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.
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.
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.