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C I V I L E N G I N E E R I N G

Keywordsdisaster engineering; ailures;

geotechnical engineering

doi: 10.1680/cien.2009.162.5.44

Serious landslides have occurred in Hong Kong rom time to time with

loss o lie and signiicant socio-economic impact on the community.The systematic landslide investigation programme implemented by

the Hong Kong governments geotechnical engineering oice has

proved to be an important asset-management tool in contributing

to enhanced landslide-risk management and proessional practice.

This paper reviews the evolution o orensic landslide investigations in

Hong Kong and presents the ramework o the systematic landslide

investigation programme, together with the key lessons learnt.

Forensic landslideinvestigations inHong Kong

Ken HoMSc, DIC, FGS, Eurlng,

FHKIE, MICE, CEng

is chie geotechnical engineer atthe Hong Kong governments

geotechnical engineering ofce

Tony LauMEng, MHKIE, MICE, CEng

is senior geotechnical engineerat the Hong Kong governmentsgeotechnical engineering ofce

Jonathan LauBEng, MHKIE, MICE, CEng

is geotechnical engineer at the HongKong governments geotechnicalengineering ofce

Proceedings of ICE

Civil Engineering 162 May 2009Pages 4451 Paper 800053

Hong Kong has a mountainous terrain with

natural hillsides covering about 65% of the

land area, approximately half of which have

gradients greater than 30 (Figure 1). Rainfall

intensities can be high, with 50 mm/h and

250 mm/day not uncommon.

The significant increase in population

post-World War II necessitated major urban

developments involving extensive cutting

and filling for civil engineering and building

works. Of the 57 000 sizeable man-made

slopes registered in the governments slope

catalogue, about 39 000 were formed before

the introduction of territory-wide geotechnical

control in 1977.

The acute slope-safety problems faced by

Hong Kong are the result of dense develop-

ment in a hilly terrain, high seasonal rainfall

and a large number of vulnerable man-made

slopes and marginally stable natural hillsides.

Figure 1. Dense urban development on hilly terrain in Hong Kong natural hillsides cover 65% of the landarea, approximately half of which have gradients greater than 30

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2/845ISSN 0965 089 X PRoCEEDINGS oF THE INSTITUTIoN oF CIVIL ENGINEERS CIVIL ENGINEERING, 2009, 162, N. CE5

FoRENSIC LANDSLIDE INVESTIGATIoNSIN HoNG KoNG

Geolog nd lndslides in Hong Kong

On average, about 300 landslides arereported in Hong Kong every year. Given thedense development, even a small landslide inclose proximity to buildings or roads can resultin casualties. Since the late 1940s, the deathtoll arising from landslides has been over 480(Figure 2).

The government set up the geotechnicalcontrol office in 1977 renamed geotechni-cal engineering office (GEO) in 1994 as thecentral body to regulate slope safety in Hong

Kong. Over the years, the GEO has developedand implemented a comprehensive slope-safetysystem to manage landslide risk.1

Geology

The geology of Hong Kong is describedin detail by Sewell and Campbell.2 The twodominant rock types associated with areas ofhigh relief in the urban areas of Hong Kong aregranitoids and volcanic rocks (Figure 3). Thegranite is extensively weathered, with highlyvariable depths of silty-sandy saprolites, oftenwith corestones left behind in the matrix. Thevolcanic rocks consist mainly of coarse tuffs,fine tuffs and rhyolite, which are usually fine

grained and have a blocky structure. Relictjoints are preserved in the saprolitic zone, withjoint spacings of typically 0.51 m in graniteand less than 0.25 m in volcanic rocks. Steeplydipping intrusions in the form of dykes (e.g.dolerite, porphyry) are not uncommon, as arekaolin veins that may be a product of weather-ing or hydrothermal alteration.

Extensive bodies of colluvium may be presentat the lower hill-slopes as a result of accumula-tion of debris from past mass movements. Thecolluvium is up to 30 m thick in places, whichcommonly consists of boulders, cobbles andgravel in a matrix of sand, silt and clay, and isprone to the formation of soil pipes as a resultof seepage erosion.

Failure of man-made slopes

Systematic records of reported landslideshave been kept by the GEO since 1984. Thecorresponding landslide database contains some6500 reported landslides on man-made slopesthat have been inspected by professionals fromGEO as part of the emergency-response system.

The nature and failure mechanisms oflandslide hazards in man-made slopes arediscussed by Wong et al.3 and Martin.4 Themajority of the landslides in Hong Kong areshallow, typically less than 3 m deep. On aver-

age, about 90% of the reported failures areless than 50 m in volume.

Natural terrain landslides

The natural terrain in Hong Kong is suscep-tible to shallow landslides, which are liable to

develop into mobile debris flows along drain-age lines or local topographic depressions. Forthe purposes of risk assessment and risk man-agement, the common types of natural terrainfailures can be distinguished as

n open hill-slope debris avalanchen channelised debris flown mixed debris avalanche/flow along topo-

graphic depression.

In the 1990s, an inventory of about 30 000landslides on natural hillsides in Hong Kong

was compiled by the GEO on a geographicinformation system (GIS) platform, based on

Establishment ofgeotechnicalcontrol office (GCO)

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Figure 2. Over 480 people are known to have died in Hong Kong as a result of landslides since the late 1940s

Figure 3. Simplified geological map of Hong Kong

even a smalllandslide inclse prximity

t buildings rrads can result

in casualties

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Ho, LAU AND LAU

interpretation of high-level aerial photographs,taken at 2400 m or above. A separate inventorycomprising about 1900 large historical naturalterrain landslides has been compiled from inter-pretation of aerial photographs and hillside geo-morphology. Recently, GEO compiled a morecomprehensive enhanced natural terrain land-slide inventory (ENTLI) using low-level aerialphotographs, taken at lower than 2400 m,which contains information on about 105 000

natural terrain landslides (Figure 4).As hillside failures can occur over a con-

siderable distance from a given facility, agood appreciation of the characteristics andcauses of the different types of hillside fail-ures, together with the mechanisms involvedin landslide initiation and debris run-out, isimportant when assessing risk. An improvedunderstanding of natural terrain landslides hasbeen achieved through investigation of indi-

vidual landslides as well as through regionallandslide studies. The available landslide datahave proved invaluable in developing rainfall-landslide correlations for natural terrainlandslides5 and provided the basis for landslidesusceptibility analyses6 and back-analyses ofthe mobility of landslide debris.7 Studies ofnatural terrain landslides have also allowedsite-specific as well as global quantitative riskassessment to be carried out.8,9

Wong and Ho10 illustrated the diverse rangeof modes and mechanisms of slope failuresand debris run-out associated with different

hillside instabilities. The complexity of failureinitiation and debris run-out of natural ter-rain landslides is further highlighted by thesevere rainstorm in early June 2008, whichtriggered some 1500 natural terrain landslides.Some of these caused havoc to the community(Figure 5).

The 4 h duration rainfall of 384 mm had acalculated return period of over 1000 years.According to the preliminary informationobtained from satellite imagery as well asaerial and ground surveillance, approximately110 mobile channelised debris flows withdebris run-out distances greater than 200 moccurred. The severity of the rainstorm is

illustrated by the fact that some 20% of thesemobile debris flows had run-out distancesgreater than 500 m, with the run-out distancesof the four most-mobile landslides beinggreater than 1 km. In comparison, the ENTLIrecords only one debris flow with run-out dis-tance exceeding 1 km and 12 cases with run-out distances between 500 m and 1 km for theperiod 19242006.

Evolution of lndslide investigtions

Forensic landslide investigations in HongKong have long played a key role in advancingthe governments knowledge of slope perform-ance and landslide mechanisms in weatheredrocks. However, the arrangements for con-ducting landslide investigations have evolvedwith time.

In the 1960s, local academics were involvedin studying landslides. For example, the severerainstorm in June 1966 triggered some 500landslides resulting in significant loss of lifeand socio-economic consequences, and anoverview was carried out by So.11 Lumb12,13conducted technical investigations of selectedserious landslides in the 1960s and 1970s.Additionally, consultants were engaged by thegovernment to investigate specific landslides,

such as Ching Cheung Road in 1972.The landslide disasters in the 1970s prompt-ed a different approach to landslide investiga-tion. The government appointed a commissionof inquiry to investigate the two fatal land-slides in 1972. A consultant was also engaged

Figure 4. Example screen shot of the enhanced natural terrain landslide inventory, which has information onaround 105 000 landslides

Figure 5. Approximately 110 landslides with debris run-out distances greater than 200 m occurred in theJune 2008 rainstorm seen in this case reaching a highway

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to assist in the technical investigation of the1972 Po Shan Road landslide (Figure 6). Inthe case of Sau Mau Ping in 1976, the govern-ment convened an independent review panelcomprising six international landslide expertsto conduct the forensic investigation.

Since its establishment in 1977, the GEOhas played a leading role in conducting inves-tigations of serious landslides using in-housestaff. These included studies of selected fatallandslides, such as the boulder fall at KingsRoad in 1981, for submission to the coronerscourt, and landslides triggered by severe rain-

storms, for example May and August 1982,May 1992 and November 1993. The TsingShan channelised debris flow in 1990 was anear-miss and highlighted the potential riskposed by natural terrain landslides to the com-munity (see next section).

In the early to mid-1990s, the GEO contin-ued to carry out forensic investigations of allfatal landslides, at Baguio Villas and KennedyRoad in 1992,14 Cheung Shan Estate in 1993,14Kwun Lung Lau15 and Castle Peak Road in1994,14 and Fei Tsui Road16 and Shum WanRoad in 1995.17 For the forensic investigationsof the 1994 Kwun Lung Lau landslide and thetwo fatal landslides in 1995, an external inde-

pendent reviewer was engaged to review thetechnical investigations that were undertakenby GEO.1820

The Kwun Lung Lau disaster aroused con-siderable public concern (see later section). InOctober 1994, the legislative council voted toset up a select committee to inquire into thecircumstances of the landslide and the relatedissues, only the second select committee inHong Kongs 150-year colonial history,21 inparallel with GEOs technical investigation.

The systematic landslide-investigation pro-gramme was introduced by the GEO in 1997and it has been integrated with the govern-ments HK$25 million landslip-preventive-measures programme22 since 2000. Theaverage annual cost of systematic landslideinvestigations is about US$ 3 million, which isapproximately 3% of the annual programmeexpenditure. The programme is being imple-mented with the assistance of consultantsmanaged by the GEO.

Some of the milestone landslides in recentyears that prompted much improvement tolandslide risk management and slope engi-neering practice in Hong Kong are presentedbelow.

Tsing Shn chnnelised debris flow, 1990

On 11 September 1990, a massive chan-nelised debris flow occurred on the easternflanks of Tsing Shan (Figure 7). The failurewas triggered by relatively light rainfall with areturn period of less than 3 years. The initial

failure was in the form of a relatively small-scale (350 m) rock topple and rock/soil slidein the source area on the steep upper reachesof the natural hillside. The failed materialtravelled over an exposed sheeting joint andentered the streamcourse below, developinginto a channelised debris flow along the steepdrainage line that was infilled with loose boul-dery colluvium.

Given significant entrainment of loose

material along the drainage line, the volumeof landslide debris reached approximately19 000 m. The run-out distance of the land-slide was approximately 1 km, with the debrisencroaching on a vacant building platform.Had the housing development proceeded as perthe original plan, the consequences could havebeen very serious. The findings of the landslideinvestigation are documented by King.23

The event highlighted that the scale of a

Figure 7. The 1990 Tsing Shan channelised debris flow travelled I km and reached a volume of 19 000 m3

fortunately it destroyed only building plots rather than completed buildings

Figure 6. The 1972 Po Shan Road landslide, in which 67 people died

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Ho, LAU AND LAU

natural terrain failure may not necessarily berelated to the rainfall severity, especially wherethe site setting is unfavourable and vulnerable tosignificant entrainment. It also emphasised thedifficulty in predicting the locations and mobil-ity of natural terrain landslides and the need toconsider low-frequency, high-magnitude naturalterrain landslide hazards in risk assessments.

The mobility of the debris flow has beenback-analysed using different numerical modelsby a number of parties over the years, includ-ing Hungr et al.24 and Kwan and Sun25 (Figure8). Considerable advances have been made inrespect of the understanding of the mechanicsand dynamics of natural terrain landslide initia-tion, processes and debris movement.

Kwun Lung Lu lndslide, 1994

The 23 July 1994 Kwun Lung Lau landslide(Figure 9) involved the sudden collapse of a100-year old masonry wall that was in goodmaintenance condition and previously subjectedto a safety-screening study by the GEO. Thefull height of the masonry wall, together withthe slope above, failed. Some 1000 m of debriswas released, killing five people and seriouslyinjuring three others on the footpath below. Themasonry wall had a maximum height of 10.6 mand a base width of 0.8 m (which was only

about one-fifth of that shown in the drawingsapproved by the Building Authority in 1965),that is a slenderness ratio of more than 13. Thiswas exceptionally slender compared with typicalmasonry walls of a similar construction in HongKong (i.e. slenderness ratio of less than 4 forwell-proportioned masonry walls).

The findings of the technical investigation arepresented by Wong and Ho.26 The investigationestablished that the failure involved bucklingand brittle collapse of the thin masonry wall.The collapse was triggered by subsurface infil-tration from defective buried drainage systems,which saturated and weakened the soil mass.The state of knowledge at that time was that old

masonry walls would fail in a ductile mannerfollowing deformation for some time. However,the Kwun Lung Lau masonry wall was in agood condition and yet it failed suddenly withlittle sign of deformation or distress. The actualfailure mechanism of the slender masonry wallwas very different from the previous under-standing and the assumptions made in conven-tional stability analyses.

Numerical analyses using the distinct-elementcomputer program Udec predicted that themasonry structure would fail in a complex mode(Figure 10). The masonry wall was found tobulge initially at about mid-height, accompaniedby overturning of the portion of the masonrybelow this level. Such deformation modes com-bined to lead to tensile failure and consequentialsudden reduction of the shear strength of themortar joints. Once failure of mortar joints wasinitiated, the wall deformed rapidly with insta-bility developing in an uncontrolled manner,resulting in brittle fracture of the masonry walland failure of the ground behind. Such a com-plex failure mechanism was not considered inconventional retaining-wall analysis, which wasunsafe in the case of a thin masonry wall. As aresult, local professional practice for assessingstability of old masonry walls was duly revised.

The Kwun Lung Lau landslide also high-

lighted the critical importance of leakage fromunderground water-carrying services on slopestability. Transient seepage analyses were car-ried out to assess the contribution of the differ-ent sources of water in saturating the groundbehind the masonry wall. The analyses estab-

Figure 9. The 1994 Kwun Lung Lau landslide involved the sudden collapse of a 100-year-old masonry wall,killing five people

Figure 8. Simulation of the 1990 Tsing Shan channelised debris flow using 3d-DMM

Debris deth: m

0.5 1.0 1.5 2.0 2.5 3.0 3.5

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lished that the subsurface seepage led to thewetting up of the loose fill behind the masonrywall. The induced settlement of the loose fillprobably led to distress or rupture of a foul-water sewer running across the upper part ofthe landslide area. This resulted in substantialsaturation of the retained groundmass leadingto the collapse. Following the incident, a newcode of practice on inspection and maintenanceof water-carrying services was issued by the gov-ernment to upgrade professional practice in theinvestigation and maintenance of undergroundwater-carrying services.27

Professor N. R. Morgenstern18, the independ-ent reviewer engaged by the Hong Kong govern-ment, advocated that, among other things, amore integrated perspective should be adoptedfor slope stability studies. In response to thisrecommendation, the GEO launched a system-atic landslide investigation programme in 1997.

Ching Cheung Rod lndslide, 1997

In July and August 1997, three successivelarge failures occurred on a soil cut slope aboveChing Cheung Road, which is a major urbantrunk road. The first two collapses occurred inJuly 1997, which culminated in the major failure

on 3 August 1997. The scales of the failureswere 500, 700 and 2000 m respectively. TheAugust 1997 landslide (Figure 11) completelyblocked a 50 m section of the road and trappeda vehicle, fortunately with no casualties. Thetrunk road was subsequently closed for aboutone month until completion of temporary stabi-lisation works to the slope.

The slope comprised up to six batters witha maximum height of 47 m and a typical slopeangle 4550. The geology was complex andcomprised predominantly weathered granite,with basalt dyke intrusions up to 1.3 m thick,which were weathered to clayey silt and havea relatively low permeability. Extensive erosionpipes, up to 250 mm in height, were presentthroughout the weathered profile. The hydro-geology is complex piezometric monitoringprior to the 1997 landslides showed that thegroundwater response in the weathered profiletook weeks to develop fully following heavyrainfall.

The subject slope had a history of past insta-bility prior to the 1997 landslides. A majorlandslide of about 7500 m in volume occurredon the cut slope in 1972 during road widening,and a shallow landslide occurred on the upperpart of the slope in August 1982. Both the1972 and 1982 landslides were delayed failures

in that instability occurred several days follow-ing cessation of rainfall. The slope was sub-sequently upgraded by the government to therequired safety standards in 1992. The upgrad-ing works comprised local slope trimming anddrainage improvement.

Figure 11. The 2000 m3

Ching Cheung Road landslide in 1997 closed a trunk road for a month

Figure 10. Numerical analysis of the 1994 Kwun Lung Lau landslide revealed a complex bulging failure notconsidered in previous stability assessments: (a) at calculation cycle no. 25 000; (b) at calculation cycleno. 30 000; (c) at calculation cycle no. 60 000; (d) at calculation cycle no. 75 000

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Ho, LAU AND LAU

The investigation established that the 1997landslides were probably caused by the build-upof adverse transient groundwater pressure, fol-lowing the severe rainfall in early July 1997 andthe preceding months.28 The failures occurredat the location of a buried drainage line below asizeable upslope catchment where concentratedgroundwater flow was likely to have resultedin higher groundwater levels than that in thesurrounding area. The previous instabilitiesprobably resulted in some local weakening of thegroundmass and contributed to the developmentof the extensive erosion pipe system in the slope.

The landslides highlighted the inherent uncer-tainties in ground investigation and stabilityanalyses of slopes in weathered profiles. Theyalso emphasised the vulnerability of unsupport-ed soil cuts to adverse geological features andhydrogeological conditions, which are difficultto account for fully in the engineering geologicalmodel. A pragmatic approach of adopting morerobust design solutions, such as soil nailing andretaining structures, is called for.29

Lndslide-investigtion rogrmme

The main goals of the systematic landslide-investigation programme30 are

n identification of slopes in need of earlyattention under the slope retrofitting pro-gramme before the situation deteriorates toresult in a serious problem

n improvement in the knowledge on landslidecauses and mechanisms

n reviewing the performance of the govern-ments slope safety management system toidentify areas that warrant improvement

n providing evidence in respect of seri-ous landslides that may involve coronersinquests, legal action or financial dispute.

Since the introduction of the programme in1997, some 3000 landslide records have beenexamined, and approximately 200 landslidestudies have been completed.

In addition to the study reports for the indi-vidual landslides, an annual diagnostic reportis prepared which augments the annual factualreport on rainfall and landslides. The diagnosticreport seeks to make observations on overalltrends, consolidate key findings and make rec-ommendations to enhance the slope engineeringpractice. Thematic studies are also carried out,such as reviews of soil-nailed slope failures andlandslides during construction.

Enhancement of technical knowledgeThe key findings and lessons learnt areavailable from the governments slope safetywebsite (http://hkss.cedd.gov.hk).

Examples of advances in technical knowledgeas a result of landslide studies include

n improved understanding of the progressivenature of many large-scale landslides

n influence of movement mechanisms ondebris run-out

n role of subsurface water for exampleperched water, seepage pressure and dam-ming and uncontrolled surface runoff intriggering landslides

n significance of adverse geological structuresand hydrogeological features

n the need to cater for local minor failures inslope design and detailing

n the importance of a robust slope design to

cater for inherent uncertainties in geologicaland groundwater conditions for example,soil nails are more robust than an unsup-ported cut.

In addition, landslide investigations havecontributed to making technical advances inthe assessment of debris mobility, evaluationof annual failure rates of different slope typeswhich serve as inputs to quantitative riskassessments,31 and refinement of the rainfall-landslide correlations for landslip warnings.

Lessons learnt from slope failures

A key observation of the systematic landslide

investigation programme was that the failurerate of engineered slopes that is slopes thathave been processed by the slope-safety system is not as low as previously perceived.32

With regard to major failures of 50 m ormore of engineered slopes that are not suf-ficiently robust, such as unsupported cuts, thekey contributing factors are as follows

n use of inadequate geological models leadingto unrealistic stability analyses, for examplestability-critical features such as adverserelict discontinuities, sheeting joints, andkaolin-rich seams not properly considered

n adoption of inadequate hydrogeologicalmodels in slope design leading to under-prediction of transient groundwaterpressures

n progressive slope deterioration and dis-placement not duly considered.

The key problems with respect to minorfailures (less than 50 m) of engineered slopesare as follows

n uncontrolled surface water flown inadequate slope maintenancen poor detailing of slope surface protection

and surface drainage provisionsn

local weaknesses in the groundmass.

The above diagnosis emphasises the need toimprove design practice and slope detailing andthus enhance the reliability of engineered slopes.Key areas that warrant attention include

n inadequate engineering geological inputduring investigation, design and construc-tion review leading to the adoption ofinadequate geological and hydrogeologicalmodels

n insufficient attention to the history ofinstability

n inadequate consideration of overall site set-ting in an integrated perspective

n uncontrolled surface runoffn poor detailing in slope drainage provisionsn inadequate slope maintenance.

Imrovement in sloe-engineering rctice

Lessons learnt from systematic landslideinvestigations have contributed to enhancingthe professional practice in respect of both man-made slopes and natural hillsides.

Examples of improvements made in man-made slopes include improved detailing forsurface and subsurface drainage provisions,soil-nailed slopes, together with improvedtechnical guidance to enhance the robustness ofengineered slopes.29

Landslide investigations have also led to animproved understanding of the complexity ofmodes and mechanisms involved in natural

terrain landslides.32 The improved technicalknow-how has in turn contributed to enhanc-ing natural terrain landslide risk management.Improved technical criteria and guidance forstudy and mitigation of natural terrain land-slides hazards in Hong Kong are summarisedby Wong.6

A new rolling programme, known as thelandslip prevention and mitigation programme,was launched by the GEO in 2007, to dovetailwith the current programme upon the comple-tion of its current phase in 2010. Under theprogramme, expanded efforts will be made totackling systematically vulnerable natural-terraincatchments posing a risk to the community in arisk-based priority-ranking order, in addition toman-made slopes.

Discussion nd conclusions

The systematic landslide-investigationprogramme has some similarities to the UKStanding Committee on Structural Safety(Scoss), which was established by the Institutionof Civil Engineers and Institution of StructuralEngineers in 1976.33 The common theme forboth programmes is on identifying the key les-sons to be learnt and promulgation of the les-sons to the code drafters and profession.

GEOs programme covers all reported land-slides in Hong Kong and thus the totality ofthe problem is better appreciated, with suitablefocus being directed to those significant failuresthrough detailed investigations. Experience hasshown that the lessons learnt are not necessarily

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confined to technical aspects but can also coverrisk management, as well as the administrativeand regulatory framework. It is of the essencethat the post-failure investigations are under-taken by personnel experienced in forensic engi-neering to ensure that the studies are sufficientlyrigorous and insightful.

Much has been learnt from landslide inves-tigations in advancing the understanding ofslope failures and enhancing the slope-engi-neering practice. Whereas slope failures can

be a bitter experience, landslides have provedto be an important asset from which engineerscan learn how to better prevent similar failuresin future.

The key is to have a suitable framework forthe profession to capture the lessons that needto be learnt. This includes the undertaking ofrigorous investigations, promulgation of thefindings and prompt updating of technical guid-ance and professional practice. The systematiclandslide investigation programme has proved

to be an important asset management tool toensure that maximum benefit can be derivedfrom studies of failures.

acknowledgements

This paper is published with the permissionof the head of the geotechnical engineeringoffice and the director of civil engineering anddevelopment, Government of the Hong KongSpecial Administrative Region.

Wht do ou think?If ou would like to comment on this er, lese emil u to 200 words to the editor t [email protected] ou would like to write er of 2000 to 3500 words bout our own exerience in this or n relted re of civil engineering, the editor will be h to rovide n hel or

dvice ou need.

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