Indonesian Journal on Geoscience Vol.
12 No.
3 December 2025: 367-382
INDONESIAN JOURNAL ON GEOSCIENCE
Geological Agency Ministry of Energy and Mineral Resources Journal homepage: h ps://ijog.
ISSN 2355-9314, e-ISSN 2355-9306 Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
Hulu Terengganu.
Malaysia Sofea Razali2.
Muhd Nur Ismail Abdul Rahman1,2*.
Siti Syaza Aiman Seh Wali2.
Dony Adriansyah Nazarudin2.
Muhamad Zaki Zulkifli2.
Ahmad Nor Zaimie Roslan3, and Effi Helmy Ariffin4 Aqua Heritage Research Interest Group.
Faculty of Science and Marine Environment.
Universiti Malaysia Terengganu 2023 Kuala Nerus.
Terengganu.
Malaysia ROCK Research Interest Group Faculty of Science and Marine Environment.
Universiti Malaysia Terengganu 2023 Kuala Nerus.
Terengganu.
Malaysia Mineral and Geoscience Department Malaysia PT3102K.
Jalan Sultan Sulaiman, 20200 Kuala Terengganu.
Terengganu.
Malaysia Institute of Oceanography and Environment.
Universiti Malaysia Terengganu.
Kuala Nerus.
Terengganu 21030.
Malaysia *Corresponding Author: nur.
ismail@umt.
Manuscript received: September, 15, 2024.
revised: July, 28, 2025.
approved: September, 24, 2025.
available online: October, 30, 2025 Abstract - Riverbank erosion has become a serious problem in Hulu Terengganu River recently.
In some locations, the failed riverbank is particularly hazardous, especially near recreational and plantation areas.
Therefore, this study aims to assess the lithological characteristics controlling the failed riverbank in Ajil.
Hulu Terengganu.
Malaysia.
Two localities were chosen based on the hazardous conditions and severity of erosion along the riverbanks.
Both riverbanks were vertically logged to differentiate lithological units.
Field observations identified four lithological horizons, designated as Litho A-D, based on in-situ characteristics .
structure, plant roots, grain siz.
Samples were taken from selected horizons for Particle Size Analysis (PSA) and Atterberg limit testing.
The results of the investigation from PSA indicate that the samples from localities L1S1.
L2S1.
L1S2, and L2S2 exhibit positive skewness, with fine sediment sizes identified as silt-clay for L1S1 and L1S2, and sand-silt for L2S1 and L2S2.
Conversely, samples L1S3 and L2S3 demonstrate a trend of negative skewness, indicating coarse sediment sizes.
The results indicate that the sand-silt composition suggests a decrease in water flow energy from high to low.
The coefficient of uniformity (C.
in this study ranged between 3.
81, and the coefficient of curvature (C.
ranged from 0.
63 to 1.
The Cc values for L1S3 and L2S3 are below 1.
indicating well-graded soil, while the Cc values for other samples are above 1.
0 indicating poorly graded soil.
Keywords: Hulu Terengganu, lithological unit, particle size analysis (PSA).
Atterberg limits, coefficient of uniformity (C.
, coefficient of curvature (C.
A IJOG - 2025 How to cite this article:
Razali.
Rahman.
Wali.
Nazarudin.
Zulkifli.
Roslan.
, and Ariffin, , 2025.
Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
Hulu Terengganu.
Malaysia.
IndoAnesian Journal on GeoAscience, 12 .
, p.
DOI: 10.
Introduction Background Several rivers in Malaysia are prone to hazards due to factors such as deforestation, flooding, and sand mining.
These factors can lead to riverbank failure, where the banks experience hydraulic erosion (Julian and Tores, 2006.
Huang et al.
, 2021.
Duan et al.
, 2.
and geotechnical instability (Islam, 2008.
Taha et al.
, 2022.
Razali et al.
, 2.
Indexed by: SCOPUS
PUBLISHED IN IJOG
Indonesian Journal on Geoscience.
Vol.
12 No.
3 December 2025: 367-382 Hydraulic instability can be caused by erosion beneath a somewhat stable bank, unpredictable floods, sediment debris, removal of bank vegetation, loosening of coarse sediment .
by wave action, secondary currents, and other factors (Darby and Thorne, 1996.
Ahmed, 2001.
Dury et , 2020.
Hao et al.
, 2.
Toriman and CheAo Lah .
explain that erosion, transportation, and sedimentation in rivers are natural processes that help shape a river balance.
River balance is influenced by time factors according to their respective The rate of river erosion occurs faster in larger retivers compared to smaller river channels.
According to Lemnitzer et al.
, the mechanism of riverbank erosion produces extensive undercutting effects at the bottom of steep banks, resulting in landslides when the water level is high.
Riverbank erosion can occur due to a single factor or a combination of several factors simultaneously, such as the loss of soil particles in the bank structure by river flow and soil slippage (Toriman and CheAoLah, 2.
Slippage is caused by an increase in the slope of the bank due to natural erosion or engineering works.
Additionally, erosion beneath the riverbank caused by turbulent currents, as well as lateral erosion, can lead to riverbank failure.
In Malaysia, the detailed causes of riverbank erosion are still not well understood, especially when it comes to the morphology of the river, the strength of the soil, and the overall stability of the banks (Toriman, 2005.
Chassiot et al.
, 2020.
Razali et al.
, 2.
Most local studies focus on general factors such as rainfall, the amount of sediment carried by the river, and water flow (Gasim et al.
, 2005a.
Lemnitzer et al.
, 2.
In other parts of the world, however, riverbank erosion has been widely studied, especially near river mouths and deltas where erosion problems are most serious (Toriman, 1997.
Baracos and Lew, 2003.
Duan et al.
, 2.
Many of these studies are from temperate and polar regions (Chassiot et al.
, 2.
, but research in tropical areas like Malaysia is still limited.
This gap is important, because the type of soil and rock plays a big role in how riverbanks behave.
In Malaysia, riverbanks are mostly made of sand, silt, clay, and ferric oxide, which are very different from the loess, glacial till, or frozen soils found in colder These differences show why detailed soil and rock studies are needed in tropical rivers to better understand and prevent riverbank failures.
This study will primarily focus on the Berang River, which serves as the main water drainage system in the Ajil area, delineated by latitudes N 5A5' and N 4A50', and longitudes E 102A50' and E 103A10' (Figures 1a and .
The river stretches approximately 30 km from The Belukar Bukit waterfall to the main Terengganu-Jenagor River Junction.
It plays a crucial role in the hydrological cycle of the Ajil area.
Given that the surrounding area is inhabited by residents and contains oil palm plantations, in addition to being a popular recreational spot, the river also functions as an agricultural drainage channel.
However, the river presence also contributes to cliff erosion, and poses a risk of flooding during heavy rains.
This study will focus specifically on two localities, labeled as Locality 1 and Locality 2, chosen based on the hazardous conditions and severity of erosion along the riverbanks.
These areas were selected due to severe erosion observed following continuous heavy rainfall and landslides in February 2022.
a result of this incident, small buildings, pedestrian walkways, access roads, and trees were washed Given these circumstances, the study aims to examine the lithological aspects contributing to riverbank failure.
General Geology Geology of the Hulu Terengganu area comprises metasedimentary rocks, igneous rocks, and alluvial plains (Sharoum et al.
, 2015.
Rahman et , 2.
Geological information from the Ajil and Kuala Berang areas indicates that these regions consist of Carboniferous-aged sedimentary rocks .
pproximately 350 million years ol.
and intrusive igneous rocks such as granite and gabbro .
pproximately 259 - 250 million years ol.
The metamorphosed sedimentary rocks in the Hulu Terengganu area constitute the oldest rock sequences extensively exposed in the southwestern part of the studied area.
These rock fragments PUBLISHED IN IJOG Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
Hulu Terengganu.
Malaysia (S.
Razali et al.
Legend Road Contour River Locality Elevation High : 1295.
Low :-1.
103o 4' N 103 6' N Ajil
5o 4' N 5o 4' N Kuala Berang
Symbol
Dyke intrusive Tanggol granite 5o 2' N 5o 2' N UndiAerentiated sediment
Mempelas gabbro
Ajil granite Lerek granite Binjai granite Payong granite Inferred fault
Main road
103 4' N
5 Km 103 6' N Figure 1.
Base map showing the drainage system and also the topography of the Ajil area.
Map of Peninsular Malaysia showing the location of the studied area in a blue box.
Geological map of the Ajil area consisting mostly of granite rocks .
odified from Badruldin et al.
, 2.
PUBLISHED IN IJOG
Indonesian Journal on Geoscience.
Vol.
12 No.
3 December 2025: 367-382 are widely exposed as rock blocks to gravel-sized grains in the Belukar Bukit waterfall area in Ajil.
Fragments of these sedimentary rocks are also found within the Berang River channel.
While granite rocks dominate in the east west area of Ajil.
These rocks are also exposed in the waterfall area, and also form hills in several places in the Ajil area.
This igneous rock is included in the Tanggol granite igneous rock that connects the Kapal granite on the high hilly strip in the Lake Kenyir and its surrounding area.
The granite from Ajil area according to Badruldin et al.
comprised predominantly of Jerangau Forest Pluton (Figure 1.
The pluton is further divided into Lerek.
Binjai, and Payong granites.
Lerek and Binjai granites are characterized by pinkish colour, medium- to coarse-grained phaneritic texture.
addition, numerous mafic dykes were intruded and exposed in the western part of Lerek and Binjai granites along 10 km distance from KM 390 to KM 400 of East Coast Expressway.
Limited study has been conducted on the Quaternary geology, with most of the information derived from the authors' observations and analyses prior to completing the research.
The alluvial plain exposed in Ajil can be classified into recent alluvium and old alluvium.
Recent alluvium is prominently visible near lowland areas, especially along the river, and partially covers the sandbar This recent alluvium predominantly comprises various unsorted grains, with a dominant clay-silt composition.
In contrast, old alluvium is primarily distributed along the riverbank sections.
The alternating horizontal sedimentary sequences exhibit distinct lithological evidence.
They are slightly compacted, with a variety of grain sizes ranging from clay to gravel, forming the natural structure of the riverbank.
Gravel displays imbrication and is supported by unsorted coarse to fine sand.
Additionally, signs of root decay and oxidation processes are evident on the sedimentary bed.
The old alluvium is older than the recent alluvium, as indicated by the preservation of its sedimentary layers.
In Ajil, a significant portion of the old alluvium undergoes pedogenesis on a large scale due to its interaction with water.
Methods and Materials Sedimentological Methods Two riverbanks were vertically logged to differentiate the lithological units.
These units were visually identified from the dominant lithology observed in the outcrop.
The logged profile was used to assess the connectivity of the lithological units between these riverbanks.
Furthermore, the interpretation of the observed riverbanks and recent features will provide additional information about the condition of the riverbank after the The thickness of each bed was measured, and samples were taken from each lithology.
The samples were then processed in the laboratory for further analysis.
Particle Size Analysis Particle size determination analysis is a laboratory procedure aimed at determining the proportion of soil particles of varying sizes in a given This analysis comprises two methods:
wet and dry.
A 250-gram sample will be weighed and then subjected to the dry sieve technique.
The specimen will undergo dehydration in an oven set at 100 0 C.
The process of dry sieving will involve the use of sieves with specific dimensions: 4.
mm, 2.
00 mm, 1.
00 mm, 0.
710 mm, 0.
500 mm, 355 mm, 0.
250 mm, 0.
125 mm, 0.
063 mm, and <0.
063 mm.
The mass of the sample for each sieve size will be measured and documented.
The specimen will undergo the sieving process using a sieve with a pore size of 63 micrometres.
The sieve findings with diameters smaller than 63 micrometres will be used for wet analysis, utilizing an Anton Paar apparatus for particle size analysis.
Samples containing more than 10 % fine sediment will be analyzed using the laser scattering method.
The PSA device utilizes laser scattering technology, directing the laser at sediment samples contained within a glass beaker.
This method is suitable for determining fine particles present in soil samples.
Statistical data of the sediment will be displayed on a computer using the Kalliope programme.
Detecting light
at specific angles offers different benefits in light
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Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
Hulu Terengganu.
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Razali et al.
scattering measurements.
"Back angle" or "back scatter" measures light scattered back toward the incident laser beam, typically at 175A, while "side scatter" refers to light scattered 90A perpendicular to the beam.
"Forward scatter" detects light scattered at 15A in the same direction as the beam.
Atterberg Limit The Atterberg limits test is performed to determine the plastic limit and liquid limit of soils with a high proportion of fine particles.
Assessing the engineering features of soils affected by moisture content is of utmost importance, as it pertains to the correlation between the amount of moisture present and the engineering characteristics of the soil.
The Atterberg limit test consists of three primary analyses: liquid limit, plastic limit, and shrinkage limit.
Liquid Limit.
LL (ASTM D 4.
A sample is placed within a ceramic vessel.
The sample used is one that has successfully passed through a 0.
425 mm sieve and has been A small quantity of distilled water was added to the sample to create a paste.
The specimen is protected to avoid desiccation.
Subsequently, the sample was introduced into a Casagrande liquid limit apparatus and leveled.
The sample was partitioned using a groove tool, creating a distinct groove with no physical interaction between the two sides.
The liquid limit test commenced by determining the number of impacts (N) necessary for the soil specimen to make contact with itself over a 13 mm .
/2-inc.
A small fraction of the sample was extracted, transferred to a porcelain dish, and weighed.
The specimen was then dehydrated in a furnace for 16 hours at a temperature of 1000 C.
This method was repeated four times by incorporating distilled water.
The range of blows consists of values of 10, 20, 30, 40, and The goal of incrementally taking blows at ten intervals is to achieve a graph that is both smooth and easily interpretable.
Plastic Limit.
PL (ASTM D 4.
The plastic limit determination is an extension of the liquid limit test, where the soil sample obtained from the liquid limit test is reused.
After being combined with water, the sample was transformed into a compact and unified substance resembling a slender thread measuring 3.
2 mm in diameter .
quivalent to 1/8 inc.
The thread-like sample was further extended until it showed signs of fracture.
Subsequently, the sample was placed into a receptacle, weighed, and the data was This technique was repeated multiple times to derive an average value.
The specimen was then placed in a furnace and subjected to a drying process lasting 16 hours.
Once dried, the sample was weighed again, and the weight was The plasticity index (PI) was calculated by subtracting the value of the plastic limit (PL) from the liquid limit (LL).
This study allows for the plotting of LL vs.
PI data on a plasticity chart to ascertain the soil type.
Result and Discussion Field Observation and Sedimentology Characteristics Riverbank 1 The riverbank exhibits a height of 5.
2 m following a significant river overflow (Figure 2.
The logged section reveals various lithological units, which can be categorized into three main types: Litho B.
Litho C, and Litho D.
Litho B, 2 m thick, is distinguished by prominent cross-bedded features.
It consists of unconsolidated gravel arranged in an imbricated pattern along the upper portion of the Samples obtained along Riverbank 1 were designated with the codes L1S1 to L1S3.
Field observations indicate occasional mixing of coarse sand and silt within this area.
Litho B is believed to experience high-energy processes that transport larger sediments, and produce cross-bedding features along the riverbank.
Cross-bedding occurred due to the sideways and downstream movement of a channel bar, with the inclined layers being steep and slopy (Casnedi and Giulio, 1999.
Saikia and Laskar, 2.
The imbrication of gravel suggests the direction of PUBLISHED IN IJOG Indonesian Journal on Geoscience.
Vol.
12 No.
3 December 2025: 367-382
Riverbank 1
Litho B
L1S3
L1S2
Meter
Litho C
Litho D L1S1
Litho C
LEGEND
Cross bedding/trough cross bedding with gravel
Coarse sand
Silt-sand with oxidized plant root
Laboratory Analysis Particle size analysis Atterbergh
(PSA)
Litho A Riverbank 2
Litho B
L1S3
Litho D Litho C
L1S2
L1S1
Litho C
LEGEND
Imbricated gravel/occasional coarse sand Laboratory Analysis Cross bedding/trough cross bedding with gravel Particle size analysis (PSA) Coarse sand Atterbergh Silt-sand with oxidized plant root Figure 2.
Riverbank 1 exhibits height of 5.
2 m following a significant river overflow, categorize by three main types:
Litho B.
Litho C, and Litho D.
Riverbank 2, recorded at 7 m above water level and categorized to four distinct lithological units: Litho A.
Litho B.
Litho C, and Litho D.
Note that the sample identification is located on the riverbank as L1S1AeL2S3.
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Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
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water flow.
Litho C, situated beneath Litho B and above Litho D, varies in thickness from 1 to 2 m, and displays unsorted coarse grains.
This unit is interpreted as part of the river coarse sediment deposition during periods of water overflow, particularly during floods.
Litho D represents the lowest section in the logged profile with 1.
m thick, lying below Litho C.
It is characterized by its thinner nature compared to the other lithological divisions, and consists mainly of sand-silt material.
Litho D shows a slightly more compact bank compared to Litho B and Litho C.
The sand-silt composition suggests a decrease in water flow energy from high to low.
During high-energy floods and low-energy dry seasons, the water flow varies, leading to the settlement of silt.
Within Litho D, numerous oxidized plant root remnants are also present, indicating past vegetation in the area.
Small plants, especially marshes and bushes, can grow along the riverbank when the sand is exposed due to low river water levels.
This cycle repeats each year, leaving oxidized plant material on the sand.
2, is thicker than in riverbank 1.
It measures 82 m in thickness, and is characterized by cross-bedding structures, such as trough crossbedding, with granule grains partially covering the area of the riverbank.
Cross-bedding forms from the lateral and downstream movement of a channel bar, with steep, sloping layers (Casnedi and Giulio, 1.
Litho B features an erosional base, indicating river scour due to swift water flow during floods.
Litho C, situated below Litho B and marked by an erosional base, ranges from 1 to 3 m in thickness.
Its characteristics are similar to those of riverbank 1, consisting of unsorted coarse sand.
Litho D at riverbank 2 is thinner compared to its exposure at riverbank 1, and is composed of a variety of sand and silt compositions, with a notable abundance of oxidized plant roots (Figure .
Imbricated gravel Trough cross bedding Riverbank 2 This riverbank was recorded at 7 m above water level, based on the logged profile (Figure The riverbank was clearly exposed when the floodwaters receded, leaving behind the characteristic features of a modern river.
The bank is divided into four distinct lithological units:
Litho A.
Litho B.
Litho C, and Litho D.
Samples obtained along Riverbank 2 are designated with the codes L2S1 to L2S3.
Unlike the profile of riverbank 1.
Litho A is absent, possibly eroded away during a major flood event.
Litho A in the logged profile is up to 1.
32 m thick, consisting of gravel and coarse-grained sand.
The gravel at the lower portion of Litho A is aligned in an imbricated pattern, transitioning to a mix of scattered gravel and fine-to-coarse sand grains toward the upper part.
This indicates an influx of river sediment during periods of high-water Imbricated gravel can serve as strong evidence of the water flow direction in a modern river channel.
Litho B, also found in riverbank
L2S3
L2S2
Scoured surface Coarse grains L2S1
Coarse grains with plant root
Scoured surface L2S3
L2S2
L2S1
Figure 3.
Lithology and erosional features on riverbank 2.
There is trough, imbricated gravel, and scoured surface indication for high energy of water current.
Note that the sample identification is located on the riverbank as L1S1AeL2S3.
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3 December 2025: 367-382 Physical Properties of Soil Grain Size Analysis (ISO 13.
Soil samples L1S2 and L2S2 showed a high percentage of sand (>90 %), while the other samples had higher proportions of silt and clay (Figure The obtained grain size distribution data were presented in a graph, and the results classified each sample differently, although silt dominated the percentages (Figure 4.
Each sample was also analyzed to determine the uniformity coefficient (C.
and the coefficient of curvature (C.
to assess the grading quality of the particles.
The calculations were based on cumulative distribution values at particle sizes D10.
D30, and D60, as shown in the cumulative distribution graph (Table .
From the calculations, the uniformity coefficient (C.
ranged from 3.
6 to 9.
The coefficient of curvature (C.
for samples L1S1.
L1S2, and L2S1 ranged from 1.
05 to 1.
66, while samples L1S3 and L2S3 showed values of 0.
Based on this information, samples in the L1S1.
L1S2.
L2S1, and L2S2 areas are categorized as well-graded, while samples in the L1S3 and L2S3 areas are categorized as poorly graded (Casagrande, 1948.
Das and Sobhan, 2.
According to White .
, well-graded soil refers to soil where the spaces between the coarser and more uniform particles .
are filled by a range of fine-sized particles .
, forming a denser pore Poorly graded soil may indicate issues such as excessive voids, inadequate compaction, and diminished stability.
The high silt content can be considered a natural defensive barrier on the riverbank, in addition to being bound by other clay minerals .
articularly kaolinit.
The percentage of fine particles, along with the silt content in the samples, indicates active weathering processes due to Malaysia tropical climate.
Atterberg Limits (ASTM 4318-.
Atterberg limits refer to the moisture levels in soil at which the soil consistency changes from one state to another, marking the transition between silt and clay to other specific phases.
The test involves determining the liquid limit and plastic limit of the soil.
The results are then Finer (%) Grain Size Analysis Particle Size (AA.
Figure 4.
Grain size analysis chart of five samples.
Particle size distribution graph showing positive-negative skewness comprising coarse-fine to silt-clay grains samples.
Plasticity Chart plotted of each sample.
Percentage of plasticity limits for each sample in the studied area.
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Table 1.
Percentage of Plasticity Limit for Each Sample in the Studied Area
Sampel
L1-S1
Particle size distribution Clay
Sand (%) Silt (%) (%) D10
D30
D60
Coefficient of uniformity.
Cu, [D60/D.
Coefficient of Soil quality curvature.
Cc, (USCS) [(D.
/ D60xD.
L1-S2
Well
L1-S3
Poor
L2-S1
Well
L2-S2
Well
L2-S3
Poor plotted on a plasticity chart to identify the soil plasticity and organic properties in fine-grained soils for soil classification.
Liquid Limit The liquid limit test results revealed that sample L1S3 exhibited the highest liquid limit percentage at 62 %, followed by sample L1S1 with 54 %.
Samples L2S1 and L2S2 recorded the lowest liquid limit percentages at 36 % and 34 %, respectively (Table .
The high percentage in sample L1S3 suggests it consists of residual granite soil, which according to Tan .
, contains higher water content.
This elevated liquid limit for residual granite soil can be attributed to particle size distribution.
The liquid limit represents the water content at which soil transitions from a plastic to a liquid state.
Soil with water content above the liquid limit may experience structural failure when it can not sustain stress under high moisture conditions (Jiang and Cui, 2022.
Wang et , 2.
Samples L1S2 and L2S1 showed lower liquid limits, possibly because they originate from residual metamorphic soil.
Soils with lower liquid limit values have a reduced capacity to handle There is a clear relationship between clay Well content and liquid limit, showing mutual dependence.
Liquid limit values play a critical role in slope design as they help predict soil strength under stress.
Considering Malaysia tropical climate and high annual rainfall, the liquid limit study assists engineers in designing stable slopes.
Plastic Limit Plastic limit tests indicated that sample L1S3 had the highest percentage at 34 %, followed by sample L1S1 at 31 %, sample L2S2 at 26 %, sample L2S3 at 25 %, and samples L1S2 and L2S1 at 24 % and 20 %, respectively (Figure Table 2 details the plasticity index values for each sample type.
Calculations show plasticity indexes ranging from 10 % to 28 %, with notably high plasticity indexes in samples L1S1.
L1S3, and L2S2.
Soil classification was based on data plotted on the plasticity chart.
According to the Unified Soil Classification System (USCS), analysis indicated that samples L1S2 and L2S2 fell above the A-line, and were classified as clay.
Meanwhile, samples L1S1.
L1S3, and L2S1, falling below the A-line, were classified as silt.
Samples L1S2.
L2S2, and L2S3 were categorized as CI, indicating clay with moderate plasticity.
Table 2.
Information on Liquid Limit.
Plastic Limit.
Plasticity Index.
And Soil Classification for Each Sample Sample Liquid limit.
LL (%) Plastic limit.
PL (%) Plasticity index.
PI (%) Soil classification system (USCS.
ASTM D2.
L1S1
L1S2
L1S3
L2S1
L2S2
L2S3
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Indonesian Journal on Geoscience.
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3 December 2025: 367-382 Given their liquid limit below 50 % per ASTM D2487 classification, they are regarded as clayey Samples L1S1 and L1S3 were classified as MH, referring to silt with high plasticity.
Sample L2S1 was categorized as ML due to its liquid limit falling within the 34 % range, denoting a low plasticity index (Figure 4.
Soil failure occurs when it reaches its maximum stress level at a specific strain value.
Plastic soil can endure an increase in strain within the soil.
Soil failure due to peak stress may continue to bear stress, albeit with diminishing stress levels.
According to Holtz and Kovacs .
, soil that is semi-solid or brittle can withstand maximum applied stress, but will fail immediately afterward.
Riverbank Erosion River bank erosion has been observed along the riverbank of the Belukar Bukit recreation centre (Locality .
Erosion affects an area approximately 100 m horizontally, causing the banks, pedestrian infrastructure, trees, and built structures on the riverbanks to erode (Figures 5a and .
Furthermore, the erosion caused a small landslide scarp after the flood (Figure 5.
The height from the base to the surface of the eroded cliff reaches up to 3 m (Figure 5.
Additionally, the water has dredged up to 2 m towards the riverbank.
Such severe erosion is likely to occur when the water flow is fast, and the water level surpasses the original riverbanks.
This is evidenced by several uprooted trees, collapsed road and clay sediment in the riverbank toe due to affected by the water flow (Figure .
According to Lemnitzer et al.
, the erosion mechanism of the riverbank results in wide scouring at the bottom of the steep bank, leading to landslides during high water levels.
Another contributing factor is that the area of Belukar Bukit is closest to the water source, experiencing the highest pressure compared to downstream areas.
Increased water velocity exerts stronger pressure on finesized soil types compared to river sediments such as pebbly rocks, making them more susceptible to erosion by water.
Analysis of the eroded area reveals that the soil at the bottom of the cliff .
L2S.
consists dominantly of silt (Table .
Mass Movement /Wasting The phenomenon of mass movement/riverbank failure on the sandy cliffs/slopes occurs along the riverbanks in the Belukar Bukit Village area (Locality .
Mass movement that occurs around this river .
uspect to occur during the monsoon season of December to Marc.
shows that the banks are unstable, and are easily eroded by water This is because riverbanks always experience fluvial hydraulic erosion and geotechnical instability .
gravitational instabilit.
processes that will lead to bank failure (Surian and Rinaldi.
Islam, 2008.
Zhao et al.
, 2022.
Zhou et al.
This two process often linked together.
Fluvial erosion is the scouring of materials from bank toe by flowing water once the hydraulic force of the river exceeded the resistance threshold of the bank material, thus increasing bank angle and reduce bank stability (Langendoen and Simon Langendoen et al.
, 2.
Sediment transport will be initiated when the shear stress overcomes bank material strength that causes the soil particles to be entrained.
This happens because shear stress increases as flow increases, while bank material strength normally decreases as bank becomes saturated.
This erosion leads to unstable bank, because the forces supporting the weight of the upper bank are reduced (Baracos and Lew, 2003.
Lemnitzer et , 2.
Gravitational failures are frequently preceded by fluvial erosion, which is also responsible in transporting the debris created by gravitational Bank sediments experience varying levels of shear stress due to differences in water pressure along the slope where sediment particles located at the bottom of the slope encounter higher hydraulic head and consequently, more stress than those located higher up the bank where the hydraulic head is lower (Staley et al.
, 2006.
Zhao et al.
This results in a gradient of shear stresses across the bank.
The difference in shear stress also leads to the formation of a zone where the bank erodes at a faster rate, particularly at the bottom of the slope, which eventually creates steep or overhanging slopes.
The greatest erosion suggested occurred due to flowing water from the nearest dam .
Sultan Mahmud Da.
occupy high energy will be con-
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Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
Hulu Terengganu.
Malaysia (S.
Razali et al.
Figure 5.
A house hanging near the riverbank due to erosion at locality 1.
Road damage caused by water overflow during the flood at locality 1.
A small scarp formed by a landslide on the steep riverbank after the flood at locality 1.
Erosion occurring in the riverbank area at locality 1.
Tarred road fragments at the foot of the bank due to the riverbank .
An effect of erosion on the steep riverbank, eventually causing a landslide from a different angle.
Vegetation fell due to a landslide on the riverbank, caused by erosion at Locality 1.
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Indonesian Journal on Geoscience.
Vol.
12 No.
3 December 2025: 367-382 Mud Origin Imbricated Flow current eAects Riverbank Roots River sediment Road Water body Mud Figure 6.
Anatomical sketch of erosion in the Belukar Bukit Waterfall area.
Ajil after the rise in water level and also the rapid water current.
Clearly observed the effect of waterways through the arrangement of imbrication of pebbles on the Clay sediment as well as broken roads can be seen in the lower part of the river.
sidered as a main factor for erosion.
The erosion will take place on the lower part of riverbank by scouring and dreging the bank.
The observation has been taken on the riverbank slide.
The estimated distance of the landslide from field observations can reach more than 50 m horizontally (Figures 5e and .
The ruins of this riverside slope originally had a tarred road used by local Now, fragments of the road and vegetation can still be seen on the lower riverbank (Figures 5f and .
The original distance between the riverbank and the shoulder of the road is 6 m, and when the collapse occurred, part of the road in the middle collapsed (Figure .
The collapse of the slope occurs vertically, indicating that the slope is very steep.
Slope measurements .
were recorded in the studied area with relatively high slope values of 60A to 70A and a slope direction of 305A.
Using the friction angle, , of 31A, a simple kinematic model has been made using a stereonet to determine the type of failure (Figure .
When a flood recedes, the planar type of riverbanks is usually affected by significant erosion due to several factors.
These include.
the reduction of the hydrostatic New crack Road slide Figure 7.
Sketched area of the broken road due to the occurrence of a landslide, also clearly observed the breakdown of the tar road at the bottom of the slope.
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Analysis of Lithological Aspects and Their Influence on Slope Stability of the Failed Riverbank in Ajil.
Hulu Terengganu.
Malaysia (S.
Razali et al.
Joint/crack .
o Friction angle .
Upper slope Tension crack Slope face Slope face .
) Sliding plane .
o Figure 8.
A field observation analysis of the steep river slope in Sungai Berang at locality 2.
A geometric sketch was drawn for the purpose of making it easier to observe mass movement on the river slope.
Next to the picture is a geometric sketch showing a kinematic model that is analyzed as a planar failure of the soil due to the sliding plane .
riction angl.
< the crack .
liding surface angl.
< the slope face angle .
lope face angl.
and the crack plane is parallel to the slope face.
force/pressure caused by the decrease in the water level (Pasha et al.
, 2025.
Eskandarinejad et al.
, .
the time delay between the lowering of the water level in the river and the underground water level which generates a significant uplift force (Wang et al.
, 2020.
Ren et al.
, 2.
, and .
the decrease in suction force and apparent cohesion caused by the infiltration of rainwater or groundwater seepage (Rinaldi and Casagli.
When a slope is steep, the failure is likely to happen on a smooth and level surface where the block slides into the channel without any rotation.
Conclusions Observations at the initial stage of the study found that some study localities have experienced river erosion and also quite severe landslides, especially in the area of Belukar Bukit Waterfall and river in the area of Belukar Bukit Village.
addition, the slope angle of the river bank plays a very important impact in the scouring/dredging process by the water current.
The bigger the slope, the bigger the acting current, the more unstable the slope and the more soil loss.
The riverbanks in Hulu Terengganu show significant erosion and mass movement, with layers of lithological units that reflect dynamic sedimentary processes driven by fluctuations in river flow, particularly during flood events.
Riverbank 1 shows a sequence of cross-bedded gravel (Litho B), unsorted coarse grains (Litho C), and compacted sand-silt with oxidized plant roots (Litho D), indicating varying energy conditions.
Riverbank 2, standing higher and showing more extensive erosion, features similar layers but with thicker cross-bedding and a more prominent erosional base, suggesting stronger flood influences.
Physical test analysis reveals a predominance of sand with variations in silt and clay, affecting soil stability, with wellgraded soils contributing to stability and poorly graded soils indicating potential compaction Severe erosion and mass movements, especially during the monsoon, have significantly impacted the riverbanks, leading to infrastructure damage and highlighting the area vulnerability to further landslides and erosion due to the steep slopes, hydraulic forces, and soil characteristics.
Acknowledgments This research was funded by the Terengganu
Strategic and Integrity Institute (TSIS) research
The authors also thank Mohamad Hakim
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Indonesian Journal on Geoscience.
Vol.
12 No.
3 December 2025: 367-382 Mohd Shukri.
AAoli Naufal Allias.
Muhammad Thareq A.
Aziz.
Muhammad Fauzi Rodzee, and Muhammad Hafiz Jamaludin for their assistance in the field.
References