S2 Geo 1 Landslide Mechanism

Contents/ syllaby

• Introduction (Definition, Scope of study, Objective of study) – First week

• Rock mass classification & index properties (week 2 – 4)

• Rock strength and Failure criteria (week 5 – 6)

• Initial Stress & the measurement ( wee8 – 9)

• Planes of weakness in rock or rock discontinuity (week 10)

• Rock deformability (week 11)

• Applied rock mechanics ;– Rock slope engineering (week 12)

– Foundation Engineering (week 13)

– Underground excavation (week 14)

ROCK SLOPE STABILITY

(Teuku Faisal Fathani)

References:

- Introduction to Rock Mechanics (Goodman,

R.E., 1980)

- Rock Slope Stability (Giani, G.P., 1992)

- Rocks and Rocks Minerals (Dietrich, R.V.,

1979)

Topics

1. Landslide mechanism and causes

2. Rock strength

3. Rock slope engineering

4. Shear strength and failure criteria

5. Rock slope stability

6. Peak ground acceleration

7. Movement prediction

Mechanism and Causes of

Landslides

Landslide blocked both the river stream and the highway.

Jizukiyama

Landslide,

Japan

Many houses were

buried by the slide.

Sueling Landslide

Cibangkong Landslide, Banyumas Province

Type of Movement Type of Material

Bedrock Engineering soils

Predominantly coarse Predominantly fine

Falls Rockfall Debris fall Earth fall

Topples Rock topple Debris topple Earth topple

Slides: Rotational

A few units Rock slump Debris slump Earth slump

Slides: Translational

A few units Rock block slide Debris block slide Earth block slide

Many units Rock slide Debris slide Earth slide

Lateral spreads Rock spread Debris spread Earth spread

Flows Rock flow (deep

creep)

Debris flow (soil

creep)

Earth flow (soil

creep)

Complex Combination of two or more principal types of movements

Geological formations are subdivided into bedrock, debris soil and earth

soil. Slope movements were classified by Varnes (1978) into 18 types.

The abbreviated classification of slope movements:

Type of

Movement

Identification Type of Movement Identification

Rockfall Debris slide

Rock topple Debris spread

Rock slump Debris flow

Rock slide Earth fall

Rock spread Earth topple

Rock flow Earth slump

Complex Earth slide

Debris fall Earth spread

Debris topple Earth flow

Debris slump

Frequency of the Varnes classification movement types and identification

difficulty degree in the Italian geological environment (Carrara et al., 1985)

Slope movement types :

Large diffused slope; Average diffuse; Rare diffused

Easily identifiable slope; Difficult to identify; Unlikely identifiable

Rate Definition term

> 3 m/s Extremely rapid

> 3 m/min Very rapid

> 1.5 m/day Rapid

> 1.5 m/month Moderate

> 1.5 m/year Slow

> 0.006 m/year Very slow

< 0.006 m/year Extremely slow

Slope movement scale (after Varnes, 1978)

HEAVESLIDE

DRY

WET FAST

SLOW

FAST SLOW

Rock Slide

Landslide

Earth Flow

Mud Flow

Debris

FLOW

Carson and Kirkby (1972) proposed the classification of slope

movements based on water content of sliding mass and slope

movement velocity

Difference between landslide and slope failure

Landslides Slope Failures

Geology Occur in places with particular

geology or geological formation

Slightly related to geology

Soils Are mainly active on cohesive

soil such as slip surface

Frequently occur even in sandy

soils

Topography Occur on gentle slopes of 5° to

20°Frequently occur on the slopes

steeper than 30°

Situation of

activities

Continuous, or repetitive

occurrences

Occur suddenly

Moving velocity Low at 0.001 to 10 mm/day High speed > 100 mm/day

Masses Have little disturbed masses Have greatly disturbed mass

Provoking causes Greatly affected by groundwater Affected by rainfall intensity

Scale Have a large scale between 1

and 100 ha

Have a small scale. Average

volume is about 440 m3

Symptom Have cracks, depressions,

upheavals, groundwater

fluctuation, before occurrence

Have few symptoms and

suddenly slip down

Gradient 10° to 25° 35° to 60°

Some terms describing a landslide

(Cruden and Varnes, 1996)

Some terms describing a landslide

(Cruden and Varnes, 1996)

Causes of Landslide

• Rainfall or storm rainfall the rising of

groundwater level

• Construction works Earthwork, Cutting,

Filling, Tunnel construction,

• Reservoir induced landslide the rising and

drawdown of reservoir level

• Earthquake horizontal acceleration gx, gy

The main causative factors of Landslides

(Anagnostopoulos, 2005)

1. Climatic conditions

2. Topography

3. Lithology and distribution of soil and

rock formations (Geological Conditions)

4. Past and recent tectonic activity

(Seismicity)

5. Vegetation

6. Human activities

Rainfall – Storm Rainfall

• The magnitude of the absolute amount of annual

rainfall is not always related to the occurrence

rate of landslides

• Because the landslide occurrence related to

difference factors such as lithology-geology,

topography, vegetation, human interfere,

amount of rainfall vs duration of the event

A

ccum

ula

ted r

ain

fall

am

ount

Hourly r

ain

fall

am

ount

(in C

hoshi)

The n

um

ber

of

occurr

ence o

f la

ndslid

es

in c

liffs

Om

igaw

a-m

achi

Fig. 5.1.2 The Relationship between the Landslides in the Cliffs inOmigawa-machi

in Chiba Prefecture and the Rainfall Amount (Original by Hosono)

(The time when the Typhoon No. 25 hit in September 1971)

The relationship between landslides in the cliffs in

Omigawamachi-Chiba Prefecture and the rainfall amount

Accum

ula

ted r

ain

fall

am

ount

Landslid

es in

clif

fs

（S 42）1967

Hourly r

ain

fall

am

ount

Accum

ula

ted

rain

fall

am

ount

Landslid

es

in c

liffs

Hourly r

ain

fall

am

ount

Num

ber

of

occurr

ence

Fig. 5.1.3 The Relationship between the Rainfall Amount in City of Kobe and

the Time at which Landslides in the Cliffs Occurred (Original by Hosono)

The relationship between the rainfall amount in Kobe city and

the time at which landslides in the cliffs occurred

8th, 9th, 10th, 11th, 12th, 13th

September 1976

The Rainfall Amount in Ichinomiya

The landslide occurred in Nukiyama

Fig. 5.1.4 The Status of the Rainfall in Ichinomiya-machi in Hyogo Prefecture

Groundwater Level and Landslide

Movement

• Many reports presented about the correlation between

rainfall and the groundwater level in the landslide site,

and the relationship between the groundwater and

landslide displacement.

• Watari describes that there is a close relationship

between the water level of ponds and landslide

movement in Takizaka landslide (Fukushima Pref.)

• Taniguchi performed a soil mechanic analysis on the

groundwater level and the landslide movement velocity

in Kamiya landslide (Niigata Pref.).

Fig.5.1.6 The Relationship between the Daily Rainfall Amount and

the Displacement Velocity in Mt. Chausu (by Fukuoka)

The relationship between the daily rainfall amount and

displacement velocity in Mt. Chausu)

Fig.5.1.9 The Relationship between the Pore Water Pressure at the Sliding Surface or

Groundwater Level and the Velocity (by Fukuoka)

The relationship between pore water pressure at the sliding

surface or groundwater level and the velocity of landslide

Landslide due to construction works

• Recently, there are increasing cases of landslides in

mountainous areas, caused by large-scale cuttings and fillings

associated with construction of roads, tunnels, or large-scale

land developments.

• These landslides jeopardize execution of the project that has

caused the landslide, or oblige the project to be greatly

modified, or seriously damage, or threaten the safety of

houses and important facilities in the surroundings.

• Most of these landslides could have been avoided if a detailed

study had been done and appropriate preventive measures

based on the results of the detailed study had been taken

Road cutting

Human-induced landslide

Subsidence

Deformed sliding surface

Loosened zone

Case of A): Occurrence of

loosening and subsidence

Sliding surface with reduced stability

Loosened zone

Case of B): Loosening

Potential sliding surface

Transited sliding surface

Case of C): Transition of sliding surface, tunnel drilling too

close to the sliding surface

Fig.5.2.8 Tunnel Drilling beneath Sliding Surface

Tunnel Drilling beneath

Sliding Surface

Sliding cliff

Landslide mass Subsidence

Tunnel

Block

Sliding surface

Fig.5.2.9 Loosening by Tunnel Drilling and Occurrence of Landslide

Loosening by Tunnel Drilling and Occurrence of Landslide

Reservoir induced landslides

• The countermeasures of landslides around dam

reservoirs have been carried out to date, especially

after the occurrence of landslide at the Vaiont Dam in

Italy 1963 that resulted in 2,600 deaths

• The reservoir-induced landslides include those

occurring with the rise of reservoir level and those

occurring with rapid drawdown of the reservoir level

(Yoshimatsu, 1981).

Dam (Reservoir) construction Human-induced landslide

Landslide at A dam reservoir

Location : A dam, A1 area, Shikoku Island, Japan.

Geological feature:

Site investigation:

1. Tiltmeter

2. Piezometers installed in the boreholes

Weathered slate and schalstein (Mesozoic & Paleozoic)

Fracture-zone type landslide

Reservoir level, tiltmeter fluctuation and rainfall

at A dam area

Precipitation (mm)

Cracks found

Typhoon

No. 13

Typhoon

No. 19

1 m/day

1 m/day

0.5 m/day

Rese

rvo

ir level (m

)

210

200

190

180

170

160

150

Tiltm

ete

r fl

uctu

ati

on

(sec

)

4/1 5/1 6/1 7/1 8/1 9/1Time

5040302010

50

0

-50

-100

1 c= 0 kN/m2 ; =34.25o

2 c= 5 kN/m2 ; =32.15o

3 c=11 kN/m2 ; =29.50o

4 c=15 kN/m2 ; =27.65o

5 c=20 kN/m2 ; =25.26o

6 c=25 kN/m2 ; =22.77o

7 c=30 kN/m2 ; =20.19o

8 c=62.54 kN/m2 ; =0o

FS change by the rising of reservoir level

0 10 20 30 40 50 60 70 80 90

Distance (m)

160

170

180

190

200

210

220

230

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Safety factor

Res

erv

oir

lev

el (

m) a

V

III

II

I

IV

Sliding mass

Bedrock

875 61 2 3 4

I Low level

II Limiting level

III Normal level

IV Failure level

V Surcharge level

Distance (m)

Safety factor

Rese

rvo

ir level (m

)

40

60

80

100

120

140

160

180

0 20 40 60 80 100 120 140 160 180 200

Distance (m)

Re

se

rvo

ir le

ve

l (m

)

Landslide at E dam reservoir

c - tan

FS vs cc=20 kN/m2 ; =33.69o

c=30 kN/m2 ; =31.93o

c=40 kN/m2 ; =30.10o

c=42.1kN/m2 ; =29.71o

c=45 kN/m2 ; =29.16o

c=50 kN/m2 ; =28.20o

c=80 kN/m2 ; =22.05o

t=18 kN/m3

sub=8 kN/m3

Actual slip surface

1 2 3 45 6

7

1

2

3

4

5

6

7

3

1

2

45

6

7

20

30

40

50

60

70

80

0.40 0.45 0.50 0.55 0.60 0.65 0.70

tan

Co

he

sio

n, c

(k

N/m

2)

7

654

2

1

3

0.96

0.97

0.98

0.99

1.00

20 30 40 50 60 70 80

Cohesion, c (kN/m2)

Sa

fety

Fa

cto

r

Chi-chi Earthquake (1999)

• The Chi-chi Earthquake (Sept 21st, 1999), the biggest quake

on Taiwan in this century, has a Richter scale magnitude of

7.6. The peak ground acceleration greater than 1g was

recorded. Nearly 2400 people were dead and more than

10,000 were injured. Total damage 9,200 million USD.

• Across the Central Mountain Range of Taiwan, at least 7000

landslides hit an area of several thousand square kilometers.

There were 16 places where individual landslide area

exceeds 10 ha. There were two gigantic landslides of an order

of magnitude of 108 m3, at Tsaoling and Chiufengershan.

Earthquake induced landslides

E-W

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

0 20 40 60 80

Time(Second)

Acceleration(gal)

Earthquake magnitude = 7.6 R or 7.3 R (BMG Taiwan)

Epicenter depth = 7.5 km

The maximum acceleration of earthquake motion = 989gal(s)

(near Sun Moon Lake of a 魚池 basin, EW ingredient

Maximum speed, the shake is observed for a long time very

greatly with about 40 seconds,

Prof. Mori (DPRI-Kyoto Univ):

車籠埔 (Che-lum) Fault moved in

the direction of north and south in

about 25 seconds as a mechanism

of this earthquake covering the full

length of about 60 km

Damage of 石岡 Dam on September 21st 1999

Damage in mountain slope

Tsaoling Landslide

Induced by 1999 Chi-

chi Earthquake,

Taiwan

Volume: 1.4 x 108 m3

Affected area: 698 ha

Total length: 4 km

Source area:

Length: 1.5 km

Width: 2 km

Depth: < 200 m

Destruction of 5

houses, resulting in 29

deaths

Chiufengershan Landslide

Induced by 1999 Chi-

chi Earthquake

Volume : 3 x 107 m3

Affected area : 180 ha

Total length : 1.2 km

Width: 1.1 km

Average depth:

30~50m

Destruction of 21

houses, resulting in 41

deaths.

The landslide blocked

the river along 1 km,

and 2 small lakes

have been formed at

the upstream.

気象庁：平成16年(2004年)新潟県中越地震についての報道発表資料（2004年10月23日19時10分発表）

Chuetsu Earthquake, Niigata Pref, Japan(M=6.8)

Landslides induced by the Chuetsu Earthquake, Japan

Landslide dam caused by

the Chetsu Earthquake

ランドスライドダムを形成した地すべり

梶金地すべり

More than 150 events of landslides occurred with various

dimension and mechanism in response to Bantul

earthquake May 27, 2006

Aims:

Most of the landslide susceptible areas were formed by

steep volcanic rocks such as interbeded tuff sandstone –

pumice breccia and andesitic breccia.

• Addresses factors controlling the occurrence and

mechanism of landslide

• Potential impact to the safety of surrounding

environment (empirical analysis)

Bantul Earthquake (2006)