Slope Management

and

Landslip Preventative Measures

Development of the Hong Kong System

Major Landslide at Maracas, Trinidad - Late December 2002.

Prevention of Landslides•Database of Slopes

•Type of Slope

•Condition•Consequences

•Cut Slopes

•Fill Slopes

•Retaining Walls

•Hybrids: Cut/Retaining Wall / Fill/Retaining Wall

•“Natural” Slopes - slopes where there has been no anthropogenic activity, or where such activity is causes small changes to the geometry of the slope so that the Factor of Safety is largely unaffected.

Development of Slope Registration Scheme

Began in late 1970’s based on geographic map area

e.g. 11SW-A

followed by a code

C - Cut Slope : F - Fill Slope

R - Retaining Wall : N - Natural Slope

CR - Cut Slope with Retaining Wall

FR - Fill Slope with Retaining Wall

followed by a number

i.e. 11SW-A/R188

refers to Retaining wall 188 in Map area 11SW-A

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

NW NE

SW SE

A B

C D

Map Referencing

Scheme

11SW-A

Hong Kong Slope Catalogue

Cut Slopes and Fill Slopes

e.g. North Coast Road

Fill Slope

Retaining Wall

“Natural” Slope

Cut Slope

Fill Slope

Fill Slope

Home Page for Hong Kong Slope Information System

http://hkss.ced.gov.hk/hkss/

Main Menu for Slope Information System

Click once on Slope to display data for 11SW-A/CR175

Slope Catalogue: Slope 11SW-A/CR175: Po Shan Road

Centred Map for Cut Slope 11SW-A/CR175: Po Shan Road

Failure of slope in June 1972

Aerial Photograph of Slopes

11SW-A/CR175 and

11SW-A/FR30

Photographs of Slope 11SW-A/CR175: Po Shan Road

Background Information And Related DocumentFeature Reference Number 11SW-A/CR 175

GIU Cell Ref. 11SW7D8 - NKT’s Classification still in use!

Location 18M NORTH OF 21 PO SHAN ROAD

Sift Class C1

Map Sheet Reference (1:1000) 11SW- 7D

Coordinates Easting : 832718 Northing : 815676

Aerial Photos Photo No. Year

1824-5 1972

Nearest Raingauge Station(Station Number)

Knowles Building, Hong Kong University (H04)

Data Collected On 11/14/97Date of Construction, SubsequentModification & Demolition

N/A

Related Reports/Files or Documents File/Report Ref. No

Previous Instability 16/6/72 63/11SW-A, 64/11SW-A

Previous Instability17/6/72 66/11SW-A, 18/6/7269/11SW-A

Development GCI 2/B7/113

BP ReportsIIB Po Shan Rd Area 3/76, MidLevel Rpt 3/86

GEO Mid Levels Study Rpt

LPM Considered in 91 Selected

Pre-SIRST Field Sheet

DH-Order (To Be Confirmed with Buildings Department)

NONE

Advisory Letter (To Be Confirmed with Buildings Department)

NONE

Remarks N/A

Follow Up Actions N/A

Maintenance Responsibility Government

Responsible Government Department - HyD

(Based on SIMAR results from Lands Department) IMPORTANT - READDisclaimer

Feature Registration Form for Po Shan Road Slope

11SW-A/CR175

Major Disaster in June 1972

Landslide Preventative

Measure

FIELD OBSERVATION Feature Reference No : 11SW-A/CR 175

Location : 18M NORTH OF 21 PO SHAN ROAD

Consequence Category : 1 Toe Elevation (mPD) : 184

Inspected On : 11/21/97 Weather : Mainly Fine

SLOPEMaterial Soil (Decomposed volcanic)

Max Height(m) : 110 Length (m) : 130 Angle (deg) : 35 Berm No. : 13 Width (m) : 1.5

fair fair

% Bare : 50 % Sealed : 0 % Vegetated : 50

Slope Face Conditions &Coverings

Seal Type : N/A

Condition beyond Crest& Covering % Sealed : 0

Type Size (mm) Spacing Condition FlowCondition

Weepholes/ HorizontalDrain

N/A N/A N/A N/A N/ASeepage No signs of seepage

Sign of Leaking Services Presence of potentially leaky services but no signs of leakage

Sign of Distress Reasonable (near crest, mid-portion, at toe)

Past Instability Inferred : None Confirmed : None

U-Channels Type Size (mm) Condition FlowCondition

Berm 251-300 partially blocked dry

Crest 251-300 partially blocked dry

Stepped 251-300 blocked dry

WALLType of Wall Dressed block (Wall at toe)

Max Height(m) : 2 Length (m) : 45 Angle (deg) : 87 Berm No. : 0 Width (m) : 0

Type Size (mm) Spacing Condition FlowCondition

Weepholes/ HorizontalDrain

N/A N/A N/A N/A N/ASeepage No sign of seepage

Sign of Leaking Services Presence of potentially leaky services but no signs of leakage

Sign of Distress None

Past Instability Inferred : None Confirmed : None

U-Channels Type Size (mm) Condition FlowCondition

Toe 201-250 partially blocked dry

Crest facility Undeveloped green belt Distance(m) 0Affected Facilities

Toe facility Residential Distance(m) 18

Service Conduits Water main found at crest

Observations N/A

http://hkss.ced.gov.hk

Field Observation Form for Po Shan Road Slope

11SW-A/CR175

Example of Fill Slope from Slope Catalogue: Street Reference

Fill Slope 11SW-A/FR 30: Reconstructed slope from lower part of 1972 Po Shan Road Failure

Failure of slope in June 1972

Aerial Photograph of Slopes

11SW-A/CR175 and

11SW-A/FR30

Photographs of Slope 11SW-A/CR175: Po Shan Road

Example of Retaining Wall from Slope Catalogue: Street Reference

Peel Street:

Wall: 11SW-A/R188

before LPM in 1984

Same Wall in 1998 - from Slope Catalogue

Landslip Preventative Measures

NK Tovey inspecting Walls

11SW-A/R5311SW-A/R64

Oct. 15th 1982

Po Hing Fong: LPM 33

LPM: Wing Lee Street (Bridges Street Market)

Wall in 1982 prior to LPM

Note: N.K. Tovey with assistants

Wall in 2001

Landslide Preventative Measures - Step 1Slope catalogue - data collected quickly - Stage 1

Very Simple Ranking System

NOT necessarily accurate BUT gives a Rational Way to move forward.

Evaluate a score system

e.g. (3 * height + drainage factor )* consequence factor

drainage factor say 0 if good > 5 if poor)

consequence factor - say

1 for little consequence : 2 for minor roads

3 for major roads : 4 for buildings

Slopes with High Rank Number SUGGESTS they are MORE CRITICAL

Landslide Preventative Measures - Step 2Limited Financial Resources available in any one year

so

From the thousands of slopes in catalogue, select the 20 with the highest RANK Number for more careful analysis.

e.g.

•Check initial catalogue data is correct

•probe depth of weep holes in retaining walls

•make a thorough check for signs of distress

•do a simple slope stability analysis

shortlist (say 6) most critical based on this Stage 2 study for detailed study as Stage 3

Landslide Preventative Measures - Step 3

Arrange for Site Investigation of shortlisted Slopes

•trial pits

•soil sampling - shear testing

•bore holes

•install piezometers

•monitor piezometers

•Check History of Site

•Check responsibility

•Full Stability Analysis

•If public responsibility move to design Stage - otherwise serve notice on person or authority liable for maintenance.

Landslide Preventative Measures - Step 4

•Design Preventative Measures

•Check designs

•Working Drawings

•Tender Process

•Contractors on Site

•Preventative Work complete

•Begin Step 2 for next group of slopes for next year.

After a number of years, most critical slopes should now be safe.

Some recent Landslides in Trinidad

•There is NO proactive system in place

•Everything is Reactive

Landslide at Maracas

December 2002

December 2004 – note the slide is much more

extensive

December 9th

Landslide

3 km beyond Las Cuevas as seen on TV

half of road blocked

Landslide 11th December 2004 at

approximately 13:00

1 km before Las Cuevas

half of road blocked

Slope before failure at Couva

Slope after Landslide

Slide by Derek Gay, UWI