Reactivation of old landslides: lessons learned from a case-study in the Flemish Ardennes (Belgium)

The number of human-induced landslides is increasing worldwide, but information on the impact of human intervention on slope stability is often lacking. Therefore, this study analyses the Hekkebrugstraat landslide, the best-recorded landslide in the Flemish Ardennes (Belgium). Information obtained from local inhabitants, aerial photographs and newspaper articles enabled a 50-year reconstruction of both the landslide history and the land-use changes at or close to the landslide site. The reconstruction suggests that anthropogenic preliminary factors such as: (i) the absence of well-maintained drainage ditches in the affected area; (ii) the elevation of the surface of the road, i.e. a sunken lane, in the affected area; (iii) increased surface runoff from the drainage area; (iv) the creation of ponds; and (v) the removal of the lateral support at the landslide foot have played an important role in the reactivation of the Hekkebrugstraat landslide. After the reactivation of February 1995, landslide movement was observed for more than 5 years and caused damage to houses, and other infrastructure. However, also natural factors, such as the presence of an impermeable clay layer at limited depth, springs and relatively steep slopes (i.e. 0.14 m m(-1)), and above normal antecedent rainfall have contributed to the reactivations. Comparison of our reconstruction of the reactivation with precise Digital Terrain Models (DTMs) of 1952, 1973 and 1996, produced by digital stereophotogrammetry, indicated that the reported movements correspond well with the uplifted and collapsed zones found on the DTMs. Hence, this analysis provides valuable information for land-use planners in areas with old, apparently stable, landslides.