Abstract
Rock mass classification system used to determine stability of rock slope mainly in highways passing through rock cuts. Several rock mass classification systems developed to know stability against rock slope conditions. In a region of Saudi Arabia, slope cuts located in uneven rocky area is considered. Different empirical methods have been applied to 22 rock cuts chosen based on failure mechanism and discussed limitations of empirical classification methods and future research directions.
Introduction
In this case study, five rock mass classification methods are chosen based on RMR system of classification.
- Original Slope Mass Rating (SMR)
- Chinese Slope Mass Rating (CSMR)
- Continuous Slope Mass Rating (CSMR)
- Graphical Slope Mass Rating (GSMR)
- Hazard Index (HI) method
Basically Rock Mass Rating is used in determining underground rock stability like tunnels. Parameters considered in RMR system are
- UCS (Unconfined Compressive Strength) of intact rocks
- RQD (Rock Quality Designation) test
- Spacing between discontinuities
- Discontinuity conditions
- Conditions of Ground water
Hence fourth parameter on discontinuities is made to determine slope stability condition on surface of rock slopes. From this discontinuity factor addition with excavation factor five empirical methods are developed.
These failure cuts are divided into two types they are
- Structurally Controlled Failure and
- Non-structurally controlled failure
Structurally controlled failures are due to discontinuities and high weathering conditions and Non-structurally controlled failure due to external forces like ground water. HI method is used to analyze stability condition for both types of slope cuts. Other empirical methods used only for structurally controlled failures. These structurally controlled failures are having mechanisms 1) planar failure, 2) wedge failure and 3) overtopping failure.
Literature Review
The material properties of rock slope, Height, Face angle and joint orientation plays a major role in stability of rock slopes. Additionally Road curvature in rugged terrains may also be influenced. (Hoek and Bray, 1981)
Empirical methods represent an important tool used to preliminary assessment of engineering behaviors of rock mass. (Duran and Douglas, 2000)
Stereo net projection is used to protect the orientation of discontinuities and information about dip and dip direction of a joint. (Price and Cosgrave, 1990)
DRMR is different users deriving different RMR score, depending on their own experience and selection of discrete values. Modified RMR system by converting discrete ratings to continuous rating function with RMR values estimated by different users can be controlled within 10 % (Bieniawski, 1989)
Methodology
Considered site is located in south western part of Saudi Arabia called Jazan region. 22 road cuts are taken for case study along five main roads having structurally controlled failure sites are 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 18, 19 and 22. Non -structurally controlled failure sites are 8, 9, 15, 20 and 21.
First step is field investigation and data collection. This includes weathering conditions, RQD, joint spacing, joint surface conditions, slope height, face dip and dip direction. The GSI (Geological Survey Index) is an important factor especially in the assessment of highly weathered effect of water mainly in case of Non-structurally controlled failure. Laboratory testing is done by taking rock samples from site to determine physical and mechanical properties.UCS test is carried out.
Next step, In case of structurally controlled failure kinematic analysis and RMR classification is done. In kinematic analysis DIPS software for stereographic projection is used. From this planar failure, wedge failure and toppling failure in rock cuts is determined. RMR test is performed using physical and laboratory testing values. From this all types of slope mass rating classification is done. Original SMR, Chinese SMR, Continuous SMR and Graphical SMR classification is used for failure due to discontinuities only (structurally controlled failure). In case of non-structurally controlled failure HI method of classification is considered. HI (Hazard Index) has two parameters Normal condition and triggering mechanism.
Result
In case of structurally controlled failure:-
RMR system’s analysis indicated that rock cuts sites 11 and 12 are classified as “very good”. Rock cuts sites 1, 2, 3, 4, 6, 16, 17 and 18 are classified as “fair”. The four locations 5, 7, 10, 14 and 13 are classified as “good”. Site 19 gives poor rock quality. KINEMATIC Analysis results indicate that there is potential for
Planar failure in 5 locations, nine locations shows potential for wedge failure mechanism, for toppling failure, five sites show potential for flexural toppling and five locations indicate potential for block toppling.
In case of Non-structurally controlled failure:-
This can be only determined by HI method. Results indicate sites 8, 9, 15, 20 and 21 are classified as poor rock cuts.
Discussion
SMR methods can only be used for structurally controlled failure rock slopes. HI method is used for both structurally and non- structurally controlled failure slopes. By using CRMR function over DRMR it gives a distinctive score for each RMR parameter. Graphical SMR gives result in graphical representation by means of great circles. A line plots as a point and a plane plots as a great circle. All elements plotted pass through the center of hemisphere and their projection reflects where the plane or line intersects the outer hemisphere surface.
Finally, all empirical methods used in this study are all developed for linear structures. They are not designed for mountainous roads which are curved having rock cuts of concave and convex surface shapes. The above mentioned points are subject of research currently.
Conclusion
Original and Graphical SMR methods shows similar results and the resultant stability conditions are almost the same because the underlying calculation formulae of F1, F2, F3 and F4 are the same. However, application of the technique is different. The Chinese SMR method is not suitable for rock cuts below 80 m it should be further improved by adding factor of slope height below 80 m and by analyzing the effect of pore water on stability behavior of rock slopes. The HI method is superior as it takes into account the effect of water. The influence of slope curvature should also be considered in mountainous regions such as jazan area in kingdom of Saudi Arabia.
References
- Anbalagan R, Sharma S, Raghuvanshi TK. Rock mass stability evaluation using modified SMR approach. In: proceedings of the 6th National Symposium on Rock Mechanics; 1992. p. 258e68.
- Bieniawski ZT. Engineering classification of jointed rock masses. Civil Engineer in South Africa 1973;15(12):335e43.
- Duran A, Douglas K. Experience with empirical rock slope design. In: proceedings of ISRM International Symposium; 2000.
