1.3 Methodology of Slope Instability Assessment
Monitoring and predicting slope instability in mountainous and rugged terrain usually is difficult and dangerous. Slope instability in natural terrains is usually unnoticed and encountered during periodic or routine visits to the jungle, such as transmission tower inspections. Determining the slope's stability is critical because slope failures can cause transmission towers to collapse, affecting the public electricity supply. Thus, in order to ensure long-term slope stability, subsurface information such as soil profile and groundwater level must be obtained. A detailed desktop study integrating geology, topography, elevation, and slope angle was practical in regional analysis to locate potential slope instability. This paper discusses the applicability of a spatially integrated mapping framework that uses spatial data, geological mapping data, and laboratory result testing to determine slope instability and influence factors contributing to landslides in the study area.
(a) Typical Site Investigation
Preparation, which includes desk study and walkover survey, is one of the key performance indicators in geotechnical site investigation, according to Baharuddin et al. (2016), and Zhao et al. (1994). This key performance indicator combines soil information obtained from the surface and subsurface at the regional and site-specific levels. A detailed and thorough desk study can be conducted at regional and site-specific scales. The latter helps to optimize the identification of borehole locations and the number of boreholes required. Three to five boreholes with 10 m intervals are typically placed in a straight line up the slope to assess slope stability. If the area is extensive, more than one line with approximately 50 m intervals is required. Boreholes should be drilled up to 6 m into the rock before being stopped when they reach a competent hard stratum. This requirement is to create a detailed subsurface soil profile to indicate the boulders or dumping sites. Due to the anisotropy and complexity of soils, the main disadvantage of conventional site investigation using a borehole is that the subsurface information obtained can be classified as localized information. Thus, an additional step of the geophysical survey would be advantageous to determine the subsurface profile of a large area before borehole drilling.
Another essential factor in slope stability analysis that is frequently overlooked is the influence of geological features such as relict discontinuities. In weathered rock profiles, researchers confirmed the importance of relict geologic discontinuities as contributing factors to many landslides. Weathering occurs at relict discontinuities, which may be partially filled with transported clay and other sediments, most commonly associated with rock mass dilation. Relict discontinuities posed a significant and uncertain risk, regardless of whether the slopes were temporary or permanent. Relict discontinuities are challenging to detect using standard site investigation procedures. Potential occurrences are ignored until apparent movements by mass shear strength or failures along these planes occur. In slope failures, relict discontinuities cause a near vertical release surface and an extensive basal slip plane (Baharuddin et al. 2016). Although slope failure due to geological features is only 6%, it is suggested that these features play an essential role in residual soil formation, particularly in sedimentary formation (Baharuddin et al. 2016). It is also recommended that confirmatory slope mapping on exposed areas be carried out.
(b) Slope Instability Mapping Framework with Spatial Integration
Geological mapping necessitates mastering a wide range of skills, including observational and interpretive abilities, a thorough understanding of rocks and geological processes, and navigational and cartographic abilities (Baharuddin et al. 2016). Over the last decade, new methods for creating detailed and accurate digital topographic models of the Earth's surface have been developed and proven extremely useful in geological mapping (Baharuddin et al. 2016).
A new framework known as spatially integrated slope instability mapping will be introduced based on advancements in geological mapping technology. Spatially integrated slope instability mapping is a forecast model that identifies slope failures by analyzing triggering variables (i.e., discontinuities and relicts joint) at regional and site-specific scales. This model is intended to use geological features such as relict discontinuities in slope stability prediction for weathering grades V and VI. Analyses were performed to identify slope instability in an area by utilizing all information from a large regional scale up to a site-specific scale (Baharuddin et al. 2016).
A regional desktop study includes landslide hazard assessment, terrain mapping, and geological mapping research. A geographic information system transforms secondary data from geological maps, land use maps, and topography maps into spatial data. These spatial data were layered and weighted to create a landslide hazard map and a terrain map. Cross-sections are used in the study area's geological map to interpret the geology state, particularly the fault occurrence or others discontinuity. The combination of terrain map, landslide hazard map, and geological mapping results indicate the condition of the area, i.e., the location of potential slope stability or the area that should be investigated for back analysis assessment. These parameters (fault, erosion, stream, lineament, and landslide area) have been identified as disrupting factors in slope stability. As a result, this process can serve as an initial indicator in site visit planning. A site visit was made to the location identified during the regional scale analysis to obtain information such as discontinuities and soil profiles through discontinuities mapping and geophysical survey. This data was later used to analyze on a site-specific scale. A kinematic analysis and slope stability were performed using SLOPE/W software to determine the type of slope failures and the factors influencing the stability of the study area. Figure 1 depicts a spatially integrated slope instability mapping framework.
1.4 Mapping using Spatially Integrated Slope Instability
1.4.1 Desk Study
The analysis begins with selecting a study area on a regional scale, as shown in Fig. 1. The study area's data was analyzed using Geographic Information System (GIS), and spatial data such as geology map, land use, and topography were overlayed and weighted to create a landslide hazard map. The topography map was used to identify the study area's boundaries: Pergau, Kelantan, and Kenyir, Terengganu. The river basin is used to justify the study area boundary. Pergau and Kenyir slope, on the other hand, was observed and identified because the slope had previously failed, and remediation works such as guniting and soil nailing was used to stabilize the slope. Recent site visits and observations revealed significant evidence of new soil movements. As a result, an integrated slope stability study was carried out to investigate the cause of slope failure. The problematic slope's location was integrated with the geological map, topographic map, and land use map to produce a digital elevation model, aspect map, slope map, and later producing landslide hazard map using geographic information system (GIS) concept, as shown in Fig. 2 (a), (b), (c), (d), (e), (f), (g), (h) and (i) depict the location of potential slope stability.
Some slope orientations are more sheltered (shaded) from sunlight than others as shows in Fig. 2c, Fig. 2d, Fig. 2e and Fig. 2f. A slope that is protected from the direct rays of the sun is more remarkable, can retain more moisture, and may have more plants. Moisture, soil, and plants all promote chemical weathering, so a slope shaded from the sun has more soil than one that is not. Rainfall runs off faster on steep slopes, and runoff may wash away weathering products. Slopes with loose soil and other materials can also slide down. Because chemical weathering is slower on a steep slope, the soil may be less developed, and hillslopes may be rockier and barren as in Fig. 2c, Fig. 2d, Fig. 2e and Fig. 2f. Weathering products can accumulate on gentle slopes, and water may remain in contact with rock for extended periods, resulting in faster weathering rates. Once formed, soil and lose pieces stay in place for extended periods.
The slope aspect, or orientation of the slope, is an essential factor in weathering. Aside from receiving more or less rain, slopes that face the Sun receive more light and heat than those that face away. As a result, sunny surfaces are warmer and drier, with more evaporation and less chemical weathering, soil, and plants than slopes facing away from the Sun. On sun-facing slopes, physical weathering is more critical. On the slope facing away from the Sun, chemical weathering will most likely be dominant. According to the analysis, most of the slopes are sun-facing, contributing to the highly weathered grade as Fig. 2g and Fig. 2h.
1.4.2 Location of potential slope stability and the probability of failure
The location of potential slope stability and the probability of failure is identified, and the area is identified as being in a low to medium landslide hazard zone as Fig. 2i and Fig. 2j. The detailed assessment using site-specific analysis is continued as Fig. 1 flow. In order to evaluate the condition of the slope using site specific concepts as Fig. 1, the combination of geological investigation technique i.e. geophysics’ survey, macroscopic and microscopic rock identification were used along with laboratory testing, rock quality designation (RQD %) and N value from standard penetration test (SPT).
Geological mapping and borehole in Pergau, Kelantan shows that the area is dominated by a metamorphic rock which is phyllite. Phyllite is an intermediate-grade, foliated metamorphic rock type that resembles its sedimentary parent rock, shale, and its lower-grade metamorphic counterpart, slate. This rock mostly has a high grade of weathering from grade IV to V. The geological structure, such as foliation still prominent in a particular area, and the reading of strike/dip of the foliation is 076ᵒ/50ᵒ. At the southeast part of this map have mica schist outcrop as shown in Fig. 3. Faults can be found near granite bedrock. This rock was highly weathered at grade IV to V and still has foliation, and the reading of strike/dip of the foliation is 076ᵒ/50ᵒ. Figure 4 shows the geological mapping and cross-section at Pergau, Jeli, and Kelantan.
Rock Mass for Kenyir, Terengganu area shows the area consists of sedimentary rocks from the Carbonaceous age, mainly sandstone and shale intruded by granitic masses as Fig. 5. Sandstone is a sedimentary rock composed mainly of quartz, feldspar and rock fragments with sand-size minerals or rock grains. Kenyir, Terengganu, on the other hand, is dominated by igneous rock, specifically an acidic intrusive granitic rock, as shown in Fig. 6. Granite is a light-colored intrusive igneous rock with visible grains to the naked eye. It is formed due to the slow crystallization of magma beneath the Earth's surface. Granite primarily comprises of quartz and feldspar, with trace amounts of mica, amphiboles, and other minerals. Granite with this mineral composition is typically red, pink, grey, or white, with dark mineral grains visible throughout the rock. The predominant soil type in this area was sandy soil with clay from weathered phyllite and acid intrusive (existence of quartz vein). Because of environmental factors, most of the area has already weathered between grades IV and VI. The main geological structure in this area mainly consists of the relict joint. This structure has the potential to impact landslides related to slope failure significantly.
The result from the borehole indicates that Pergau consists of Sandy CLAY and weathered phyllite. This area's primary type of soil consists of sandy SILT soil from weathered phyllite above the groundwater level. Sandy CLAY layers were discovered at 5.0 m to 10.0 m depth below the ground water, consisting of the firm to very stiff clay layers with SPT-N values 7 to 29. Whereas the slope at Kenyir, Terengganu consists of soil and rock. This area's primary type of soil consists of sandy soil with clay from weathered granite. A Boulder of granite sized 25.0m x 10.0m was discovered in this area.
Landslide hazard maps shown in Fig. 2i and Fig. 2j are used as a based map to produce site-specific landslide map as Fig. 4 and Fig. 5. The maps are developed based on information from the borehole, geological and cross-section maps, landslide maps, and field investigation by mapping landslide signs, scarp area, accumulation areas, and verification of landslides mapped through image interpretation and classification as results shows in Fig. 4 and Fig. 6.
1.4.3 Microscopic Analysis
Microscopic analysis sample from Pergau, shows the sample consists of very fine-grained rock, made up of an intimate association of very fine-grained elongated quartz grains (Q) and very fine-grained laths of muscovite (M). Quartz and muscovite grains form relatively strong foliation in the more or less E-W direction. Iron-oxide mineral grains (O) are evenly distributed in this rock. The composition of phyllite is Quartz (35%), muscovite (55%), and iron oxides (10%). The mineral texture is differentiated based on the type of minerals. Quartz occurs as tiny grains, ranging from a few tenths of µm to 0.1 mm, with an average of 0.05 mm. Most grains are anhedral and elongated, orientated parallel to the rock foliation. The width-to-length ratio of quartz grains is 1:1 to 1:3, with 1:2 as the average. Most grains are free of inclusion and show sharp extinction. Muscovite occurs as long laths, 1–3 mm in size. The width-to-length ratio is 1:8 − 1:10. The muscovite shows some degree of deformation, as shown by its crumpled nature. The crumpled muscovite grains texture indicates that the rock has undergone regional metamorphism. Sericite occurs as tiny grains, a few tenths of µm to 0.1 mm, with an average of 0.05 mm. The width-to-length ratio of Sericite is 1:3. Sericite occurs due to the weathering process of these phyllite rocks. Iron-oxides occur as anhedral grains, 0.1 mm in size. Rock texture shows the rocks are rather strongly foliated, formed by a specific arrangement of the intimate association of muscovite and quartz. Due to the very fine-grained nature of the rock, the accurate term for the foliation is slaty cleavage. Fractures 0.5–1 mm wide parallel to the rock's foliation are common. Figure 7 shows the condition of the phyllite sample in the study area. However, grades V and VI, or residual soil, are differentiated based on the boulder occurrence. The soil name is classified based on distribution analysis names as sandy SILT and sandy CLAY.
Macroscopic observation within this area indicates that the sample was intact but a moderately weathered block of massive rock. The sample lacks a bedding plane or foliation plane. It is very pale brown in color (HUE 10YR 8/4 on Munsell Color Charts). The rock is fine-grained; no minerals can be recognized by the naked eye. Mineral texture such as Quartz occurs as anhedral grains, a few tenths of µm to 0.6 mm (average is 0.25 mm). Quartz occurs in two states: As monocrystalline Quartz and as polycrystalline Quartz. Most monocrystalline grains maintain their sub-rounded to a rounded shape, inherited from the original clastic sedimentary rock, but smaller grains lose their granular nature – instead, it shows interlocking features. All quartz grains show sharp extinction, and almost all grains are fractured. Muscovite also occurs as tiny grains (10–20 µm) surrounding quartz grains. This type of muscovite is more appropriately called sericite. Although granoblastic texture has been observed, indicating recrystallization and deformation, the deformation intensity applied to the rock does not reach a point where the original clastic sedimentary rock begins to form foliation. The formation of foliation is always delayed in rocks where equidimensional quartz grains are the main constituent – a high degree of deformation is required to elongate and orientate the quartz grains. It is suggested that the original rock within this area is clean sandstone (arenite) that has undergone a low degree of metamorphism to form the meta-arenite. Figure 8 shows the location of Quartz (Q) with sharp extinction and tiny grains of muscovite (M), 10–20 µm in size. The image also presents a perfect granoblastic texture (G) shown by a polycrystalline quartz grain. Most quartz grains are fractured (F), and evidence of foliation is lacking.
Kenyir, Terengganu, on the other hand, is dominated by igneous rock, specifically an acidic intrusive granitic rock, as shown in Fig. 9. Granite is a light-colored intrusive igneous rock with phaneritic texture and medium in grain size. Granite primarily comprises quartz (Q), plagioclase (P) and feldspar (F), with trace amounts of muscovite and biotite (mica), and other minerals. Granite with this mineral composition is typically grey, or white, with dark mineral grains visible throughout the rock. Mineral texture such as Quartz occurs as anhedral grains, while plagioclase shows the perthitic texture. Muscovite also occurs as tiny grains (10–20 µm) surrounding quartz grains. This type of muscovite is more appropriately called sericite.
1.4.4 Resistivity and Seismic Survey Test
Geophysics surveys were carried out on slopes in Pergau, Kelantan (Fig. 10) and Kenyir, Terengganu (Fig. 11). Geophysics surveys, including resistivity and seismic, were carried out along the same line covering the original and landslide area as shown in Fig. 10 and Fig. 11. The testing equipment for the characterization of soil electrical resistivity in this study is known as ABEM Terrameter LUND Imaging System. The testing procedure was carried out according to the recommendation of ABEM Instruction Manual Terrameter SAS 1000/4000. The ABEM SAS4000 Terrameter in the study is shown in Fig. 12(a). This equipment used electrical imaging to produce continuous images of the variation in properties in the subsurface. Resistivity data were collected by a 2D electrical resistivity imaging technique (ABEM Terameter) using the Schlumberger method. Electrical imaging was undertaken using an insulated multi-core cable with several fixed intervals take-off points to which electrodes are connected. The cable was connected to the resistivity meter, which in turn, connected to a laptop computer with relevant software to run the process. A parameter file was written to instruct the computer which sets of 41 electrodes to be used and the currents to apply with 5.0 meters or 2.5 meters spacing. The data captured by electrical imaging was raw data that was needed to depict the soil profile at the site. In order to determine the result with higher accuracy, the raw data from the resistivity test were transferred to the software program RED2DINV to inverse the data and get the type of soil profile, depth, contour, and groundwater level. The resistivity images were recorded in 2-dimensional axes form, where the y-axis is the elevation from sea level, and the x-axis is the length of the survey line where the electrodes were buried. This survey was able to detect the type of soil and rock layers to a depth of 40.0 m from ground level.
Seismic refraction is based on the first arrival of a signal that travels through a layer with a higher velocity. The equipment to carry out this survey is shown in Fig. 12(b), which includes 24 channels ABEM Terraloc Mark 6, geophones for S-wave and P-wave and a hammer. Seismic methods are based on the measuring of an elastic wave (also: seismic, shockwave, or acoustic wave) travelling through the sub-surface. It measured the time taken by seismic waves to travel from the source through the medium and back to the source, which reflects the type of medium. The data was collected by using Interpex Seismic Refraction Interpretation Software (IXRefraX). The maximum seismic length is 120 meters with 5.0 meters of geophone spacing. In this survey, the researcher used a hammer to produce pressure waves. Energy radiates out from the shot point, either travelling directly through the upper layer (direct arrivals) or travelling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on the surface using a linear array of geophones. Observation of the travel times of the refracted signals provides information on the depth profile of the refractor. The seismic image is recorded in the form of 2-dimensional axes, where the y-axis is the travel time of seismic wave (ms− 1) and depth below ground level (m), and the x-axis is the length of the survey line where the geophones were buried.
Based on a resistivity survey performed at Pergau, Kelantan, the existence of groundwater levels has been found at a depth of 10.0m from ground level for both lines. Generally, the subsurface is made up of sandy SILT soil from weathered phyllite above the groundwater level, which a range of resistivity value from 1311–2780 Ohm-m for line 1 and 251–260 Ohm-m for line 2. A boulder of weathered phyllite grade IVa sized 10.0m x 5.0m was discovered at 5.0 m below the ground surface for line 1, and a boulder of weathered phyllite grade III sized 5.0m x 10.0m above the ground water level for line 2 which range of resistivity value more than 2771–3390 Ohm-m. The firm and very stiff layers of Sandy CLAY were interpreted below the groundwater level with a range of resistivity value 31–150 Ohm-m at line 1. Figure 13 and Fig. 14 show the results of the resistivity survey of Pergau for line 1 and line 2, respectively.
The Standard Penetration Test (SPT-N) and Rock Quality Designation (RQD) for soil and rock types in Table 1 have been interpreted at 1.5 m depth from the surface level based on the depth of the borehole. This area consists entirely of weathered rock with RQD values of 25–50% and only a quarter of the area with RQD values of 50–75% for both lines. There are boulders of rock that have high RQD values of 50–75% at lines 1 and 2. The Sandy CLAY soil in this area, which has SPT-N values from 7 to 29, was discovered below the groundwater level. This soil can be categorized as firm to very stiff clays. Therefore, lines 1 and 2 detected weathered phyllite grade IVb with RQD values of 25–50% at 1.5m depth. The summary of the resistivity survey test for lines 1 and 2 at Pergau, Batu Melintang, is shown in Table 1.
Table 1
Summary of resistivity survey test for Pergau, Batu Melintang
|
Line
|
Point
|
Max elevation obtained
(m, BGL)
|
Min elevation obtained
(m, BGL)
|
Water level
(m, BGL)
|
Range of ohm at 1.5m depth
|
SPT-N value at 1.5m
(if soil)
|
RQD value at 1.5m (%)
(if rock)
|
Hard layer level, N > 50 or RQD > 1%
(m, BGL)
|
|
1
|
A
|
176.0
|
140.0
|
10.0
|
1311–1320
|
-
|
25–50
|
NSL
|
|
B
|
175.0
|
149.5
|
5.0
|
1311–1320
|
-
|
25–50
|
NSL
|
|
C
|
177.5
|
150.0
|
10.0
|
2771–2780
|
-
|
50–75
|
NSL
|
|
2
|
A
|
165.0
|
129.0
|
10.0
|
251–260
|
-
|
25–50
|
NSL
|
|
B
|
172.5
|
145.5
|
10.0
|
1031–1040
|
-
|
25–50
|
NSL
|
|
C
|
165.0
|
142.5
|
10.0
|
251–260
|
-
|
25–50
|
NSL
|
Note:
BGL = below ground level; NSL = near surface level; Hard layer occurs when ohm value is more than 250 ohm which indicates rock with different weathering grade.
|
From the resistivity survey conducted with this Kenyir, Terengganu, the existence of groundwater levels has been found at a depth of 5.0m from ground level for both lines, line 1 and line 2. Generally, the subsurface is made up of sandy soil with clay from weathered granite grade V and IV, which has a resistivity value from 411–1310 Ohm-m for line 1 and 1221–2650 Ohm-m for line 2. Boulder of weathered granite grade I sized 25.0m x 10.0m with the resistivity values is more than 10000 Ohm-m was discovered at this site as shown in Fig. 15 and Fig. 16.
The Standard Penetration Test (SPT-N) and Rock Quality Designation (RQD) for soil and rock types in Table 2 have been interpreted at 1.5m depth from the surface level based on the depth of borehole. This area consists entirely of weathered rock with RQD values of 25–75%. There are several boulders of rock that have high RQD values of 75–90% and also more than 90%. Therefore, line 1 and 2 detected weathered granite grade V and IV with RQD values of 25–50% and 50–75% at 1.5m depth. The summary of resistivity survey test for line 1 and line 2 of Kenyir, Terengganu is shown in Table 2.
Table 2
Summary of resistivity survey test for Kenyir, Terengganu
|
Line
|
Point
|
Max elevation obtained
(m, BGL)
|
Min elevation obtained
(m, BGL)
|
Water level
(m, BGL)
|
Range of ohm at 1.5m depth
|
SPT-N value at 1.5m
(if soil)
|
RQD value at 1.5m (%)
(if rock)
|
Hard layer level, N > 50 or RQD > 1%
(m, BGL)
|
|
1
|
A
|
347.5
|
312.5
|
5.0
|
1301–1310
|
-
|
50–75
|
NSL
|
|
B
|
351.0
|
337.5
|
5.0
|
411–420
|
-
|
25–50
|
NSL
|
|
C
|
348.5
|
325.0
|
5.0
|
411–420
|
-
|
25–50
|
NSL
|
|
2
|
A
|
347.5
|
314.0
|
5.0
|
1221–1230
|
-
|
50–75
|
NSL
|
|
B
|
350.0
|
335.0
|
5.0
|
1221–1230
|
-
|
50–75
|
NSL
|
|
C
|
349.0
|
327.5
|
5.0
|
2641–2650
|
-
|
50–75
|
NSL
|
Note:
BGL = below ground level; NSL = near surface level; Hard layer occur when ohm value is more than 250 ohm which indicates rock with different weathering grade.
|
Based on the seismic analysis for Pergau, Batu Melintang, the subsurface is made up of sand with gravel and clay for both lines. Overall, there are five layers of soil type, sand with gravel with a thickness of 21.3 m for both lines and followed by two layers of clay which are about 10.6–10.9 m thick. This study area shows that the lower ground is denser than the upper part of the soil, which is medium. The sand with gravel shows there is a fracture zone in this area, as shown in the detailed seismic result in Table 3.
Table 3
Summary of resistivity survey test for Pergau, Batu Melintang
|
Line
|
Thickness of first layer (m)
|
Velocity of second layer
(ms− 1)
|
Type of soil
|
Consistency1
|
Allowable Soil Pressure2 (kPa)
|
SPT-N Value3
|
|
1
|
0-(-2.5)
2.5
|
524.48
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-2.5-(-7.6)
5.1
|
585.94
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-7.6-(-13.8)
6.2
|
662.25
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-13.8-(-15.2)
1.4
|
763.36
|
Sand with gravel
|
Dense
|
280–470
|
30–50
|
|
-15.2-(-21.3)
6.1
|
898.20
|
Sand with gravel
|
Dense
|
280–470
|
30–50
|
|
-21.3-(-25.4)
4.1
|
1094.89
|
Clay
|
Stiff
|
280–470
|
8–15
|
|
-25.4-(-32.2)
6.8
|
1395.35
|
Clay
|
Stiff
|
280–470
|
8–15
|
|
2
|
0-(-2.5)
2.5
|
536.67
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-2.5-(-7.6)
5.1
|
597.61
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-7.6-(-13.8)
6.2
|
674.16
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-13.8-(-15.2)
1.4
|
775.19
|
Sand with gravel
|
Dense
|
280–470
|
30–50
|
|
-15.2-(-21.3)
6.1
|
909.09
|
Sand with gravel
|
Dense
|
280–470
|
30–50
|
|
-21.3-(-25.4)
4.1
|
1102.94
|
Clay
|
Stiff
|
280–470
|
8–15
|
|
-25.4-(-31.9)
6.5
|
1395.35
|
Clay
|
Stiff
|
280–470
|
8–15
|
1, 2, 3 Modification from Friedel et al. (2006) for integration.
The seismic test for Kenyir, Terengganu analysis results within this study area indicate that the subsurface is made up of loose sand, sandy CLAY and sand with gravel. The first and second layer of soil is loose sand which is about 9.4 m thick, with its velocity ranging from 300 ms− 1 to 335 ms− 1. Then, it’s followed by 8.1 m of sandy CLAY (third and fourth layer) and ends up with 13.2 m of sand with some gravel resulting from the weathering of Granite. The upper parts of the soil are soft and loose at depths from 0 to 25.4 m and change to medium at depths 25.4 m and below. For line 2, the topsoil is made up of loose sand, about 1.7 m thick, with a velocity of 321.89 ms− 1. The second and third layers of soil are made up of sandy CLAY, about 16.0 m thick, followed by 6.1 m of sand. In this area, the type of soil ends up with 12.9 m of sand with gravel. Overall, the consistency of the soil is loose at the upper part (at depth 0–23.8 m) and changed to medium (at depth 23.8 m and below) on the lower part of the soil.
Table 4
Summary of seismic survey test for Kenyir, Terengganu
|
Line
|
Thickness of first layer (m)
|
Velocity of second layer
(ms− 1)
|
Type of soil
|
Consistency1
|
Allowable Soil Pressure2 (kPa)
|
SPT-N Value3
|
|
1
|
0-(-4.2)
4.2
|
300.30
|
Loose Sand
|
Loose
|
2–60
|
0–4
|
|
-4.2-(-9.4)
5.2
|
335.20
|
Loose Sand
|
Loose
|
2–60
|
0–4
|
|
-9.4-(-12.7)
3.3
|
378.79
|
Sandy CLAY
|
Soft
|
25–50
|
0–2
|
|
-12.7-(-17.5)
4.8
|
435.41
|
Sandy CLAY
|
Soft
|
25–50
|
2–4
|
|
-17.5-(-25.4)
7.9
|
512.82
|
Sand with gravel
|
Loose
|
0–80
|
4–10
|
|
-25.4-(-30.7)
5.3
|
622.41
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
2
|
0-(-1.7)
1.7
|
321.89
|
Loose Sand
|
Loose
|
2–60
|
0–4
|
|
-1.7-(-7.8)
6.1
|
362.76
|
Sandy CLAY
|
Soft
|
25–50
|
0–2
|
|
-7.8-(-17.7)
9.9
|
416.09
|
Sandy CLAY
|
Soft
|
25–50
|
0–2
|
|
-17.7-(-23.8)
6.1
|
487.8
|
Sand
|
Loose
|
0–80
|
4–10
|
|
-23.8-(-30.4)
6.6
|
588.24
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
|
-30.4-(36.7)
6.3
|
742.57
|
Sand with gravel
|
Medium
|
80–280
|
10–30
|
1, 2, 3 Modification from Friedel et al. (2006) for integration.
1.4.5 Slope Stability Analysis by Using SLOPE/W
Slope stabilization methods must be understood and modelled in realistic ways. An understanding of geology, hydrology and soil properties is needed before applying slope stability principles correctly. Analyses must be based on a model that accurately represents site subsurface conditions, ground behavior and applied loads. For soil type of slope, Bishop Method is used for stability analysis. The bishop method is one of the applications in the stability analysis inside the SLOPE/W Software. The bishop method is an iterative solution with a Factor of safety (FOS) on both sides of the equation, as in Eq. 1, including the horizontal interslice forces in the solution of the moment equilibrium. However, it ignores the interslice shear forces and horizontal equilibrium.
Generally, the FOS equation is:
$$FS=\frac{1}{{\sum }_{i}{W}_{i}.\text{sin}{\alpha }_{i}}{\sum }_{i}\frac{\left({c}_{i}.{b}_{i}+\left({W}_{i}-{u}_{i}.{b}_{i}\right)\text{tan}{\phi }_{i}\right)}{\text{cos}{\alpha }_{i}+\frac{\text{tan}{\phi }_{i}. \text{sin}{\alpha }_{i}}{FS}}$$
1
where;
ui pore pressure within block
ci, φi effective values of soil parameters
Wi block weight
αi inclination of the segment of the slip surface
bi horizontal width of the block
A factor of safety (FOS) can mean either the fraction of structural capability over that required or a multiplier applied to the maximum expected load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. Appropriate factors of safety are based on several considerations. Prime considerations are the accuracy of load, strength, and wear estimates, the consequences of engineering failure, and the cost of over-engineering the component to achieve that Factor of safety.
In slope design, generally in the area of geotechnical engineering, the factor which is very often in doubt is the shear strength of the soil. The loading is known more accurately because it usually merely consists of the self-weight of the slope. The FOS is therefore chosen as a ratio of the available shear strength to that required to keep the slope stable. Table 5 shows the guideline for the limit equilibrium of slope and different factors of safety obtained from the calculation (Friedel 2006).
Table 5
Guideline for limit equilibrium of a slope (Friedel 2006)
|
Factor of safety
|
Details of slope
|
|
< 1.0
|
Unsafe
|
|
1.0–1.25
|
Questionable safety
|
|
1.25–1.4
|
Satisfactory for routine cuts and fills, Questionable for dams or where failure would be catastrophic.
|
|
> 1.4
|
Satisfactory for dams / slope.
|
Results for slope stabilization was analyzed by type of soil based on observations at the site and laboratory test, SLOPE/W analysis and factor of safety (FOS) for slope. The type of soil can be classified by particle size distribution using a sieve analysis test, and shear strength parameters (i.e. cohesion value and friction angle) were by direct shear box test or triaxial unconsolidated undrained (UU) test. The summary of laboratory test result is shown in Table 6.
|
Type of test
|
Result (Pergau)
|
Result (Kenyir)
|
|
Soil Classification Test
|
Particle Size Distribution
|
Gravel (> 2mm)
|
0.3
|
0.0
|
|
Sand (0.06-2mm)
|
33.0
|
36.8
|
|
Silt + Clay (< 0.06mm)
|
66.7
|
63.2
|
|
Moisture Content (%)
|
7.6
|
37.6
|
|
Specific Gravity
|
2.63
|
2.65
|
|
Units Weight (kN/m³)
|
16.00
|
16.25
|
|
Soil Classification Based on BSCS
|
Sandy SILT
|
Sandy Clay
|
|
Mechanical Properties Test
|
Triaxial Test
|
Friction Angle (°)
|
20
|
8
|
|
Cohesion (kN/m²)
|
6
|
32
|
SLOPE/W analysis that was carried out near the tower showed the factor of safety (FOS) for slope Pergau, Batu Melintang is 0.48, which is an unstable slope. The exact place and factor of safety for the critical slope of Pergau, Batu Melintang is shown in Fig. 17, and Table 7 indicates the description of layers at the slope.
The results of the SLOPE/W analysis for Kenyir, Terengganu, on the other hand, showed the factor of safety (FOS) equal to 0.76, which is in unsafe categories. Therefore, appropriate stabilization methods also should be adopted. The exact place and factor of safety for the critical slope of Kenyir, Terengganu is shown in Fig. 18, and Table 8 indicates the description of layers at the slope.
Table 7
Description for layer at slope of Pergau, Batu Melintang
|
No. of layer
|
Type of soil
|
Note
|
Unit weight
|
Cohesion
|
Friction angle
|
|
1.
|
Sandy SILT
|
Above ground water level
|
16.00kN/m³
|
6.0kN/m²
|
20°
|
|
2.
|
Saturated Sandy CLAY
|
Below ground water level
|
18.00kN/m³
|
10.0kN/m²
|
18°
|
Table 8
Description for layer at slope of Kenyir, Terengganu
|
No. of layer
|
Type of soil
|
Note
|
Unit weight
|
Cohesion
|
Friction angle
|
|
1.
|
Sandy CLAY
|
Above ground water level
|
16.25 kN/m³
|
8.0 kN/m²
|
30°
|
|
2.
|
Saturated Sandy CLAY
|
Below ground water level
|
18.0 kN/m³
|
8.0 kN/m²
|
32°
|