Rehabilitation requires mixed approach

Geography, population density and a tight footprint all pose serious challenges when choosing the best technique for upgrading Hong Kong's water mains, reports KL Wong, Technical Director of Maunsell Consultants Asia Ltd.


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The history of water supply in Hong Kong goes back a mere 150 years. Piped

supply was restricted initially to individual urban areas and small supply systems

were then slowly constructed to meet community needs. As the economy began to

boom after World War II, the rapid growth in population, industry and commerce

and the improvement in living standards necessitated massive expansion of the

water supply and distribution systems. Piped water is now available to virtually

everyone in Hong Kong via over 6,000kms of water mains, many requiring refurbishment.

20-year renewal plan

In early 1990s, the Water Supplies Department (WSD) of Hong Kong conducted an

Underground Asset Management Study which revealed that many of the existing

water mains would come to the end of their service life in about 20 years. Consequently

in 1996, an asset management plan recommended a 20- year renewal plan for some

3,000kms of the old water mains in the territory at an estimated cost of HK$10BN.

Stage 1 of the plan has already begun, covering about 350kms split into 3 large

districts, each of which is the subject of feasibility studies. Last year, Maunsell

Consultants Asia Ltd (MCAL) and Hyder Consulting Ltd Joint Venture, led by MCAL

were appointed to undertake the feasibility study for Kowloon and New Territories

South. The objective was to examine each alignment of water main requiring refurbishment

and determine the method of construction – open trench replacement, on-line

rehabilitation (lining), or other trenchless techniques.

The main thrust was to identify cost effective solutions to minimise traffic

and environmental impacts, inconvenience and disruption to water consumers.

The feasibility study also included preliminary designs, an implementation programme,

estimated costs, contract and public consultation strategies.

The district covers about 150kms of fresh and salt water mains of between 20mm-1,200mm

dia. Pipe materials include galvanized steel, PVC, asbestos cement, cast iron,

ductile iron and mild steel.

The water mains are mostly laid under heavily trafficked carriageways with a

few small size mains in backlanes or narrow village roads. The areas in which

works are to be carried out are also the most heavily populated and closely

developed, where the effect on pedestrian traffic is also a key issue to be

addressed.

Complex impacts

The study area has been divided into 22 sub-areas on a geographical basis each

of which includes on average about 3 schemes each with a group of water mains

requiring investigation.

Each scheme was studied in detail. The traffic condition was surveyed and the

likely impacts arising from the works assessed. Where traffic impact is unacceptable,

it becomes the single factor which renders open trench method of construction

unfeasible. Environmental and social impacts were also assessed and mitigation

measures formulated. Where environmental or social impacts are unacceptable,

trenchless methods would be recommended even if open trenching is also technically

feasible.

Ground investigations in terms of boreholes and trial pits were also carried

out to ascertain the geotechnical conditions along the proposed alignments of

the water mains.

Most carriageways in Hong Kong are packed with other utility services, drains

and sewers, gas mains and electric and telephone cables. Often there is insufficient

space underground to allow for even one extra water main. Utility mapping was

therefore conducted to identify the underground space constraints.

In Hong Kong, the open trench method has by far been most commonly adopted

for laying of water mains. In the early days when traffic and environment impacts

were far less severe, this method of construction offered the low cost solution

compared with other rehabilitation methods. The margin of cost saving is now

becoming less, due to the costs incurred for implementing mitigation measures

to alleviate traffic and environmental impacts.

Choice of construction method is based on a combination of factors besides

cost. Other constraints render on-line rehabilitation unfeasible or inappropriate

– e.g. large water mains which cannot be taken out of service or mains with

sharp bends or too many lateral connections. All rehabilitation techniques need

careful evaluation as to their suitability for application in Hong Kong.

Making the right choice

Rehabilitation techniques can be broadly classified as non-structural and structural,

the latter further classified into semi or fully structural.

In the feasibility study, all these techniques were reviewed and a decision

flowchart was specially designed for the purpose of selecting the most suitable

or preferred technique for each section of the water mains to be refurbished.

Trenchless techniques considered include spray-on epoxy or cement lining; lining

by pipe insertion; replacement by pipe bursting or splitting; lining by cured-in-place

pipe; replacement by pipe ramming; pipe jacking; and directional drilling. Spray-on

lining is non structural and is not recommended for use in mains without prior

condition assessments of the pipes to be rehabilitated.

Other options are semi-structural or fully structural and their suitability

has been considered case by case.

These rehabilitation techniques have been widely practised in UK, Europe and

USA but their use in Hong Kong is very limited. WSD has therefore proceeded

with pilot tests of some of these techniques – cured-in-place lining, swagelining

(a form of pipe insertion) pipe ramming, pipe bursting and directional drilling

– before committing to large scale adoption. Another key element for successful

delivery of the rehabilitation programme is good management of public consultation.

During the study, MCAL assisted WSD in this and a consumers’ care strategy has

been developed.

Contracts and contractors

In view of the large volume of work, a survey of the local contractors’ capability

was also conducted. The results indicated that there are established and experienced

contractors in Hong Kong capable of undertaking the work, but many will link

with specialist overseas contractors.

In terms of contracting, various forms including term contract, re-measurement

contract, supply and lay, lay-only and design and build contracts have been

considered. The recommended form of contract will consist of a detailed engineering

specification for traditional open trench main laying, and a performance-based

specification for trenchless replacement and rehabilitation techniques. The

latter will allow contractors’ designs or proprietary designs for trenchless

techniques as long as the requirements in the specification are met. The construction

contracts would take the form of ‘supply and lay’ and work done would be paid

on a re-measurement basis. Pre-qualification of contractors has also been recommended.

With the feasibility study now complete, consultancy for detailed design has

been invited by WSD and construction work is expected to start in 2003.

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