THE STABILITY OF METASEDIMENTARY ROCK IN RANAU,...

Ismail Abd Rahim & Baba Musta ICG 2015

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THE STABILITY OF METASEDIMENTARY ROCK IN RANAU,SABAH, MALAYSIA

Ismail Abd Rahim*& Baba MustaNatural Disasters Research Centre, Faculty of Science &Natural Resources,Universiti Malaysia Sabah,

Jalan UMS 88400 Kota Kinabalu, Sabah, MalaysiaPhone: 088 320000 (5737/5999)

Fax: 088 435324E:Mail:[email protected]

Abstract

The aim of this paper is to determine the stability of slopes and to propose preliminary rock cut slope protection andstabilization 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 withquartz vein (metagreiwacke), metamudstone, shale, slate, sheared sandstone and mudstone. The rock unit can bedivided into four (4) 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 andprescriptive measure were used in this study. Results of this study conclude that the potential modes of failuresareplanar andwedge. Terrace, surface drainage, weep holes, horizontal drain, vegetation cover, wire mesh, slopereprofiling 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

INTRODUCTIONThis studyarea is located in southeast of Ranau

town which covering theMarakau-Kinapulidan-Kigiok road alignment (Figure 1). Generally, thetopography of the study area can be divided intohilly and low land areas with the elevation are above500m and below 500m, respectively. Hillytopography is situated at the west, east and southparts of the study area. The topography alsorepresented the northeastern-southwestern ranges inRanau area. The highest elevation is about 900 –1000 m. The low land area situated at the middlepart of the study area and mainly controlled by Sg.Liwagu. This river provides alluvial plain duringflooding event and concentrated at the western partof the study area.This low land area is usually usedfor agricultures activity such as paddy cultivation.

The study area is underlain by the TrusmadiFormation andQuaternary alluvium deposit(Figure1). The age of the Trusmadi Formation isPalaeocene to Middle Eocene (Jacobson, 1970;Sanudin & Baba, 2007). Formerly, this formationconsists of interbeddedmudstone and sandstoneunits of sedimentary rock with various thicknessesfrom thin to very thick. The mudstone is moredominant then sandstone.

Trusmadi Formation is also experiencing lowgrade metamorphism into green schist facies. Themetamorphismcaused the Trusmadi Formationaltered into metasedimentary and/or metamorphicrock unit. Typical characteristic of Trusmadiformation is the appearances of foliation (slatycleavage) and quartz veins.


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Figure 1Geological map and sampling station of the study area.

The fieldwork observation showed that theoutcrops of metasedimentary rock consistofmudstone, metamudstone, shale andmetasandstone with quartz vein. Themetamudstoneof rock is represented byslate.TrusmadiFormation were faulted and folded bymoderate to high degree of deformations. Thesemoderate and high degree deformations areresponsible for the formation of metasedimentaryand metamorphic rock units, respectively. Highdegree of deformation in soft rock unit was alsocontributed to the formation of fault zone and shearzones.

Quaternary depositswith Pleistocene andHolocene ages aredistributed in low land area alongthe main rivers and flood plain in study areas. Fieldobservation shows that Pleistocene alluvium depositischaracterized by unconsolidated and poorly sortedsedimen. The materialconsist of eroded andtransported

materials such as clay, sand, organic, pebble,gravel, boulder etc.Maximum thickness of this

alluvium deposit is approximately 50m (Hutchison,2005).

Slope failure is a main factor in causing trafficinterference, maintance coast to increase, propertydamage as well as lost of lives especially in weakand deformed rock formation such asmetasedimentary Trusmadi formation. Thereforethemain objective of this study are to assess thestability, to propose protection and stabilizationmeasures of rock cut slope along the road in thestudy area.

METHODOLOGY

Geological mapping, discontinuity survey andstereographic analysis have been used in this study.Geological mapping includes lithological andstructural identification, measurement andinterpretation. Discontinuity survey has beenconducted using random method.

In stereographic analysis, Dips V5.013computer program (Rocscience, 2003) of lowerhemisphere spherical projection was used toperform the pole plot of discontinuity. The resultobtain were used for the clustering of discontinuity


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and identification of discontinuity set or averageorientations of discontinuity set.

To assess the relative stability and potential forfuture rock failures in the study area, graphicalstereo net kinematic analysis by Markland (1972)has been used. This Markland test allows theorientation of joints, bedding planes and fractures tobe analysed at numerous stations to discriminatewhich discontinuities are likely to provide failuresurfaces for future rock failures. This methodcompares the orientation of the slope withorientations of rock discontinuities and internalfriction angle (frictional component of shearstrength) of the rock mass to see which fractures,joints or bedding planes render the rock masstheoretically unstable.

Slope orientation, discontinuity sets orientationand friction angle of the Trusmadi Formation are themain parameters which required in performingMarkland test. Data of the strike and dip values ofslope has been obtained from discontinuity surveyand discontinuity sets from pole plot. The slope faceis shown as a great circle and friction angle isrepresented by an interior circle. A representativevalue of 30 degrees was selected for the internalfriction angle along discontinuities in themetasedimentary Trusmadi Formation followingHoek & Brown (1981).Selections of slopeprotection and stabilization measures wereconducted by using prescriptive measures (Yu et al.,2005) as well kinematic analysis result.

SLOPE STABILITY ASSESSMENTS

Slope stability assessment were conducted ontwelve (12) slopes along the road (Photo 1A,1B, 1C,1D, 1E, 1F, 1G, 1H, 1I, 1J, 1K and 1L). In this slopestability assessment, the rock unit (TrusmadiFormation and alluvial deposit) are also divided into

five (5) geotechnical units i.e. arenaceous unit,argillaceous unit, interbedded unit, sheared unit andalluvium unit (Figure 2). The first four (4) units arecollected from Trusmadi Formation. Arenaceousunit consist of sandstone with quartz vein(metagreiwacke)(Photo 2A & 2B), argillaceous unitof metamudstone and shale (Photo 2C & 2D),interbedded unit of metagreiwacke and slate (Photo2E), sheared unit of sheared sandstone andmudstone (Photo 2F, 2G & 2H) and alluvial unit ofalluvium deposit (Photo 2I). Some slopes arelabeled as ‘a’ and ‘b’ due to the occurrence of thickargillaceous unit in a slope such as S2a and S2b(thick argillaceous unit); however for overallslopethe unit were represented by numbers.

Slope geometry and geotechnical unit

Summary of slope geometry and geotechnicalunit are shown in Table 1. Slope S2 (55.6m) andslope S11 left (3m) are the highest and lowers cutslopes in the study area, respectively. The thickestarenaceous unit, argillaceous unit, interbedded unit,sheared unit and alluvial unit were found in slopesS1 (70m), S4 (55.8m), S4 (111.6m), S3 (81.3m) andS9 (20m), respectively.

Kinematic analysis

Discontinuities sets and mode of failuresfor theslopes are shown in Table 2 and Figure 3. Theslopes consists of three (3) to six (6) sets ofdiscontinuities but undetermined for sheared unit(S3 and S5), argillaceous unit (S11 right) andalluvial unit (S9 and S10).

It is found that there are potential wedge failure inslope S2a, S4a and S6a; but none for S1, S7, S8 andS11 left. Circular failures were found to be occurredin slope S3, S5, S9, S10 and S11 right


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Photo1Slope sections. A- slope 1; B- slope 2; C- slope 3; D-slope 4; E-slope 5; F- slope 6; G- slope 7; H- slope 8; I- slope 9; J- slope10; K- slope 11 left; L- slope 11 right.


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Figure 2Geotechnical rock units along the road alignment.

Table 1Slopes geometry, geotechnical units and thicknesses.

Slope Geotechnical Unit

Strike (o) Dip (o) Height (m) Type Thick (m)S1 221 35 23.4 1 70

S2 185 41 55.61; 2; 1; 4;1; 3; 2; 1

99.3; 19; 15.4; 22.4;43.3; 13; 14.6; ?

S3 186 58 29.8 4 89.3

S4 215 45 24.5 3; 2; 3 111.6; 55.8; 70.9

S5 185 48 11.9 4 155.8

S6 198 37 14.4 1; 2; 1 59; 34; ?

S7 173 40 21.1 1 54.2

S8 206 40 34 3; 1 50.3; 94

S9 208 53 3 5 20

S10 295 50 4 5 10

S11Left 301 35 10.8 3 50

Right 121 35 3 4 50Note: Type 1 - arenaceous unit; 2 - argillaceous unit; 3 - interbedded unit; 4 - sheared unit; 5 - alluvium unit


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Photo 2Geotechnical units. A & B- arenaceous unit; C & D- argillaceous unit; E- interbedded unit; F, G & H- sheared unit; I-alluvial unit.

Table 2Discontinuities and mode of failures.

SlopeDiscontinuities Mode of

failureSet strike/dip (o) UnfavourableS1 J1; J2; J3; J4 76/50; 244/78; 280/78; 174/69 Nil Nil

S2a J1; J2; J3; J4; J5; J6 60/61; 132/70; 156/44; 227/67; 273/69; 330/72 J3J5 Wedge

S2b Nil Nil Nil Circular

S3 Nil Nil Nil Circular



S5 Nil Nil Nil CircularS6a J1; J2; J3; J4; J5; J6 170/16; 244/50; 272/79; 15/35; 79/83;308/79 J2J3 WedgeS6b Nil Nil Nil CircularS7 J1; J2; J3; J4; J5; J6 78/64; 283/39; 145/28; 217/42; 199/73; 237/82 Nil NilS8 J1; J2; J3; J4 142/41; 264/58; 290/53; 61/84 Nil NilS9 Nil Nil Nil Circular

S10 Nil Nil Nil CircularS11 L J1; J2; J3 234/50; 83/46; 339/78 Nil Nil

Note: J1- discontinuity 1; J2- discontinuity 2; J3- discontinuity 3; J4- discontinuity 4; J5- discontinuity 5; J6-discontinuity 6; J3J5- intersection of discontinuity 3 and discontinuity 6.


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Photo 2Geotechnical units. A & B- arenaceous unit; C & D- argillaceous unit; E- interbedded unit; F, G & H- sheared unit; I-alluvial unit.

Table 2Discontinuities and mode of failures.

SlopeDiscontinuities Mode of

failureSet strike/dip (o) UnfavourableS1 J1; J2; J3; J4 76/50; 244/78; 280/78; 174/69 Nil Nil



S3 Nil Nil Nil Circular



S5 Nil Nil Nil CircularS6a J1; J2; J3; J4; J5; J6 170/16; 244/50; 272/79; 15/35; 79/83;308/79 J2J3 WedgeS6b Nil Nil Nil CircularS7 J1; J2; J3; J4; J5; J6 78/64; 283/39; 145/28; 217/42; 199/73; 237/82 Nil NilS8 J1; J2; J3; J4 142/41; 264/58; 290/53; 61/84 Nil NilS9 Nil Nil Nil Circular

S10 Nil Nil Nil CircularS11 L J1; J2; J3 234/50; 83/46; 339/78 Nil Nil

Note: J1- discontinuity 1; J2- discontinuity 2; J3- discontinuity 3; J4- discontinuity 4; J5- discontinuity 5; J6-discontinuity 6; J3J5- intersection of discontinuity 3 and discontinuity 6.


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Slope stability

The stability of cut slopes in the study area issummarized in Table 3. Based on this study, eight(8) out of twelve (12) slopes are determine as stableslopes.

Slope protection and stabilization measures

The proposed slope protection and stabilizationmeasures are summarized in Table 4. Most ofterraces and surface drainage are suggested for highslopes. The slopes that are dominated by arenaceous

unit and alluvial unit are suitable for vegetationcover. Weep hole is proposed to argillaceous,sheared and alluvial unitshowever should be addedwith horizontal drain for higher slopes. Wire meshis suggested to be installed in fails slopes. Finally,soil nail is suggested for fail and high slope. Theentire fail

slope must be cuts according to proposed optimumslope angle but the maintaining for those stableslopes should be carried on.

J K L

M N OFigure 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; J4- discontinuity 4; J5- discontinuity 5; J6- discontinuity 6; SF- slopeface; mottled area- critical zone.

DISCUSSIONS

Based on the result from the field study and dataanalysis, it was found that most of the slopes alongroad alignment are potential for the wedge failureand circular failures except for slope S1, S7,S8andS11 left. Generally, the slopes are consideredstable because except three slope has potential forwedge failures. The expected circular failure slopesof argillaceous and sheared units arehigh butlocalized in nature, low in height and small volume

therefore will not much affectedto theinfrastructures. The stability of the slopes in theroad alignmentisinfluenced by the types ofgeotechnical rock units, geological structures,weathering, erosion, groundwater and slopegeometry.

Trusmadi Formation consists of sandstoneinterbedswith quartz vein (metagreiwacke),metamudstone, shale, slate, sheared sandstone andmudstonecan be divided into four (4) geotechnical


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units i.e. arenaceous unit, argillaceous unit,interbedded unit and sheared unit, but alluvium

deposit as an alluvial unit

Table 3Slope stability.

Slope Stability1 Stable due to underlying arenaceous unit, favourable discontinuity and absent of water seepage2 Unstable due to potential wedge failure, occurrence of thick sheared rock mass unit and water seepage2a Stable because expected circular failure is localized3 Unstable due to occurrence of water seepage and thick sheared unit which potential for circular failure4 Unstable due to the occurrence of potentially wedge failure, water seepage and thick argillaceous unit4b Stable because expected circular failure is localized5 Stable because oflow slopealthoughexpected for circular failure but the material (debris) volume is small6 Unstable due to potential wedge failure and thick argillaceous unit6b Stable because expected circular failure is localized7 Stable because no potential failure8 Stable because no potential failure9 Stable because of low slope although expected for circular failure but the material (debris) volume is small10 Stable because of low slope although expected for circular failure but the material (debris) volume is small11R Stable because no potential failure and the slope is lowalthough expected for circular failure but the material

(debris) volume is too small.11L Stable because no potential failure and the slope is low.

Table 4Protection and stabilization measures

Slope

Protection and stabilization measuresTerrace&Surface drainage

Vegetationcover

Weep holes Horizontaldrain

Wiremesh

Rockbolt

optimumslopeangle

1 √ √ 35o

2 √ √ √ √ √ 35o

3 √ √ √ √ 45o

4 √ √ √ √ 45o

5 √ √ √ √ 45o

6 √ √ √ √ 37o

7 √ √ 40o

8 √ √ 40o

9 √ √ 45o

10 √ √ 45o

11R √ √ 35o

11L √ √ √ 35o

Deformation has causing to the formation ofjoints in the rock mass of Trusmadi formation. Thejoints (discontinuities) that daylight on the slopeface is potential to form planar, wedge or toppling

failures. Arenaceous units with four (4) to six (6)discontinuities sets arepotentially toformwedge

failure such as in slope S2a and S6a. These slopemust be designed by terrace, surface drainage and

vegetation cover, weep hole and wire mesh buthorizontal drainage is vital if seepage activitiesoccurs such as in slope S2b.


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The occurrence of water seepage on the slopeshows the infiltration of water from the fractures(joint or fault) in the rock mass. The infiltration haspotential to enlarge the fractures and shortening thesaturation period. High density of fractures andoccurrence of water will increase the pore pressureas well as unit weight. Therefore, it will reduce thestability of slope material especially during the rainysession. Interbedded rock unit with three (3) to six(6) discontinuities sets have potential to form wedgefailure such as in slope section no. 4. The slopes forthis geotechnical unit must be designed byconsidering the terrace, surface drainage, weep hole,vegetation cover, horizontal drainageand wire mesh.

The deformation is also caused by the folded,jointed and faulted of Trusmadi Formation. Thefault zone has been change into shear zone aftermultiple deformation acts especially in argillaceousrock unit. In shear zone, the rock unit is foundbroken and fractured which is naturally weak andunstable.Discontinuities sets cannot be identified insheared unit such as in slope S3, S5 and S11R. Therock material is loose, weak and highly weathered.The characteristics have potentialto be circularfailureonce it highly saturated with groundwater.Terrace, surface drainage, wire mesh, vegetationcover and horizontal drain are recommended forhigh slope (S3) but without horizontal drain andwire mesh for low slopes(S5 and S11R).

Exposed argillaceous rock unit on the earthsurface will break into small particle and producethick weathered material or soil layer. The thicknessof soil and saprolite (weathered material)approximately up to 20m. The weatheredargillaceous rock unit will produce clayey materialwhich is characterized by low permeability and beable to hold maximum water content hence risingthe total weight. The occurrences of lowpermeability and heavy weight are becometriggering factor for slope failure. The potentialmode of failure in this weathered material is circularfailure.

In order to form circular failure, argillaceousunit must have more than 10m thick. This has been

found in slope S2b, S4b and S6b. Discontinuitiessets cannot be determined because of sheared andweathered natures of rock mass. Terrace, surfacedrainage, weep hole, wire mesh and horizontal drainare recommended mitigation measures for high (S2band S4b) slopes but without horizontal drain andvegetation cover for low (S6) slopes.

Discontinuities sets are cannot be determined inalluvial unit due to the loose nature of rock massmaterial such as in slope S9 and S10. This willcontribute to small scale of circular failure. Rocknetting or wire mesh, vegetation cover and weephole arerecommended mitigation measures for theseslopes.

Potential fail slopes are also must be cutsaccording to the optimum slope angle to ensure theirstabilities. The proposed optimum slope angleranges from 35o to 40o but most of the slopes are45o.

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 measuresare terrace, surface drainage, vegetation cover,weep hole, horizontaldrain, wire mesh andoptimum slope angle.

REFERENCES

Collenette, P. (1958). The geology and mineral resources of theJesselton–Kinabalu area, North Borneo. MalaysiaGeological 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 RockMechanics (ISRM) Suggested Methods. Pergamon,Oxford. 211 pp.

Jacobson, G. (1970). Gunung Kinabalu area, Sabah, Malaysia.Geological Survey of Malaysia, Report, 8.


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Markland, J. T. (1972). A Useful technique for estimating thestability of rock slopes when the rigid wedge slide type offailure is expected. Imperial College Rock MechanicsResearch reprint, no. 19.

Rocscience (2003). Dips 5.1: Graphical and statistical analysisof orientation data. http://www.rocscience.com.

Sanudin, T. and Baba Musta. (2007). Pengenalan KepadaStratigrafi. Universiti Malaysia Sabah.

Tongkul, F. (1991). Tectonic evolution of Sabah, Malaysia.Journal of Southeast Asian Earth Sciences, 6 (3), pp. 395–405.

Yin, E. H. (1985). Geological map of Sabah, East Malaysia. 3rd

ed. 1:25 000 scale, Geological Survey of Malaysia.

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