A 30 km portion of a mountain road that lies along Al-Baha descent suffers from frequent rock failures, mainly in rainy seasons. Failures in both natural and man-made rock slopes are formed mainly of wedge, planar, and toppling failures, and rockfalls. Slope stability analyses were performed to identify the stability/instability zones. Probability analyses of casualties along the road have been performed using the rock slopes factor of safety and probability of failure analyses. Areas of high probability of failure have high probabilities of casualties. The probability of a vehicle running into a landslide is calculated at rush hours. The results indicate that an increase of support measures will decrease the likelihood of failure and casualties in the areas that are prone to failure. The highest risk sections along the road are those characterized by the lowest factor of safety, and lowest support measures, particularly at rush hours on rainy days.
Landslides occur frequently on mountain roads, in rugged terrains. One of the most difficult terrains in western Saudi Arabia is Al-Baha descent, which is characterized by sharp cliffs and elevations that reach above 2000 m a.s.l. The descent lies between longitudes 41º25'E and 41º29'E and latitudes 19º47'N and 20º01'N (Fig. 1). The Al-Baha escarpment 50 km-road starts southwest of Al-Baha city, and runs through Al-Baha descent. The road connects the highlands where Al-Baha city is located with the lowlands at Al-Mukhwah town. Along this distance, natural slopes in addition to many man-made slopes and engineering structures are located. Al-Baha descent includes three distinct geomorphologic terrains: a dissected upper plateau of low mountains and hills, a precipitous escarpment, and a low-lying coastal plain (Fig. 2). The most prominent of these features is the northwesterly trending Asir or Tihama escarpment, a structure that is traceable for some 1500 km.