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Influence of discontinuities on the stability of cutslopes in weathered meta-sedimentary rocksJ.I. Nkpadobia, J.K. Raja & T.F. Nga

a Department of Geology, Faculty of Science, University of Malaya, Kuala Lumpur, MalaysiaPublished online: 02 Feb 2015.

To cite this article: J.I. Nkpadobi, J.K. Raj & T.F. Ng (2015): Influence of discontinuities on the stability of cutslopes in weathered meta-sedimentary rocks, Geomechanics and Geoengineering: An International Journal, DOI:10.1080/17486025.2014.987701

To link to this article: http://dx.doi.org/10.1080/17486025.2014.987701

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Influence of discontinuities on the stability of cut slopes inweathered meta-sedimentary rocks

J.I. Nkpadobi*, J.K. Raj and T.F. Ng

Department of Geology, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia

(Received 13 November 2013; accepted 11 November 2014)

The meta-sedimentary rocks along Pos Selim Highway in Perak State, Malaysia showed a gradational weathering profile based on differencesparticularly in textures, hardness, lateral changes in colour and consistency of material extension. Both large and small scale discontinuities observedin the investigated rocks reduce the physical and mechanical properties of the rocks, and provide slip surfaces for failures. Rock and soil sampleswere tested using established standards to determine their characteristics and responses under a wide variety of disturbances. Kinematic analyseswere also carried out to determine the modes and likely modes of failures.

Petrographic analyses revealed associated micro-structures, and the implications of these micro-structures showed shearing components along planesof weakness. From the determined index properties of the tested soil samples, the weathered quartz mica schist is not suitable for structural support.Further study involving unconsolidated undrained direct shear box tests carried out under total stress, revealed a non-uniform response of the rocks toshearing disturbance along discontinuity planes, and the rate and depth of deformation. The shear strength components of the investigated rocks werethereby prescribed in terms of cohesion and friction angle. From the kinematic analyses across this extended cut slope, there are possibilities ofwedge and planar failures.

Keywords: weathering; rock index properties; micro-structures; shear box tests; discontinuity planes; kinematic analyses

1. Introduction

In the actual climatic conditions of the Earth, prolonged weath-ering in the hot and humid tropics and subtropics such asMalaysia produce thick weathered profiles which may be up to100 m or more in thickness. In turn, weathering profile is avertical assemblage of weathering zones (subsurface zones ofalteration differing physically, chemically or mineralogicallyfrom adjacent zones) from the surface soil to the unaltered bed-rock (Pinti 2011). Considering the heterogeneity of some rockmasses, Raj (1983) defined weathering profile as the verticalprofile that illustrates the transition from fresh and unweatheredbedrock at depth to the thoroughly weathered material at theground surface. Weathering plays a major role in the failure ofEarth materials especially in the soil zone and where rock’sstructural deformation involves ravelling of the rock blocks.According to Arikan and Aydin (2012), both chemical andmechanical processes work together to break down rocks andminerals to smaller fragments or to minerals more stable near theEarth’s surface. Considering various weathering agents, theeffects of weathering on schist have been studied by Raj (2000),Yamasaki and Chigira (2006) and Wells et al. (2008), and theyoutlined the relationships between weathering and failures.

Discontinuities increase the surface area available for attackby weathering agents such as water. With the developed

structures and micro-structures observed during field investiga-tions and laboratory analyses respectively, this paper aims togive deformational histories of the original bedrock, and alsoanalyse the attitudes and implications of these structures andmicro-structures as planes of weakness. In most cases, informa-tion from field investigations is limited in defining the implica-tions of these structures. Therefore, relative to the instability ofthe investigated rocks, there is need for laboratory analyses inorder to pictorially and graphically analyse the components ofthese structures and micro-structures along planes of weak-nesses. Petrographic descriptions in this work started with fieldnotes at the outcrop and include megascopic description of handspecimens, down to preparation of thin sections and ultimatelythe use of a polarised microscope to reveal the micro-structures.From these petrographic analyses, the distinctive structuresrevealed include foliations, clasts of mylonites and folds.There is planned construction of settlement at location 010which might extend to other locations. This study thereforeaims to identify the suitability of this weathered profile forstructural support. Field observed structures on the extendedcut slope within the meta-sedimentary rocks include faults,foliations and joints. Other identified structures such us quartzand granite veins also intruded into these meta-sedimentaryrocks at various locations. These veins are extremely usefulstrain markers as they often enable easy separation of different

*Corresponding author. Email: ikjohnbosco@yahoo.com

© 2015 Taylor & Francis

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episodes of deformations (Engelder and Delteil 2004, Nkpadobi2009). Most of these veins were, however, readily weatheredand as such no further analysis was carried out on them. In orderto identify the influence of these structures on the strength of theinvestigated rocks, unconsolidated undrained shear box testswere carried out to determine the strength of the rocks alongplanes of weakness and the characteristics of the discontinuitieswhen subjected to shearing disturbance.

To determine the modes and likely modes of failuresalong the studied discontinuities, considerable efforts weremade through field investigations and kinematic analyseswere carried out employing joints and foliations as disconti-nuity planes. Considering the large displacements of thefaults observed in this study, and as a result of inaccessi-bility to measure the parameters of these thrust faults and

strike-slip fault, their data were not employed in the stabilityanalyses.

2. Geology of the investigated area

2.1 Geological setting

The study area which is part of Pos Selim Highway is locatedin northeast Perak State, Malaysia. Different colours and geo-logical features distinguished between the schist and granitewhich are two major geological formations in this study area.However this study is centred on the schist, and the cut slopesin the schist are designated as locations (Figure 1). In order of

Figure 1. Geological map of the study area.

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abundance, this schist comprises quartz mica schist and graphi-tic schist, as well as quartzite which occurred both within thequartz mica schist and graphitic schist, although in most loca-tions, the quartzite is reduced to isolated quartz due to weath-ering. According to Zakaria and Chow (2003), the age of thesemeta-sediments are put mainly between Ordovician and SilurianPeriods. The colour variations and relict discontinuities in theschist show that the material is heterogeneous and highly vari-able over a small area. The quartz mica schist is well foliated,heavily jointed, locally faulted and sheared. Both the graphiticschist and quartzite occurred mostly within the fault zones andshowed little lateral continuity. The well preserved sedimentarylaminations in the graphitic schist are prominently marked bystringers of fine-grained sulphides, and this graphitic schistexhibits a shiny but highly contorted foliation. The quartzitehowever exhibits crystalline character and colour ranging fromwhite to slightly grey, appearing as thick layers and also thinlens and intercalations at various outcrops.

2.2 Weathering profile over quartz mica schist

Most rocks were originally formed under high temperature andpressure, devoid of air and water, once these rocks are exposedto or located near the Earth’s surface, they enter disequilibriumwith the low temperature, low pressure, and presence of air andwater. As a result of this disequilibrium, the rocks undergovarious changes including textural, structural, mineralogicaland chemical changes (Raj 1983). Although the rock materialsof the graphitic schist which outcrop at location 015 are easilybroken with fingers along foliation planes, there was no failureobserved, and the cut slope in this graphitic schist looksrelatively stable. Field investigations revealed occurrence of

failures in the weathered quartz mica schist extending throughthe entire weathered horizons.The weathering profile over this quartz mica schist is broadly

differentiated into three zones by identifying lateral changes incolour, texture, hardness and consistency of material extension.Based on differences particularly in textures and structures ofthe original bedrock as well as degree of preservation of theconstituent minerals, these three broad zones are further differ-entiated into thinner characteristic horizons. Further advantageof these thinner horizons is in their aid in delineating rock massweathering grades (BSI 1981), as well as in the study of stabilityof slope analyses since failures of cut slopes are often related tothese morphological zones. Field photograph and schematicdiagram of weathering profile of this quartz mica schist atlocation 010 is shown in Figure 2. The entire thickness ofzone I is less than 8 m, corresponding to grade 6, and furthersub-divided into IA, IB and IC. Horizons IA and IB are brown-ish to a reddish brown pedological soil profile whereby IAwhich is less than 1 m is friable sandy clay, and IB which isless than 2 m is firm sandy clay. Horizon IC is completelyweathered and mostly stiff yellowish brown sandy clay devoidof any distinct discontinuity plane. As a result of extensivethickness, complexity, and of course easy accessibility, greaterattention was focused on zone II. With thickness measuring upto 72 m, this zone II comprises about 24% of higher horizon IIA(corresponding to grade 5) of highly weathered brownish unitdevoid of distinct discontinuity plane, but with intercalations ofrelicts of moderately weathered units. The lower horizon IIB isabout 55% of the entire zone II, comprising moderately weath-ered greyish coloured quartz mica schist. This horizon IIBcorresponds to grade 4, exhibits conspicuous quartzite veins,and distinct joint and foliation planes, but an indistinct faultplane. Horizon IIC is the lowest horizon of zone II, and itcorresponds to grade 3. With thickness of about 21% of the

Figure 2. Field photograph and schematic diagram of weathering profile over quartz mica schist.

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entire zone II, this slightly weathered dark grey unit comprisesdistinct relict discontinuity planes and unweathered core-boulders which are very prominent. The thickness of zone IIIencompasses grades 2 and 1 corresponding to IIIA and IIIBhorizons respectively. Weathering manifested itself in this zonewhich is schematically represented only by morphological zoneIIIA by opening up of discontinuities instead of weathering ofthe rock mass itself.

3. Materials and methodology

3.1 Materials

Sampling strategy depends on the type of material and thepurpose of the investigation. A thorough field investigation ishelpful not only to visualise the overall condition in place, butalso to locate the lithotype boundaries, mark the locations forsampling from the different lithotypes, document the environ-mental conditions, and observe the extent and severity ofchanges in rock composition.

In this work, more than 200 block samples were collectedwith sledge hammer and chisel from 10 different locations inthe studied extended cut slope. From these block samples, 97thin sections were prepared which represent every megascopi-cally studied lithotype in the study area. For each representa-tive rock sample, there were two thin sections cut orthogonallyto each other (one is perpendicular and the other is parallel tothe polished surface). Petrographic and mineralogical studieswere conducted with the use of LEICA DM LB microscope.

To understand the indications of the physical properties ofthe weathered quartz mica schist relative to their suitability forstructural support, their bulk densities, particle densities andporosities were determined from location 010. As shown inFigure 3, from the top of this cut slope, twelve soil sampleswere collected at different depths numbering twelve sampling

points using two self-fabricated stainless cylindrical soil sam-plers of 254.22 cm3 each, twelve plastic bags weighing 2.8 geach, measuring tape, flat bladed knife, sledge hammer andspade were used for the sampling.Measurement of the shear strength of the rock samples was

carried out through unconsolidated undrained direct shear boxtest. Only representative rock samples of different dimensionsmeasured with ruler and Vernier calliper from locations 006,009, 010 and 015 were tested using shear box of dimension150 mm x 150 mm x 100 mm, applying suitable lubricants andencapsulating compound, and the shear pressure was appliedby a 20 tons capacity hydraulic pump. These tests were per-formed using TDS-302 data logger (Serial No. 0050135).In order to determine the likely modes of failures on the ten

investigated locations, the dip and dip direction of the jointsand foliations, as well as the slope angle and slope directionwere measured with clinometers for kinematic analyses.

3.2 Methodology

The essence of testing for rock properties is to generate ques-tions and predictions using field observations and laboratoryanalysis, and then explaining the recorded and derived data forthe intended purposes. These tests can be index tests, funda-mental tests, or combination of the two. The microscopicanalysis of the thin sections from the investigated rocksinvolved the elaboration of the thin sections according to BSI(2007) standard. These analyses carried out in terms of mineralphase content and textural and micro-structural considerationswere done under 5x magnification.At location 010, the bulk densities, particle densities and

porosities of the weathered quartz mica schist were determinedbased on ASTM (1970) special procedures for testing soil androck for engineering purposes, whereas particle size distribu-tion and Atterberg Limits were determined using ASTM(1998) and ASTM (2000) standards respectively. For the deter-mination of these physical properties, soil samples were col-lected at different depths of uniform spacing, numberingtwelve sampling points. Before each soil sample was collected,the surrounding of the sampling point was cleared of plantsand a horizontal surface on the ground was prepared. The twoself-fabricated stainless cylindrical soil samplers were earlierlubricated for easy penetration into the soil. Soil samples werecollected from points devoid of plants` roots or with minimalpresence of grasses and shrubs roots to ensure the soil samplerwas full, thereby using the volume of the soil sampler as thesoil volume. Prior to driving the sampler into a chosen point,the materials on the sampling point were excavated to somedepth of 0.5 m to 0.7 m in order to minimise the effect ofsurface disturbance. The first sampler was then gently driveninto the soil with a sledge hammer to fill up the inner cylinderuntil it flushed with the ground surface, ensuring that compac-tion did not take place in course of driving the sampler into theground. The second sampler of same dimension was thenplaced on top of the first sampler and further driven deeper

Figure 3. Soil sampling points and morphological zones within the weatheredquartz mica schist at location 010.

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into the soil with the sledge hammer. The samplers were care-fully removed by digging around and beneath them with aspade so as to preserve the soil sample. The two samplerswere then separated, retaining the soil in the inner sampler.Excess soil was being removed with flat bladed knife bytrimming the soil sample to flush with each end of the sampler.Then the soil was pushed out of the sampler into a plastic bagand labelled.

Concept of direct shear box test involves shearing inducedby actuators that apply a load perpendicular and parallel to thefracture plane. With the representative rock blocks, unconsoli-dated undrained direct shear box tests were carried out employ-ing ASTM (2008) standard under constant normal force. Theentire samples used in this direct shear box tests have fullytight discontinuities, and the cross-sectional areas of the repre-sentative samples were calculated applying geometric func-tions after measuring the dimensions with ruler and Verniercalliper. Based on the value of the cross-sectional area, the sizeof the normal load to be used was decided. The value of thisinitial measured cross-sectional area was used in determiningboth the normal stress and shear stress because the continualchange in the contact area is not considered during the testingof samples. The samples were sheared along planes of discon-tinuities of pre-existent fully tight cracks, and they wereoriented such that they lie in the shear plane. Both horizontaland vertical transducers for displacements have 200 µ/mmconstant, and the shear load was at 9 kN/µ constant. In orderto restrain dilation, the normal load was increased from 400kPa to 800 kPa, 1000 kPa, 1200 kPa, and finally to 1600 kPawhere applicable and relative to each tested sample. During thetest, the shear strength was determined at various appliedstresses normal to the sheared plane and at various displace-ments. The shear mechanism thereby involves the shearing offof the asperities in relation to the effect of loading.

In view of the importance of joints in slope stability ana-lyses, measurement of joint parameters in this study areaencompasses joint spacing between discontinuities, types,orientation, persistence, aperture, natural infilling by minerals,hydraulic condition and roughness. Although these measuredparameters enhanced the understanding of the characteristics ofdiscontinuities in these meta-sedimentary rocks, orientation ofjoints and foliations were employed in the kinematic stabilityanalyses based on Markland’s test as described in Hoek andBray (1981). In these kinematic analyses, the dip direction/dipcompletely defined the attitude of the inclined planes. Anothermeasured parameter in the field needed for the analyses wasthe slope face orientation, recording its angle and direction. Ateach investigated cut slope, more than one hundred data ofdiscontinuities were measured using random survey method.Relevant discontinuity (joints and foliations) data were thenanalysed using GEOrient 9.5.0 version.

Firstly, equal area projection of poles were plotted to obtainthe poles concentration. From the contour plots using 1% perarea standard, individual discontinuity sets were thus identifiedand the average or representative plane of discontinuity foreach set was plotted. Pole intensity greater than 4% was

regarded as a major discontinuity. The option of plottingmajor discontinuity planes allowed the view of planes onlyon a clean stereonet, without poles or contours. The geometryof the slope face (slope direction and slope angle) was plottedalongside the major discontinuity planes, and the friction circlewas plotted (red coloured dotted line in the plots) assumingthat friction angle along the discontinuities in the rocks of thestudy area was averaged at 25°. This value was consideredappropriate for the friction angle because field observations aswell as shear box tests revealed that sliding took place alongvery smooth and shiny foliated plane. Output result of theseplots enables the definition of the critical sets of discontinuityand the likely failure modes. The statistics reported by thisGEOrient software are based on procedures of statistical ana-lysis of spherical data from Fisher et al. (1987).

4. Results and applications

Microscopic analyses show that the schist is foliated except thequartzite. Millimetre scale microscopic analysis confirms thatmost of the quartz-mica schist are mylonitised. In general, therepresentative samples of the quartz-mica schist are marked bya consistent mineralogical composition having abundance ofquartz (40–60%), mica (30–50%), plagioclase (1–15%), andassociated with varied percentages of clay mostly in weatheredsamples. The light brown quartz mica schist (whereby biotite isreplaced by chlorite and muscovite) varieties are retrogres-sively altered than the mylonitised and dark brown varieties.Mylonites form in response to high rates of strain within deepductile shear zones, which are the extensions at depth of sur-face faults, thrusts and fault breccias (Trouw et al., 2010).These are rocks that have undergone modification of originaltextures by predominantly plastic flow due to dynamic re-crystallisation. The mylonitised quartz mica schist is found inrock samples from locations 007 and 008 with different asso-ciated micro-structures. Under cross polarised light they exhi-bit similar mineral contents except the presence of plagioclasefeldspar in the samples from location 008 (Figure 4a). Theyalso show few individual quartz crystals within the alignedmatrix of alternating muscovite and fine grained quartz.Muscovite is the dominant silicate mineral in both photomicro-graphs. Considering the mineral content of both rocks, they arebetter termed quartz muscovite schist. The non-mylonitisedsamples of quartz mica schist were collected from parts oflocations 004, 006, 009, 010, 011 and 014. Their analysedphotomicrographs show mineralogical and textural similarities.Under crossed polarised light, they show composite quartzgrains in which the crystal boundaries within an aligned matrixof fine grained quartz and muscovite are sutured and are alsoelongated in a preferred direction. These are sheared quartzgrains with biotite appearing as small flakes. The slight miner-alogical difference found in the samples from location 009 isthat the individual quartz grains are embedded in matrix ofmuscovite, quartz, including kaolinite which evidently formed

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from biotite, and chlorite which formed as product of chemicalweathering of other silicate minerals.

All rock samples collected from location 005 exhibited micro-folding as a dominant structural deformation (Figure 4b). Thesevarieties of quartz mica schist thus attest to the fact that meta-morphic rocks can always undergo several deformation episodes.Both biotite and muscovite are almost of equal percentage inthese rocks. They are also rich in subhedral plagioclase whichare seen interlocking with the anhedral and granular quartz. Itsstructure shown in photomicrograph is crenulation cleavagewhich is a special kind of cleavage that forms as a result ofshortening at a low angle to a pre-existing cleavage. Undercross polarised light, the photomicrographs from microscopicinvestigation of the quartzite, revealed uniform mineralogyamong different outcrops in locations 004, 007, 008 and 010,whereby they consist of a mosaic of irregular shaped quartz grainswith very little amount of biotite. As shown in Figure 4c, thephotomicrograph of the quartzite from location 006 showsannealed quartz having presence of muscovite within and at itsgrain boundaries, with some interstitial plagioclase, and fewrecognised fine grained detrital zircon which predates most ofthe minerals in this rock.

The graphitic schist has variable mineralogy and renownedmicrostructure (Figure 4d). Microscopically, it is locallyfolded. The thin-section contains an anticline-syncline withwell-developed axial-plane crenulation cleavage. Furtherimplication of this flexural folding is the formation of extrusivewedged-shaped domains as a result of inhomogeneous

shortening which in turn implies minor shearing componentsalong the cleavage planes (Hobbs et al. 1982, Price andCosgrove 1990, Williams 1990). Another distinctive featureis its crystallinity which increases with increase in grade ofmetamorphism and remains unaffected during retrogression,provides degree of graphitisation and temperature of graphiteformation (Rawat and Sharma 2011). The photomicrographshows graphite and biotite associated with variable amount ofquartz and plagioclase. The biotite is intergrown with graphiteand plagioclase. It is observed that the biotite align along thecleavage planes which are parallel to the axial planes of thefold. The graphite which exhibited both anhedral and euhedralshapes formed minute flakes. Further microscopic study showsthat the fine grained anhedral quartz is concentrated in lamina-tions along the cleavage.In this work, all graphical relationships were plotted using

Microsoft Excel 2010 graphs. Determined bulk densities of thesoil samples range from 1 g/cm3 to 1.5 g/cm3 which fall withinBrady (1990) range of typical bulk density thresholds for clay(<1.1 g/cm3), silt (<1.4 g/cm3), and sand (<1.6 g/cm3). Thesedetermined bulk density values which are desirable for plantgrowth confirm field observation of dense vegetation cover insome parts of the weathered zones, which in turn protectsthe soil from harmful effects of rain drops and erosion.Planar failures observed within coarse sandy soil of 1.3 g/cm3

to 1.5 g/cm3 in the moderately weathered zone at location 10were possibly induced by the steep slope face angle of 68°. Asexpected, the values of particle density ranging from 2.10 g/cm3

Figure 4. Photomicrographs of representative samples.

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to 2.72 g/cm3 were far greater than bulk density, hence indicat-ing the presence of pore spaces. Porosities of the soil samplesranging from 35% to 63% were determined. In the course of thisanalysis, the relationship between bulk density and porosity ofsoil shows a strong negative correlation, hence both soil indexproperties are inversely associated.

Semi logarithmic graphs for the particle distribution curveswere plotted. From the grading curves, it is observed that mostof the soil samples are gap graded except soil samples fromsampling points 17.5 m, 35 m and 38.5 m elevations that arewell graded. Representative particle size distribution curves forwell graded soil at a depth of 17.5 m and gap graded soil at31.5 m depth are shown in Figures 5a and 5b respectively.Although it is accepted that gap graded soils have been shownto have better drainage and also allow root penetration evenwhen compacted, they pose compaction problems in mostengineering structures. On the other hand, well graded soil iseasily compacted and suitable for most structural support.

Representative flow curves for the liquid limits from 7 metresand 10.5 metres elevations are shown in Figures 5c and 5drespectively. The low values of the liquid limits ranging from21% to 47% as a result of cohesion (hydrogen bonding) andadhesion (attraction of water molecule to a non-water molecule)characteristics of the tested soil samples, show that short intensemillimetres of minimal rainfall can easily induce debris flow andslump in the weathered zone. Although the determined plasticlimits fell below non-critical value of 40, the variations in theplastic limit of the different soil samples collected at variedelevations reflect the pockets of failures within these weatheredzones. Considering the entire values of the determined plasticityindex ranging from 1 to 21, the weathered profile can be regarded

as low plastic. Summary of these determined index properties ofthe analysed soil samples are presented in Table 1.Relative parameters determined from unconsolidated

undrained direct shear box tests include vertical deformation,horizontal displacement, shear force, shear stress, normal forceand normal stress. Determination of the strength of rock materialsinvolves quantitative evaluation of the mechanical properties ofjoint. Summary of the results of the direct shear strength tests onthe representative rock samples is shown in Table 2. Considerablenon-linear relationships between shear stress and horizontal dis-placement as shown in Figure 6a revealed the weakness alongplane of sliding and they clearly displayed the characteristics ofthe natural discontinuities of the investigated rock samples. Toascertain the rate and depth of deformation mechanism, plotswere made to show the relationship between vertical deformationand horizontal displacement (Figure 6b). These plots show non-uniform internal weakness of the rock materials which are surelyrelated to constituent minerals, micro-structure and weathering.The strengths of these representative samples were determined interms of total stress as the pore pressure was not measured. Hencethe shear strength components are prescribed in terms of cohesionand friction angle. As shown in Figure 6c, both peak (φp) andresidual (φr) friction angles for all tests were obtained by plottingthe peak and residual strengths against the normal stress. Thepeak (Cp) and residual (Cr) cohesions were evaluated as inter-cepts on peak and residual axis, whereas the friction angles wereestimated as the gradient of the lines.The output of the results of the kinematic analyses carried out

on the investigated ten outcrops enabled the definition of criticalsets of discontinuity and likely modes of failure. Summary of themajor discontinuity data and slope face orientation is presented in

Figure 5. Representative particle size distribution curves for well graded and gap graded soils, and flow curves for liquid limits.

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Table 3. The kinematic analyses of the discontinuity sets revealedthe possibility of both wedge and planar failure. At location 004,analysis shows intersection of J2 and J3 along 267°/27° lineorientation. Four intersections within the critical zone wererecorded at location 005; J2 and J3 intersected along 280°/34°line orientation, J3 and J4 intersected along 265°/30° line orienta-tion, J2 and J4 intersected along 266°/46°, and J3 and J5 inter-sected along 136°/54° line orientation. Location 006 recordedthree intersections; J1 and J4 intersected having orientation ofline of intersection as 49°/29°, J1 and J3 intersected along 128°/37°, while J1 and J2 intersected along 155°/27°. At location 007,only one intersection between J1 and J3 along 53°/31° wasrecorded. The same single intersection was also recorded at loca-tion 008 between J1 and J3 along 58°/32°. J1 and J3 intersectedalong 242°/54° at location 009. There was also only one inter-section between J2 and J4 at location 010 along 48°/65°. J3 andJ4 only intersected along 262°/56° at location 011, while J1 and

J3 intersected along 222°/61° at location 014. Analysis on thegraphitic schist at location 015 yielded only one intersectionbetween J3 and J4 having orientation of 326°/28° line of inter-section. As shown in Figure 7, five out of the ten investigated cutslopes showed possibility of only wedge failure while fiveshowed possibilities of both wedge and planar failures. Hencethere are possibilities of only wedge failures at locations 004, 010,011, 014 and 015, thenwedge and planar failures at locations 005,006, 007, 008 and 009.

5. Discussion

Failures occurred both in the weathered and unweatheredzones. Types of failures observed on the slopes reflect thechanges in the geotechnical properties of the rocks, and the

Table 2. Summary of the results of the direct shear strength tests on the representative rock samples.

Specimen No. 6 1st test 2nd test 3rd testCross-sectional area (m2) 0.006727 0.006727 0.006727Normal Stress (kPa) 800 1200 1600Peak Stress (kPa) 628.8 923.1 1043.6Horizontal Strain at failure (mm) 12.54 12.33 10.91Residual Stress (kN/m2) 588.7 869.6 1003.4

Specimen No. 9 1st test 2nd test 3rd testCross-sectional area (m2) 0.003584 0.003584 0.003584Normal Stress (kPa) 800 1200 1600Peak Stress (kPa) 502.2 753.3 853.8Horizontal Strain at failure (mm) 4.78 10.01 3.2Residual Stress (kN/m2) 452 728.2 803.6

Specimen No. 10 1st test 2nd test 3rd testCross-sectional area (m2) 0.002807 0.002807 0.002807Normal Stress (kPa) 1000 1200 1600Peak Stress (kPa) 1250.4 1442.8 1571.1Horizontal Strain at failure (mm) 0.12 1.19 3.75Residual Stress (kN/m2) 1026 1250.4 1346.6

Specimen No. 15 1st test 2nd test 3rd test 4th testCross-sectional area (m2) 0.007748 0.007748 0.007748 0.007748Normal Stress (kPa) 400 800 1200 1600Peak Stress (kPa) 313.6 545.9 766.6 1010.6Horizontal Strain at failure (mm) 2.0 1.8 3.8 10.5Residual Stress (kN/m2) 267.2 476.3 673.7 859.6

Table 1. Summary of index properties of the quartz mica schist.

Sampling elevationfrom top of slope (m)

Bulk density(g/cm3)

Particle density(g/cm3) Porosity (%)

LiquidLimit (%)

PlasticLimit (%)

Plasticityindex

Silt andclay (%) Sand (%) Gravel (%)

0 1.2 2.29 49 47 26 21 44 53 23.5 1 2.43 59 40 34 6 37 59 47 1 2.45 59 24 19 5 33 64 310.5 1 2.72 63 37 36 1 26 69 514 1 2.48 58 39 22 7 27 68 517.5 1.3 2.14 41 32 31 1 13 85 121 1.3 2.25 44 33 30 3 15 82 224.5 1.4 2.25 38 32 29 3 12 83 428 1.5 2.23 35 38 21 17 21 76 331.5 1.1 2.10 48 21 18 3 36 60 335 1.2 2.10 42 22 21 1 10 86 438.5 1.2 2.10 43 33 29 4 9 86 5

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causes of these failures are relative to the chemical and physi-cal state of the bedrocks. Wedge failures occurred only withinthe unweathered bedrock zones whereas planar failure, slumpand earth flow occurred within the weathered bedrock. Fieldinvestigations showed that ground movement affected the

entire cut slope, thereby developing cracks and opening updiscontinuities in the rocks. Both large scale and small scalediscontinuities in general reduce the strength of rocks andinfluence the stability of cut slopes. They play a major rolein the inter-relationship between physical and chemical

Figure 6. a) Plots of horizontal displacement vs shear stress for representative samples, b) plots of horizontal displacement vs vertical deformation forrepresentative samples, c) plots of peak and residual stress vs normal stress.

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weathering. These discontinuities however, were visuallyobserved in the field as well as through microscope. As a resultof differences in morphological features within a small area asshown in Figure 2, meticulous field investigations were carriedout to determine the trend of failures on the failed slope. Fieldinvestigations showed that wedge failures involved ravelling ofloose blocks along discontinuity planes. Further study on theplanar sliding within the moderately weathered zones showedthat the failure planes are sub-parallel to the slope face follow-ing the trend of foliations, and the rock mass slides down alonga relatively planar failure surface. The observed failure sur-faces are discontinuities having an overlying layer of highly tocompletely weathered rock. At most of the outcrops, jointscombine to allow multiple dish-like failures especially in themoderately weathered zone. Measurement of discontinuitiesshows the foliations dipping into the slope, and developedjoints are roughly at right angles to the foliations. This arrange-ment is critical to the instability of these failed slopes. There iscontinual deterioration of the slopes destroying the designedberms, cascade drains, and concrete sprays on the slope facewhich are employed as stability measures.

Petrography, as the name implies, is a pictorial-baseddescriptive science. Distinctive structures which are foliations,clasts of mylonites, folding and annealing found in the repre-sentative samples through the microscope revealed discontin-uous poly-deformation. Their implications showed that they

have shearing components along planes of weakness. From thethin section analysis the mylonites are prevalent within therock units located along the major fault zones. As it is thecase of the graphitic schist samples microscopically analysed,the original rock is folded into a series of anticlines andsynclines with fold axes perpendicular to the direction ofmaximum compressional stress. Since the axial planes areoriented perpendicular to the maximum compressional stressdirection, microscopic observation shows that the foliationdeveloped along these directions. Unlike the micro-foldingfound in quartz mica schist at location 005, the fold layersrestricted to the graphitic schist at location 015 are howevermylonites. The history of quartzite is certain to greater extent,limiting its sedimentary origin to regional metamorphism ofsandstone or chert, thereby enabling the investigatingGeologist to concisely focus on the very purpose of investiga-tion. Based on the output of this study, and considering theoccurrence of these micaceous quartzites, it is the opinion ofthe writer that the schist must have been derived from sedi-mentary rocks as muds and clays having shales, slates andphyllite as intermediary steps.As a result of the dominant ratio of the gap graded soil

compared to well-graded soil from these sampling points, theweathered zone of the quartz mica schist is not suitable forstructural support. Higher values of shear stresses were deter-mined from the direct shear box tests. The higher values of the

Figure 6. (Continued).

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Table3.

Sum

maryof

theorientations

ofslopelocatio

nsandmajor

discontin

uities.

Sum

maryof

theinvestigated

discontin

uities

Slope

face

orientation

Cut

slopes

Elevatio

n(m

)Discontinuity

data

Dip

(°)

Dip

Directio

n(°)

Discontinuity

Set

Intensity

Rem

ark

Slope

angle(°)

Slope

direction(°)

Location00

410

03115

32110

J1>8%

Foliatio

n63

236

6019

2J2

>8%

Joint

7818

2J3

>4%

Joint

Location00

510

6112

050

98J1

>8%

Foliatio

n63

246

6021

0J2

>8%

Joint

3630

6J3

>4%

Joint

8535

2J4

>4%

Joint

4627

0J5

>4%

Joint

Location00

61114

119

4010

0J1

>8%

Foliatio

n63

110

6223

2J2

>8%

Joint

7821

0J3

>8%

Joint

6633

2J4

>4%

Joint

4831

0J5

>4%

Joint

Location00

71129

120

4010

0J1

>8%

Foliatio

n63

100

4426

0J2

>4%

Joint

8032

8J3

>4%

Joint

5832

2J4

>4%

Joint

Location00

81155

110

4010

2J1

>8%

Foliatio

n63

9874

260

J2>8%

Joint

8033

4J3

>4%

Joint

6432

0J4

>4%

Joint

Location00

912

0610

556

260

J1>8%

Joint

6325

020

190

J2>4%

Foliatio

n66

295

J3>4%

Joint

Location01

012

14117

8630

8J1

>8%

Joint

6860

8613

0J2

>8%

Joint

8823

0J3

>4%

Joint

8034

0J4

>4%

Joint

6626

0J5

>4%

Joint

Location011

1275

120

2022

8J1

>8%

Foliatio

n70

250

7836

J2>8%

Joint

6631

2J3

>4%

Joint

7819

0J4

>4%

Joint

Location01

413

4810

282

145

J1>8%

Joint

6325

018

335

J2>8%

Foliatio

n67

180

J3>8%

Joint

8824

0J4

>8%

Joint

4020

J5>4%

Joint

Location01

513

6610

180

147

J1>8%

Joint

7025

020

337

J2>8%

Foliatio

n86

238

J3>8%

Joint

3818

J4>4%

Joint

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shear stress are related to the increase in normal stress which inturn increases the contact area. When the normal force is notsufficient to restrain dilation, then the shear mechanism involvesthe overriding of the asperities, and this can be applicable totesting of non-planar joints or discontinuities. In the case ofthese tested pervasive planar discontinuous schist samples, thenormal load was increased with subsequent tests in order torestrain dilation. For example, 400 kPa normal stress was firstapplied to sample 15A, the next normal stress was 800 kPa atwhich point most of the asperities have been broken, therebyensuring increase in contact area. This phenomenon produceshigher frictional resistance. Therefore the rate at which this

frictional resistance increases with normal stress is expressed asfriction angle. These friction angles are however evaluated fromlinear regression analysis based on the relationship between peak/residual strengths and normal stress and are in other words theangles of shearing resistance in degrees.The purpose of using the kinematic method in the analysis

of the rock cuts was to identify critical planes of weaknessesand determine likely modes of failure along the extended cutslope. Based on the kinematic analyses of these discontinuitysets, it is anticipated that all cut slopes in the investigated areaare kinematically unstable with regards to wedge and planarfailures. It however appears that toppling is not a major

Figure 7. Stereographic projections for investigated outcrops.

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concern from the results of this kinematic analysis, althoughfield observation at location 004 shows this probability.

6. Conclusion

This work tried to evaluate the macro and micro roles ofdiscontinuities in the instability of rock slopes along PosSelim Highway in Perak State, Malaysia. These were achievedthrough field investigations relative to failure trends, and theimplications of the shear components of the micro-structuresrevealed through microscopic analyses. The roles of thesediscontinuities were also evident in the non-linear deformationcharacteristics of the tested rock samples deduced throughdirect shear box tests, and the likely modes of failures fromkinematic analyses. The computation of results, schematic andgraphical representations were aimed at encouraging the needfor the use of expensive and wider in situ and laboratorytesting equipment in order to reasonably predict the geotech-nical properties of these rocks before any further infrastructuraldevelopment in the area is carried out.

During routine fieldwork, contractors and developers hadstarted erecting make-shift bungalows for agricultural officesat two locations at the foot of this extended cut slope. In viewof the influence of discontinuities to stability of these rockslopes, it is hereby advised that broader laboratory and in situmeasurements of the engineering properties of the lithotypesshould be carried out to determine the suitability of thisweathered extended cut slope for structural support. In thison-going research, a model of groundwater flow regime withinthese failed slopes should be constructed from results of obser-vations of water pressure and water flows in order to under-stand the ground conditions. Also variable stability measuresshould be applied on the weathered and unweathered bedrockzones relative to their failure prognosis.

Funding

Special thanks to the University of Malaya for sponsoring this research throughPostgraduate Research Fund, grant number: PS362/2010B.

References

Arikan, F. and Aydin, N., 2012. Influence of weathering on theengineering properties of dacites in Northeastern Turkey. ISRNSoil Science, 2012, 218527, doi:10.5402/2012/218527.

ASTM (American Society for Testing and Materials), 1970. SpecialProcedures for Testing Soil and Rock for Engineering Purposes.5th ed. West Conshohocken, PA: ASTM Special TechnicalPublication.

ASTM (American Society of Testing and materials), 1998. StandardTest Method for Particle-Size Analysis of Soils. ASTM D 422–98.

West Conshohocken, PA: Annual Book of ASTM Standards, vol.14(3),10–17.

ASTM (American Society for Testing and Materials), 2000. TestMethod for Liquid Limit, Plastic Limit, and Plasticity Index ofSoils. West Conshohocken, PA: Annual Book of ASTMStandards, vol. 4(8),546–558.

ASTM (American Society for Testing and Materials), 2008. StandardTest Method for Performing Laboratory Direct Shear StrengthTests of Rock Specimens under Constant Normal Force. ASTMD5607-08. West Conshohocken, PA: ASTM International.

Brady, N.C., 1990. The nature and properties of soils. New York:Macmillan.

BSI (British Standards Institution), 1981. Code of Practice for SiteInvestigations. BS 5930:1981. London: BSI.

BSI (British Standards Institution) 2007. Natural stone test methods –Petrographic examination. EN 12407:2007. London: BSI.

Engelder, T. and Delteil, J., 2004. The orientation distribution of singlejoint sets. In: J.W. Cosgrove and T. Engelder, eds. The Initiation,Propagation, and Arrest of Joints and Other Fractures. London:Geological Society of London, issue 231, 281–298.

Fisher, N.I., Lewis, T. and Embleton, B.J.J., 1987. Statistical Analysisof Spherical Data. Cambridge: Cambridge University Press.

Hobbs, B.E., Means, W.D. and Williams, P.F., 1982. The relationshipbetween foliation and strain: an experimental investigation.Journal of Structural Geology, 4, 411–428.

Hoek, E. and Bray, J.W., 1981. Rock Slope Engineering. London: TheInstitute of Mining and Metallurgy.

Nkpadobi, J.I., 2009. Stability of slope cuts in the dinding schist of theUkay Perdana area, Selangor Darul Ehsan, Malaysia. Thesis(MSc). University of Malaya.

Pinti, D., 2011. Weathering profile. In: M. Gargaud, ed. Encyclopediaof Astrobiology. Berlin: Springer, 1778.

Price, N.J. and Cosgrove, J.W., 1990. Analysis of GeologicalStructures. Cambridge: Cambridge University Press.

Raj, J.K., 1983. A study of residual soils and their cut slope stabilityin selected areas of Peninsular Malaysia. Thesis (PhD).University of Malaya.

Raj, J.K., 2000. Weathering and weathering profiles; short course onEngineering Geology. Malaysia: The Institution of Engineers, 2–3.

Rawat, R. and Sharma, R., 2011. Features and characterization ofgraphite in Almora Crystallines and their implication for thegraphite formation in Lesser Himalaya, India. Journal of AsianEarth Sciences, 42 (1–2), 51–64.

Trouw, R.A.J., Passchier C.W. and Wiersma, D.J., 2010. Atlas ofMylonites and Related Micro-structures. Berlin: Springer-Verlag.

Wells, T., Willgoose, G.R., and Hancock, G.R., 2008. Modelingweathering pathways and processes of the fragmentation of saltweathered quartz-chlorite schist. Journal of GeophysicalResearch, 113, F01014. doi:10.1029/2006JF000714

Williams, P.F., 1990. Differentiated layering in metamorphic rocks.Earth Science Review, 29, 267–281.

Yamasaki, S. and Chigira, M., 2006. Weathering processes andmechanisms of pelitic schist, providing basic conditions for land-slides. Disaster Mitigation of Debris Flows, Slope Failures andLandslides, 2, 393–401.

Zakaria, M. and Chow, W.S., 2003. Geological terrain mapping inCameron Highlands district, Pahang. Bulletin of the GeologicalSociety of Malaysia, 46, 69–73.

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