THE STABILITY OF METASEDIMENTARY ROCK IN RANAU, SABAH, MALAYSIA

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1 THE STABILITY OF METASEDIMENTARY ROCK IN RANAU, SABAH, MALAYSIA Ismail Abd Rahim*& Baba Musta Natural Disasters Research Centre, Faculty of Science &Natural Resources,Universiti Malaysia Sabah, Jalan UMS 8800 Kota Kinabalu, Sabah, Malaysia Phone: (5737/5999) Fax: Abstract The aim of this paper is to determine the stability of slopes and to propose preliminary rock cut slope protection and stabilization measures for Paleocene to Middle Eocene Trusmadi Formation along Marakau-Kigiok inranau, Sabah, Malaysia. The rock of Trusmadi Formation is slightly metamorphosed and dominated by interbeds of sandstone with quartz vein (metagreiwacke), metamudstone, shale, slate, sheared sandstone and mudstone. The rock unit can be divided into four () geotechnical unit s namely arenaceous unit, argillaceous unit, interbedded unit and sheared unit. Twelve(12) slopes were selected for this study. Geological mapping, discontinuity survey, kinematic analysis and prescriptive measure were used in this study. Results of this study conclude that the potential modes of failures areplanar andwedge. Terrace, surface drainage, weep holes, horizontal drain, vegetation cover, wire mesh, slope reprofiling and retaining structure were proposed protection and stabilization measures for the slope in the study area. Keywords: Trusmadiformation, Ranau, metasedimentary rock, slope stability, mode of failure INTRODUCTION This studyarea is located in southeast of Ranau town which covering themarakau-kinapulidankigiok road alignment (Figure 1). Generally, the topography of the study area can be divided into hilly and low land areas with the elevation are above 500m and below 500m, respectively. Hilly topography is situated at the west, east and south parts of the study area. The topography also represented the northeastern-southwestern ranges in Ranau area. The highest elevation is about m. The low land area situated at the middle part of the study area and mainly controlled by Sg. Liwagu. This river provides alluvial plain during flooding event and concentrated at the western part of the study area.this low land area is usually used for agricultures activity such as paddy cultivation. The study area is underlain by the Trusmadi Formation andquaternary alluvium deposit(figure 1). The age of the Trusmadi Formation is Palaeocene to Middle Eocene (Jacobson, 1970; Sanudin & Baba, 2007). Formerly, this formation consists of interbeddedmudstone and sandstone units of sedimentary rock with various thicknesses from thin to very thick. The mudstone is more dominant then sandstone. Trusmadi Formation is also experiencing low grade metamorphism into green schist facies. The metamorphismcaused the Trusmadi Formation altered into metasedimentary and/or metamorphic rock unit. Typical characteristic of Trusmadi formation is the appearances of foliation (slaty cleavage) and quartz veins. 69

2 Figure 1Geological map and sampling station of the study area. The fieldwork observation showed that the outcrops of metasedimentary rock consist ofmudstone, metamudstone, shale and metasandstone with quartz vein. The metamudstoneof rock is represented by slate.trusmadiformation were faulted and folded by moderate to high degree of deformations. These moderate and high degree deformations are responsible for the formation of metasedimentary and metamorphic rock units, respectively. High degree of deformation in soft rock unit was also contributed to the formation of fault zone and shear zones. alluvium deposit is approximately 50m (Hutchison, 2005). Slope failure is a main factor in causing traffic interference, maintance coast to increase, property damage as well as lost of lives especially in weak and deformed rock formation such as metasedimentary Trusmadi formation. Thereforethe main objective of this study are to assess the stability, to propose protection and stabilization measures of rock cut slope along the road in the study area. Quaternary depositswith Pleistocene and Holocene ages aredistributed in low land area along the main rivers and flood plain in study areas. Field observation shows that Pleistocene alluvium deposit ischaracterized by unconsolidated and poorly sorted sedimen. The materialconsist of eroded and transported Geological mapping, discontinuity survey and stereographic analysis have been used in this study. Geological mapping includes lithological and structural identification, measurement and interpretation. Discontinuity survey has been conducted using random method. materials such as clay, sand, organic, pebble, gravel, boulder etc.maximum thickness of this 70 METHODOLOGY In stereographic analysis, Dips V5.013 computer program (Rocscience, 2003) of lower hemisphere spherical projection was used to perform the pole plot of discontinuity. The result obtain were used for the clustering of discontinuity

3 and identification of discontinuity set or average orientations of discontinuity set. To assess the relative stability and potential for future rock failures in the study area, graphical stereo net kinematic analysis by Markland (1972) has been used. This Markland test allows the orientation of joints, bedding planes and fractures to be analysed at numerous stations to discriminate which discontinuities are likely to provide failure surfaces for future rock failures. This method compares the orientation of the slope with orientations of rock discontinuities and internal friction angle (frictional component of shear strength) of the rock mass to see which fractures, joints or bedding planes render the rock mass theoretically unstable. Slope orientation, discontinuity sets orientation and friction angle of the Trusmadi Formation are the main parameters which required in performing Markland test. Data of the strike and dip values of slope has been obtained from discontinuity survey and discontinuity sets from pole plot. The slope face is shown as a great circle and friction angle is represented by an interior circle. A representative value of 30 degrees was selected for the internal friction angle along discontinuities in the metasedimentary Trusmadi Formation following Hoek & Brown (1981).Selections of slope protection and stabilization measures were conducted by using prescriptive measures (Yu et al., 2005) as well kinematic analysis result. SLOPE STABILITY ASSESSMENTS Slope stability assessment were conducted on twelve (12) slopes along the road (Photo 1A,1B, 1C, 1D, 1E, 1F, 1G, 1H, 1I, 1J, 1K and 1L). In this slope stability assessment, the rock unit (Trusmadi Formation and alluvial deposit) are also divided into five (5) geotechnical units i.e. arenaceous unit, argillaceous unit, interbedded unit, sheared unit and alluvium unit (Figure 2). The first four () units are collected from Trusmadi Formation. Arenaceous unit consist of sandstone with quartz vein (metagreiwacke)(photo 2A & 2B), argillaceous unit of metamudstone and shale (Photo 2C & 2D), interbedded unit of metagreiwacke and slate (Photo 2E), sheared unit of sheared sandstone and mudstone (Photo 2F, 2G & 2H) and alluvial unit of alluvium deposit (Photo 2I). Some slopes are labeled as a and b due to the occurrence of thick argillaceous unit in a slope such as S2a and S2b (thick argillaceous unit); however for overall slopethe unit were represented by numbers. Slope geometry and geotechnical unit Summary of slope geometry and geotechnical unit are shown in Table 1. Slope S2 (55.6m) and slope S11 left (3m) are the highest and lowers cut slopes in the study area, respectively. The thickest arenaceous unit, argillaceous unit, interbedded unit, sheared unit and alluvial unit were found in slopes S1 (70m), S (55.8m), S (111.6m), S3 (81.3m) and S9 (20m), respectively. Kinematic analysis Discontinuities sets and mode of failuresfor the slopes are shown in Table 2 and Figure 3. The slopes consists of three (3) to six (6) sets of discontinuities but undetermined for sheared unit (S3 and S5), argillaceous unit (S11 right) and alluvial unit (S9 and S10). It is found that there are potential wedge failure in slope S2a, Sa and S6a; but none for S1, S7, S8 and S11 left. failures were found to be occurred in slope S3, S5, S9, S10 and S11 right 71

4 Photo1Slope sections. A- slope 1; B- slope 2; C- slope 3; D-slope ; E-slope 5; F- slope 6; G- slope 7; H- slope 8; I- slope 9; J- slope 10; K- slope 11 left; L- slope 11 right. 72

Figure 2Geotechnical rock units along the road alignment. Table 1Slopes geometry, geotechnical units and thicknesses. Slope Strike (o) Dip (o) 221 35 S2 185 1 55.

5 Figure 2Geotechnical rock units along the road alignment. Table 1Slopes geometry, geotechnical units and thicknesses. Slope Strike (o) Dip (o) S S3 S S5 S6 S7 S8 S9 S Type 1 1; 2; 1; ; 1; 3; 2; 1 3; 2; 3 1; 2; 1 1 3; S1 S11 Left Height (m) 23. Geotechnical Unit Thick (m) ; 19; 15.; 22.; 3.3; 13; 1.6;? ; 55.8; ; 3;? ; Right Note: Type 1 - arenaceous unit; 2 - argillaceous unit; 3 - interbedded unit; - sheared unit; 5 - alluvium unit 73

6 Photo 2Geotechnical units. A & B- arenaceous unit; C & D- argillaceous unit; E- interbedded unit; F, G & H- sheared unit; Ialluvial unit. Table 2Discontinuities and mode of failures. Slope Set Discontinuities strike/dip (o) Unfavourable Mode of failure S1 J1; J2; J3; J 76/50; 2/78; 280/78; 17/69 S2a 60/61; 132/70; 156/; 227/67; 273/69; 330/72 J3J5 S2b S3 Sa 226/20; 297/75; 32/66; 53/70; 11/52; 160/0 J1J6 Sb S5 S6a 170/16; 2/50; 272/79; 15/35; 79/83;308/79 J2J3 S6b S7 78/6; 283/39; 15/28; 217/2; 199/73; 237/82 S8 J1; J2; J3; J 12/1; 26/58; 290/53; 61/8 S9 S10 S11 L J1; J2; J3 23/50; 83/6; 339/78 Note: J1- discontinuity 1; J2- discontinuity 2; J3- discontinuity 3; J- discontinuity ; J5- discontinuity 5; J6discontinuity 6; J3J5- intersection of discontinuity 3 and discontinuity 6. 7

Photo 2Geotechnical units. A & B- arenaceous unit; C & D- argillaceous unit; E- interbedded unit; F, G & H- sheared unit; Ialluvial unit. Table 2Discontinuities and mode of failures.

7 Photo 2Geotechnical units. A & B- arenaceous unit; C & D- argillaceous unit; E- interbedded unit; F, G & H- sheared unit; Ialluvial unit. Table 2Discontinuities and mode of failures. Slope Set Discontinuities strike/dip (o) Unfavourable Mode of failure S1 J1; J2; J3; J 76/50; 2/78; 280/78; 17/69 S2a 60/61; 132/70; 156/; 227/67; 273/69; 330/72 J3J5 S2b S3 Sa 226/20; 297/75; 32/66; 53/70; 11/52; 160/0 J1J6 Sb S5 S6a 170/16; 2/50; 272/79; 15/35; 79/83;308/79 J2J3 S6b S7 78/6; 283/39; 15/28; 217/2; 199/73; 237/82 S8 J1; J2; J3; J 12/1; 26/58; 290/53; 61/8 S9 S10 S11 L J1; J2; J3 23/50; 83/6; 339/78 Note: J1- discontinuity 1; J2- discontinuity 2; J3- discontinuity 3; J- discontinuity ; J5- discontinuity 5; J6discontinuity 6; J3J5- intersection of discontinuity 3 and discontinuity 6. 75

8 unit and alluvial unit are suitable for vegetation cover. Weep hole is proposed to argillaceous, sheared and alluvial unitshowever should be added with horizontal drain for higher slopes. Wire mesh is suggested to be installed in fails slopes. Finally, soil nail is suggested for fail and high slope. The entire fail Slope stability The stability of cut slopes in the study area is summarized in Table 3. Based on this study, eight (8) out of twelve (12) slopes are determine as stable slopes. Slope protection and stabilization measures The proposed slope protection and stabilization measures are summarized in Table. Most of terraces and surface drainage are suggested for high slopes. The slopes that are dominated by arenaceous slope must be cuts according to proposed optimum slope angle but the maintaining for those stable slopes should be carried on. J K L M N O Figure 3bResult of the Markland test. Note: J - slope S7; K -slope S8; L slope S9; M slope S10; N - slope S11L; O - slope S11R; J1- discontinuity 1; J2- discontinuity 2; J3- discontinuity 3; J- discontinuity ; J5- discontinuity 5; J6- discontinuity 6; SF- slope face; mottled area- critical zone. DISCUSSIONS Based on the result from the field study and data analysis, it was found that most of the slopes along road alignment are potential for the wedge failure and circular failures except for slope S1, S7,S8 ands11 left. Generally, the slopes are considered stable because except three slope has potential for wedge failures. The expected circular failure slopes of argillaceous and sheared units arehigh but localized in nature, low in height and small volume 76 therefore will not much affectedto the infrastructures. The stability of the slopes in the road alignmentisinfluenced by the types of geotechnical rock units, geological structures, weathering, erosion, groundwater and slope geometry. Trusmadi Formation consists of sandstone interbedswith quartz vein (metagreiwacke), metamudstone, shale, slate, sheared sandstone and mudstonecan be divided into four () geotechnical

9 units i.e. arenaceous unit, argillaceous unit, interbedded unit and sheared unit, but alluvium deposit as an alluvial unit Table 3Slope stability. Slope 1 2 2a 3 b 5 6 6b R 11L Stability Stable due to underlying arenaceous unit, favourable discontinuity and absent of water seepage Unstable due to potential wedge failure, occurrence of thick sheared rock mass unit and water seepage Stable because expected circular failure is localized Unstable due to occurrence of water seepage and thick sheared unit which potential for circular failure Unstable due to the occurrence of potentially wedge failure, water seepage and thick argillaceous unit Stable because expected circular failure is localized Stable because oflow slopealthoughexpected for circular failure but the material (debris) volume is small Unstable due to potential wedge failure and thick argillaceous unit Stable because expected circular failure is localized Stable because no potential failure Stable because no potential failure Stable because of low slope although expected for circular failure but the material (debris) volume is small Stable because of low slope although expected for circular failure but the material (debris) volume is small Stable because no potential failure and the slope is lowalthough expected for circular failure but the material (debris) volume is too small. Stable because no potential failure and the slope is low. Slope R 11L Terrace& Surface drainage Table Protection and stabilization measures Protection and stabilization measures Vegetation Weep holes Horizontal Wire cover drain mesh Rock bolt Deformation has causing to the formation of joints in the rock mass of Trusmadi formation. The joints (discontinuities) that daylight on the slope face is potential to form planar, wedge or toppling failures. Arenaceous units with four () to six (6) discontinuities sets arepotentially toformwedge optimum slope angle 35o 35o 5o 5o 5o 37o 0o 0o 5o 5o 35o 35o failure such as in slope S2a and S6a. These slope must be designed by terrace, surface drainage and vegetation cover, weep hole and wire mesh but horizontal drainage is vital if seepage activities occurs such as in slope S2b. 77

10 The occurrence of water seepage on the slope shows the infiltration of water from the fractures (joint or fault) in the rock mass. The infiltration has potential to enlarge the fractures and shortening the saturation period. High density of fractures and occurrence of water will increase the pore pressure as well as unit weight. Therefore, it will reduce the stability of slope material especially during the rainy session. Interbedded rock unit with three (3) to six (6) discontinuities sets have potential to form wedge failure such as in slope section no.. The slopes for this geotechnical unit must be designed by considering the terrace, surface drainage, weep hole, vegetation cover, horizontal drainageand wire mesh. found in slope S2b, Sb and S6b. Discontinuities sets cannot be determined because of sheared and weathered natures of rock mass. Terrace, surface drainage, weep hole, wire mesh and horizontal drain are recommended mitigation measures for high (S2b and Sb) slopes but without horizontal drain and vegetation cover for low (S6) slopes. The deformation is also caused by the folded, jointed and faulted of Trusmadi Formation. The fault zone has been change into shear zone after multiple deformation acts especially in argillaceous rock unit. In shear zone, the rock unit is found broken and fractured which is naturally weak and unstable.discontinuities sets cannot be identified in sheared unit such as in slope S3, S5 and S11R. The rock material is loose, weak and highly weathered. The characteristics have potentialto be circular failureonce it highly saturated with groundwater. Terrace, surface drainage, wire mesh, vegetation cover and horizontal drain are recommended for high slope (S3) but without horizontal drain and wire mesh for low slopes(s5 and S11R). Potential fail slopes are also must be cuts according to the optimum slope angle to ensure their stabilities. The proposed optimum slope angle ranges from 35o to 0o but most of the slopes are 5o. Exposed argillaceous rock unit on the earth surface will break into small particle and produce thick weathered material or soil layer. The thickness of soil and saprolite (weathered material) approximately up to 20m. The weathered argillaceous rock unit will produce clayey material which is characterized by low permeability and be able to hold maximum water content hence rising the total weight. The occurrences of low permeability and heavy weight are become triggering factor for slope failure. The potential mode of failure in this weathered material is circular failure. In order to form circular failure, argillaceous unit must have more than 10m thick. This has been 78 Discontinuities sets are cannot be determined in alluvial unit due to the loose nature of rock mass material such as in slope S9 and S10. This will contribute to small scale of circular failure. Rock netting or wire mesh, vegetation cover and weep hole arerecommended mitigation measures for these slopes. CONCLUSION 1. Modes of failure are wedge and circular failure. 2. Generally the slopes in the study area are stable. 3. Proposed protection and stabilization measures are terrace, surface drainage, vegetation cover, weep hole, horizontaldrain, wire mesh and optimum slope angle. REFERENCES Collenette, P. (1958). The geology and mineral resources of the Jesselton Kinabalu area, North Borneo. Malaysia Geological Survey Borneo Region, Memoir, 6. Hoek, E. & Bray, J.W.(1981). Rock slope engineering. 3rd Ed. Institution of Mining and Metallurgy. Hutchison, S.C. (2005). Geology of North-West Borneo (Sarawak, Brunei and Sabah). Elsevier, Amsterdam ISRM. (1981). Rock Characterization, Testing and Monitoring. In: Brown, E.T. (Ed.). International Society for Rock Mechanics (ISRM) Suggested Methods. Pergamon, Oxford. 211 pp. Jacobson, G. (1970). Gunung Kinabalu area, Sabah, Malaysia. Geological Survey of Malaysia, Report, 8.

11 Markland, J. T. (1972). A Useful technique for estimating the stability of rock slopes when the rigid wedge slide type of failure is expected. Imperial College Rock Mechanics Research reprint, no. 19. Rocscience (2003). Dips 5.1: Graphical and statistical analysis of orientation data. Sanudin, T. and Baba Musta. (2007). Pengenalan Kepada Stratigrafi. Universiti Malaysia Sabah. Tongkul, F. (1991). Tectonic evolution of Sabah, Malaysia. Journal of Southeast Asian Earth Sciences, 6 (3), pp Yin, E. H. (1985). Geological map of Sabah, East Malaysia. 3rd ed. 1: scale, Geological Survey of Malaysia. 79

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