electric field intensities mentioned above exceed the control value

which would speed up the corrosion rate of coke and reduce the service life of grounding electrode. Therefore

the construction of the grounding electrode must strictly accord with the construction standard

that is

the backfilled soil should be tamped compactly with fine and well-distributed particles.