Stability Analysis of Rock Slopes and Rock Falls, Al Hada Descent, At Ta’if Area, Kingdom of Saudi Arabia

SGS-TR-2003-1 Hasan Basahel, Ghassan S. Al-Sulaimani, Mohammed M. Al-Katheeri, and Sultan B. Al-Bahloul
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ABSTRACT

The Al-Hada road descent west of the city of At Taif, is a major connecting highway in the western part of the Kingdom of Saudi Arabia.
It is one of a series of descents connecting towns close to the edge of the Kingdom’s western escarpment, with the coastal plain below.
Al Hada, the highest point of the descent, is 2000 m above sea level.
The Al Hada Descent connects Makkah al Mukarramah (approximately 600m asl in the vicinity of ‘Arafat) and the port and city of Jeddah, on the Red Sea coast, with At Taif city and then on to cities farther south along the escarpment.
This 22-km long stretch of highway is the oldest paved descent in the Kingdom, having been constructed by the Bin Laden Company during the 1960s.
Since that time, the road has been susceptible to frequent slope failures; rock falls and accidents occur to traffic passing along the highway, particularly during and following rain storms, which cause periodic closures of the road.
Several studies were done which examined the road slopes to determine the critical failure locations and to suggest adequate solutions, but these studies used conventional techniques, and ignored the water factor, which is very important in the stability analysis of rock slopes.
In this study, geomechanical methods and newly developed software packages were used.
The analyses took into consideration the effect of water to predict the worst potential failure conditions and to suggest remedial measures.
A stability analysis of the rock slopes was made using Rock-Mass Rating (RMR) and Slope-Mass Rating (SMR) procedures, and DIPS software to determine possible slope failures and the types of failure.
The results of this analysis pin point critical sites that may warrant further investigation.
When high potential failure is predicted, the surface with the minimum safety factor is identified, and its safety factor is determined under dry and saturation conditions using numerical modeling with SLIDE software.
The rockfall hazard is analyzed using the Rockfall Hazard Rating System (RHRS) procedure and ROCFALL software.
The results indicate that many parts in the middle section of the descent are unstable and require systematic or spot supports such as anchors, wire nets, and shotcrete.
The results of the RHRS indicate the presence of around 20 locations along the road that are considered dangerous, including three deemed to be high risk, and in need of immediate action.
As an optimal solution, it is suggested that the middle section of the road be rebuilt using the same bench-blasting and pre-splitting techniques used to reconstruct the upper and lower sections of the road.

 

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Basahel, H., Al-Sulaimani, G.S., Al-Katheeri, M.M., and Al-Bahloul, S.B., 2004, Stability analysis of rock slopes and rock falls, Al Hada descent, At Ta’if area, Kingdom of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2003-1, 25 p., 14 figs., 16 tables, 6 apps, 5 plates.