The trenchless challenge in urban Hong Kong

Dec 07, 2007

The underground environment in Hong Kong Installing and maintaining underground utilities can be a challenge anywhere, regardless of whether conventional open trench or trenchless methods are adopted. However, every location and environment imposes specific constraints and difficulties, and Hong Kong is a particularly difficult environment in which to work underground.

The combination of the hilly natural terrain and the high-rise built environment, epitomised by the well known cityscape of Victoria Harbour, make Hong Kong unique. The total area is 1,100 square km but only 17 per cent of the land area is developed, and much of this is actually reclaimed from the sea. With nearly 7 million people living in such a small area, there is severe congestion both above and below ground. The rapid economic growth of the last two decades has compounded the problem as old districts have been torn down and dramatically redeveloped with huge high-rise office and apartment blocks. Because land is at a premium and already congested, it has been extremely difficult to expand the road and pavement capacity to cope with either the above or below ground needs.
The dense and crowded environment of Hong Kong’s streets is therefore mirrored underground, where road openings invariably reveal a crowded spaghetti-like complex of utility services. It is common to find utilities running alongside each other with little or no clearance between them. This means that there is intense competition for space between the utility services, all of which are planning to upgrade expand their underground services in the coming years. Good communication and continuing liaison between utilities and their contractors and consultants is essential if project interface problems are to be avoided and nuisance to the public minimised. The lack of available underground space also means there is a significant risk of damaging nearby utilities when installing new services, or rehabilitating existing ones with techniques such as pipe bursting, for example. The situation is aggravated by the fact that plans and records of existing services are often incomplete, inadequate, or insufficiently accurate.
Other utilities are not the only obstacles to the installation of new services. Ground conditions in Hong Kong are notoriously unpredictable and factors such as high water tables, rapidly varying ground conditions, and natural obstructions such as large boulders can cause major problems when using techniques such as pipe ramming, pipe jacking, and directional drilling. A further factor is that much of the urban area of Hong Kong is built on reclaimed or made up land, which can contain a whole range of potential hazards to the trenchless contractor. Underground obstacles, such as old sheet piles and buried sea walls, are not uncommon and have caused several problems with machines becoming stuck or damaged underground.
In order to improve the co-ordination and control of road opening works and to reduce traffic impact, all underground utility projects are subject to traffic impact assessments during the design stage. For heavily trafficked roads and major strategic highways, it may take several months to obtain the necessary excavation permits. In many cases these permits will include restrictions on daytime working, meaning that all work has to be carried out during the night and, in some cases, at weekends. During the construction stage, all proposed works must be discussed with representatives from the Police, Transport Department, Highways Department, and other stakeholders. In addition, contractors must develop temporary traffic management schemes which are subject to trial runs before the works commence. Whilst trenchless techniques can substantially reduce the requirement for traffic lane closure, encroachment on traffic lanes for the construction of launching and receiving pits is often inevitable. So, although trenchless methods can reduce the overall impact on traffic, obtaining all the permits and approvals imposes a major constraint on planning and programming trenchless works in Hong Kong.
In order to reduce the risks, costs and time associated with trenchless construction, the use of advance contracts has recently been introduced to gather information necessary for identifying the most suitable location of shafts and the most appropriate trenchless method to use. The work in these advance contracts includes utility surveys, geotechnical investigations, pipeline condition surveys, and trial runs of temporary traffic schemes.
Although not as severe as for open trench work, there are always environmental impacts associated with trenchless work and these can present a constraint and added difficulties for the implementation of a rehabilitation program. In particular, it is very difficult to obtain approval to cut down roadside trees or shrubs in Hong Kong, and this is often a major consideration when identifying suitable locations for access pits for trenchless work. As mentioned above, it may be necessary to carry out the works at night. But there are noise restrictions imposed by the Hong Kong authorities, particularly for working in residential areas at night. This can make excavation of access pits difficult, if not impossible in some cases, and can sometimes rule out the use of techniques such as pipe ramming, if noise levels are unacceptably high.
Another constraint when rehabilitating water mains is that interruptions to supply are limited to a maximum of eight hours. As there are no on-line rehabilitation techniques that can be completed within such stringent shut-down constraints, temporary bypass systems or alternative supplies must be installed for all relining work in Hong Kong. This is difficult in Hong Kong’s busy, crowded, and densely populated urban areas and adds to the project time and cost, particularly for the larger diameter pipelines.
Trenchless Technology in Hong Kong
Some trenchless methods, such as pipe jacking and tunnelling have been used in Hong Kong for a number of years. For example, many kilometres of large diameter (up to 5.6 m in diameter) sewer tunnel have been constructed using Tunnel Boring Machines (TBMs) at depths of up to 150 m below ground level.
In the late 1990s, two major directional drilling projects were instigated. The first included the installation of two 300 mm diameter pipelines, crossing under the 1,100 m wide, and 45 m deep Ma Wan channel. This work involved drilling a total distance of around 1.4 km though difficult and complex ground with granite and volcanic tuffs, intersected by faults, and overlain in the shallower sections by marine deposits. Although the internal diameter of each pipe is 315 mm, the actual bore was 800 mm in diameter, in order to meet conditions laid down by Hong Kong’s Water Supplies Department (WSD).
At the time it was one of the largest and most difficult hard rock directional drilling projects ever undertaken anywhere. The second directional drilling was to install twin ducts for electricity supply cables across the same channel, a few hundred metres away from the WSD project. This was made even more complex by the fact that the alignment was curved in plan.
On the rehabilitation front, a number of trenchless relining projects were successfully completed for clients such as the MTRC (underground railway), the Drainage Services Department (DSD) and the Hong Kong Airport Authority, some dating back to 1990.
However, the widespread use of Trenchless Technology did not really take off until the WSD commenced a massive program to replace or rehabilitate almost half of all the water mains in the Hong Kong Special Administrative Region. This is probably one of the largest pipeline rehabilitation programs in the world, and approximately 3,000 km of water mains have been targeted for renewal or rehabilitation in four stages at an estimated total cost of approximately HK$11 billion (US$1.4 billion). The work commenced in 2000 and is scheduled for completion by 2015. A range of trenchless techniques have been used so far in the program, including pipe jacking, pipe ramming, directional drilling, pipe bursting, slip lining, swagelining, fold and form close fit lining, and CIPP.
At the same time, the use of trenchless methods by all the other utilities is also growing. DSD is currently installing more large diameter deep sewers in very congested urban areas using pipe jacking. DSD have also used a variety of pipe rehabilitation methods including CIPP, folded liners, and spirally wound liners. The two electric companies in Hong Kong (CLP and HK Electric) have both used trenchless methods for the installation of cables. The gas supply company, Towngas, has been particularly pro-active in promoting Trenchless Technology and has formed its own wholly owned specialist Trenchless Technology contractor who are now carrying work throughout Hong Kong for not only gas but also most of the other utilities as well.
Meeting the trenchless challenge
Although the use of trenchless methods in Hong Kong is difficult and subject to a variety of constraints, the benefits are recognised and its use is on the increase. There have already been a number of achievements and local contractors are gaining experience in a variety of trenchless methods. Hong Kong faces some even greater challenges ahead including the rehabilitation of many large diameter water mains located under heavily trafficked roads in a congested urban environment. In spite of these difficulties, there continues to be a huge commitment to using trenchless methods, and engineers throughout Hong Kong are rising to meet this challenge.

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