Supporting Report
6. Evaluation of Slope Stability
6.1. General
The following three methods are indicated as the slope stability estimation methods in the
“Manual for Zonation on Seismic Geotechnical Hazards” by TC4, ISSMFE (1993).
1) Method Grade 1: simple and synthetic analysis by using seismic intensity or
magnitude without information of geological condition
2) Method Grade 2: rather detail analysis with geological information by using site
reconnaissance result or existing geological information
3) Method Grade 3: detail analysis by using geological investigation result and numerical
analysis
For evaluation of the slope failure, many characteristics are to be considered. Especially the
following parameters are basic factors for stability of slope: scale of slope, shape of slope,
geological condition, groundwater condition, type, shape or scale of failure, strength of
ground. There are varieties of slope characteristics in the Study area. It is difficult to take
all these parameters into account for every slope. Procedure applied in this Study
corresponds to above-mentioned Grade 2 to Grade 3 method.
6.6. Present Topographic Condition and Slope Stability Condition(1) Present Topographic Condition
50m grid DTM data are used in calculation. Distribution maps of slope area ratio for
gradient over 10% and 30% are compiled. These data are summarized by each district and
slope gradient are calculated. Districs Adalar, Beykoz, Sariyer shows most slope prevailing
area. Slope area ratio of gradient less than 10%, shows 30% in these districts.
(2) Slope Stability Condition
Kutay Özaydın(2001) summarized general condition of slopes as follows:
In areas where surface geology is Güngören Formation and Gülpnar Formation,
landslide take place in many places. This sliding phenomenon is conspicuous for 1)
once ground surface gradient exceeds 30%, 2) once cut and fill work are undertaken
and 3) change of groundwater level occurs.
Erdoğan Yüzer (2001) summarized general condition of slopes as follows:
Evaluation of Slope Stability 1
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
In Asian side, surface geology is mainly rock and “landslide” is not obvious. In
European side, “landslide” is observed alongside coast lines and its adjacent areas.
This phenomena is observed far beyond Silivri District. Scale of slide is complex of
50 to several 100m sliding block. Especially eastside slope of Büyükçekmece Lake,
south coast of Avcılar District and southwest coast of Küçükçekmece lake.are typical
area of landsliding. In these area, soil strength are considered as residual conditions.
JICA Study team also observed some surface failures of slope in rock formation. In these
areas, slope gradient shows over 100% and there are residential buildings in front of and
top of failure surfaces.
Typical examples are shown in Figure 6.2.1, Figure 6.2.2 and Figure 6.2.3.
(3) Types of Slope Failure
Considering the above mentioned slope conditions, types of major slope failure are
classified as:
Area of Rock Formation
Surface failure of weathered zone or talus is considered. Large rock mass failure, of
which size exceeds several hundreds meters, are not considered. Stability of these
kinds of large failure must be examined based upon detail indivisual investigation.
Area of Tertiary Formation
Güngören Formation and Gülpnar Formation distributing areas are always suffered
from landslide activities. Ground strength is considered as residual condition. Surface
failure of weathered zone or talus is considered in other Tertiary prevailing area.
Area of Quaternary Formation and Fill Material
General circular slip is considered.
2
Supporting Report
Figure 6.2.1 Landslide in Eastside Slope of Büyükçekmece Lake
Note: Many residential buildings have been damaged.
Figure 6.2.2 Surface Failure in Üsküdar District
Note: Residential building exists in front of failure
Figure 6.2.3 Surface Failure in Pendik District
Note: Sliding is observed alongside of river slope. Horizontal length reaches to several hundreds meters. Some building exists at top of the slope.
Evaluation of Slope Stability 3
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
6.7. Method of the Slope Stability Evaluation(1) Procedure for Slope Stability Proposed By B. Siyahi
Siyahi and Ansal studied procedure of slope stability for microzonation purpose. This
procedure is introduced in “Manual for Zonation on Seismic Geotechnical Hazards” by
TC4, ISSMFE (1993) as Grade 3 method. Applicability of the procedure was confirmed
against earthquake occurred in 1967 at Akyokus Villedge, in Adapazarı region, Turkey.
Bilge Siyahi (1998) revised this procedure. The method originally proposed by Koppula
(1984) was a pseudo-static evaluation of slope stability utilizing a seismic coefficient A to
account for the earthquake induced horizontal forces. The variation in shear strength s with
depth is assumed and potential failure surface is taken as a circular arc as shown in Figure
6.3.4.
Figure 6.3.4 A Typical Section of Slope
Source: Siyahi (1998)
Parameters , , , and n are related to the geometry of the slope and configuration of
sliding surface. Shear strength is defined as s. Then safety factor, Fs, ca be defined as:
If it is assumed that shear strength changes linear with depth, and c0=0 for normally
consolidated soil, then the shear strength of soils is represent as follows:
4
Supporting Report
Then safety factor is calculated as
(eq. 6.3.1)
Thus the safety factor depends on the angle of shear strength and stability number N1
representing the configuration of the slope and failure surface. The minimum value of the
stability number are determined by carrying out a parametric study in terms of , , and
n to find out the most critical failure surface as given in Figure 6.3.5. The variation of
minimum N1 can be expressed as a function of (slope angle) and A (earthquake
acceleration). It becomes possible at this stage to calculate minimum safety factor Fs, if
value can be determined or estimated.
Horizontal axis: Slope gradient (degree)Vertical axis: Minimum shear strength stability indexA: Acceleration g: Gravitational acceleration
Figure 6.3.5 Relationship between Slope Gradient, Seismic Coefficient and Minimum Shear Strength Stability Number
Source: Siyahi (1998)
(2) Consideration of Analysis Procedure
There are varieties of slope characteristics in the Study area and it is difficult to identify
slope failure parameters for every slope in detail. Therefore, it is required that slope
stability is qualitatively evaluated assuming slope failure categorization.
Siyahi’s procedure introduced idea for obtaining minimum safety factor for various shapes
of failure surface and slope shape. And it assumes circular arc failure and normally
consolidated soil. Only slope gradient and shear strength are required data for calculation.
Evaluation of Slope Stability 5
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
Furthermore, as results of the parametric approach, this procedure is considered to extend
to not only circular surface failure but also another type of slope failure to some extent.
Slopes and failure types in the Study area are not always that of assumed in Siyahi’s
procedure. However the characteristics of the procedure acts advantageous for considering
the slope failure categorization.
In this Study, Siyahi’s procedure is applied to evaluate slope stability for small analysis
unit. And each result of evaluations is aggregated into microzonation units.
(3) Procedure of Analysis and Evaluation of Stability
The outline of the evaluation method is described below and shown in Figure 6.3.6.
Figure 6.3.6 Flowchart of Slope Failure Evaluation
Source: JICA Study Team
6
Supporting Report
a. Slope Stability Evaluation for 50m Grids
The slope gradient for each 50-m grid, that covers all of the Study area, is calculated at
first. Then the slope stability of each point is judged, using Siyahi’s equation (eq. 6.3.1)
taking the peak ground acceleration value and strength of soil into account. Score Fi = 0 for
a stable point (Fs > 1.0) or Fi = 1 for an unstable point (Fs < 1.0) is given.
b. Slope Stability Evaluation for 500m Grids
There are total 100 of 50m-grids in every 500m grid and the stability score for 500 m grid
is determined as follows:
If all 50m grids are evaluated as unstable, then Score (500m grid) is calculated as 100. If all
50m grids are evaluated as stable, then Score (500m grid) is calculated as 0. This score
directly represents how much percent of 59m grids in each 500m grid is judged as unstable.
Finally the results are represented by risk for each 500m grid, as shown in Table 6.3.1.
Table 6.3.1 Evaluation of Risks on Slope Stability for 500m Grid
Unstable Score (500m Grid) Risk Evaluation for 500m Grid
0 Very low
1-30 Low
31-60 High
61-100 Very high
6.8. Parameters for Calculation(1) Slope Gradient
Details are mentioned in the Main Report.
(2) Ground Motion
Scenario earthquake model A and model C are considered because these two scenarios is
considered to represent the most general idea of the hazard conditions.
(3) Shear Strength of Ground
Shear strength is the most important parameters for calculation. Available data on shear
strength for soil is limited and do not cover for all the geological formation. Therefore the
values are estimated considering existing two references. One is “Strength of Sliding
Evaluation of Slope Stability 7
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
Surface for Weathered Rocks”, quoted in “Design Guideline for Road Construction, Slope
Treatments and Stabilization”, Japan Road Association, 1999 (Table 6.4.3). Another one is
“Strength of Sliding Surface for Weathered Rocks”, quoted in “Slope Stability and
Stabilization Methods”, L. Abramson et al., 1996 (Table 6.4.4). Determined strength of
each formation and considered failure type are summarized in Table 6.4.2.
Table 6.4.2 Applied Angle of Shear Strength for Slope Stability Calculation
Type of Ground
Geological Formation Angle of shear Strength (Degree)
RemarksGeological Map Formation
Rock IBB 1:5,000 Kuf, Af, Gf, Df, Kf, Tf, Blf, Trf, Bg, V 25 Considering surface failure of weathered zone or talusMP 1:50,000 Kuf, Af, Gf, Df, Kf, Tf, Blf, Trf, Kz, Saf
MTA 1:25,000 tsk, ts, tq, ptq
Tertiary Sediments
IBB 1:5,000 Sf, Cf, Baf 25 Considering surface failure of weathered zone or talusMP 1:50,000 Sf, Cf, Baf
IBB 1:5,000 Cmlf 15 Same with Güf , Gnf
IBB 1:5,000 Sbf, Çf, Saf 30 Considering surface failure of weathered zone or talus. Gravelly condition are taken into account.
MP 1:50,000 Çf,
MTA 1:25,000 m2m3-19-k
IBB 1:5,000 Güf , Gnf 15 Landslides are occurring in these formations. Residual strength is considered.
MP 1:50,000 Güf , Gnf
MTA 1:25,000 e3-ol1-10-s, ebed-20-s, ebed-8-s, m3-pl-18k, ol2-18-k, ol2m1-19-k, ol-8-s,pgg
Quaternary Sediments
IBB 1:5,000 Ksf, Qal, Ym 25 General slope failureSame with weathered zoneMP 1:50,000 Oa, Q
MTA 1:25,000 Q-21-k
Fill IBB 1:5,000 Yd, Sd 25Source: JICA Study Team
Table 6.4.3 Strength of Sliding Surface for Weathered Rocks
Rock Type Number of Samples Cohesion (kN/m2) Angle of Shear Strength (degree)
Metamorphic Rocks 6 0 – 2 (1) 20 – 28 (26)Igneous Rocks 8 0 (0) 23 – 36 (29)Sedimentary Rocks Paleozoic Strata 7 0 – 4 (0) 23 – 32 (29)
Mesozoic Strata 6 0 – 10 (5) 21 – 26 (24)Palaeogene Strata 4 0 – 20 (7) 20 – 25 (23)Neogene Strata 32 0 – 25 (20) 12 – 22 (12.5)
Source: Japan Road Association (1999)Note: () shows average value
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Supporting Report
Table 6.4.4 Shear Strength of Residual Soils, Weathered Rocks and Related Minerals
Soil/Rock/Mineral Type Degree of Weathering Strength Parameters Kg/cm2 Degrees
Igneous Rocks Granite Partly weathered (Zone IIB) r = 26 – 33 Granite Relatively sound (Zone III) r = 29 – 32 Quartz diorite Decomposed; sandy, silty c=0.1 = 30 + Diorite Weathered c=0.3 = 22 Rhyolite Decomposed ’ = 30 Metamorphic Rocks Gneiss (micaceous) Decomposed (Zone IB) c = 0.3-0.6 = 23 – 37Gneiss Decomposed (Zone IC) = 18.5 Gneiss Decomposed (fault zone) c=1.5 = 27
Much decomposed c=4.0 = 29Medium decomposed c=8.5 = 35Unweathered c = 12.5 = 60
Schist Weathered (mica-schist soil) = 24.5 Partly weathered c=0.7 = 35
Schist Weathered = 26 – 30Phyllite Residual soil (Zone IC) c=0 = 18 – 24 Sedimentary rocks London clay Weathered (brown) c' = I.2 ’ = 19 – 22
r = 14 Unweathered c’ = 0.9 – 1.8 ’ = 23 – 30
r = 18 –24Keuper Marl Highly weathered c’< 0.l ’= 25 – 32
r = 18 –24Moderately weathered c' < 0. l ’ = 32 – 42
r = 22 – 29 Unweathered c’ < 0.3 ’ = 40
r = 23 – 32 Shale Shear zones = 10 – 20 MineralsKaolinite Minerals common in residual
soils and rocksr = 12 –22
Illite r = 6.5 –11.5Montmorillonite r = 40 – 11 Source: Lee Abramson, Tom Lee, Suil Sharma, Glenn Boyce,. 1996.
6.9. Slope Stability(1) Slope Stability Risk
The result of the slope stability estimation is shown in Figure 6.5.7 and Figure 6.5.8.
Generally most of the Study areas are evaluated as “very low risk”.
In case of Model A, “Very High Risk” grids exist in Adalar and Silivri. These correspond
to steep cliff and not residential area. “Low Risk” grids exist in Avcılar and
Küçükçekmece, Büyükçekmece. These correspond to residential area.
Evaluation of Slope Stability 9
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
In case of Model C, “Very High Risk” grids extend to Avcılar. , “High Risk” grids prevail
in Büyükçekmece. These correspond residential area. “Low Risk” grids extend to
Bahçelievler, Bakirköy, Güngören. These correspond to residential area.
10
Supporting Report
Figure 6.5.7 Risk on Slope Stability: Model A
Evaluation of Slope Stability 11
Fig
ure
6.5
.1R
isk
on
Slo
pe
Sta
bil
ity:
Mo
del
A
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
Figure 6.5.8 Risk on Slope Stability: Model C
12
Fig
ure
6.5
.2R
isk
on
Slo
pe
Sta
bil
ity:
Mo
del
C
Supporting Report
(2) Slope Stability Condition for each District and Geological Formation Unit
Slope risks are examined more detail level. Unstable score are summarized for each District
and each geological formation.
The stability score for each district is determined as follows:
At first, slope stability for each 50m grid is calculated. Next, number of unstable grids in a
distict is calculated. Then, area ratio for these grids is calculated. This score directly
represents how much percent of area for each district is judged as unstable.
The stability score for each geological unit is determined as follows:
At first, slope stability for each 50m grid is calculated. Next, number of unstable grids in
each geological formation is calculated. Then, area ratio for these grids is calculated. This
score directly represents how much percent of area for each geological formation is judged
as unstable.
Unstable scores are summarized for each district and for geological formation unit. Results
are shown in Table 6.5.5 and Table 6.5.6 respectively.
In Büyükçekmece district, areas of “low risk” and “high risk” are prevailing. Unstable
scores are about 3% for Model A and about 7% for Model C, respectively. This area is
characterized by landslide. Unstable area is concentrated in eastside slope of
Büyükçekmece Lake. Low strength of Güf formation is a reason of high damage ratio; even
slope gradient is not steep.
In Adalar district, areas of “high risk” and “very high risk” exist in southern part of
Büyükada Island. The area is closest to source fault. Unstable scores are about 2% for
Model A and about 5% for Model C, respectively. Unstable area concentrates in Büyükada
Island because this district is closest to earthquake source fault.
In Avcılar dıstrıct, areas of “high risk” and “very high risk” exist in southern part of the
district. Unstable scores are about 1% for Model A and about 4% for Model C,
respectively. This area is also characterized by landslide. Unstable area concentrates in
Evaluation of Slope Stability 13
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
southern coast area where Gnf formation is prevailing. Some unstable areas exist in
districts Bahçelievler, Bakirköy, Güngören, Çatalca and Silivri.
Table 6.5.5 Results of Slope Stability Analysis by DistrictDistrict Name Calculation
Points (50m grid)
Model A Model CUnstable Points
(50m grid)Unstable Score
(Average Unstable Area
Ratio %)
Unstable Points(50m grid)
Unstable Score (Average
Unstable Area Ratio %)
Adalar 3786 75 1.98 185 4.89Avcilar 15358 140 0.91 608 3.96Bahçelievler 6638 26 0.39 111 1.67Bakirköy 11678 49 0.42 95 0.81Bağcilar 8768 0 0.00 8 0.09Beykoz 15208 0 0.00 0 0.00Beyoğlu 3487 0 0.00 0 0.00Beşiktaş 7217 0 0.00 0 0.00Büyükçekmece 5520 166 3.01 402 7.28Bayrampaşa 3840 1 0.03 14 0.36Eminönü 2001 0 0.00 0 0.00Eyüp 20208 0 0.00 1 0.00Fatih 4157 3 0.07 23 0.55Güngören 2880 6 0.21 24 0.83Gaziosmanpaşa 22680 0 0.00 0 0.00Kadiköy 16304 0 0.00 0 0.00Kartal 12462 0 0.00 0 0.00Kağithane 5778 0 0.00 0 0.00Küçükçekmece 47949 59 0.12 256 0.53Maltepe 22038 0 0.00 0 0.00Pendik 18822 0 0.00 0 0.00Sariyer 11040 0 0.00 0 0.00Şişli 14161 0 0.00 0 0.00Tuzla 19641 0 0.00 0 0.00Ümraniye 18252 0 0.00 0 0.00Üsküdar 15059 0 0.00 0 0.00Zeytinburnu 4583 0 0.00 2 0.04Esenler 15552 0 0.00 16 0.10Çatalca 21054 50 0.24 144 0.68Silivri 15262 116 0.76 141 0.92Total 391383 691 0.18 2030 0.52Source: JICA Study Team
14
Supporting Report
Table 6.5.6Results of Slope Stability Analysis by Geological Formation Unit
Covering Geological
Map
Formation Name
Calculation Points
(50m grid)
Model A Model CUnstable Points
(50m grid)Unstable Score
(Average Unstable Ratio
%)
Unstable Points
(50m grid)
Unstable Score (Average
Unstable Ratio %)
IBB 1:5,000
MP 1:50,000
Gnf 18562 259 1.59 1063 6.69Çmlf 3284 1 0.03 18 0.55Güf 1991 24 1.21 77 3.87Tf 2104 3 0.14 3 0.14Af 4497 52 1.16 144 3.20Kuf 24427 16 0.07 31 0.13V 436 4 0.92 7 1.61
MTA 1:25,000
ebed-8-s 908 25 2.75 73 8.04ol2-18-k 19289 282 1.46 544 2.82ol-8-s 488 24 4.92 60 12.30pgg 1026 1 0.10 10 0.97
Total 391383 691 0.18 2030 0.52Source: JICA Study Team
Evaluation of Slope Stability 15
The Study on a Disaster Prevention/Mitigation Basic Plan in Istanbul including Seismic Microzonation in the Republic of Turkey
0
1
2
3
4
5
6
7
8
BÜYÜ
KÇEK
MEC
EAD
ALAR
AVCI
LAR
BAHÇ
ELİE
VLER
SİLİ
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GÜN
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BAKI
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YÇA
TALC
AFA
TİH
KÜÇÜ
KÇEK
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YRAM
PAŞA
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YTİN
BURN
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ŞİKT
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AZİO
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HANE
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İKSA
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RŞİ
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TUZL
AÜM
RANİ
YEÜS
KÜDA
R
District
Unst
able
Sco
re (m
ax 1
00)
Model A
Model C
Figure 6.5.9 Unstable Score (Area Ratio) of Slope by District
Souce: JICA Study Team
0
2
4
6
8
10
12
14
ol-8-s ebed-8-s Gnf Güf Af ol2-18-k V pgg Çmlf Tf Kuf
Formation Name
Unst
able
Sco
re (
max
100
) Model A
Model C
Figure 6.5.10 Unstable Score (Area Ratio) of Slope by Geological Formation
Souce: JICA Study Team
16
Supporting Report
Acknowledgement
The slope stability analysis in this Chapter was conducted under close discussions with Dr.
Prof. Kutay Özaydın, Yıldız Technical University, Fuculty of Civil Engineering,
Department of Engineering, Geotechnical Division, Dr. Prof. Erdoğan Yüzer, Istanbul
Technical University, Faculty of Mining, Geological Engineering Department, Dr. Assoc.
Prof. Bilge G. Siyahi, Boğaziçi University, Kandilli Observatory and Earthquake Research
Institute, Department of Earthquake Engineering. The Study Team expresses special thanks
to their collaboration for the Study.
Reference to Section 6
Bilge G. Siyahi, 1998, Deprem Etkısindeki Normal Konsolide Zemin Şevlerinde Yari-
Statik Stabilite Analizi, İMO Teknik Dergi, Yazı 112, 1525-1552.
Erdoğan Yüzer, 2001, Privarte Interview.
ISSMFE, 1993, Manual for Zonation on Seismic Geotechnical Hazards, Technical
Committee for Earthquake Geotechnical Engineering, TC4, International Society
of Soil Mechanics and Foundation Engineering.
Japan Road Association, 1996, Japanese Design Specification of Highway Bridge (in
Japanese).
Japan Road Association, 1999, Design Guideline for Road Construction, Slope Treatments
and Stabilization, pp. 352. (in Japanese)
Kutay Özaydın, 2001, Private Interview.
Lee Abramson, Tom Lee, Suil Sharma, Glenn Boyce, 1996, Slope Stability and
Stabilization Methods, John Willy & Sons, pp94.
Evaluation of Slope Stability 17