Failure mechanisms in weathered meta-sedimentary rocks

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Environmental Earth Sciences ISSN 1866-6280 Environ Earth SciDOI 10.1007/s12665-014-3725-4

Failure mechanisms in weathered meta-sedimentary rocks

J. I. Nkpadobi, J. K. Raj & T. F. Ng

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ORIGINAL ARTICLE

Failure mechanisms in weathered meta-sedimentary rocks

J. I. Nkpadobi • J. K. Raj • T. F. Ng

Received: 10 April 2014 / Accepted: 18 September 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract The weathering profile of the extended cut

slope within the quartz mica schist along Pos Selim

Highway in Perak State, Malaysia shows three broad

morphological zones; bedrock zone, intermediate zone, and

residual soil zone. Further thinner horizons within these

broad morphological zones were differentiated based on

differences particularly in textures and structures of origi-

nal bedrock and degree of preservation of the constituent

minerals. Both large and small scale discontinuities

observed in the schist provide slip surfaces for failures, and

field investigation showed evidence of failures in this

extended cut slope. The implications of the micro-struc-

tures revealed through petrography showed shearing com-

ponents along planes of weakness. From the determined

index properties of the weathered profile, the quartz mica

schist is not suitable for structural support. This is also

evidenced by the high values of sand over silt and clay

deduced from the sieve analysis. In the bedrock zone,

weathering manifests itself by opening up of discontinu-

ities within the rock mass rather than weakening of the

material itself. From the kinematic analyses carried out on

the rock cuts, there are possibilities of wedge, planar and

toppling failures across this extended cut slope.

Keywords Slope failure � Kinematic analysis � Rock

index properties � Weathering profile � Petrography �Micro-structures

Introduction

Weathering plays a major role in the failure of earth

materials especially in the soil zone and the rock zones

involving ravelling of the rock blocks. According to Arikan

and Aydin (2012), both chemical and mechanical processes

work together to break down rocks and minerals to smaller

fragments or to minerals more stable near the earth’s sur-

face. The effects of weathering on schist have been studied

by Raj (2000), Yamasaki and Chigira (2006), and Wells

et al. (2008), considering various weathering agents and

outlining the relationships between the weathering and

failures. There is little or no publication on the engineering

properties and weathering profiles over the schist formation

along Pos Selim Highway, which is a vast extent of the cut

slopes. Apart from broadened studies on weathering in

Peninsular Malaysia, published data on weathering and

weathering processes in this investigated area were carried

out by Omar et al. (2009) who rather researched on the

degree of weathering in the granite unit.

In this study, significant understanding on the impact of

weathering and weathering processes on the quartz mica

schist was gained through intermittent fieldworks. Dis-

continuities increase the surface area available for attack by

weathering agents such as water. With the developed

structures and micro-structures observed during field

investigations and laboratory analyses, respectively, this

paper aims to analyze the attitudes and implications of

these structures and micro-structures as planes of weak-

ness. Petrographic descriptions in this work started with

J. I. Nkpadobi (&) � J. K. Raj � T. F. Ng

Department of Geology, Faculty of Science,

University of Malaya, 50603 Kuala Lumpur, Malaysia

e-mail: [email protected];

[email protected]

J. K. Raj

e-mail: [email protected]

URL: http://www.researcherid.com/rid/B-8716-2010

T. F. Ng

e-mail: [email protected]

URL: http://www.researcherid.com/rid/B-9234-2010

123

Environ Earth Sci

DOI 10.1007/s12665-014-3725-4

Author's personal copy

field notes at the outcrop and include megascopic

description of hand specimens, down to preparation of thin

sections and the use of polarized microscope. Also, to

identify the effects of weathering on the stability of the cut

slopes, and to determine the failure mechanisms, consid-

erable efforts were made through field and laboratory

investigations.

Geological setting of the investigated area

The study area is located in Perak state, Malaysia along Pos

Selim Highway. Mapping of this investigated area was

carried out using Garmin GPS, and Fig. 1 shows the

location and geological setting. Coverage of these cut

slopes extends to 17.7 km beginning at latitude 4�3304400

and longitude 101�1808600 traversing through Titiwangsa

main range mountainous terrain, reaching an elevation of

1587 m above sea level at Gunung Pass and terminating at

latitude 4�3509500 and longitude 101�2008000 whi ch stood at

1420 m above sea at Perak/Pahang boarder. Although this

study focused on schist, the study area traversed through

two different major geological formations; granite which

covers about 60 % of the area and schist covering about

40 %. Tajul (2003) reported that the schist, which gener-

ally occurred as roof pendant to the granite, might probably

be of Upper Palaeozoic age and was intruded by the

younger main range granite. The color variations and relict

Fig. 1 Geological setting of the

investigated area

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discontinuities in the schist indicate that the material is

heterogeneous and highly variable over a small area. The

variable relief observed in the field is a result of varying

degrees of resistance to weathering and erosion of the

underlying bedrock.

Different outcrops of this road cuts are designated as

locations. In order of abundance the schist consists of

quartz mica schist, and the graphitic schist, with the

thick quartzite layers which occur both within the

quartz mica schist and the graphitic schist. The quartz

mica schist is characterized by conspicuous coarse

texture. It is well foliated, heavily jointed, locally

faulted and sheared. Nkpadobi (2009) described folia-

tions as those structures that develop within a rock as a

consequence of the alignment of crystals and minerals

during metamorphism and the intrusion of igneous

bodies. The foliated graphitic schist and the non-foli-

ated quartzite occur mostly within the fault zones and

show little lateral continuity. Stringers of fine-grained

sulphides prominently mark the well-preserved sedi-

mentary laminations in the graphitic schist and it

exhibits a shiny but highly contorted foliation. The

quartzite exhibits a crystalline character and color

ranging from white to slightly gray, appearing as thick

layers and also as thin lens and intercalations in various

locations.

Recent failures at cut slopes

Major identified failure types across the investigated area

include planar sliding, wedge failure, and complex failures

characterized by slump and earth flow which occurred

along the Gunung Pass axis. Planar sliding occurred at

locations 006, 007 and 010 within moderately weathered

morphological zones. These zones possess relief forces that

provide insignificant resistance to sliding. On 29th October,

2013, it was observed that combination of earlier investi-

gated closely spaced shallow slips at location 007 gave rise

to defined planar failures (Fig. 2a). Planar sliding at these

locations has a common trend whereby the failure planes

are sub-parallel to the slope face, and the rock mass slides

down along a relatively planar failure surface. The

observed failure surfaces are overlain by highly to com-

pletely weathered rock. The wedge failure occurred at

location 004 (Fig. 2b). Field observations show strong

evidence that ground movement and exposure to atmo-

spheric attack are prevalent and having adverse effects on

the bedrock. This continuing ground movement initiates

the cracking and further mechanical breakdown of the rock

materials, therefore inducing the opening up of disconti-

nuities by slip, extension, and rotation.

Slump and earth flow characterize the complex failures

at locations 012, 013, 014 and 016; and these four cuts

Fig. 2 Some of the failed slopes

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constitute the Gunung Pass. These cuts bear similar min-

eralogical composition and weathering profile, but differ in

degree of deformation. The exposed rock masses have been

subjected to intense chemical weathering, reducing the

rocks into very weak soil-like materials. The slopes are

actively unstable and deteriorations occur continually,

destroying the designed berms, cascade drain, and shot-

crete (Fig. 2c). The failed rock materials from the com-

pletely weathered morphological zones as well as the

pedological soil slump downwards, sometimes as coherent

mass or silty sands and remain on the slope. Some of the

movements noticed occurred after short and prolonged

intense rainfall, saturating the pedological soil and the

weathered morphological zones of the outcrops, thereby

inducing flow (Fig. 2d). The summary of the field observed

failures is presented in Table 1.

Weathering profile developed over quartz mica schist

In the actual climatic conditions of the earth, prolonged

weathering in the hot and humid tropics and subtropics

produce thick weathered profiles which may be up to

100 m or more in thickness. In turn, weathering profile is a

vertical assemblage of weathering zones (subsurface zones

of alteration differing physically, chemically, or mineral-

ogically from adjacent zones) from the surface soil to the

unaltered bedrock (Pinti 2011). Considering the heteroge-

neity of some rock masses, Raj (1983) defined weathering

profile as the vertical profile that illustrates the transition

from fresh and unweathered bedrock at depth to the thor-

oughly weathered material at the ground surface. Deter-

mination of morphological zonation of weathering profile

of rocks of Peninsular Malaysia has been carried out

notably by Eswaran and Wong (1978), Raj (1983, 1985,

1993, 1998, 2010), Zauyah (1986), and Zauyah and Stoops

(1990). Though slightly varying in nomenclature and

thickness, all these published schemes have clear differ-

entiation of uppermost zone that constitutes the layer of

active soil formation which comprises completely weath-

ered bedrock materials with indistinct relict bedrock tex-

ture, which have been subjected to pedological processes.

Field investigations revealed occurrence of failures in

the weathered quartz mica schist extending through the

entire weathered horizons. A distinct weathering profile of

this quartz mica schist is exposed at chainage 21,000

(location 010), and the investigation was based on BSI

(1981) standard. By identifying the lateral changes in color,

texture, hardness and consistency of material composition,

the weathering profile is differentiated into three major

zones: bedrock zone, intermediate zone and residual soil

zone. Based on differences particularly in textures and

structures of original bedrock as well as degree of preser-

vation of the constituent minerals, these three broad zones

are further differentiated into thinner horizons. Both field

photograph and schematic diagram of this weathering

profile are presented in Fig. 3, and broadly divided into

zones I, II, and III. Zone I is the residual soil zone with

thickness of less than 8 m and is mainly sandy clay, cor-

responding to grade 6. It is further sub-divided into IA, IB,

and IC. Horizons IA and IB are brownish to reddish brown

pedological soil whereby IA, which is less than 1 m, is

friable sandy clay, whereas IB, which is less than 2 m, is

firm sandy clay. Horizon IC is completely weathered, stiff

yellowish brown sandy clay and devoid of any distinct

discontinuity plane.

Greater attention was focused on the intermediary zone

II because of its extensive thickness, complexity, and easy

accessibility. It encompasses slightly, moderately, and

highly weathered units. With thickness measuring up to

72 m, this zone II comprises about 24 % higher horizon

IIA (corresponding to grade 5) of highly weathered

brownish unit devoid of distinct discontinuity plane, but

with intercalations of relicts of moderately weathered units.

The lower horizon IIB is about 55 % of the entire zone II,

comprising moderately weathered gray colored quartz mica

schist and corresponding to grade 4. It exhibits conspicuous

quartzite veins, joint and foliation planes, but indistinct

fault plane. Horizon IIC is the lowest horizon of zone II,

and it corresponds to grade 3. With thickness of about

21 % of the entire zone II, this slightly weathered dark gray

unit comprises distinct relict discontinuity planes and

unweathered core boulders which are very prominent.

The bedrock zone III which is schematically represented

only by morphological zone IIIA as the only exposed unit

is unweathered bedrock which experiences the effect of

weathering only along and between structural discontinuity

planes. The thickness of this particular zone III encom-

passes grades 2 and 1 corresponding to IIIA and IIIB

horizons, respectively, although IIIB is not shown in the

schematic diagram as it is not exposed in the field.

Weathering profile developed over graphitic schist

Although the graphitic schist at location 015 is easily

broken with fingers along foliation planes, there was no

failure observed, and the cut slope looks relatively stable.

This outcrop towering about 80 m is finely inter-layered

with lens and intercalations of micaceous quartzite which

could be traced for only a few meters. Employing field

investigative standard of BSI (1981), this cut slope exposes

a weathering profile of only one broad morphological zone

(Fig. 4). Considering the morphological features, it is

designated zone II, and is summed up into highly, mod-

erately, and slightly weathered units, which corresponds to

thinner horizons IIA, IIB, and IIC, respectively. In horizon

IIA, which corresponds to grade 5, more than half of the

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Table 1 Summary of the orientations of slope locations, major discontinuities, and failure types

Cut slopes Slope

height

(m)

Failure type Summary of the investigated discontinuities Slope face

orientation

Discontinuity

data

Dip (�) Dip direction

(�)

Discontinuity

set

Intensity

(%)

Remark Slope

angle (�)

Slope

direction

(�)

Location 004 35 Wedge failure 115 32 110 J1 [8 Foliation 63 236

60 192 J2 [8 Joint

78 182 J3 [4 Joint

Location 005 20 None 120 50 98 J1 [8 Foliation 63 246

60 210 J2 [8 Joint

36 306 J3 [4 Joint

85 352 J4 [4 Joint

46 270 J5 [4 Joint

Location 006 80 Planar failure 119 40 100 J1 [8 Foliation 63 110

62 232 J2 [8 Joint

78 210 J3 [8 Joint

66 332 J4 [4 Joint

48 310 J5 [4 Joint

Location 007 35 Planar failure 120 40 100 J1 [8 Foliation 63 100

44 260 J2 [4 Joint

80 328 J3 [4 Joint

58 322 J4 [4 Joint


74 260 J2 [8 Joint

80 334 J3 [4 Joint

64 320 J4 [4 Joint

Location 009 70 None 105 56 260 J1 [8 Joint 63 250

20 190 J2 [4 Foliation

66 295 J3 [4 Joint

Location 010 100 Planar failure 117 86 308 J1 [8 Joint 68 60

86 130 J2 [8 Joint

88 230 J3 [4 Joint

80 340 J4 [4 Joint

66 260 J5 [4 Joint


78 36 J2 [8 Joint

66 312 J3 [4 Joint

78 190 J4 [4 Joint

Location 014 60 Complex failure 102 82 145 J1 [8 Joint 63 250


67 180 J3 [8 Joint

88 240 J4 [8 Joint

40 20 J5 [4 Joint

Location 015 80 None 101 80 147 J1 [8 Joint 70 250


86 238 J3 [8 Joint

38 18 J4 [4 Joint

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rock materials are disintegrated with pronounced reddish

brown color. The joint and foliation planes of this some

4-m thick horizons are not pronounced. Horizon IIB is

about 73 m thick and is made of moderately weathered

black to dark gray colored graphitic schist. The joints and

foliation planes in this horizon corresponding to grade 4 are

pronounced, and the rocks are easily broken with finger

nails along its foliation plane. Horizon IIC corresponding

to grade 3 varied widely with thick bands of less weathered

rocks and thin bands of slightly weathered rocks. The

layers are black colored and showed less sign of deep

weathering compared to quartz mica schist and quartzite

units at its boundaries. Discolouration of parts of the rocks

occurred along the discontinuity planes.

Fig. 3 Field photograph and schematic diagram of weathering profile over quartz mica schist at location 010

Fig. 4 Field photograph and schematic diagram of weathering profile over graphitic schist at location 015

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Materials and methodology

Kinematic stability analysis was applied to identify critical

planes of weakness opened up as a result of physical

weathering, and thus determines the likely modes of failure

in the bedrock zone of the rock slopes comprising the

quartz mica schist and graphitic schist. Strike and dip as

well as dip direction were measured with clinometers. The

slope face orientation was also measured. In this analysis,

the dip direction/dip completely defines the attitude of the

inclined planes. At each investigated cut slope, more than

one hundred data of discontinuities were measured using

random survey technique. In random survey technique, the

dominant structures (joint sets, faults, etc.) at the slope face

are first identified and relevant discontinuities data (joints

and foliations) were measured, and then analyzed using

GEOrient 9.5.0 version. The statistics reported by this

GEOrient software are based on procedures of statistical

analysis of spherical data from Fisher et al. (1987). Firstly,

equal area projection of poles was plotted to obtain the pole

concentration. All natural discontinuities have certain

variability in their orientation that results in scatter of the

pole plots. However, by contouring the poles using 1 % per

area standard, most highly concentrated areas of the poles

representing the dominant discontinuity sets were identi-

fied. From the contour plots, individual discontinuity sets

were identified and the average or representative plane of

discontinuity for each set was plotted. Pole intensity greater

than 4 % was regarded as a major discontinuity. The

geometry of the slope face (slope direction and angle) was

plotted alongside the major discontinuity planes, followed

by plotting of the friction circle using friction angle of 25�.

A value of 25� was considered appropriate for the friction

angle because field investigations, as well as tilt tests and

shear box tests revealed that sliding took place along very

smooth and shiny foliated plane. From the stereographic

plots, likely modes of failure are analyzed according to

stipulations in Markland’s test as described in Hoek and

Bray (1981).

Rock samples were collected from 10 different cut

slopes in the quartz mica schist and graphitic schist, and

weathered samples of the quartzite. Out of more than 200

samples collected, 97 thin sections were prepared which

represent every megascopically studied lithotype. The

microscopic analysis involved the elaboration of the thin

sections according to BSI (2007) standard. For each rock

sample, there were two thin sections cut orthogonally to

each other (one is perpendicular and the other is parallel to

the polished surface). The analyses were carried out in

terms of mineral phase content, textural and micro-struc-

tural considerations under 59 magnification.

Most of the cut slopes in the quartz mica schist were

cordoned off by Public Works Department Malaysia, and

some were covered by thick canopy of vegetation hinder-

ing greater accessibility. There is planned construction of

settlement at location 010 which opened up accessibility.

To understand the failure mechanisms of the cut slope in

weathered quartz mica schist at this location, the bulk

densities, particle densities, porosities, particle size distri-

bution, and Atterberg limits were analyzed. These physical

properties relate to the suitability of the weathered quartz

mica schist for structural support. Soil samples were col-

lected at different depths from location 010 totalling twelve

sampling points as shown in Fig. 5. Before every soil

sample was collected, the surrounding of the sampling

point was cleared of plants and a horizontal surface on the

ground was prepared. Two self-fabricated stainless cylin-

drical soil samplers of 254.22 cm3 dimensions were used.

They were earlier lubricated for easy penetration into the

soil. Efforts were made to collect samples from points

devoid of plant roots or with minimal presence of grasses

and shrubs roots to ensure the sampler was full, thereby

using the volume of the soil sampler as the soil volume.

Prior to driving the sampler into a chosen point, the

materials on the sampling point were excavated to some

shallow depth to minimize the effect of surface distur-

bance. The first sampler was then gently driven into the soil

with sledge hammer to fill up the inner cylinder until it

flushes with the ground surface, ensuring that compaction

did not take place in course of driving the samplers into the

ground. The second sampler of the same dimension was

then placed on top of the first sampler and further driven

deeper into the soil with the sledge hammer. Then the

samplers were carefully removed by digging around and

beneath them with a spade so as to preserve the soil sam-

ple. Subsequently, the two samplers were separated,

retaining the soil in the inner sampler. Excess soil was

removed with flat-bladed knife by trimming the soil sample

to flush with each end of the sampler. The soil was then

pushed out of the sampler into a plastic bag of known

weight, and labeled.

Taking the soil samples to the laboratory, the bulk

density, particle density, and porosity of the collected

samples were determined using ASTM (1970) special

procedures for testing soil and rock for engineering pur-

poses. To determine the particle size distribution, the col-

lected soil samples were tested according to ASTM (1998)

standard, whereas the Atterberg limits were measured

according to ASTM (2000) standard. Prior to sieve ana-

lysis, the soil samples were oven dried and weighed. Stack

of sieves prepared for these tests includes 5, 2, 0.6, 0.425,

0.212, 0.15, and 0.063 mm. The vibrating machine was set

at 15 min, and thereafter the mass of soil retained on each

sieve, cumulative percent, and percentage finer were

determined. Then semi-logarithmic graphs of particle dis-

tribution curves were plotted to grade the soil. Oven dried

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samples that passed through sieve number 40 (0.425 mm)

were used to determine liquid limit, plastic limit, and

plasticity index. The Casagrande liquid limit device was

used to determine the liquid limit. Four tests were per-

formed with each soil sample, then flow curves were

plotted and moisture content at 25 blows was determined.

To determine the plastic limit, the remaining parts of the

soil samples that passed through sieve number 40 were

used. Three tests were performed for each sample and the

average of the moisture contents was calculated to deter-

mine the plastic limit. The plasticity index was calculated

as the difference between liquid limit and plastic limit.

Applications

The kinematic analyses were carried out on nine outcrops

within the quartz mica schist and one outcrop within the

graphitic schist. Critical sets of discontinuities and the

likely modes of failures were determined through stereo-

graphic plots. Summary of the orientations of the slope

locations and major discontinuities are presented in

Table 1. Analysis of every slope shows one or more

intersection of the discontinuities within the critical zone.

As represented in Fig. 6, considering the modes of failures,

there are possibilities of wedge, planar and toppling fail-

ures. There is possibility of both wedge and planar failures

at locations 005, 006, 007, and 009, while location 008

shows possibility of wedge, planar and toppling failures.

The analyses show possibility of only wedge failures at

locations 004, 010, 011, 014, and 015. Likely modes of

failures are analyzed according to stipulations in Hoek and

Bray (1981).

At location 004, analysis shows intersection of J2 and J3

along 267�/27� line orientation. Four intersections within

the critical zone were recorded at location 005; J2 and J3

intersected along 280�/34� line orientation, J3 and J4

intersected along 265�/30� line orientation, J2 and J4

intersected along 266�/46�, and J3 and J5 intersected along

136�/54� line orientation. Location 006 recorded three

intersections; J1 and J4 intersected having orientation of

line 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� was recorded. The same single intersection was

also recorded at location 008 between J1 and J3 along 58�/

32�. J1 and J3 intersected along 242�/54� at location 009.

There was also only one intersection between J2 and J4 at

location 010 along 48�/65�. J3 and J4 only intersected

along 262�/56� at location 011, while J1 and J3 intersected

along 222�/61� at location 014. Analysis on the graphitic

schist at location 015 yielded only one intersection between

J3 and J4 having orientation of 326�/28� line of

intersection.

Clasts of mylonites, folds, and foliations are distinctive

micro-structures found in the representative thin sections

Fig. 5 Soil sampling points and

morphological zones within the

weathered quartz mica schist at

location 010

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using LEICA DM LB microscope. Millimeter scale

microscopic analyses confirm that most of the quartz mica

schists are mylonitized. In general, the representative

samples of quartz mica schists are marked by a consistent

mineralogical composition having abundance of quartz

(40–60 %), mica (30–50 %), plagioclase (1–15 %), and

Fig. 6 Stereographic plots defining likely wedge, planar, and toppling failure modes

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associated with varied percentage of clay mostly in

weathered samples. The light brown quartz mica schist

varieties (whereby biotite is replaced by chlorite and

muscovite) are retrogressively altered than the mylonitized

and dark brown varieties.

Mylonitized quartz mica schist was found in rock sam-

ples from locations 007 and 008 with different associated

micro-structures. Representative photomicrographs of

mylonitized quartz mica schist taken under cross-polarized

light as presented in Fig. 7a exhibit similar mineral con-

tent. There are few individual quartz crystals within the

aligned matrix of alternating muscovite and fine-grained

quartz. Muscovite is the dominant silicate mineral; hence,

considering the mineral content of rock samples, they are

better termed quartz muscovite schist.

Non-mylonitized quartz mica schist was collected from

locations 004, 006, 007, 010, 011, and 014. Samples col-

lected from these locations exhibit mineralogical and tex-

tural similarities. They show composite quartz grains

viewed under cross-polarized light, in which the crystal

boundaries within an aligned matrix of fine-grained quartz

and muscovite are sutured and are also elongated in a

preferred direction. These are sheared quartz grains with

biotite appearing as small flakes. Slight mineralogical dif-

ference observed is that in highly weathered samples, the

individual quartz grains are embedded in matrix of

muscovite, quartz, including kaolinite which evidently

formed from biotite, and chlorite which formed as product

of chemical weathering of other silicate minerals.

Micro-folding was observed within quartz mica schist

sample collected from location 005 (Fig. 7b). This quartz

mica schist attests to the fact that metamorphic rocks can

always undergo several deformation episodes. In this

sample, both biotite and muscovite are almost of equal

percentage. This sample is also rich in subhedral plagio-

clase, which is seen interlocking with the anhedral and

granular quartz. Its distinctive structure is crenulation

cleavage which is a special kind of cleavage that forms as a

result of shortening at a low angle to a pre-existing

cleavage.

Microscopically, the graphitic schist is locally folded.

The thin section reveals an anticline-syncline with well-

developed axial-plane crenulation cleavage (Fig. 7c). Fur-

ther implication of this flexural folding is the formation of

extrusive wedged-shaped domains as a result of inhom-

ogenous shortening which in turn implies minor shearing

components along the cleavage planes (Hobbs et al. 1982;

Williams 1990; Price and Cosgrove 1990). Another dis-

tinctive feature is its crystallinity which increases with

increase in grade of metamorphism and remains unaffected

during retrogression, and provides degree of graphitization,

and temperature of graphite formation (Rawat and Sharma

2011). The photomicrograph shows graphite and biotite

associated with variable amount of quartz and plagioclase.

Fig. 7 Photomicrographs of representative samples

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The biotite is intergrown with graphite and plagioclase. It

is observed that the biotite aligned along the cleavage

planes which are parallel to the axial planes of the fold. The

graphite which exhibited both anhedral and euhedral

shapes form minute flakes, and the fine-grained anhedral

quartz which concentrated in laminations along the

cleavage.

The micaceous quartzite covers a very small fraction of

the study area. Under cross-polarized light, the represen-

tative photomicrograph (Fig. 7d) revealed uniform miner-

alogy among different outcrops in locations 004, 007, 008,

and 010, whereby they consist of a mosaic of irregular

shaped quartz grains with very little amount of biotite. This

photomicrograph of the quartzite in location 006 shows the

presence of muscovite within and at the grain boundaries of

the annealed quartz, with some interstitial plagioclase, and

few recognized fine-grained detrital zircon which predates

most of the minerals in this rock. Quartzite encountered in

most locations is often white or gray but the photomicro-

graph of quartzite found at the graphite boundary at loca-

tion 015 has yellow and orange-colored quartz grains, and

this might be due to certain elemental impurities.

As earlier revealed by the weathering profile of quartz

mica schist shown in Fig. 3, the result of index properties

of the weathered quartz mica schist to a depth of 38.5 m,

presented in Table 2 shows vertical variations of earth

materials. Determined bulk densities of the soil samples

from different elevations ranging from 1 to 1.5 g/cm3 fall

within Brady (1990) range of typical bulk density thresh-

olds for clay (\1.1 g/cm3), silt (\1.4 g/cm3), and sand

(\1.6 g/cm3). These determined bulk density values, which

are ideal for plant growth, confirm field observation of

dense vegetation cover in the weathered zones which

protect the soil from harmful effects of rain drops and

erosion. As expected, the value of the particle density

ranging from 2.10 to 2.72 g/cm3 was far greater than bulk

density, indicating the presence of pore spaces. Porosities

of the soil samples ranging from 30 to 63 % were deter-

mined. Differences in the volume of pore spaces of the

sampled soil covering a vertical depth of 38.5 m are

indicative of the pockets of failures observed in the quartz

mica schist. It is noted that bulk density and porosity are

inversely associated, whereby porosity increases with

decrease in density.

Representative particle size distribution curves for well-

graded soil at depth of 17.5 m and gap-graded soil at

31.5 m depth are shown in Fig. 8a and b, respectively.

From the result of sieve analyses of the twelve soil sam-

ples, eight are gap-graded and four are well graded. The

liquid limit tests produced low values from 21 to 47 % and

the plastic limits fell below non-critical value of 40 %.

Representative flow curves for the liquid limits from depths

of 7 and 10.5 m are shown in Fig. 8c and d, respectively.

Considering the average of the entire values of the deter-

mined plasticity index which range from 1 to 21, the

weathered profile can be regarded as low plastic (Gopal

and Rao 2007). Therefore, in view of the analyses of the

tested soil samples, the weathered zone is not suitable for

structural support.

Discussion and conclusion

Gunung Pass stretches from locations 012–016 and is

actively unstable except location 015 which is graphitic

schist outcrop that looks stable. When the weathered

schistose materials were exposed due to excavations in

course of constructing the berm slopes, they experienced

physical changes, thereby became disaggregated and fria-

ble. Slips, slumps and earth flows characterize the failures

Table 2 Summary of index

properties of the weathering

profile at location 010

Sampling

elevation

from top of

slope (m)

Bulk

density

(g/cm3)

Particle

density

(g/cm3)

Porosity

(%)

Liquid

limit (%)

Plastic

limit (%)

Plasticity

index

Silt and

clay (%)

Sand

(%)

Gravel

(%)

0 1.2 2.29 49 47 26 21 44 53 2

3.5 1 2.43 59 40 34 6 37 59 4

7 1 2.45 59 24 19 5 33 64 3

10.5 1 2.42 63 37 36 1 26 69 5

14 1 2.48 58 39 22 7 27 68 5

17.5 1.3 2.14 41 32 31 1 13 85 1

21 1.3 2.25 44 33 30 3 15 82 2

24.5 1.4 2.25 38 32 29 3 12 83 4

28 1.5 2.23 35 38 21 17 21 76 3

31.5 1.1 2.10 48 21 18 3 36 60 3

35 1.2 2.10 42 22 21 1 10 86 4

38.5 1.2 2.10 43 33 29 4 9 86 5

Environ Earth Sci

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at the moderately to completely weathered quartz mica

schist. At most of the outcrops, joints combine to allow

multiple dish-like failures especially in the moderately

weathered zone suggesting that the failures were largely

controlled by discontinuities rather than strength of the

rocks. The foliations dip into the slope and almost at right

angles to the developed joints. This arrangement is critical

to the instability of these failed slopes. Because of the high

degree of weathering, a section of the cascade drain located

within the moderately weathered zone at location 014 was

completely destroyed by multiple failures, and washed

down by erosion, followed by shallow slip of surficial

materials. At location 014, these surficial materials washed

down the slope accumulate at the foot of the slope forming

alluvial deposit (Fig. 2d).

From field observation, the failures at locations 006,

007, and 010 occurred as planar sliding within moderately

weathered zone with an overlying layer of highly to com-

pletely weathered rock. At location 010, the planar failures

within coarse sandy soil in the moderately weathered zone

having bulk density of 1.3–1.5 g/cm3 were possibly

induced by the steep slope face angle of 68� and suscep-

tibility of sand to erosion. There was a typical wedge

failure at location 004 within the bedrock zone involving

gradual ravelling of loose blocks as a result of physical

weathering.

Petrography as the name implies is a pictorial-based

descriptive science. Choice of LEICA DM LP light

polarizing microscope stems from the fact that it offers

exemplary stability as a research grade microscope, par-

ticularly for photomicrography or digital imaging. The

observed micro-structures are evidence of subsequent epi-

sodes of re-crystallization. The clasts of mylonites are

prevalent within the rock units located along the major

fault zones. Foliations apply to both weathered and

unweathered samples and even among the individual grains

of the quartzite as well as the folded graphitic schist. Also

annealing found in the representative samples revealed

discontinuous poly-deformation. In the case of the gra-

phitic schist samples, the original rock is folded into a

series of anticlines and synclines with fold axes perpen-

dicular to the direction of maximum compressional stress.

The axial planes are oriented perpendicular to the maxi-

mum compressional stress direction, and foliation devel-

oped along these directions. Unlike the micro-folding

found in quartz mica schist at location 005, the fold layers

restricted to the graphitic schist at location 015 are mylo-

nites. The history of quartzite is certain to greater extent,

limiting its sedimentary origin to regional metamorphism

of sandstone or chert. Considering the occurrence of these

micaceous quartzites, it is the opinion of the authors that

this schist must have been derived from sedimentary rocks.

Fig. 8 Representative particle size distribution curves for well-graded and gap-graded soils, and flow curves for liquid limits

Environ Earth Sci

123


From the percentage of grading of the soil as presented

in Table 2, it is shown that sand has greater percentages

than gravel, silt and clay. Greater percentages of silt and

clay decreased down the slope and increased at sampling

point 31.5 m elevation which fall within the moderately

weathered zone where pockets of planar failures were

observed. Increase in the percentages of silt and clay in this

zone might also be attributed to failed residual soil and

materials from completely weathered zone which slump

downwards and remain on the slope within the moderately

and slightly weathered zones. These higher values of silt

and clay from this sampling point reflected significantly on

the reduced value of its determined bulk density. From the

particle size distribution analysis, total of four soil samples

were well graded, and eight samples were gap graded.

Although it is accepted that gap-graded soils have better

drainage and also allow root penetration even when com-

pacted, they pose compaction problems in most engineer-

ing structures. On the other hand, well-graded soil is easily

compacted and suitable for most structural support. Con-

sidering the dominant gap-graded soil samples compared to

well-graded samples, this cut slope at location 010 is not

suitable for structural support. The low values of liquid

limits as a result of cohesion and adhesion characteristics

of the tested soil samples revealed the presence of organic

materials, suggesting that short intense rainfall can easily

induce debris flow and slump in the weathered zone.

Although the determined plastic limits fell below non-

critical value of 40, the variations in the plastic limit of the

different soil samples collected at varied elevations reflect

the pockets of failures within these weathered zones. The

low plasticity index which reflects the abundance of sand,

predicts the possibility of failures even on gentler slopes.

In view of the failure mechanisms studied in this

extended cut slope within the quartz mica schist, charac-

teristics of weathering and engineering applications are

essential, and should be integrated in the guidelines for

application of mitigation measures to varied failure prog-

nosis. Further studies should construct a model of

groundwater flow regime within these failed slopes from

results of observations of water pressure and water flows to

carry out wider investigations and ensure better under-

standing of the behavior of the recorded failures.

Acknowledgments Special thanks to University of Malaya for

sponsoring this research through Postgraduate Research Fund, Grant

number: PS362/2010B.

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