H/V ambient noise for a rapid assessement of an unstable zone geometry : road slide case in Lakhdaria (Algeria)
Résumé
Following torrential rainfall that struck the area of Lakhdaria during the winter of 2006, a slide road slope carried away half of the road on forty meters length, leaving a five meters high scarp. The remaining part of the road and houses located upstream were threatened by a potential second slip as showed by the cracks and the déformations observed around the scarp. Ambient vibration recordings were carried out 3 days after this event, with the aim of circumscribing the unstable zone around the scarp, characterized by a strongly disturbed soil. The results of this study showed that in the investigated area, H/V curves exhibit a frequency peak around 6 Hz behind the edge of the scarp and another one around 2 Hz, along the road, on both sides of the scarp. The first peak is related to the disturbed and unstable section of the soil, which is about 6m thick (slip surface depth), in good agreement with field observations and confirmed by geotechnical investigations. The second peak is related to a deeper geological interface. Upstream of the road, where there are some houses, H/V curves are flat, indicating the absence of the strongly disturbed soil section. These results allowed us to rapidly evaluate the lateral extension and thickness of the unstable zone. This study shows the reliability of H/V ambient vibration method to characterize the geometry of "fresh" sliding zones or unstable disturbed soil masses.