Historical structures without detailed information about inner material and architecture represent a special challenge for condition monitoring with non-destructive testing methods. The measurements presented here were carried out on a retaining wall of a former coal mine in Germany. The 9 m high and 286 m long wall shows different cover materials like concrete, masonry or quarry stone. Information about reconstruction and extension steps is not available. Three vertical lines along the wall were selected in representative parts to evaluate the usefulness of non-destructive testing methods. Therefore, the retaining wall has been investigated using Ground Penetrating Radar (GPR) with two low frequency antennas (200 and 400 MHz) and Large Aperture UltraSound (LAUS) to gather information about the condition and inner structure of the wall. In addition, infrared thermography is planned for further information about the wall condition. The LAUS results showed the layer structure at one line where the wall was enforced by a concrete shell and were unspecific regarding the inner structure beyond the first layer. GPR results could be collected much faster and showed some internal features. Penetration was limited to 2-3 m due to the high attenuation in the material. The discussed measurements and results reflect the high requirements for NDT of such objects. Furthermore, the area of effective measurement differs for each method and also the time and effort required by the respective method.

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