Hong Kong is not unfamiliar with tunnel projects of almost all kinds, from transport to utility services and deep foundation excavations. But though large, stand-alone rock cavern developments have been rare enterprises, that may be about to change.
The scarcity of expensive surface land, and the opportunity – the chance – to regain pockets for redevelopment, has led the Hong Kong special administrative region (SAR) Government to pursue studies on the strategic transfer of some key facilities and services below some of the hills. The Government’s civil engineering and development department (CEDD) is exploring the case for caverns, the efforts being led by its geotechnical engineering office.
Earlier this year, a study carried out by Arup for CEDD highlighted the potential for major cavern excavation to house some public works facilities that could be relocated underground within the granitic and volcanic rocks of the hilly terrain. The work – supported by subconsultants Norconsult and Urbis – reviewed the use of underground space locally and globally, and benchmarked Hong Kong. Suitable areas for possible rock cavern development were identified, and recommendations made to take such developments forward. In addition, the consultants looked at the challenge of transferring existing services, and three facilities were selected to demonstrate the broad viability of transferring those facilities underground and to explore the main issues involved.
“This was essentially a feasibility study that reviewed the technical and planning aspects of cavern development,” says Mark Wallace, Arup director and project manager on the study.
With project sizes dependent on the public works or other processes to be housed underground, the Arup study says smaller cavern schemes might be excavating volumes of 100,000 to 200,000m3 while the largest might be up to 700,000 to 1Mm3.
In terms of support, Wallace adds, “Realistically, a shotcrete and rockbolt approach would be the most cost effective but there may be a preference to consider cast in-situ linings for those facilities where groundwater control and water tightness is of priority. Final lining requirements will depend on the end-user requirements.”
Despite the costliness of such major tunnelling schemes, it is expected the cavern schemes will be cost-effective as they would, in effect, be trading relatively cheap underground acreage for more valuable surface land in one of the world’s most expensive stretches of real estate.
At the end of June, Arup was awarded a HKD 18.64M (USD 2.4M) consultancy contract by CEDD to follow-up on the initial planning and technical study. The second study is to look at land supply – leveraging the benefits of underground rock caverns as well as looking at reclamation potential. In addition, the contract includes a sizable public consultation element. The work is due to finish by April 2013 and is to explore a further eight prospective cavern sites to re-locate some government facilities.
In exploring the potential for rock cavern projects, Arup’s main challenges are to establish an overall strategy and identify opportunities and facilities in a planned manner. “Technical issues are common to any underground scheme,” says Wallace, “but all these are dependent on the type of facility to be placed underground.”
Underground strategies
Hong Kong’s famous congested and spiked skyline is bound in tight swathes around the edges of the hilly terrain of the city and in places around many of the surrounding islands. With a population of about 7.1 million and ever-decreasing availability of flat land to occupy, many of the buildings are high-rise blocks. Every square metre that can be found or recaptured is prized.
Much non-residential infrastructure development is underground, especially metro tubes and other transport in addition to the conventional placement of utilities, and much more is planned. Hong Kong, from the start of the last century to 2009 had constructed about 58km of drainage and sewerage tunnels (Pang et al, 2009). Almost the same again – 49km – is either in planning, design or to be constructed by 2019. For rail tunnels it is a similar story – 87km by 2009 and a further 76km planned by 2019. These categories cover approximately 70 per cent of the tunnel projects planned or in construction for the remainder of this decade.
Current tunnel projects include drill and blast excavation as well as TBM boring that is being carried out on metro projects, such as the West Island Line (WIL) extension and the new South Island Line – both on Hong Kong Island. Among other developments have been varied sewerage, road and Slope stabilisation tunnels.
The largest project is construction of 26km-long high-speed twin tubes for the Hong Kong section of the Express Rail Link (XRL) to Shenzhen and Guangzhou.
The Government has been clearly supportive of the needs and provisions for underground solutions to infrastructure demands, and no small example is the protection of the XRL corridor out of Kowloon terminus. To this strategic perspective is added the possibility of new caverns being built to take suitable existing public facilities, not least those that may not be too favoured for how close they are to residents and businesses, such as refuse centres or sewage plants.
Earlier this year, just before the second consultancy study was awarded and while visiting some existing underground facilities, Donald Tsang, Chief Executive of the HKSAR Government, said, “Hong Kong needs to create land resources to cope with the demand for housing and development of various industries. However, our land is limited.”
Tsang toured the recently completed western salt water underground service reservoirs in Pokfulam, in the west of Hong Kong Island. The caverns are around 16m wide, 14m high and 40m long, and the lining is partly cast-insitu lining with a SCL crown. The site also has covered fresh water reservoirs, following some recent reconfiguration and relocation of the various reservoirs, releasing land for the nearby University of Hong Kong.
The Pokfulam site is slightly to the south of where some new, fresh water service caverns might be built, in the Mount Davis/Kennedy Town area, which was identified as a potential project from the consultants’ first study. The report also suggested, in outline concept, much larger projects, such as putting the Sha Tin sewage treatment works, northeast of central Hong Kong, underground in an adjacent hillside.
“It is crucial that we think outside the box in creating new land,” says Tsang, adding that rock cavern development is an option worth considering.
Caverns – past and planned
Yet the studies are not the first time Hong Kong has looked at strategic rock cavern development. A couple of decades ago, Arup with Norconsult undertook two of the early studies – the Study of Potential Use of Underground Space (SPUN) and the Cavern Project Studies (CAPRO), in 1990 and 1991 respectively.
“We have a good relationship with Norconsult and have collaborated on a few projects in other parts of the world,” says Wallace.
During the 1990s there were various cavern related studies undertaken by CEDD’s geotechnical engineering office, including Cavern Area Studies (CAS) from 1992-98, and which covered Lantau Island, Hong Kong Island and Kowloon Peninsula by identifying potential cavern areas in those areas.
Also, from 1990 until the latest studies began, there were Preliminary Engineering Geology Studies (PEGS) for a variety of government facilities, such as sewage treatment, fuel storage, abattoir and water.
About 60 per cent of the land in Hong Kong is considered suitable for rock cavern development, though addressing land ownership is a separate, and crucial, issue according to the latest studies. Arup concludes there is significant potential for cavern development, especially as many hillsides have shallow rock cover and reserves of good rock mass.
Arup anticipates that future caverns housing transferred public works facilities could have dimensions of, typically, lengths of 100m to more than 250m, widths of up to 20 to 25m and heights of 10 to 18m. Access tunnels might be two-way or oneway routes, and could be 10 to 12m wide, and about 6 to 8m high, depending on truck, vehicle and ventilation requirements of each type of facility.
Wallace says that cavern options were considered in the past, and there are some successful examples, such as the Island West refuse transfer station and the Western salt water service reservoir in the west of Hong Kong Island. On the south side of Hong Kong Island, the Stanley sewage treatment plant is housed in a cavern. Underground storage is also used for explosives at the government depot at Kau Shat Wan, on Lantau, while an underground explosives magazine has been successfully constructed for WIL metro project on Hong Kong Island. In addition, MTR’s expanding metro network will see more station caverns.
Among other underground, non-cavern related facilities reviewed by the consultants in their first, recently completed study are: the drainage service department’s (DSD) Tai Hang Tung flood storage tank and pumping station; Crown Wine Cellars’ use of former air raid bunkers on Hong Kong Island; numerous examples of deep excavations, such as cut and cover for metro stations and highrise buildings, and an underground pumping plant for Stonecutters Island sewage station and various works for the current HATS Two A project.
The longest caverns built so far in the city are for MTR lines with MTR’s Tai Koo station still the longest at 250m. Two other cavern stations are being constructed on WIL with similar dimensions to Tai Koo. The cavern for the Island West Transfer refuse station is the widest, built in the late 1990s with a span of approx 27m in the main tipping hall. The highest cavern, at 17m, is the main hall of the Stanley sewage treatment works, higher than Tai Koo by only 1m; the Stanley cavern is half the length of Tai Koo.
Yet, while there have been numerous tunnelling works in Hong Kong, and perhaps it is a recognised nexus for the industry, Wallace notes that the cost-benefit assessments for public works have not been a level playing field in weighing underground against surface options. He says ‘land allocation is not considered as part of the project cost, which tends to support above ground schemes.’
Cavern Study: 2010-11
In addition to gaining land, other benefits to be won from going below ground include more stable temperatures and security, though there would be added engineering obstacles to overcome, such as ventilation, lighting, fire safety and evacuation. Matters of land ownership, too, will play no small part in filtering the options for caverns.
To drive forward its latest initiative for rock caverns, CEDD launched the work in March 2010 with the study: ‘Enhanced Use of Underground Space in Hong Kong’ [Agreement CE66/2009 (GE)] study, which Arup finished in March this year.
Key points covered by the study, in addition to reviewing what had been studied and built before, were to develop a cavern suitability map and identify five strategic cavern areas and potential government sites to transfer underground.
Going into some local detail to showcase how existing facilities could be transferred underground, the study also considered three potential cavern projects and looked into a wider context by placing Hong Kong’s caverns and land uses in an international setting. Also in Asia, in a similar manner to Hong Kong, Singapore is considering developing further projects and plans for major underground facilities.
“It appears that Singapore has a similar land supply problem and maybe even a more acute problem,” says Wallace. “They have less available steep terrain to enter the ground and hence the majority of any cavern development would involve driving down into the ground, rather than in Hong Kong where the facility could lie close to existing ground level at the portal or to drive up into the steeper hillside areas.”
International perspective
The study noted the significant underground facilities for strategic oil, gas and fuel storage built since the late 1970s in South Korea. Recently this has been undertaken in Singapore as well. There are also a variety of facilities in Japan, many of them commercial environments with underground shopping and retail areas. There is also an underground bicycle park and ride complex at a suburban rail station.
Numerous underground space projects are also planned in China, as the study noted from ‘The Use of Underground Space as an Unexpected Solution for Promoting Sustainable Development’ presentation at a joint UN-ITA workshop in late 2007. The presentation foresees about 60Mm2 of underground ground space development by 2020, principally in the Beijing and Shanghai areas and other major cities.
Further afield, underground schemes looked at included a number in northern Europe, including Helsinki, Finland, the first city to completely map its use of underground space.
Wallace says, “The Helsinki underground masterplan is considered by many people as a good example of strategic use of rock caverns and underground space in the world.”
Cavern development in Sweden and Norway, has included parts of the metro in Stockholm and, in Oslo, placement of the national archives underground in a complex which is being expanded in stages. Norway also holds one of the largest underground spaces in the famous 60m-span of the Gjovik Olympic Hall built almost 15 years ago for the Winter Olympics at Lillehammer.
Norway also has other underground structures, such as the 16 storage caverns of the Ekeberg Warehouse and adjacent petroleum product facilities; a pair of unlined caverns for LPG storage near Mongstad; and, a swathe of cold storage facilities, mostly built over 1956-82 though most recently there is the International Seed Bank on the Svalbard archipelago in the Arctic. The country also has some water treatment facilities below ground, such as the Oset plant, opened in 2008 outside Oslo.
Wallace notes that while Hong Kong does not have a current policy for underground civil defence shelters, such as in Singapore, he says, “Many of the previous cavern schemes in Scandinavia were supported by grants and subsidies to develop mixed use shelters for local communities.”
Other examples where underground planning is being considered are in The Netherlands, at Zwolle and Arnheim, and in Dallas and Montreal in North America.
Globally, and while touched upon in the review, one of the common needs for cavern construction is in the energy sector, especially the power and transformer halls for the hydropower and pumped storage facilities that exist in many countries, and many more are to be constructed around the world.
In the energy sector, there are also questions related to cavern construction and geological competence under examination as well as prolonged debate for the long-term burial of nuclear waste. In Norway, one of the examples cited in the study is the Himdalen waste facility.
Separately in the energy sector, there is also increased use of pre-mined salt chambers for storage of pressurized natural gas.
ITA’s Committee on Underground Space (Itacus) Working Group Four collected global data showing that, proportionally, civil applications dominate the use of tunnels with a 45 per cent share. With shares of almost about a third each – 35 per cent and 30 per cent – other categories of use are goods storage and waste disposal, respectively. The remaining 10 per cent of their study identified experimental laboratories as other types of uses, such as the Deep Underground Science and Engineering Laboratory (Dusel) in South Dakota, in the US; as well as the research facilities at Boulby Potash mine in the UK, and the huge particle physics facilities in the Alps.
Map and facilities
In the study, almost two-thirds of Hong Kong’s land area was assessed to have medium-to-high suitability for cavern development based on analyses of set of spatial data, including ground conditions, existing underground infrastructure and other constraints.
A map was developed and it was noted that those government facilities seen as particularly suited to transfer underground are those close to the most favourable zones and others on edges of urbanised areas.
The study looked at more than 400 government facilities and developed a set of preliminary criteria to rank them for potential to be transferred underground. It was noted, however, that developing the rock caverns to take some government facilities underground would require an environmental impact assessment (EIA) – that covered both the construction and operation of the facility.
In looking at land uses with potential for rock cavern development, the study proposed to add a number to those listed already in the current planning guidelines, such as food/wine storage, warehousing, archives, data centres, research laboratories, science park, bicycle park and ride, car park, crematorium, refuse collection, maintenance depot, underground quarries and substations.
“Another challenge is the location of the portal and the accessibility of the underground scheme to transport networks, although there are many areas where there is good connectivity to nearby roads and highways,” says Wallace. He adds, “In Hong Kong, we have a world class approach to dealing with landslide hazards and this will be utilised in mitigating the risk to the rock cavern scheme, in particular at portal locations.”
Sample case studies
The first study also considered the challenges of transferring some government facilities to caverns by selecting three to examine on a preliminary feasibility basis.
The existing facilities that were examined in the study were: Sha Tin sewage treatment works; Mount Davis and Kennedy Town water service reservoirs; and, at Mui Wo, a sewage works and refuse transfer station to be housed in a multi-function facility. Relocating them underground into grids of long galleries, as conceptual layouts only, would release a total of approximately 32.5 hectares (0.33sqkm) of land. The selection was made to also look at different ranges of benefits to the wider and local communities, and review the technical challenges, constraints and possible solutions.
Sha Tin
This scheme was chosen to demonstrate the feasibility of moving a major facility underground – in this case a sewage treatment works – from a prime location at the mouth of the Shing Mun River. If done, the transfer of the large facility would release about 28 hectares (0.28sqkm) of waterfront land, reduce environmental and visual impacts and enhance local land value. The feasibility of transferring such a large facility has precedent overseas, and it could be further expanded underground, the report says.
Mount Davis and Kennedy Town
If done, it would see relocation of fresh water service reservoirs to rock caverns, and release about two hectares (0.02sqkm) of prime land. It would improve security and integrity of the water tanks, and also allow future expansion without extensive cutting open-cut works on the hillside.
Mui Wo
This scheme was envisaged as moving a sewage treatment plant, refuse transfer station, bus depot and some other facilities into an integrated cavern complex, releasing about 2.5 hectares (0.025sqkm) of waterfront land with 500m of coastline. Improvements would result for the general environment and appearance of the area.
The key recommendations from first study were to include the cavern option in government project planning, develop a strategy to relocate government facilities underground, protect potential cavern sites from development, and develop a policy for private sector involvement.
Cavern study: next step
The follow-on study, now underway, is ‘Increasing Land Supply by Reclamation and Cavern Development’ [Agreement CE 09/2011]. CEDD said the 21-month study is a ‘major milestone’ in the strategy to identify potential reclamation sites outside Victoria Harbour and also suitable rock caverns.
In parallel with the study, there will be a two-stage public consultation, the first to start before the end of this year and lasting six months, and the second to commence in the third quarter of 2012.
Following the two stages, the process will deliver, by early 2013, a list of 18 publicly accepted sites – eight rock cavern sites, seven reclamation sites and three handling sites for contaminated sediment.
Figure 1, Hong Kong’s strategic areas for rock caverns and study sites Government visit to cavern in Western Salt Water Service Reservoirs Cavern hall in Island West refuse transfer station Figure 2, comparative sizes of existing caverns in Hong Kong Figure 3, cavern concept for Mount Davis/Kennedy Town reservoirs Figure 4, cavern concept for Sha Tin sewage treatment works Figure 5, cavern concept for Mui Wo multi-function public facilities
