6 Evaluation of Slope Stability

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.



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.



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



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

8

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.



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


Fig

ure

6.5

.1R

isk

on

Slo

pe

Sta

bil

ity:

Mo

del

A


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



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

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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



0

1

2

3

4

5

6

7

8

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Unst

able

Sco

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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.


6 Evaluation of Slope Stability

Documents

stability of slope

slope gradient

scale of slope

shape of slope

types of slope failure

slope prevailing area

evaluation of slope

surface failures of