Rock Slope Instability Analysis Along Jabal Fayfa Main Roads, Jazan Area, Kingdom of Saudi Arabia

SGS-TR-2009-1 Waheed S. Baamer, Ahmed M. Youssef, Yaser A. Zabramawi, Mohammed M. Al-Katheeri, Mohammed F. Baamer, Hasan M. Basahel, and Hussam J. Al-Ahmadi
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ABSTRACT


The study area is located in the middle of the western part of the Jabal Fayfa quadrangle in Jazan geographic province of southwestern Arabian Shield, Kingdom of Saudi Arabia.  It is located between lat 17°11’09.38”N and 17°18’00.20”N. and long 43°01’47.34”E and 43°09’56.64”E. The Jabal Fayfa area covers about 181 Km².  The area is dissected by three main roads including Road Twenty, Eight, and Twelve, which are constructed across the rugged topography of the western side of Kingdom of Saudi Arabia.  These roads represent the backbone of the area and transportation and commercial traffic is dependent on them.  They connect most of the populated areas on Jabal Fayfa with Jazan city and the other parts of the Saudi Kingdom. The total length of these roads is ~32 km. The lowest elevation point is ~300 m above mean sea level, and is located on Road Twelve and the highest elevation point is 1660 m above mean sea level and located on Road Twenty.  These roads pass through many rock cuts and slope sections.  These rock cuts have been excavated a long time ago, many of which extend for long distances and are commonly 20 to 50 meter in height.  Each year, the combination of steep slopes, adverse geologic structures, and ongoing weathering processes leads to slope instability problems. Since the time of construction, these roads were susceptible to frequent slope failures (rock slides/rock falls).  Serious stability and rock fall issues were recognized and different types of accidents were documented along these roads, particularly during and after rainstorms.  Due to that, the slope instability problems cause periodic closure and damage to these roads, as well as injury and death to the travelers.  No previous studies were done to examine the rock cuts and rock slopes to determine the locations of the unstable sections. In this project, rock cuts and rock slopes along the three main roads were evaluated using different techniques including:- 1) Detailed geological mapping; 2) Applying the Rock-Mass Rating (RMR) and the Slope-Mass Rating (SMR); 3) Interpreting discontinuity data collected from the joint surveys using stereographic projection for assessment the modes of failures using DIPS software; 4) Applying the Rock Hazard Rating System (RHRS); 5) Applying a rock fall simulation program(s) to evaluate the potential rollout distance for the falling rocks; and 6) Identifying the optimum mitigation methods that will decrease and minimize the consequence of slope instability.  The results of these analyses revealed that some of the rock cuts and rock slopes need to be modified because of the ongoing slope instability.  These instability phenomena are related to 1) structural control (planar, wedge, and toppling mode of failures) 2) raveling mode of failure and 3) rolling and bouncing blocks. All different methods applied in this study are plotted on a map and the final three maps have been established for each road including 1) A map showing the DIPS program results, 2) A RMR and SMR map, and 3) A RHRS map.  The results of rock cut and rock slope stations along these three main roads indicate that the rock cuts are stable, partially stable, unstable, or completely unstable.  It was noticed that most of these unstable and completely unstable areas exist immediately adjacent to these roadways, and they represent a serious hazard that threatens life, vehicles and property.  The analyses have resulted in recommendations in which the stable slopes and cuts do not need any further remediation; the partially stable cuts require toe ditches and/or fences, nets, systematic bolting, anchors, and/or systematic shotcrete. These sites occasionally exhibit some falling blocks on the road. On the other hand, the unstable slopes require systematic reinforced shotcrete, retaining walls and/ or concrete re-excavation deep drainage. In addition, the completely unstable slopes require gravity or anchored wall re-excavation.

 

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Baamer, W.S., Youssef, A.M., Zabramawi, Y.A., Al-Katheeri, M.M., Baamer, M.F., Basahel, H.M., and Al-Ahmadi, H.J., 2010, Rock slope instability analysis along Jabal Fayfa main roads, Jazan area, Kingdom of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2009-1, 65 p., 51 figs., 35 tables, 10 plates