Landslides harm roads and structures in many parts of Algeria; especially in Tarzoust city; where spectacular slope failures occurs since spring 2004, causing serious disasters in the region. Our methodology uses the Vertical Electric Soundings (VES) and the Seismic Refraction Method (SRM) for the underground prospection, and the Electrical Resistivity Tomography (ERT) to support the last two methods. The result confirms the clayey nature of the terrain very often covered by a mantle of superficial colluvium formations. The depth of the bed rock and shear surface were precisely determined. ERT reveals that the terrain has already undergone instabilities in the past. Our approach proves that the combination of the geological and geotechnical data with geophysical deterministic methods can helps engineers and decision-makers for land management.