False low water content in road field compaction control using nuclear gauges: a case study

During the construction of a highway in the north of Portugal, it was verified that in a few places, the compaction control of the road fill materials presented unexpected results with the surface moisture-density gauge (SMDG). The water content was lower than measured by oven drying, and the density and compaction grade were much higher than when using the sand cone. In an effort to understand the reasons for this unexpected offset, the road fill material was tested in situ with two different brands of SMDG, extra sand cone tests were performed and samples were collected at the test sites and tested in the laboratory to determine the water content by oven drying. The local geology, mineralogy, geochemistry and radiation level of the fill materials were studied. It was concluded that the incorrect results were caused by the presence of chemical elements in the road fill materials, mainly manganese, chlorine, cadmium and boron, which are able to capture the neutrons released by the SMDG, delivering a false low water content (FLoW). An index to evaluate the susceptibility of the soil to present a FLoW is proposed. Despite the reliability of the SMDG in most fill control, it must always be used with great care, after cross-checking its results with other direct control tests.