The Red River Delta (RRD), considered as the largest center of economics, culture and agriculture of Viet Nam, has been developing rapidly. Annually, the amounts of goods, agricultural and industrial products are being produced more and more, which need to be distributed to the other parts of the country and inside the RRD. This study deals with a geotechnical-geophysical study of the clay of the RRD, especially characterization of the subsoil profiles along a number of national highways for the maitain and improve the highway network in the RRD. A very big amount of geotechnical data (laboratory and field test) were collected in this study in order to properly manage, visualize and characterize the subsoil al. One of significant outputs of this study is to perform successfully a 1-km long survey line of electric imaging with three kinds of electrode configurations from Km 165.5 to Km166.5 along the National Highway 21. This geophysical study has certainly value-added the investigation of the highway by giving a continuous imaging of the subsoil between two boreholes and indicating the pocket of weak soil in it.