It will take nearly 20 years and $1.25bn to upgrade the congested tangle of water mains under Hong Kong's crowded streets. Over 3,000km of pipes supplying both fresh and salt water (for toilet flushing) will have to be repaired or replaced.
New and innovative techniques and technology will be needed to avoid disruption to businesses, residents and tourists.
'Our biggest challenge is striking the balance between disrupting the public now and achieving the improvements we want in our water supply, ' says Hong Kong Water Supply Department (WSD) chief engineer Kwok Hung Wong.
Most of the pipes are over 40 years old and the past decade has seen a rise in the number of leaks and bursts. Water pressure has dropped right across the region - a huge problem in Hong Kong's rugged terrain, where high water pressure is needed to service high rise flats perched on rocky hillsides.
In 1997, WSD decided to take stock of the problem before the situation got worse.
'We were finding an alarming difference between the amount of water going into the system and the amount registered by the consumer. This unaccounted water volume was getting higher and higher, ' says Wong.
He adds that the cost of repairing the leaks and bursts and dealing with the disruption caused was becoming an embarrassment to the authorities.
An assessment carried out in 1997 revealed that more than half the fresh water supply pipes and 25% of the salt water supply system needed replacing or rehabilitating.
The issue of pumping water at such high pressures (16 bars) had taken its toll on the iron and concrete mains pipes in the region and discrete repairs would not last long. Also salt water had corroded pipes in some areas, requiring whole lengths to be replaced.
Rapid development of the island and New Territories had taken its toll as piling work and utilities installing new services often hit water pipes, prompting quick patch repairs, says Wong.
He adds that, while most services are beneath the main road network, nearby development has often caused the ground to settle, exerting excessive forces on the pipes.
The result is a tangled web of inaccessible, under-performing services beneath the city. But the WSD is well aware that fresh water is in short supply and although using salt water in toilets helps the situation, minimising water loss is the only way to maintain a long term supply.
WSD initially intended to upgrade the network itself, but as the scale of the project unfolded, it realised that outside help would be required if the job was to be done with minimal disruption.
The project is split into four stages and divided between partnerships of Hong Kongbased contractors and European consultants including Black & Veatch, FaberMaunsell, Scott Wilson, MWH and Atkins.
Black & Veatch is implementing part of the stage one works, led by project manager Ian Vickridge.
'Everything is exaggerated here - there's much less space to carry out work, more people could be affected, there's more traffic and utilities underground than on any other project I've worked on, ' he says. Lack of reliable data recording the location of utilities has also been a huge problem.
'We have to do a lot of co -ordination on site because we have no records of where the buried services are, not even our own in some cases, ' admits Wong. 'The moment of truth comes only when you look underground. If you fi nd a gas pipe, you call the gas people.' Much of the work is carried out in intensive night time possessions or during weekends.
To limit disruption, trenchless pipe laying techniques are being employed.
But even trenchless techniques involve building launch pits and use noisy machinery. Vickridge admits that there is no single solution which satisfies all the constraints. The Hong Kong Environmental Protection Department (EPD) imposes strict conditions of work regarding noise vibration and traffic management, and limits shutdown of consumer supply to just eight hours.
Since pipe replacement and most trenchless rehabilitation techniques take longer than eight hours, temporary bypass systems must also be set up.
Black & Veatch has been rehabilitating a 3km stretch of mains supply pipe beneath the busy Tolo Highway near the famous Sha Tin racecourse, using swage lining techniques.
Contractor is Chun Wo U-Tech joint venture.
The process involves lining the damaged host pipe with a tight fitting polyethylene tube. Beneath the Tolo Highway, a 40mm thick lining is being installed inside the 800mm diameter freshwater pipe.
Black & Veatch resident engineer Fanny Lau explains that the thick lining is required to withstand the high water pressure.
But such thickness makes it difficult to weld sections and to form tight bends.
Getting EPD permits to work on this project has been a 'painful experience', admits Lau.
Weekend work is limited by major horseracing events, but wealthy local residents also object to noise disruption at any time of day.
She says contractors can only work 26 hours a week and much of that time is spent setting up and dismantling machinery.
Limiting noisy work to just one hour per night, says Lau, means that relining the pipes beneath the Tolo Highway will take two and a half years to complete instead of six months had the job been done continuously.
With such a disjointed working programme, the operation has to be split into 46 sections punctuated by 47 launch pits across the 2.8km length of Tolo Highway. Each launch pit is covered up with a metal plate during the day.
Ironically, local residents complain of the noise of trucks passing over the plates. The nature of objections means it can take up to six months to get EPD permits processed.
'We've successfully lobbied the EPD to assure them that every measure has been adopted. We've appealed that the background noise from traffic will often be louder than our machinery, ' says Lau.
The work being carried out over the next 20 years will extend the mains supply design life by another 50 years. Vickridge hopes that as Hong Kong contractors build up experience of trenchless techniques, such as pipe bursting and cured in place lining, costs will drop, the rate of progress will increase and residents will become more co-operative.