Development and application of underground space use in Hong Kong

Tunnelling and Underground Space Technology xxx (2016) xxx–xxx

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Development and application of underground space use in Hong Kong M.I. Wallace a,⇑, K.C. Ng b a b

Ove Arup and Partners Hong Kong Limited, Hong Kong Geotechnical Engineering Office, Civil Engineering and Development Department, Hong Kong SAR Government, Hong Kong

a r t i c l e

i n f o

Article history: Received 9 November 2015 Accepted 13 November 2015 Available online xxxx

a b s t r a c t Underground space has been utilised in Hong Kong for many decades. It has been developed through various phases of infrastructure development and improvement. The early forms of underground space construction were associated with war time protection and mining operations and this has extended with increased urban densification to numerous examples of underground basements that have incorporated car parks, retail and commercial underground spaces. Notable underground space also includes numerous road, rail and utility/service tunnels that comprise a network of over 500 km of tunnels in the city to accommodate essential services and transport. With the Mass Transit Railway (MTR) construction in the 1970s it kicked off a spate of excavation of underground space for metro stations and linking tunnel networks that has developed one of the most efficient and reliable metro systems in the world. The private sector has increasingly, over the last few decades, linked their commercial and retail properties to the MTR station network providing hubs of interest and commerce within the underground network. Since the 1990s, a few government ‘‘Not-In-My-Backyard” type facilities have been built in rock caverns to meet the needs of the community, including a sewage treatment plant, a refuse transfer station and an explosives depot. Ó 2015 Published by Elsevier Ltd.

1. Introduction The challenge of providing useable land within the city’s rugged terrain, while still trying to accommodate various competing needs and requirements, is promoting an increased awareness and interest in developing underground space. Hong Kong covers a land area of about 1110 km2, of which only about 24% (PlanD, 2014) of the land has been developed. The remainder of the land in the city comprises predominantly woodland/shrubland/grassland/wetland (about 67%) with the remaining 9% comprising agricultural, water bodies and some barren land. About 40% of the total land area is designated as country parks. As a result of various competing demands for land from different users, as well as an increased desire to protect our existing landscapes, there is a shortage of land for further urban expansion. At the end of 2014 the city had a population of 7,221,800 which combined with the current extent of developed areas in the city gives an average population density of developed land areas of around 27,000 people per square kilometre. The city has excelled at the construction of high rise buildings for living spaces and in many areas the height limits and development potential has been reached with limited scope to ⇑ Corresponding author. E-mail addresses: [email protected] (M.I. Wallace), samuelkcng@cedd. gov.hk (K.C. Ng).

build higher. In recent years, the Hong Kong Government has been actively exploring the potential of underground space development as a viable option of enhancing utilisation of land resources for a dense urban city. This paper outlines the different types of underground space in Hong Kong and how the use of underground space has evolved over times. A major constraint on normal surface development is the city’s steep terrain (Fig. 1) with hilly terrain comprising about 60% of the area of Hong Kong. Steeper terrain with slope gradients greater than 30° accounts for about 35% of the city. The underlying igneous rocks account for about 85% of the rocks found in the city. Granitic (35%) and volcanic (50%) rocks (Fig. 2) make up the major portion of the surface rock formations. There are other rock formations comprising meta-sedimentary and sedimentary rocks but these typically lie in the low lying terrain areas. With the high property values for residential buildings (Kowloon & Hong Kong Island) typically between US$14,000 and US $33,000 per square metre (end 2014) the potential to pay for and utilise underground space is becoming increasingly attractive and viable. Indeed retail rents in some of the congested urban shopping areas are in excess of US$50 per square metre. Hong Kong has a sub-tropical climate that tends towards being temperate for nearly half the year in autumn and winter. Summer months can be very hot with occasional mid to low 30 °C being encountered with high humidity (Fig. 3). The city has periods of

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Source : CEDD Web Site

Fig. 1. Terrain map of Hong Kong.

Source : CEDD Web Site

Fig. 2. Simplified geology map of Hong Kong.

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Source : HK Observatory Web Site

Fig. 3. Monthly means of rainfall and rainy days at HKO between 1981 and 2010.

heavy rain and thunderstorms and is affected by typhoons during the main summer months (Fig. 4). This tends to drive the population to use air conditioned spaces during the summer months to seek relief from the heat/humidity and inclement weather. The planning framework in the city has identified various limits to development such as height restrictions and reduced plot ratios in some areas. The country parks, green belts and conservation areas are also threatened with encroachment by surface developments and there is great concern from the public in preserving those areas from surface development (Fig. 5). Since the 1990s the Hong Kong Planning Standards and Guidelines (HKPSG) have identified the potential for cavern development with some successful projects already implemented. The Planning Department is responsible for defining the overall mix and zoning of land uses in Hong Kong. They consult with The Town Planning Board to approve and set the development and land use zoning standards and approach for the city. 1.1. Recent impetus to explore rock cavern and urban underground space development Chan and Ng (2006) remarked that while there are administrative and technical issues that need to be addressed to facilitate enhanced use of underground space, from the cases of projects using cavern solutions, the potential for more active use of caverns for infrastructure development still remains. In October 2009, the Chief Executive of the Government of the Hong Kong Special Administrative Region (HKSAR) presented the 2009–10 Policy Address on ‘‘Breaking New Ground Together”. The Development Bureau put forward a new initiative under the Policy Agenda of ‘Developing the Infrastructure for Economic Growth’ to launch strategic planning and technical studies to facilitate planned development of underground space, which are aimed at promoting the enhanced use of rock caverns as part of Hong Kong’s pursuit of sustainable development. In the 2013 Policy Address on ‘‘Seek Change, Maintain Stability – Serve the People with Pragmatism”, the Chief Executive highlighted that rock cavern development is a viable source of long-term land supply and stressed the need to conduct a study on the long-term strategy for cavern development with a view to preparing rock cavern master plans and formulating policy guidelines. In addition to moving ahead with cavern development, the 2014 and 2015 Policy Addresses highlighted the need to explore the potential for developing underground space with a view to increasing usable space and enhancing connectivity in the urban areas.

Source : HK Observatory Web Site

Fig. 4. Average monthly temperatures (°C).

1.2. Studies on rock cavern development In the late 1980s the Geotechnical Engineering Office (GEO) of the Civil Engineering and Development Department (CEDD) had been tasked by the Hong Kong Government to undertake various Cavern Development studies that explored the potential for cavern development at the time. It reviewed the overseas approach to cavern development as well as the technologies and the approach to cavern design. As a result of those studies several notable facilities in Hong Kong were placed in rock caverns to solve the problems of lack of space or to protect nearby communities from the nuisance aspects of the facilities. These are further discussed below in Section 3. Since the 1980s cavern construction has become a more established solution in various countries for a range of different types of facilities. The technology and equipment used to construct these underground spaces has shown continual improvement in its application and efficiency. Other countries have been using purposebuilt rock caverns as their preferred choice to house a variety of facilities, including water and sewage treatment works, data centre, oil and gas storage, warehousing, freight transfer, sports hall, etc.

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Fig. 5. Highrise development on Hong Kong Island (Kennedy Town).

Fig. 6. Population growth and tunnel development in Hong Kong (adapted from Pang, 2015).

2. Infrastructure based underground spaces The main development of infrastructure based underground space came in the 1970–1990s with the development of a comprehensive underground railway network with numerous underground stations and associated entrances and exists. Prior to the start of Mass Transit Railway (MTR) development there was an extensive network of water supply as well as a few road and rail tunnels in the city. Various road, rail, sewage and cable tunnels now criss-cross the city. As the population of the city has grown it has been noted by Pang (2015) that the overall supporting tunnel infrastructure has

been generally keeping pace with the growth of the population since the 1960s. Fig. 6 clearly shows various phases of development of the supporting tunnel infrastructure to serve the city. The first major phase was the construction of the water supply tunnel network including surface dams and reservoirs in the 1960s and 1970s to ensure continuity of water supply to the city. Several phases of railway tunnel expansion are also seen in the 1980s & 2000s and more recently. Drainage and sewerage tunnel networks have been progressively built over the last 10–15 years to correspond with a desire to improve water quality in Victoria Harbour as well as provide flood prevention measures in various parts of the city.

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2.1. Water supply tunnels The first tunnel known to be built for a specific purpose was a 1.5 m diameter 80 m long water supply tunnel in Pok Fu Lam in 1877. This was quickly followed by a 2.2 km long 1.5 m diameter water tunnel from Tai Tam to Wong Nai Chung to supply water to northern Hong Kong Island. This followed with a few tunnels in the 1920–1950s to divert and connect catchwater channels on the hillsides to gather surface runoff water to local reservoirs as well as supply water through Hong Kong’s hilly terrain to water treatment plants and the main population centres. The network including the addition of various tunnels has continued to be expanded to improve supply to the new towns and other development areas since the 1970s. As of 2012 water tunnels extend to over 200 km of network within the city. 2.2. Railway tunnels The first main railway tunnel, completed in 1910 by the Kowloon Canton Railway (now part of MTR), comprised the Beacon Hill Tunnel with single track tunnel of 2.2 km long, an internal width of 5.2 m and height of 5.8 m for the main railway line to Guangzhou and Hong Kong. The first underground metro railway in the city was the Kwun Tong Line in 1979 closely followed by the Tsuen Wan Line in 1982 and the Island Line in 1985. Significant

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expansion of the MTR network occurred in the 1980s and 1990s with many new lines to service new town development as well as improve connectivity within the urban centres. The Airport Core Programme during the 1990s also extended the railway to Lantau Island (Tung Chung Line & Airport Express in 1998) but many of those lines were generally above ground or at grade. The early to mid-2000s saw the expansion of the network with the construction of the Tseung Kwan O line (2002), West Rail (2003), East Rail Extension (2004), Ma On Shan Line (2004), Disneyland Line (2005) and the Lok Ma Chau Spur Lines (2007). By the end of 2012 the total route length of railway tunnels is about 88 km. Five major rail projects which involve either the significant extension of existing lines or the establishment of brand new lines will add a further 56 km of route tunnels by the end of 2020 (Fig. 7). These projects are summarised below:  West Island Line (WIL, 2009–2014) – An extension of the MTR Island Line, that runs beneath the densely populated areas of Western District on Hong Kong Island.  Express Rail Link (XRL, 2010–2018) – The 26-km long Hong Kong Section of the Guangzhou–Shenzhen–Hong Kong Express Rail Link (Express Rail Link, or XRL) runs from West Kowloon Terminus in Hong Kong to the boundary of Hong Kong and Shenzhen. The Express Rail Link will connect with the 16,000km National High-speed Railway Network.

Fig. 7. Future MTR System Map circa 2020 including Lines under construction.

Please cite this article in press as: Wallace, M.I., Ng, K.C. Development and application of underground space use in Hong Kong. Tunnel. Underg. Space Technol. (2016), http://dx.doi.org/10.1016/j.tust.2015.11.024

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Fig. 8. Existing road tunnel locations in Hong Kong (2014).

Fig. 9. Lion rock tunnel portal (Twin 2 Lane).

 Kwun Tong Line Extension (KTE, 2011–2016) – KTE extends the existing Kwun Tong Line to Southeast Kowloon.  South Island Line (SIL, 2011–2016) – The South Island Line (East) will be a medium-capacity railway connecting the MTR network at Admiralty to the Southern District of Hong Kong Island.  Shatin to Central Link (SCL, 2012–2020) – SCL links Tai Wai to Admiralty, whilst strategically linking with a number of existing railway lines. It will serve areas in East Kowloon that currently do not have any MTR service. 2.3. Road tunnels The first Lion Rock tunnel was completed in 1967 as a dual 2 lane tunnel 9 m wide by 9.2 m high and 1.4 km long. This was

Fig. 10. Route 3 Road Tunnel (Twin 3 Lane).

further supported by the second Lion Rock tunnel in 1978 of similar cross section and of similar length. There have been various toll road expansions and upgrades that have numerous highway tunnels constructed from the 1990s onwards. In 1972 a steel tube concrete lined twin immersed tube road tunnel was completed across the Victoria harbour from Hung Hom to Causeway Bay at around 1.9 km long. This was later followed by two more immersed tube road crossings of the harbour in the eastern (1989) and western (1997) areas. The eastern harbour crossing is the only one that incorporates both road and rail elements. Overall there are 16 road tunnels (Fig. 8) in Hong Kong. Up to 2012 there was around 31 km of route length road tunnels in Hong Kong with a further 22 km of tunnel routes to be added by 2020. The Discovery Bay road tunnel is a single tube dual lane tunnel,

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Wines and is acknowledged as one of the world’s finest wine cellar facilities (Fig. 11). The facility boasts six underground wine cellars and a private members clubhouse (Crown Wine Cellars, 2012). 2.5. Mining activities There have been various dates of mining of metalliferous ores in Hong Kong with the main period of active mining taking place between 1915 and 1981 (Sewell et al., 2000). There are many small mine adits and mine workings with numerous 2–2.5 m diameter tunnels at Lin Ma Hang, Needle Hill, Lin Fa Shan, Ma On Shan, West Brother Island and Sha Lo Wan Mines. These mines have over 30 km of recorded mine adits and tunnels.

Source: www.winebuzz.com.hk

2.6. Other tunnels

Fig. 11. Shouson wine cellars/clubhouse (former air raid bunker).

all the other road tunnels are twin tunnels for separated traffic movement with either two or three lanes of traffic. Two examples are shown of the two and three lane tunnels in Figs. 9 and 10. There are currently various road tunnels being constructed to ease urban traffic flows and many of those have incorporated cut and cover tunnel techniques. The Central Wanchai Bypass is a significant underground road tunnel that should ease congestion on Hong Kong Island. Other planned road tunnels include the Central Kowloon Route to aid west to east traffic flow across the Kowloon peninsula. Another road tunnel is planned to connect Lam Tin to Tseung Kwan O to improve traffic circulation from the Central Kowloon Route. The world’s largest TBM (17.6 m OD) (2105) has also been involved in tunnelling part of the road tunnel from Tuen Mun to Chek Lap Kok to construct the road connection across the sea between the Hong Kong Zhuhai Macau Bridge and the New Territories. 2.4. Disused air-raid tunnels and bunkers During the Second World War about 90 small diameter tunnels were constructed mostly by hand to provide spaces of shelter against air raids and aerial attack (GEO, 2015). These were built both by the British and the Japanese. The networks have a variety of sizes and several of them have been re-used for tourism/heritage and for electric cable routing. Some of these air raid bunkers were utilised to store rock core samples but are now used by Crown

In addition to the tunnels described above, there are various electric cable tunnels as part of the strategic power route alignments owned by the two private utility companies (China Light and Power and Hong Kong Electric), totalling about 24 km tunnel network (Fig. 12). These tunnels form major distribution network for their high voltage cables in the city and aid protection against thunderstorms and potential landslide threats that would affect equivalent overhead pylons during the wet summer months. There are also some specific purpose tunnels that have been constructed in the city. One has been constructed in the Po Shan area on Hong Kong Island to regulate regional groundwater of a hillside with a significant landslide threat to local communities with two drainage tunnels totalling about 0.44 km and associated subvertical drains (Ho et al., 2008). Other tunnels include seawater intake tunnels (0.4 km) as well as gas tunnels that are around 3.3 km long. There have been other tunnels associated with the quarrying industry to provide glory hole shaft and conveyor belt tunnels (0.18 km) to supply rock to the quarry crushing facilities. Ocean Park has a 1.3 km long funicular railway tunnel. In total there are around 63 km (2012) of other types of tunnels in Hong Kong. Specific underground uses have been developed in the urban areas with shallow cut and cover basement level facilities such as buried Mass Transit Railway (MTR) stations as well as pedestrian subways and connections and a multitude of basement shopping arcades and car parking. 3. Facility based rock cavern uses There have been a few specific types of facilities that have already been placed into rock caverns. The following are some examples of rock cavern facilities that have been developed and integrated into the urban fabric. 3.1. MTR cavern stations

Fig. 12. CLP Tsz Wan Shan cable tunnel.

With the development of the MTR and the drive to put much of the network underground it was inevitable that some stations would need to be considered to be built in rock caverns to provide convenient access to the nearby developments. This choice was mostly driven by the complexity of the existing underground obstructions and the need to avoid private lots and existing buildings while maintaining an efficient alignment for the railway. Currently, there are four operational rock cavern MTR stations in Hong Kong, namely Tai Koo (Fig. 13), Sai Wan Ho, Sai Ying Pun and University Station. A further three are currently under construction and due to be completed by 2018. Summary details of these MTR cavern stations are provided below:

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Fig. 13. Tai Koo MTR Station and ancillary retail premises. Source: Arup.

Source: Seddon, 2011 Fig. 14. Artist’s impression of the new admiralty station platform cavern.

Tai Koo: The main cavern, completed in 1985, is 251 m long, 16 m high and 24.2 m across. Shortly after construction of the reinforced concrete lining, the overburden was reduced from a maximum of 80 m to about 11 m by site formation works for the Kornhill development (Sharp et al., 1986). The station currently has around 120,000 passengers using it each day. Sai Wan Ho: This Station on the MTR Island Line was also constructed inside a cavern and was completed in 1985 at a similar time to Tai Koo. The cavern has a 16.5 m span, is 13 m high and 18 m long. Sai Ying Pun: A station of the WIL of the MTR Island Line. The station underground space is 225 m long and comprises a 187 m long cavern and a 38 m long platform tunnel extending from the north-eastern end. The cavern is located approximately 40–65 m below ground and had a span of about 23 m and a height of 16 m. University Station: Also a station of the WIL of the MTR Island Line. The station comprises a 240 m long cavern with a span of about 22 m and a height of 16 m. The overall depth of the cavern varies between 48 and 80 m below ground level. Admiralty Station: Admiralty Station will be expanded to become a four-line interchange, the first of its type in Hong Kong and likely will become one of the busiest train stations in the world with a projected station patronage of around 900,000 per day in 2030. An artist’s impression of the new station cavern section is shown in Fig. 14.

Lei Tung: This forms part of the SIL and will be constructed within a rock cavern and separated into a two level platform station (World Tunnelling, 2009; Parsons Brinckerhoff, 2012). Ho Man Tin: This will be an interchange station with the new SCL, an extension to the existing Kwun Tong Line. The Kwun Tong Line portion of the interchange station will comprise two large cavern sections with spans up to 25 m to accommodate the Kwun Tong Line platform. 3.2. Underground research facility A small laboratory is located in one of the cross-passages of the Aberdeen Tunnel (Fig. 15). The laboratory has been utilised by students from various universities in Hong Kong for the detection of cosmic rays and neutrinos. Whilst access to the laboratory is physically easy, it can only be accessed on certain nights of the week. This little-known-about facility is conducting far field neutrino research based upon the Daya Bay Reactor Neutrino Experiment and demonstrates a creative use of some of the underground spaces in Hong Kong. 3.3. Underground explosives depot/magazine 3.3.1. Kau Shat Wan explosives depot Due to the large urban development planned on the West Kowloon Reclamation close to the then explosives complex on

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Stonecutters Island, a new explosives storage facility was planned in the 1990’s to alleviate the potential threat from the facility on the new development. The selected site at Kau Shat Wan (Fig. 16) offered a remote, secure site with limited accessibility by land. The Kau Shat Wan underground explosives depot comprises one loop access tunnel (6.5 m wide, 5.5 m high, over 800 m long) with 10 underground explosives chambers that are 21 m long, 6.8 m high and 13 m wide running off the access tunnel. The underground portion of the facility was built within predominantly fine-to medium-grained granite that was intruded with east–west trending feldsparphyric rhyolite dykes. The explosives depot has a ground cover of more than 52 m. The facility was completed in 1997 and has been the principal store of explosives in Hong Kong serving the majority of explosives and blasting agents deliveries for numerous infrastructure projects as well as firework displays.

Source: www.cpr.cuhk.edu.hk Fig. 15. The underground laboratory in Aberdeen Road Tunnel.

3.3.2. MTRC WIL temporary explosives magazine A small cavern scheme which was constructed by MTR in 2010 to act as an explosives store that could hold up to 4000 kg of explosives for the construction activities relating to the WIL (Fig. 17). The magazine was designed in the configuration of a horseshoe shaped access adit (325 m long) that has eight separate niches at the far end of the horseshoe. Each niche (8.6 m long, 5.5 m wide and 4.2 m high) was designed to hold about 500 kg of explosives. The niches are located underground with sufficient ground cover to provide safe storage of the explosives.

Fig. 16. Kau Shat Wan explosives magazine site.

Source: Arup Fig. 17. Explosive storage niches (left), section of the access adit at the WIL explosives magazine (right).

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3.4. Island West Refuse Transfer Station

3.5. Western salt water service reservoir

The Environmental Protection Department (EPD) planned to develop Refuse Transfer Stations on Hong Kong Island to handle the waste generated, which were required to be placed near the waterfront to allow sea transport to ship the containerised waste more economically to the landfill sites. The cavern scheme was adopted based on the work undertaken in the SPUN & CAPRO Studies (see Section 5 below) as there were problems in identifying a suitable site for the refuse transfer station within the Central and Western District. The Island West Transfer Station (IWTS) project involved the construction of a waste transfer facility in a man-made rock cavern in the Western District of Hong Kong Island (Fig. 18). The cavern layout and arrangement include a 27 m span cavern that is 60 m long and 12 m high. This is the largest span rock cavern constructed in Hong Kong to date. The cavern facility was successfully procured through a build, operate and transfer contract with private sector involvement. Thorough examination and evaluation was therefore required by EPD (the client) to approve the design. The original conceptual design envisaged the excavation of a single, large cavern with tunnels extending two to three floors down to reach a huge cavern. The operator significantly reduced the capital cost of the project by invoking two major changes. First, traffic was designed to descend from the access road to site level by a spiral ramp which was cheaper to construct as well as less visually intrusive. Second, the layout of the underground facilities was designed in such a way that it was able to offset the two floors – the tipping hall and the compactor room – to achieve a much smaller volume of excavation. The rock within the area was principally a strong to very strong volcanic tuff, offering favourable conditions for the creation of rock caverns. The design criteria specified a 100 year design life. The construction for IWTS started in August 1995 and operations commenced in April 1997 (Littlechild et al., 1997).

The Millennium Master Plan (MMP) of the University of Hong Kong (HKU) called for an expansion of the HKU campus to celebrate its Centenary in 2011 and to prepare for the implementation of the four-year undergraduate degree curriculum in 2010. (Note: Universities in Hong Kong adopted the three-year undergraduate programme before 2010.) The proposed site was occupied by existing fresh water and salt water service reservoirs and associated waterworks facilities managed by the Water Supplies Department (WSD). Initially an open cut site formation was selected as the preferred option. During the study HKU adopted sustainability as one of the guiding principles for the Centennial Campus development and an innovative solution was proposed to re-provision the salt water service reservoirs in caverns, and release the area for relocating the fresh water service reservoirs (Chung and Chan, 2008). This has set an unprecedented local example of constructing service reservoirs in rock caverns. The land so released is now occupied by three academic buildings. The WSD western salt water service reservoir comprises a 62 m long, 7.8 m internal span access tunnel connecting to a 10 m long transition zone where the span increases from 7.8 m to 15 m. The facility then leads into two 50 m long, 17.6 m wide, and 17 m high salt water storage caverns. The design storage capacity of the reservoir caverns is 12,000 m3. The caverns were built within volcanic rocks comprising coarse ash tuff that had been affected by nearby granitic intrusions with some significantly higher rock compressive strengths in excess of 200 MPa. Due to the close proximity to a few graded historic buildings and the University, hydraulic rock splitting was used and allowed excavation to be extended to a 24 h operation due to minimal impact from the underground night time working. The facility was completed and operational in 2009 (Chung and Chan, 2008). The portal and some photographs from within the facility are shown in Fig. 19.

Fig. 18. Portal entrance to main tipping hall (left), main tipping hall cavern (middle), 3D isometric of the underground facility (right). Source: Arup.

Fig. 19. Portal entrance to underground service reservoir (left), main tank cavern (middle), access adit and pipe gallery (right). Source: HKIE Web Site.

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Fig. 20. Portal entrance to stanley sewage treatment works, service cavern (right). Source: DSD Web Site.

The major benefits of the cavern scheme were avoiding massive hillside excavations and extensive tree felling thus minimising the visual impact to the neighbourhood. Furthermore the cavern scheme facilitated the preservation of the three historic buildings within the site. The proposed cavern scheme minimised disruption to the natural form of the site as well as its ecological integrity. It also protected the biophysical features of the site in its natural state and reduced the amount of construction waste to be generated by about 80%. 3.6. Underground sewage treatment works 3.6.1. Stanley sewage treatment works A master plan for sewage disposal concluded that sewage treatment should be provided locally at Stanley. Due to the lack of a suitable surface site in the surrounding area to accommodate the plant, the cavern option was studied. The location selected was within the Stanley peninsula with little habitation near the proposed entrance. The study concluded that the cavern option was feasible and geotechnically suitable and relatively cheap as compared with the land surface option, with the real benefit coming from the local environment being left intact (Oswell et al., 1993). The underground sewage treatment works at Stanley serves a population of 27,000 inhabitants. The design capacity of the facility is 11,600 m3/day. It was designed with a process that minimised the size of the sewage treatment facility and was completed in 1995. The underground facility is sited in medium-grained granite and comprises access tunnels, a service cavern (130 m long, 15 m wide and 17 m high) (Fig. 20), and two treatment caverns about 90 m long, 15 m wide and 11 m high. These house aeration tanks, sludge pumps and final settlement basins as well as a small chamber for sodium hypochlorite chlorine disinfection of the effluent prior to discharge to the sea. Although the cavern support design was adequate for the overall stability of the cavern, minor rock fall was not catered for. On commissioning of the facility there was an early incident with minor rock fall which led to some safety concerns for operating staff. Subsequently remedial measures were implemented with the application of shotcrete over the majority of the exposed rock surfaces and in particular in the roof of the caverns. Despite this minor incident, the facility has been successful in minimising the impact on the local community through odour control and visual impact and effectively shielding the sewage treatment works from public views. 3.6.2. Sha Tin sewage treatment works The Sha Tin STW is the largest secondary sewage treatment works in Hong Kong and has a design capacity of 340,000 m3 serv-

ing a population up to 830,000 people from the Sha Tin and Ma On Shan areas of the city. The current facility (2015) occupies a footprint area of approximately 28 hectares and is located in a prime water front location with all round visibility to the sea and mountains, and convenient nearby transport connections with the adjacent road networks and railway. The Cavern Feasibility Study in 2011 identified that it was possible to relocate the facility into a nearby hillside to free up the land of the surface facility for residential and commercial development. The proposal has been taken forward to other stages of design development and it is expected to be taken forward into a full construction scheme by late 2017. Fig. 21 shows the concept design layout and the original location of the sewage treatment works that will be transferred underground. 3.7. Specific benefits of underground rock cavern facilities The benefits for those schemes that were implemented can be seen on the reduced impacts to the public and the community. Some of the main benefits are listed below:  Kau Shat Wan – Remote, secure area needed to house explosives and blasting agents.  Stanley Sewage Treatment Works – ‘‘Bad Neighbour” facility with odour and visual impact problems in a rural scenic area with limited above-ground space. Adopting the cavern option has effectively shielded the facility from public view.  Island West Transfer Station – ‘‘Bad Neighbour” facility for a populated area near residential areas with odour and visual impact problems. Limited space and a novel design to suit the process, has effectively shielded the facility from public view.  Western Salt Water Service Reservoir – Limited land space for the HKU campus development. Maximising the nearby space for new academic buildings, and providing environmental benefits.  MTR WIL Explosives Magazine – Secure, safe underground facility away from densely populated areas with relatively easy accessibility to the construction sites. 4. Urban underground spaces 4.1. General Hong Kong is a vertical city that has built some of the world’s leading high-rise buildings over the past 50 years. Due to the pressure for available space at the surface, underground basements for

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Fig. 21. Concept layout of the proposed sewage treatment works, original location on the top left corner.

car parking, shopping arcades and plant rooms have been incorporated into the local built environment since the late 1970s and early 1980s. It is a city synonymous for efficient use of urban spaces and many of the residential buildings exceed 40 storeys or more even in less desirable neighbourhoods. With high land prices, significant demand and a framework that supports vertical development the drive to use basement level space within building plots has become common place. In many downtown areas the city streets are narrow providing limited street parking space so car parks are becoming increasingly located in new developments (see Fig. 22). GFA exemptions for underground parking have been implemented to attract more efficient use of the building footprints. The drive to place much of the new MTR network underground in the early 1980s provided the impetus to consider underground space and connectivity within adjacent developments to the MTR stations. However, at that time there was little integration of the underground MTR network with adjacent area wide development. Furthermore, the existing narrow streets in some areas tended to preclude significant along street underground pedestrian networks making them difficult to build without affecting adjacent developments. In addition old culvert alignments and provision of subsurface drainage in the lower lying areas also hamper development of shallow pedestrian networks. One of the most significant examples of a recent (2004) area wide underground pedestrian subway system that has been built is the (Fig. 23) Tsim Sha Tsui to East Tsim Sha Tsui stations subway network that has connections to various shopping and retail complexes within the unpaid concourse areas and subways. This network is very functional and in

Fig. 22. Typical underground car park.

quite a few locations opportunities to connect to adjacent underground shopping centres has been added to adjacent private developments. The future Kowloon Cultural District will have a large underground transport interchange incorporated into the development including car parks and commercial spaces to facilitate above ground pedestrianisation. This project will also incorporate district cooling schemes for the various facilities and venues to be constructed as part of the cultural district.

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Fig. 23. Tsim Sha Tsui to East Tsim Sha Tsui Subway System with linkages to Commercial Developments. Source: Arup.

Fig. 24. Layout of harbour areas treatment scheme tunnels and shafts. Source: DSD.

Further enhancements have been made for connectivity to public facilities by the use of Essential Public Infrastructure Works (EPIW), which provides mechanisms for the Government to pay for specific links to the MTR stations that cannot be readily justified from the patronage model but they still serve a significant benefit to nearby communities. This mechanism has provided subways, passenger transport interchanges as well as above ground and below ground facilities to aid access and connectivity with the MTR. While each station is different the retail and commercial

activities within the stations has been generally limited due to space constraints. 4.2. Urban underground types of facilities There are various types of urban underground facilities in the city. The majority serve provision of supporting infrastructure as well as to allow connectivity between points of interest. In recent years, underground space in the form of storage tanks and drainage

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Fig. 25. Location of the flood prevention tunnels. Source: DSD.

Fig. 26. Tai Hang Tung underground flood storage tank with sport field above. Source: DSD.

tunnels, flood alleviation schemes have also been implemented that reduce the risks of flooding in the downtown areas. These schemes would also lead to greater security and reduction in risk for underground space development.

4.2.1. Sewage, drainage and flood protection type facilities There are many facilities built by the Drainage Services Department (DSD) that aid the operation and function of the city. Up until 2012 there was around 66 km of drainage and sewerage tunnels in Hong Kong with a further 41 km of tunnels to be completed by 2020. There are numerous underground primary sewage treatment facilities in many of the waterfront areas of the city that serve a deep underground tunnel network (see Fig. 24). Most pumping stations are underground and the network is controlled as an integrated scheme from the various catchments and sources of sewage to treat. Flood protection schemes with the provision of drop shafts to catch run-off from the nearby hillside catchments as well as local

flood storage tanks have been built in many areas and locations to prevent flooding in the downtown urban areas. These tunnels are located close to the urban areas as shown in Fig. 25. There is the usual collection of culverts, nullahs and other large scale underground buried channels that discharge the surface runoff from the built up areas into the sea. In some areas the DSD has built large underground tanks such as at Tai Hang Tung and in the middle of the Happy Valley Race Course to store surface runoff in low lying area to prevent local flooding during heavy rain and typhoons. An example of one of the tanks is show below in Fig. 26.

4.2.2. District cooling schemes There are several notable district cooling schemes in place to provide cooling capacity to various facilities in Hong Kong. The building of the Hong Kong and Shanghai Banking Corporation in Central has a tunnel and a cooling supply network running from the water front to the building. The Kai Tak Redevelopment is planned to have a District Cooling network that supplies cooling to various Government buildings in the new Kai Tak

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Fig. 27. Existing and planned underground facilities for the Kai Tak development including District Cooling Scheme. Source: CEDD.

Redevelopment area (see Fig. 27). In addition the West Kowloon Cultural District will also develop and implement a district wide cooling and utility network. 5. Underground planning 5.1. Rock cavern planning history The development and planning for underground space has in the past concentrated on exploring opportunities for rock cavern development. Various planning studies have been carried out to identify what uses and facilities could be considered for underground uses. Early considerations for rock cavern development were put forward in the late 1980s and early 1990s with the Study of the Potential Use of Underground Space (SPUN) and Cavern Project Study (CAPRO). The Hong Kong Government’s urban planning strategy for the 1990s was set out in the document ‘Metroplan, the Aims’. Investigation of potentially viable uses for large man-made underground spaces in rock caverns was an early Metroplan initiative, being a new response to the continuing demand for usable land supply, which is severely limited by Hong Kong’s hilly terrain. Concern about the growth of various environmental problems made this option even more attractive. The technology of tunnel construction had been well tested by then in Hong Kong, but had not yet been applied to the formation of underground space on as large a scale, or for as wide a variety of uses as was already common elsewhere (Arup, 1989). 5.2. SPUN and CAPRO The SPUN study was initiated under the aegis of the Metroplan Study to examine in some detail the potential use of rock caverns.

As part of the SPUN study a survey of possible sites and demand for use in both the public and private sectors led to the selection of seven schemes to test the concept of cavern development at four sites (Malone, 1993; Fig. 28). The outline feasibility study schemes examined under SPUN (Arup, 1990) were:  Commercial and Government/Institution/Community Facilities at Quarry Bay.  Container Freight Station/Warehouse at Tsing Yi Island.  Tractor and Trailer Park at Tsing Yi Island.  Storage of Oil and L.P. Gas at Tsing Yi Island.  Government Supplies Department Warehouse at Chai Wan.  Refuse Transfer Station at Mount Davis.  Sewage Treatment Works at Mount Davis. The SPUN study demonstrated that these schemes were not only technically feasible but that they could give substantial economic savings and environmental benefits to the community in general. Following the SPUN study, GEO initiated the Cavern Project Studies (CAPRO) including ground investigations for two schemes (Fig. 28). The studies, including refuse transfer station in Mount Davis (Arup, 1991a) and government warehouse in Chai Wan (Arup, 1991b), confirmed the viability to construct the two facilities in rock caverns as an alternative to above-ground schemes. 5.3. PEGS and cavern area studies Several Preliminary Engineering Geology Studies (PEGS) were carried out by CEDD between 1990 and 1993 and more recently to assess the potential for relocating various existing and planned facilities to rock caverns, including sewage treatment, fuel storage, abattoir & service reservoir facilities (e.g. Franks, 1991; Roberts,

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Fig. 28. SPUN, CAPRO and CAS locations.

1991; Cho, 2010). These studies generally reviewed the geology and physical constraints for each potential rock cavern site. A series of ‘‘Cavern Area Studies” (CAS) was initiated in the 1990s by CEDD in response to regional planning initiatives recognising the benefits of underground space use. These studies classified suitability of land for rock cavern development at three areas, viz. North Lantau Island, Hong Kong Island and the Kowloon Peninsula (e.g. Choy and Styles, 1992; Roberts, 1994; Fig. 28) based on consideration of land zoning, underground facilities, engineering geological conditions and topographical constraints.

with the cavern layout, and an effective management scheme and a suitable contingency plan are in place.

5.5. Recent studies on rock caverns

5.4. Previous specific technical studies

Hong Kong Government have sought to refresh the previous initiatives and to explore how new technologies in underground excavation and approach can further aid rock cavern development as a land supply solution in the city. Following on the policy initiative proposed in 2009/10 and 2013, two key studies have been carried out.

In parallel with the SPUN and CAPRO studies, early technical work was carried out by the Government. Geoguide 4 – Guide to Cavern Engineering by the CEDD in 1992, provides designers and regulatory authorities in Hong Kong with a guide to good cavern engineering practice. The Geoguide concentrates on the geotechnical aspects of cavern engineering and aims to provide guidance on how to produce cost-effective and safe-integrated designs. The Building Authority and the Fire Services Department issued the Guide to Fire Safety Design for Caverns in 1994, which provides planning and design guidance for cavern projects in regards to fire engineering for use by public utilities of a low population, e.g. refuse transfer station, sewage treatment works and water service reservoir. The guidelines presume that facility workers are familiar

5.5.1. Enhanced use of underground space A scoping study on ‘‘Enhanced Use of Underground Space in Hong Kong”, hereafter referred to as the ‘‘Cavern Feasibility Study”, was commissioned by the CEDD in March 2010 to take forward the policy initiative of rock cavern development as laid out in the 2009–10 Policy Agenda. The study, which was completed in March 2011, explored the opportunities to enhance the effective use of land resources in Hong Kong from a new perspective through the planned development of underground space. The study is summarised by Chan (2011) and the Executive Summary of the study (Arup, 2011) has been made publicly available at CEDD website. The key findings were as follows:

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 Hong Kong is suitable for developing rock caverns from a geological perspective. The hilly areas in the urban fringes of Hong Kong with strong rocks and convenient access are particularly suitable.  By relocating suitable Government facilities inside caverns and releasing the original surface land that it occupied as well as any adjacent previously unused land around the previous facility for housing and other uses, cavern development is a viable option to increase land supply.  Cavern development could also accommodate new infrastructure facilities which would otherwise occupy surface land. Reserving underground space could cater for future projects and expansion of underground facilities.  For those nuisance facilities like sewage treatment works or potentially hazardous installations like oil terminals, the cavern option would help reduce adverse impacts on the local environment, remove incompatible land uses and alleviate the ‘‘Not-InMy-Backyard” (NIMBY) sentiment. 5.5.2. Long-term strategy for cavern development Apart from the key findings outlined above, the Cavern Feasibility Study recommended that policy steer be provided to lay down administrative requirements for Government departments to consider cavern option in early project planning stage, a policy framework be developed to encourage private sector participation in cavern development, and a strategy be formulated to systematically relocate suitable existing Government facilities to caverns so as to gradually release occupied surface land for other beneficial uses. As highlighted in the 2013 Policy Address, CEDD commenced the Study on Long-term Strategy for Cavern Development (hereafter referred to as Strategic Cavern Study) in late 2012 to take forward the recommendations with a view to develop a long-term strategy for the planning and implementation of the cavern initiative. The key items of work as outlined in Ng et al. (2013) are summarised below:  Formulation of policy guidelines: A proactive policy will be formulated for Government departments to consider the cavern option at the initial project planning stage for suitable facilities, and to facilitate private sector participation in cavern development.  Preparation of Cavern Master Plans: A territory-wide Cavern Master Plan will be prepared to delineate areas that are considered strategic for accommodating cavern facilities, taking account of land use compatibility with the adjacent surface developments. Mechanisms will also be developed for safeguarding the development potential of these strategic cavern areas.  Formulation of a systematic relocation programme: A systematic programme will be developed for relocation of suitable Government facilities to caverns in a planned manner, thereby gradually releasing the surface land for other uses. This will take account of their current status so as to develop a workable schedule that meets the needs of Government departments and community expectations.  Development of mechanisms for private sector participation: Emphasis will be placed on means of facilitating private sector involvement, because many private facilities, such as storage, warehousing and data centres, can benefit from a stable and secure setting offered by rock caverns. If the private sector embraces this initiative, this could reduce the land take for these facilities in the future.  Review of technical issues: Several key issues that are crucial for cavern development will be considered under the study, This includes updating the Guide to Cavern Engineering which was published in 1982, preparing conceptual fire safety designs for

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high population density facilities, reviewing the key technical issues of housing water treatment works in caverns, and undertaking a Strategic Environmental Assessment for cavern development. 5.6. Recent studies on urban underground space Hong Kong has a long history of underground space development in the urban areas. Basements have been incorporated into the local built environment since the late 1970s, to place plant rooms and car parks as well as retail and commercial uses. Underground space developments are commonly found in the city and are commonly located close to/or connected to MTR Stations, such as East Point Centre in Causeway Bay, Three Pacific Place at 1 Queen’s Road East in Wan Chai; underground retail complexes, underground car parks such as Admiralty Car Park underneath the Harcourt Garden in Admiralty. Also as a result of the ongoing search by the HKSAR Government to optimise land in the urban areas the Policy Addresses in 2014 and 2015 highlighted the need to study urban areas for underground space development to aid better connectivity as well as space creation. The following are two key studies being undertaken by CEDD to explore how underground space can be optimised and used for the benefit of communities and commerce in the urban areas. 5.6.1. Territory-wide study on underground space development in the urban areas The prime objective of the Territory-wide Study is a scoping study aimed at exploring the opportunities of and constraints on implementing underground space development in the urban areas and new towns of Hong Kong with a view to enhancing the use of urban underground space resources in a systematic and holistic manner. The study commenced in December 2013 and will take about 2 years. The scope of the Territory-wide Study comprises:  Review of local and overseas examples to identify suitable measures and implementation strategies to facilitate underground space development in Hong Kong.  Identifying broadly areas with clear potential for underground space development in the urban areas and new towns of Hong Kong to establish territory-wide opportunities.  Development of conceptual schemes to demonstrate opportunities for space creation and connectivity enhancement for selected areas, and highlight any possible constraints/issues in implementation.  Review of specific topics related to underground space development: including the prospect of infrastructure reorganization, harbour-crossing pedestrian-cum-retail link development, and formulating guidelines for developing underground space in new developments areas. 5.6.2. Pilot study on underground space development in selected strategic urban areas As a step to further expedite underground space development, four urban areas (Fig. 29) which are strategic nodes for commercial, entertainment and tourism activities, viz. Tsim Sha Tsui West, Causeway Bay, Happy Valley and Admiralty/Wan Chai, have been selected for a pilot study to identify suitable priority projects for early implementation. The prime objective of the Pilot Study is to enhance the use of underground space resources more systematically by creating space for commercial and other uses and enhancing connectivity of facilities in these urban areas to meet the needs of the community, thereby improving the congested urban

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Fig. 29. Pilot study location of four strategic urban areas.

environment at ground level. The study commenced in mid-2015 and will take about 2.5 years. The scope of the Pilot Study comprises:  Evaluation of overall merits and key issues of underground space development in the four selected strategic urban areas.  Formulation of an Underground Master Plan for each selected area.  Identification of priority projects within each selected areas and preparation of conceptual design schemes for the priority projects.  Establishment of engineering feasibility of the priority projects, including planning, technical and financial assessments.  Preparation of preliminary engineering design for each priority project.  Recommendation of the way forward for implementation of priority projects. 6. Underground space planning and standards in Hong Kong Although housing facilities in caverns to a certain extent can minimise potential environmental nuisances to other users, it is not completely free from planning concerns. For example, land use compatibility might still be an issue, and, moreover, land uses, even underground, can still generate external influences such as traffic that need to be considered. Planning concerns of aboveground and underground development are to a certain extent similar in that cavern developments also have to address development intensity and compatibility, safety, traffic and financial viability during the course of development planning.

6.1. Rock cavern planning guideline The HKPSG sets out suggested planning criteria and guidelines that seek to determine the quantity, scale, location and site requirements of various land uses and facilities. The overarching purpose of the HKPSG is to provide general guidelines to ensure that, during the planning process, the Government will reserve adequate land to facilitate social and economic development and provide appropriate facilities to meet the needs of the public. The entire publication is available online at: http://www.pland.gov. hk/pland_en/tech_doc/hkpsg/index.html. Rock Cavern Development has been addressed within HKPSG under Section 2 of Chapter 12 ‘‘Miscellaneous Planning Standards and Guidelines” (PlanD, 2011). This was introduced in 1991 as a result of the SPUN and CAPRO studies. This section was later revised in 2008 following a review of the applicable land uses and government procedures for public works. It should be noted that all rock cavern developments are designated projects in Hong Kong under the Environmental Impact Assessment Ordinance (EIAO) and as such they will require an EIA to be carried out. Depending on the type of facility and the project there are usually various other studies to carry out such as Geotechnical, Traffic, Drainage and Sewage Impact Assessments to be submitted to the regulators for approval before the project can proceed. 6.2. Planning, usage strategy and practice in Hong Kong The Cavern Feasibility Study (Arup, 2011) generally found a lack of a well-defined strategy for Hong Kong, with only five

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purpose-built cavern developments being constructed since the completion of SPUN (see Section 5.2). Even though rock cavern development was included in the HKPSG since 1991, there has been limited development of caverns since. The study documented that the history of cavern development in Hong Kong can generally be categorised into two scenarios. First, caverns that were built ‘out of necessity’, to meet infrastructural and transportation requirements. These include the valve chamber of the Western District Aqueduct and the Taikoo & Sai Wan Ho MTR Station caverns which were constructed in the 1980s. Second, caverns that were built as a result of ‘overriding circumstances exist’. These purpose-built caverns were in response to government promoted planning initiatives geared toward the exploitation of underground space at specific locations to reduce the above-ground land take, e.g. the Stanley Sewage Treatment Works and Island West Transfer Station. Subsequent to the amendment of the HKPSG in 2008 to include the consideration of the cavern option in early planning process for new government projects, the Project Administration Handbook (PAH) for Civil Engineering Works was also amended to facilitate the implementation of procedures advised by HKPSG. Pursuant to Chapter 3: Main Procedures for Provision of Land it is stated that, ‘‘for a project or its associated facilities involving land use with potential for rock cavern development, the project office or its consultant shall consult GEO on the potential cavern sites within the general geographical search areas of the project, and on prefeasibility studies of potential cavern sites, prior to submitting the Site Search Form of the project to Planning Department.” The Cavern Feasibility Study ultimately concluded the nonmandatory guidelines presented under the HKPSG and within the PAH had been insufficient in encouraging extensive cavern development within the public or private sector. The study highlighted a distinct lack of policy directive to promote the systematic consideration cavern options in the early stages of Government project, and found no evidence of any policy support to encourage private sector participation. The primary evidence being that the existing cavern developments have almost exclusively been developed as a last resort, where no better alternatives could be identified.

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6.3. Rock cavern development planning Under the Cavern Feasibility Study, a task was carried out to identify the most suitable locations territory-wide where cavern development could be undertaken. Geographic Information Systems (GIS) were used to analyse, compare and display the various constraints and attributes that could be considered relevant for cavern suitability (Wallace et al., 2014). Various digital datasets were compiled, including terrain data, geological maps, various land uses, underground installations, geological structures, boreholes and private lots, to construct and define a Cavern Suitability Map for the whole of Hong Kong. The GIS analysis identified various buffers and zones over which it was considered that cavern development would be more or less constrained with an impact on cost and constructability. An extract of the detailed suitability map showing the buffering around existing tunnels and faults is shown in Fig. 30. The GIS analysis identified various suitability classes from high to very low and some areas that were considered to be not suitable. The cavern suitability map shows that about two-thirds of the land area of Hong Kong can be considered suitable for rock cavern development. The suitability classes provide a broad planning guide on areas where underground cavern formation may indirectly attract greater or lower cost and relative difficulty in underground excavation and stabilisation. Examples of the detailed and simplified cavern suitability maps are shown in Fig. 31. The study identified a broad classification for whether a cavern would be generally above or below a certain level to identify direct entry caverns to those that were generally in low lying areas subject to more geotechnical constraints. Physical and land related constraints have been identified and included in the GIS model for consideration in future planning studies. The suitability map can be used by planners to identify the various areas of the city where constraints may lie and where the rock mass and the ground conditions may be less suitable or more suitable than other areas for cavern development. The map forms the basis of developing the territory-wide Cavern Master Plan (see Section 5.5).

Fig. 30. Detailed cavern suitability map with zones and buffers shown. Source: Arup.

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Fig. 31. Detailed cavern suitability map (left), simplified cavern suitability map (right).

7. Institutional framework for underground space planning 7.1. Land use planning aspect 7.1.1. Town Planning Ordinance The Town Planning Ordinance (Cap. 131) (TPO) promulgates the prevailing statutory planning framework in Hong Kong. Two types of statutory plans are prepared and published by the Town Planning Board (TPB) under the provision of the TPO: Outline Zoning Plan (OZP) and Development permission Area Plan (DPA Plan). The OZPs cover most of the metro areas and new towns in Hong Kong. OZPs circumscribe parcels of land under specific land use zones wherein specific land uses are permitted with or without the permission of the TPB in accordance with their accompanying Notes. These Notes comprise the covering notes on the general provision of the OZPs, together with a set of schedule of uses which lists out the uses that are always permitted and uses requiring planning permission from the TPB. DPA Plans provide temporary statutory planning controls until OZPs are prepared to replace the DPA plans upon their expiry. Whilst the current statutory planning framework is primarily intended to govern surface development, the ascribed planning controls are also applicable to underground space. This means, for example, if land is zoned ‘‘Residential (Group A)” (R(A)) on an OZP, which regulates uses/developments on the surface and uses/ developments underground, uses such as ‘Industrial Use’ and ‘Warehouse’ (excluding ‘Dangerous Goods Godown’) are not allowed within the said zone. The current planning framework is instilled with flexibility to facilitate underground space developments by submitting planning applications under section 16 and applications for amendment of plan under section 12A of the TPO. 7.1.2. Hong Kong 2030: Planning Vision and Strategy The Hong Kong 2030: Planning Vision and Strategy, published in 2007, was aimed to update the preceding Territorial Development Strategy for Hong Kong. The Strategy is defined as: ‘‘a longterm planning strategy to guide future development and provision of strategic infrastructure and to help implement government policy targets in a spatial form”. It recommends, on the basis of a series of assumptions, how the spatial environment of Hong Kong should respond to various social, economic and environmental needs in the next 20–30 years, taking Hong Kong towards a shared vision. While this document does not specifically address the potential of underground development, it does indicate, with the limited land mass available in Hong Kong and combined with a

growing population and demands for a better living environment, the need to identify alternatives to surface development to enable for future growth of Hong Kong. 7.2. Building requirements 7.2.1. Buildings Ordinance The Buildings Ordinance regulates the planning, design and construction of buildings, which includes underground development. It is therefore necessary to obtain building plans approval prior to the development of any underground space. 7.2.2. The Building (Planning) Regulations The Building (Planning) Regulations (B(P)R) also directly/indirectly address underground development. For instance, under B (P)R 30, the provision of natural lighting and ventilation is mandatory for habitable rooms and offices. Also, in accordance with B(P)R 41 and Part III of Code of Practice for the Provisions of Means of Escape in Case of Fire 1996, sites of places of public entertainment are required to abut upon and have frontages to two or more thoroughfares for means of escape. This makes placing residential, office developments and venues of public entertainment underground technically difficult. Pursuant to Places of Public Entertainment Ordinance, places of public entertainment include venues that accommodate concerts, drama performance, etc. 7.2.3. Buildings Department Practice Note for Authorised Persons (PNAP) APP-2 Latest government incentives also encourage underground development. The Buildings Department PNAP APP-2: Calculation of GFA and non-accountable GFA was revised in November 2013 to differentiate between the GFA concessions granted for above ground and underground car parks, such that 100% concession is granted if the car park is located underground, while only 50% concession is granted where the car park is located above ground. This aims to reduce building bulk such that above ground development would not pose adverse visual impacts to the surrounding areas. 7.3. Land administration aspect The HKSAR Government administers all the land in the territory. Land is granted or sold to private parties by the Government under long-term leases of up to 50 years. It has long been the practice to sell Government land by public auction sale or tender, whereby the Government would sell the land to the highest bidder

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or tenderer for a premium, together with a small Government rent. Land is thus generally available for sale on an equal and competitive basis. There is however no established land acquisition mechanism for underground space. Unless expressly reserved in the Government Leases, the rights granted include the surface of the land and soil beneath it. It is noted that the land ownership would constraint the feasibility of underground space development, i.e. privately orchestrated underground development hardly be developed beneath private lots under the ownership of others. This would require amendment to the Lands Resumption Ordinance (Cap 124) to cater for resumption of underground strata of a private lot. If government were to resume land under Cap 124 for the purpose of selling or letting the same for general use, the intended purpose is not a public purpose as described in paragraphs 2(a) to 2(c) of Cap 124. If government were to resume the land for public development only the administration would need to demonstrate that it is for a public purpose which is in the general interest of the community. The following cases can be used to demonstrate that arrangement could be made to facilitate underground space development:  Felicity Garden in Sai Wan Ho, a privately-owned residential development, is located above a range of GIC facilities, including an indoor games hall, a civic centre, and a market which are held by Government on undivided shares of the lot. Such ownership arrangement was made prior to the development of the land when the Lands Department required the developer to build the GIC facilities for the Government and later transfer the ownership of the GIC facilities as part of the lease condition. Similar arrangement could therefore be made to facilitate underground development for a private use/by a third party. However, this is only likely to be feasible for newly disposed land.  The development of the New World Shopping Mall, located at 12 Salisbury Road, Tsim Sha Tsui, Kowloon was granted to a private developer for the development of 4-storey commercial premises with car parking spaces, below ground. As part of the lease conditions, the ‘‘Open Space” (O) component was developed by New World and, once operational, handed over to Government for management and maintenance as public open space. This arrangement demonstrates how such a land use arrangement could be utilised to facilitate both GIC and Commercial development. 7.4. Other aspects 7.4.1. Railways Ordinance In accordance with the provisions of the Railways Ordinance, land is resumed by the Government and disposed of as it sees fit. However, MTR or other railway developers may acquire the underground space beneath private lots with permission from the Government and the private lot owners, given that the development will be built for a public purpose and compensation is settled in accordance with the Railways Ordinance. MTR as a private company but with HK Government as a majority shareholder has accommodated various underground subway connections to private developments. These have been quite successful and have been able to improve underground connectivity to existing underground stations. The Developer would usually negotiate with MTR to add the connection to the station and would pay for the connection construction and any associated modification works to the station to accommodate the subway. Usually the MTR would take possession of the subway through the Railway Ordinance through the normal gazettal and planning process. The subway connections usually run along existing Government Land such as roads or streets owned by the Highways Department.

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7.4.2. Fire safety issues In Hong Kong, fire safety for buildings is designed to the following codes of practice in order to meet Building Ordinance, Building (Planning) Regulations and Building (Construction) Regulations:  The Provision of Means of Escape in Case of Fire (MoE).  Fire Resisting Construction (FRC).  The Provision of Means of Access for Fire Fighting and Rescue Purposes (MoA).  Minimum Fire Services Installations and Equipment (FSI). These codes are prescriptive in nature and written to address typical building fire safety design. Requirements for basements are also given in the codes to address structures directly below a building. (Note: these codes are not directly applicable to underground rail/metro stations, which are covered under the Railways Ordinance.) Since the codes of practice mentioned above are written with above-ground buildings in mind, some clauses within cannot be easily met by large underground structures. Some require a large investment in construction cost and may not be cost effective. The Guide to Fire Safety Design for Caverns (see Section 5.4) was for public utilities caverns that accommodate a low population. Some requirements stipulated within the document are relatively restrictive. For example, travel distance for single direction is limited to 18 m (similar to MoE Code). Provision of smoke extraction may also be required subject to Fire Services Department (FSD). Provision of Emergency Vehicular Access is also required with setting-down point. A full list of fire services installations are given with the actual provision to be discussed and agreed with FSD. 7.5. Overview This review of the existing institutional framework concerning underground space development in Hong Kong indicates that the current statutory planning framework offers some flexibility. Existing lands practice in Hong Kong restricts to some extent, the ability of the private sector to implement underground developments. Whilst the current lands system ostensibly allows the land owner the right to develop underground spaces (this may be subject to application), a third party would not currently enjoy similar rights under current lands practice as additional strata cannot be created beneath private land for a private use. 8. Future development Hong Kong has the appropriate ground conditions and certainly the required technical skill sets, as demonstrated by the wealth of underground development. However, systematic consideration of developing rock caverns at the urban fringes and underground space in the urban areas will require a change in mindset, not only by the Government but for all stakeholders. Ling (2011) states that ‘we should have a strategic vision to enhance a more effective use of underground space and make underground space development as an integral part of the city structure for the benefit of sustainable development of Hong Kong. A multi-disciplinary approach guided by a clear public policy to promote underground development and a set of administrative guidelines to deal with the technical and institutional issues is necessary to make such a vision come true’. 8.1. Rock cavern development It is evident that rock caverns can be cost-effective, through releasing valuable surface land for other beneficial uses, and in

Please cite this article in press as: Wallace, M.I., Ng, K.C. Development and application of underground space use in Hong Kong. Tunnel. Underg. Space Technol. (2016), http://dx.doi.org/10.1016/j.tust.2015.11.024

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some cases, yield additional environmental, safety and security benefits. Nevertheless, there are only a few purpose-built cavern facilities over the years. The Strategic Cavern Study (see Section 5.5.2), now near completion, has outlined a holistic approach in planning and implementing cavern development, so as to outcome some deficiencies in the current situation. The following are the key initiatives being addressed in the study. 8.1.1. Territory-wide cavern master plan The territory-wide Cavern Master Plan (CMP) that is being formulated entails about 50 strategic cavern areas at the urban fringes, covering a total plan area of about 45 km2 (4500 ha) in terms of geotechnical setting and the planning considerations for compatible land uses for future Government facilities and suitable private sector land uses. Special issues, such as traffic and transport, environmental and land ownership, have been highlighted in the accompanying explanatory statement and a set of information notes. Implementation mechanism is also being formulated to strive for the best use of the strategic cavern areas and facilitate integrating cavern development with other surface and subsurface developments to bring about synergy effect whenever possible. 8.1.2. Systematic relocation of suitable government facilities Currently, four pilot relocation schemes are being studied. Relocation of the Sha Tin Sewage Treatment Works, which would release about 28 ha of waterfront land, is in the detailed design stage with construction commencing in late 2017 (see Section 3.6.2). Three other schemes, namely Sai Kung Sewage Treatment Works, Sham Tseng Sewage Treatment Works, and Diamond Hill Fresh Water and Salt Water Service Reservoirs, are being studied for their technical feasibility and financial viability. Under the Strategic Cavern Study, some 30 existing Government facilities have been shortlisted as potential candidates for systematic relocation in accordance with the requirements of individual facility owners and following the land supply schedule. Broad planning, technical and financial assessments have been carried out to establish the preliminary technical feasibility and financial viability. Key issues that need to be further considered in future detailed feasibility studies have also been identified. 8.1.3. Proactive consideration of cavern option for government facilities Pursuant to the HKPSG (see Section 6.1), for all new Government facilities which have potential for cavern development, project offices would be required to assess the cavern option at the initial project planning stage. Enhancement measures are being considered to avoid overlooking the cavern option, particularly for facilities that have proven track record of successful implementation. An important element is an economic analysis for comparing cavern and surface site options on an equitable basis, taking account of factors such as the value of the surface land, cost of land formation, value of excavated materials for re-use, value of sterilised land nearby, enhancement of land value in the vicinity, opportunity cost of the surface site, cost impact on maintenance and operation, and other intangible benefits. 8.1.4. Early land use zoning of selected cavern areas Planning application is generally required before a hillside can be considered for cavern development, which may take 2–3 years for the entire process. This long lead time will inevitably affect the implementation of public works programme and impact the financial viability of private sector projects. With the availability of the territory-wide CMP, it would be possible to undertake early land use zoning of selected cavern areas that are considered to have clearly identified needs (e.g. accommodating Government facilities) or strong private sector demand (e.g. logistics). This

proactive approach of assessing the land use compatibility and the key engineering and environmental issues at an early stage can help to reduce project implementation time as well as risk exposure. This could also facilitate disposal of zoned cavern areas for private sector development projects. 8.2. Urban underground space development Whilst underground space development in the form of individuals is common in Hong Kong (see Sections 4), Ling and Chan (2013) advocate that the use of underground space is very limited and it is worth retrofitting the underground space in urban Hong Kong to provide a sustainable solution to some of its urban issues such as over-congestion. As compared to cities such as Tokyo (e.g. Nishida and Uchiyama, 1993) and Montreal (e.g. El-Geneidy et al., 2011), Hong Kong is still in its infancy in respect of considering underground space during city planning. However, the two studies (see Section 5.6) currently underway will look into integrating surface land and underground space in order to create a community that provides efficient connectivity, good amenities and improved environment for quality living. The Territory-wide Study, which is in its final stage, will help formulate a comprehensive and objective methodology for developing urban underground space, specifically the opportunities of and constraints on implementing underground space development in Hong Kong. Furthermore, the study will also identify urban areas (urban centres and new towns) with clear potential for developing underground space to address their urban issues for further consideration. The Pilot Study on four strategic urban areas, viz. Tsim Sha Tsui West, Causeway Bay, Happy Valley and Admiralty/Wan Chai, commenced in mid-2015, is more specific in nature. Under this study, development opportunities will be examined with a view to drawing up an Underground Master Plan for each strategic urban area, develop conceptual schemes, identify priority projects and establish their engineering feasibility. The study will also consider the opportunities of public and sector sectors in implementing urban underground space development. This is the first time that the concept of developing underground master plan is being considered in Hong Kong. If underground master plans are successfully developed for these urban areas, this would no doubt open a new chapter for Hong Kong’s pursuit of sustainable urban underground space development. 9. Conclusion Hong Kong is a dense urban city that suffers from significant seasonal variations with hot, humid, wet summers and occasional typhoons, and being temperate in autumns and winters. The population seeks relief during the summer months in air conditioned buildings and air-conditioned transport networks. The city is famous for its highrise landscape with many areas already height restricted especially in the key urban business and commercial areas. The Government has recognised that the supply of land is a significant issue for the continued sustainable development of the city and hence they have initiated various studies that have reviewed and identified how Hong Kong can further implement and promote a more effective use of underground space in the city. Hong Kong has utilised various forms of underground space within the city to aid its development. Rock caverns have been utilised to provide local solutions to surface land and space problems for various types of uses. Many metro stations are located in rock caverns to serve local communities. NIMBY facilities have also been housed in rock caverns to reduce the overall environmental impacts. Some underground metro stations are connected to

Please cite this article in press as: Wallace, M.I., Ng, K.C. Development and application of underground space use in Hong Kong. Tunnel. Underg. Space Technol. (2016), http://dx.doi.org/10.1016/j.tust.2015.11.024

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nearby basements and there are various interconnected subways and pedestrian networks that provide convenience and connectivity enhancements to various shopping malls and commercial properties. The unique attributes of Hong Kong with its steep terrain, high land values and excellent underground metro system present significant opportunities for cavern development at the urban fringes and underground space development in the urban areas. The HKSAR Government is actively pursuing an integrated master plan framework for better integration of surface and underground space development as a viable option of enhancing Hong Kong’s long-term land supply. Acknowledgments This paper is published with the permission of the Head of the Geotechnical Engineering Office and the Director of Civil Engineering and Development, Government of the Hong Kong SAR. References Arup, 1989. Cavern Engineering Report. Ove Arup & Partners Hong Kong Limited for Geotechnical Engineering Office, Civil Engineering Department, Government of Hong Kong. Arup, 1990. A Study of the Potential Use of Underground Space (Executive Summary). Ove Arup & Partners Hong Kong Limited for Geotechnical Engineering Office, Civil Engineering Department, Government of Hong Kong. Arup, 1991a. CAPRO: Refuse Transfer Station. Ove Arup & Partners Hong Kong Limited for Geotechnical Engineering Office, Civil Engineering Department, Government of Hong Kong. Arup, 1991b. CAPRO: Government Supplies Department Warehouse. Ove Arup & Partners Hong Kong Limited for Geotechnical Engineering Office, Civil Engineering Department, Government of Hong Kong. Arup, 2011. Enhanced Use of Underground Space in Hong Kong – Feasibility Study (Executive Summary). Ove Arup & Partners Hong Kong Limited for Geotechnical Engineering Office, Civil Engineering and Development Department, Government of the HKSAR . BA and FSD, 1994. Fire Safety Design for Caverns. Building Authority and Fire Services Department, Government of Hong Kong. Chan, R.K.S., 2011. Planning future cavern development in Hong Kong. In: Proceedings of the Hong Kong Institution of Engineers and the Hong Kong Institute of Planners Joint Conference on Planning and Development of Underground Space, Hong Kong, 23–24 September, pp. 37–43. Chan, R.K.S., Ng, K.C., 2006. Are we prepared for cavern development in Hong Kong? In: Proceedings of the Hong Kong Institution of Engineers Geotechnical Seminar 2006, Hong Kong, 12 May 2008, pp. 53–63. Cho, Y.M., 2010. Preliminary Engineering Geological Study for an Underground Service Reservoir Site at Mount Davis. Geological Report No. GR 6/2010, Geotechnical Engineering Office, Civil Engineering Department, Hong Kong. Choy, H.H., Styles, K.A., 1992. Cavern Area Study of North Lantau. Special Project Report No. SPR 9/92, Geotechnical Engineering Office, Civil Engineering Department, Hong Kong. Chung, E., Chan, C.C., 2008. Re-provision of water works facilities for proposed Centennial Campus of the University of Hong Kong: a sustainable and geological conservative solution. In: Proceedings of the Asia-Pacific Seminar on Geological Conservation and Sustainable Development, Hong Kong, 13 August, pp. 107– 120.

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Please cite this article in press as: Wallace, M.I., Ng, K.C. Development and application of underground space use in Hong Kong. Tunnel. Underg. Space Technol. (2016), http://dx.doi.org/10.1016/j.tust.2015.11.024