Abstract: By reflection of high freguency electromagnetic pulse,tcracks of buildings as well as voids or settlement underground could be detected by ground penetrating radar. Jacking pipe construction in dam foundation may cause settlement. Non-destructive testing by ground penetrating radar was carried out for tube wall and dam body on the jacking pipes during construction. The result showed that the heterogeneous structure existed in the scope of 5～10 m under the surface and void existed between the pipeline and upper soil. Combined with the geological conditions, it was found that the groundwater recession and the destruction of balanced force caused the settlement but not the underground cavity.