Geohazards Seminar on June 28 - Slope Safety (Slide 1-53)

2011/6/29

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Philip WK CHUNG, Chief Geotechnical Engineer

Becky L S LUI, Geotechnical Engineer

Geotechnical Engineering Office

Civil Engineering & Development Department

Ir Ms Becky L S LUI

Geotechnical Engineer

Geotechnical Engineering Office

Civil Engineering & Development Department

HK SAR Government

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1• Landslide History of Hong Kong

2• Causes of Landslides

3• Hong Kong Slope Safety System

4• New Challenges and Initiatives

5• Conclusion

Outline of PresentationOutline of Presentation





5• Conclusion


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• In old days, there was very limited regulatory control of

geotechnical engineering and slope formation.

• So in 1970s, many tragic historical landslides happened.

Landslide History of Hong Kong

木ul程-of-thumb 總n slop程 d程s總gn木ap總d d程v程lopm程nt aft程處 WW師師

Landslide at Po Shan Road on 18 June 1972

• Volume = 20,000 m3

• 67 Fatalities

土程fo處程

Aft程處

Landslide Computer Animation Landslide Computer Animation

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BeforeBefore

AfterAfter

The fill slope

Landslide at Sau Mau Ping on 18 Jun 1972

• Volume = 6,000 m3

• 71 Fatalities

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Landslide Computer Animation Landslide Computer Animation

Landslide at Sau Mau Ping on 25 Aug 1976

• Volume = 5,000 m3

• 18 Fatalities

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- Exercise geotechnical checking of new development

works

- Retrofit large number of existing man-made slopes

through the “Landslip Preventive Measures (LPM)

Programme (防止山泥傾瀉計劃)”

• Set up in 1977 as HK’s geotechnical control authority:

About 300 reported landslides each year !!

1994 Kwun Lung Lau

1992

Baguio

Villas

1995 Shum Wan Road

1997 Ching Cheung Road

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1999 Shek Kip Mei

1999 Shum

Tseng

• Hourly rainfall = 145.5 mm � Historical record!!

• 24-hour cumulative rainfall = 622.5 mm

7 June 2008 Rainstorm

Max. rolling 4-hr rainfall : 384 mm

Return period: ~ 1,100 years!!

Max. rolling 4-hr rainfall : 384 mm

Return period: ~ 1,100 years!!

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North Lantau Highway

Video from YouTube

Yu Tung Road in Tung Chung, North Lantau

• Source Volume ~ 2,350 m3

• Debris Volume Reaching Road >3,000 m3

• Runout Distance ~ 600 m

• Vertical Fall ~ 230 m

• Max Velocity ~ 15 m/sec

南涌村大澳新村

橫坑村石仔埗

吉慶後街

Tai O

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山道

深屈道

昂坪

昂坪路

Lantau

Keung Shan

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5• Conclusion


Terrain of HK

60% of HK’s total land area is hilly terrain

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Hilly Terrain, but with dense

population

Wet summers with typhoons and rainstorms

Many slopes formed to

provide platforms for

building development

and roads

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Buildings & Infrastructures built on or very near to slopes

60,000 Registered Man-made Slopes

Gov程處nm程nt: 40,000P處總vat程: 20,000

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Boulder

孤石孤石孤石孤石

Rock 岩石岩石岩石岩石

Rock Cut Slope

切削石切削石切削石切削石坡坡坡坡

Soil Cut Slope

切削土坡切削土坡切削土坡切削土坡

Natural Terrain

天然天然天然天然山坡山坡山坡山坡

Platform 平台平台平台平台

Fill Slope

填土坡填土坡填土坡填土坡

How to categorize slopes?

Soil

泥土泥土泥土泥土

Retaining Wall

擋土牆擋土牆擋土牆擋土牆

Causes of Landslides (Natural Factors)Causes of Landslides (Natural Factors)

� Heavy Rainfall

• Damage to slope surface protection

• Erosion of soil

• Infiltration of water into soil or rock

• Rise in groundwater table

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� Adverse Geological Features

• Weathered Soil (Can be as deep as 100 m)

• Exposed boulders

• Weak weathered seam

Weak seam

Steeply sloping joints

Discontinuity

Exposed boulders


� Adverse Geological Features

• Weak weathered seam

Kaolinite-infill on the seam

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Causes of Landslides (Human Factors)Causes of Landslides (Human Factors)

Inadequate support during trench excavation

� Inadequate Design or

Construction

• Slope design not up to safety standards

• Slope works not

properly constructed


� Leaking Drains

• Leakage from buried water pipes or sewers could

contribute to slope failures

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� Lack of Maintenance

• Poorly maintained slopes will deteriorate and fail

• Regular maintenance is needed





5• Conclusion


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Geotechnical Engineering OfficeGeotechnical Engineering Office土力工程處土力工程處土力工程處土力工程處土力工程處土力工程處土力工程處土力工程處

Hong Kong Slope Safety System – Major Strategies

1977

Slope Safety System

set up by GEO

Landslide

risk in HK

1.Check new slopes

2.Plan land use

Contain the

increased

risk

1950 1960 1970 1990 2010

Reduce risk by

improving

slope stability

Reduce risk by

reducing

landslide

consequence

2000

3.Upgrade & maintain

government slopes

4.Promote private

slope maintenance

5.Clear squatters on

hilly terrain

6.Educate the public

to take precaution

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Checking Designs

Site Audits

1. Check New Slopes

• GEO exercises geotechnical control over both private and public

developments to ensure that they conform with the required

safety standards.

1. Check New Slopes

• GEO exercises geotechnical control also by carrying out

geotechnical studies and publish standards and guidelines

for the practitioners.

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Tuen Mun Area 19

proposed for

residential use

1990 Tsing Shan

Landslide

2. Plan Land Use

• GEO identifies geotechnical constraints at early stages of land

developments

New Tuen

Mun Centre

Land use changed to Golf Driving Range & Road

Tuen Mun Golf

Driving Range

Lung Fu Road

2. Plan Land Use

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3. Upgrade and Maintain Government Man-made Slopes

• Starting from 1977, the GEO has begun to implement the

“Landslip Preventive Measures (LPM)” to upgrade old man-

made slopes

• Details to be covered:

� Priority with respect to consequence to life

� Methods of upgrading man-made slopes

� Slope Greening

Priority with respect to consequence to life

Major Road Residential Buildings

School

Hospital

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� Soil Cut Slopes

• Cutting back

• Using retaining wall

• Soil Nails

� Fill Slopes

• Recompaction

• Soil Nails

� Retaining Wall

• Thickening

• Soil Nails

Methods of upgrading man-made slopes

• Cutting Back to a flatter slope angle – not often used today

Before After

Soil Cut Slopes

1. A lot of trees will have to be felled

2. A large amount of soil and rock will be generated

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Soil Cut Slopes

• Using Hand-dug caissons or piles as retaining walls to retain

the cut slopes – not often used today

1. Hand-dug caissons lead to a lot of fatal accidents � banned!

2. Piles � expensive

• Soil nails

Soil Cut Slopes

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Soil Nail

Potential Slip Surface

Mechanism of how soil nails work

Interaction

between soil

and soil nails

Internal Force

of Soil Nails

PLASTIC GROUT TUBE

32∅∅∅∅ GALVANIZED HIGH YIELD STEEL BAR

Typical layout of a Soil Nail

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• Length from 8 m to 20 m

• Hole diameter normally 100 mm

Construction of Soil Nails – Hole Drilling

• Steel bar diameter either 25 mm or 32 mm

• Steel bar with corrosion protection

Construction of Soil Nails – Insertion of Steel Bars

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Construction of Soil Nails – Grouting

Construction of Soil Nails – Soil Nail Head Construction

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Construction of Soil Nails in progress

Completed Soil Nailed Slopes

Geohazards Seminar on June 28 - Slope Safety (Slide 1-53)

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