The Portuguese Triassic mudstones show frequently severe geotechnical problems, changing very rapidly from a soft rock to a soil, when decompressed and in the presence of water. They are an unusual soft rock, without expansive clays, extremely weatherable after wetting, due to the very fine equidimensional network of pores, developing high capillary stress. The water turns this soft rock into a muddy soil. The more problematic situations are related to foundations and slopes following intense rainfall.