slope failures along bril roads : quantitative risk assessment and ranking

219 Pages·2005·6.74 MB·English

SLOPE FAILURES
ALONG BRIL ROADS :
QUANTITATIVE RISK
ASSESSMENT AND RANKING
GEO REPORT No. 81
ERM-Hong Kong, Ltd
GEOTECHNICAL ENGINEERING OFFICE
CIVIL ENGINEERING DEPARTMENT
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624.15136
S66 THE GOVERNMENT OF THE HONG KONG
SPECIAL ADMINISTRATIVE REGION

SLOPE FAILURES
ALONG BRIL ROADS :
QUANTITATIVE RISK
ASSESSMENT AND RANKING
GEO REPORT No. 81
ERM-Hong Kong, Ltd
BOOKS REGISTRATION ORDINANCE
Chapter 142
lJ'J 4293 H
No.: HK
This report was originally produced in November 1997
under Consultancy Agreement No. GEO 6/97
0 9 JUL 1383
i
© The Government of the Hong Kong Special Administrative Region
First published, January 1999
Prepared by:
Geotechnical Engineering Office,
Civil Engineering Department,
Civil Engineering Building,
101 Princess Margaret Road,
Homantin, Kowloon,
Hong Kong.
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PREFACE
In keeping with our policy of releasing information
which may be of general interest to the geotechnical profession
and the public, we make available selected internal reports in a
series of publications termed the GEO Report series. A charge
is made to cover the cost of printing.
The Geotechnical Engineering Office also publishes
guidance documents as GEO Publications. These publications
and the GEO Reports may be obtained from the Government's
Information Services Department. Information on how to
purchase these documents is given on the last page of this
report.
J.B. Massey
Ag. Principal Government Geotechnical Engineer
January 1999
FOREWORD
In 1993, the Geotechnical Engineering Office (GliO)
embarked on a programme of research and development studies
on landslide risk management under the R & D theme on
Quantitative Risk Assessment (QRA).
This study was carried out by Mr Venkatcsh Sourirajan
of ERM-Hong Kong Ltd under the Consultancy Agreement No.
GEO 6/97. The main objective of the study was to formulate a
methodology for the ranking of selected roads with a history of
landslips (referred to as BRDL roads) based on quantitative risk
assessment of slope failures along such roads with due regard
given to risk to life and economic losses. Mr K.K.S. Ho and
Dr D.O.K. Lo of the Special Projects Division administered the
consultancy and reviewed the Report.
In this Report, the results of the study are expressed in
terms of risk to life (viz. potential loss of life per year) and
economic loss per year due to road closure caused by landslips.
The 41 BREL road sections considered are ranked based on
potential loss of life per year and economic loss per year (due to
traffic disruption caused by landslips). The roads are also
ranked in terms of the total economic loss considering both the
cost of a statistical life and economic loss due to road closure.
P.L.R. Pang
Chief Geotechnical Engineer/Special Projects
EXECUTIVE SUMMARY
The technique of Quantitative Risk Assessment (QRA) has been adopted by the
Geotechnical Engineering Office (GEO) to assist in the formulation of landslide
risk management strategy. This study has been commissioned to apply QRA to
evaluate the risk posed by slope failures along selected roads (called BRIL
Roads), which have a high incidence of landslides and also have high traffic
density.
Slope failures along busy roads not only have the potential to cause fatalities and
injuries but also cause serious disruption to traffic. In order to evaluate the
potential for disruption to traffic, the risk assessment technique has been applied
in this study to evaluate the risk of economic loss caused by traffic disruption in
addition to risk to life. The main purpose of this study is to provide information
on the risks associated with slope failures to help prioritize and co-ordinate the
slope and road improvement programmes.
The methodology developed in this study includes the determination of
frequency of landslide incidents, assessment of the consequences of landslides in
terms of fatalities and impact on road traffic and finally calculation of the risk
levels in terms of Potential Loss of Life (PLL) and risk of economic loss.
The approach for frequency estimation, as described in Section 3, is summarised
below:
• the frequency of landslides is estimated from historical data for slope failures
along each BRIL road section for the period 1984 to 1996. Incidents reported
as landslides, washouts, rockfalls and retaining wall failures are considered
together as 'landslides' while those reported as boulder falls are considered
separately;
• the proportion of upslope and downslope failures reported along each road
section have been considered to model the consequences of failure as that
from cut slope and fill slope respectively;
• the slope height-failure volume distribution is estimated for each road section
based on the slope height-failure volume data reported in previous incidents
for the given road section and the slope height-failure volume distribution
derived from the Special Administrative Region (SAR)-wide study on slope
failures from pre-GCO man-made slopes. Slope height information from the
1977/78 Slope Catalogue has been used. A modified volume distribution is
evolved which attempts to best represent the road specific historical data
while at the same time ensuring that certain high volume incidents for the
corresponding slope heights, although may not have occurred, are
nevertheless represented with a low probability;
• for upslope failures, the slope height-volume distribution is estimated for
three slope height ranges, <10m, 10 to 20m and >20m and for five volume
ranges, <20m3,20 to 50m3,50 to 500m3,500 to 2000m3 and >2000m3. For
downslope failures, the volume distribution is estimated without any
reference to slope height for five volume ranges, <20m3/ 20 to 50m3,50 to
200m3,200 to 1000m3 and >1000m3;
- 6 -
• the slopes along BRIL Roads are mainly pre-GCO slopes which are potentially
sub-standard. A small percentage of these slopes have been improved to
current geotechnical standards. In order to account for such improvements in
the frequency analysis, a reduction factor equivalent to the proportion of total
number of slopes along each road section that have been improved to current
standards is applied. It is further assumed that the chance of failure of slopes
designed to currently applicable standards is negligible in comparison with
pre-GCO slopes;
The approach for consequence estimation, as described in Section 4, includes
estimation of fatalities and estimation of economic loss due to road closure. The
approach for estimation of fatalities from landslides and boulder falls is
summarised below:
• the methodology for consequence assessment (ie, estimation of fatalities
caused by landslides) is based on suitable adaptation of the approach evolved
in previous studies by GEO on SAR-wide Pre-GCO Man-Made Slope Failures,
Landslide Consequence Severity Classification of Roads, etc.
• the number of fatalities from landslides is expressed as a function of
vulnerability factor, scale of failure and expected number of fatalities for a
reference landslide affecting a facility which is road in this case;
• the vulnerability factors derived in the SAR-wide study as a function of
runout angle and failure volume for cut slopes failures (ie, upslope failures)
and as a function of crest distance and failure volume for fill slope failures (ie,
downsiope failures) are adopted. The proximity of the affected facility below
a slope is expressed in terms of shadow angle, which is estimated considering
three slope height ranges and the distance to the centre of each lane from the
slope toe to determine the appropriate vulnerability factor. The average of
vulnerability factors computed for each lane as the affected facility is then
considered. Similar approach, but based on crest distance, is adopted for
downsiope failures;
• the scale factor which is the ratio of the width of actual landslide to the width
of reference landslide is computed for different failure volumes as given in
the SAR-wide study;
• the expected number of fatalities for a reference landslide (of 50m3 volume) is
estimated from the nomograph which correlates fatalities with actual Annual
Average Daily Traffic (AADT) and number of lanes for a given road;
• the estimation of fatalities from boulder fall is based on a model which
evaluates the probability of boulder hitting a vehicle for various scenarios-
falling boulder hitting a moving vehicle, moving vehicle hitting a fallen
boulder etc. The probability of death of occupants in the road vehicle is
considered for each of the scenarios.
The approach for computing economic loss due to landslides is summarised
below:
• economic loss due to landslides is computed by considering the potential loss
of life in terms of Value of life1 and the loss due to road closure resulting from
landslides;
• the value of life, or the implied cost of averting a fatality is assumed as
HK$ 24 million;
• road closure following landslips may result in increase in travelling time
which may cause indirect effects on the economy due to lost time for
productive work;
• the increase in travelling time due to congestion on the roads caused by
lane/road closure is based on the initial suggestions by the Financial Services
Bureau. The increase in travelling time is estimated for different class of roads
(rural, urban, trunk roads etc) and for different proportion (or extent) of road
closure;
• the time value for additional hours lost in commuting to work(which is
higher than time value for non-working hours) is adopted from studies
undertaken for the Transport Department in Hong Kong and in the UK;
• the increase in vehicle operating costs due to additional distance travelled as a
result of congestion on roads due to road closure is considered;
• the duration and extent of lane closure caused by landslides is derived for
various failure volumes based on data from past incidents provided by the
Highways Department for the period 1994 to 1996. Data from past incidents
on duration of closure of more than one lane is limited and therefore suitable
assumptions have been made;
• the probability of different extent of closure (ie, one lane, two lanes, etc) is
derived for various failure volumes from the GEO Annual Report on Rainfall
and Landslides. Again, this data is adopted for estimating the probability of
closure of the first lane while suitable assumptions have been made for the
probability of closure of more than one lane.
Results
The results from the study are expressed in terms of both risk to life and risk of
economic loss due to road closure. Risk to life is expressed in terms of PLL
(Potential Loss of Life per year), which is a measure of societal risk.
The 41 BRIL Road sections considered under this study are ranked based on PLL
per year and economic loss per year (due to traffic disruption caused by
landslides; other costs such as cost of slope repair, etc. are not considered). The
roads are also ranked based on total economic loss (considering both cost of life
saved and economic loss due to lane/road closure caused by landslides),
expressed in units of per kilometre per year, to provide a more uniform basis for
comparison between roads given that their lengths are different.
The Castle Peak Road section ranks first both on PLL and on economic loss per
year as a result of lane/ road closure. However, when the total economic loss is
expressed in units of per km per year, the Kwun Tong Road section ranks first.
The results for PLL, economic loss and ranking are presented in the attached
Table.