Constructing mountain roads in harsh terrains is a challenge to the engineering and development of southwestern Saudi Arabia. Al-Baha 32 km-mountain road is constructed at steep slope rock masses, where many rockfalls incidents were experienced. A 100 m long portion of the road, suffers from frequent rockfalls. The rock masses are mainly igneous rocks of medium to low quality. The rock slope-cut has no bench for 20 m, 70°–90° inclination, and 30 m height. Absence of ditches and support systems aggravates the daily road conditions. The RocFall computer program was utilized to perform the risk analysis on such steep man-made and natural rock. Geotechnical parameters such as

  1. block size,

  2. seeder points of blocks falls,

  3. slope angle,

  4. restitution coefficients, and

  5. slope roughness were used to identify properties of the potential and fallen rock blocks such as:

    1. bounce height,

    2. kinetic energies (total, translational, and rotational), and

    3. translational and rotational velocities.

The program modeled the

  1. proposed location and energy of the rock blocks barrier's heights and locations, collectors, and

  2. proposed slope safer redesign. The results of the study provide a solution to prevent rockfalls, and could be applied at similar conditions.


Slope failures, landslides and rockfalls are frequently occurred on mountain roads, at rugged terrains. One of the most difficult terrains in western Saudi Arabia is Al-Baha descent, which is characterized by sharp cliffs and high elevations which reaches above 2,000 m above sea level. The highest elevation at the descent is about 2,200 m (at km 0) above sea level, where the difference in elevation from the highest to lowest elevation is about 1,750 m, the difference is about 450 m at km 32. The descent lies between longitudes 41°25"E and 41°29"E and latitudes 19°47"N and 20°01"N; see Fig. 1. Al-Baha escarpment 50km-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 slope cuts 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 geologic history indicates that the escarpment resulted from a 3000 m lowering of Tihama Coastal Plain during the Tertiary opening of the Red Sea. Al-Baha descent's road is contained almost within a structurally controlled narrow valley, called Wadi Rash. Steep tributaries start from the escarpment ridge, and radiate out from the wadi head and cut deeply into the valley sides and escarpment face. Al-Baha region is underlain predominantly by a north-south trending belt of Precambrian schistose rocks comprised predominantly of fine grained mafic to intermediate extrusives, intrusives and pyroclastics with minor intercalations from clastics and metasediments [1]. The majority of the rocks at the descent area have been subjected to greenschist and amphibolite facies metamorphism [2].

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