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Procedia Engineering 165 ( 2016 ) 705 – 716
Available online at www.sciencedirect.com
1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the scientific committee of the 15th International scientific conference “Underground Urbanisation as a
Prerequisite for Sustainable Development
doi: 10.1016/j.proeng.2016.11.768
ScienceDirect
15th International scientific conference “Underground Urbanisation as a Prerequisite for
Sustainable Development”
Strategic use of rock caverns and underground space for sustainable
urban development of Hong Kong
Ying-Ki Ho a, Ka-Wah Shum a , Jeffrey Chun-Fai Wong a,
*
a Civil Engineering and Development Department, The Government of the Hong Kong Special Administrative Region, China
Abstract
Hong Kong’s steep hilly terrain poses significant constraints on its urban development. The shortage of developable land forms a
key driver to explore other sustainable and innovative approaches to expand land resources. Rock caverns and underground
space are viable sources of land supply, which can provide solution space for a broad variety of land uses and help address
problems encountered in the congested urban environment. The enhanced use of these ‘hidden’ land resources can offer new
opportunities for Hong Kong’s planning and development gains. Since 2010, the Government of the Hong Kong Special
Administrative Region has launched a number of strategic studies and pilot projects to unleash the potential of systematic
utilization of rock caverns and underground space. The overall strategy is to formulate a holistic and long-term plan for wider
application of rock caverns at the urban fringes and underground space in the urban areas. This paper aims to present the
findings of the on-going studies to date, with particular focus on the initiatives being developed to take forward the long-term
strategy for cavern development in Hong Kong. Notable overseas examples are cited to illustrate the benefits yielded by this
innovative approach. Challenges and solutions with respect to the implementation will also be discussed.
© 2016 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 15th International scientific conference “Underground
Urbanisation as a Prerequisite for Sustainable Development.
Keywords: rock caverns, underground space, land supply, Hong Kong.
* Corresponding author. Tel.: +852-276-25652
E-mail address: jeffreycfwong@cedd.gov.hk
© 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the scientific committee of the 15th International scientific conference “Underground Urbanisation as a
Prerequisite for Sustainable Development
706 Ying-Ki Ho et al. / Procedia Engineering 165 ( 2016 ) 705 – 716
1. Introduction
Hong Kong is a world-class city, renowned for its modernized developments in the urban areas, in particular
along its harbor front (Figure 1). Despite its signature cityscape, only about one fourth of the land in Hong Kong has
been developed. Its topographical setting with steep natural hillsides significantly limits the extent for urban
expansion. For sustainable city development, further substantial land formation on the hillsides is not preferred
though it is technically feasible. In view of limited developable land, there is a pressing need to explore new sources
of land supply to support social and economic development of Hong Kong. Strategic use of underground space is
considered a viable solution that can create space for a broad variety of land uses and help address problems
encountered in the congested urban environment. The scope is not only confined to better utilizing underground
space in the urban areas, but also enhancing the planned use of rock caverns at the urban fringes (Figure 2). It is a
paradigm shift by seeking land mass from a three-dimensional instead of two-dimensional perspective, with a view
to optimizing the use of these ‘hidden’ land resources so as to bring new opportunities for Hong Kong’s p lanning
and development gains.
Figure 1. Aerial view of Hong Kong Island
Hong Kong has been using underground space for commercial, community, transport and utility facilities for
many years. Most of them are planned on individual project basis, including: (i) basements, which have been
incorporated into individual developments to accommodate plant rooms and car parks as well as provide space for
retail and commercial activities; (ii) pedestrian links, e.g. subways and pedestrian connections of Mass Transit
Railway stations; (iii) transportation, e.g. road and railway tunnels; (iv) utilities, e.g. underground networks of water,
sewerage, drainage and cable tunnels; and (v) other innovative uses, e.g. flood storage tanks, landslide mitigation
measures, etc. There are also a few government facilities purposely built in rock caverns, viz. a refuse transfer
station, a sewage treatment plant, two salt water service reservoirs and an explosives depot, which are primarily
NIMBY facilities. Wallace and Ng (2016) gave a comprehensive review of the development and application of
underground space in Hong Kong.
Underground space development is not a new concept in Hong Kong. Notwithstanding this, its applicability is
generally restricted to a narrow range of land uses and the usage has been either out of necessity, or reactive and
only considered where over-riding circumstances exist. A more proactive and holistic approach is thus called for to
enable it to become a sustainable means of increasing long-term land supply. Since 2010, the Civil Engineering and
Development Department (CEDD) of the Government of the Hong Kong Special Administrative Region (HKSAR)
has launched a number of strategic studies and pilot projects with a view to unleashing the potential of systematic
utilization of rock caverns and underground space in Hong Kong. The overall strategy is to formulate a visionary
plan for wider application of rock caverns at the urban fringes and underground space in the urban areas. This paper
aims to present the findings of the on-going studies to date, with particular focus on the initiatives developed to take
forward the long-term strategy for cavern development in Hong Kong. Notable overseas examples are cited to
illustrate the benefits yielded by different applications. Challenges and solutions with respect to the implementation
will also be discussed.
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Figure 2. Underground space development strategy in Hong Kong
2. Rock Cavern Development at the Urban Fringes
2.1 Opportunities and benefits
The hilly terrain with strong crystalline igneous rocks in Hong Kong is highly suitable for developing rock
caverns, particularly in the urban fringes as they allow easy access for construction as well as future uses. Cavern
construction is also an established technology and there is proven local experience as demonstrated by various
existing facilities built in Hong Kong. Based on overseas experience, the applicability can even go beyond simply
the accommodation of nuisance facilities and extend to a wide variety of beneficial uses that meet the development
need of the society (Figure 3). Many cavern schemes have been successfully built around the world covering a
broad spectrum of applications, such as community and recreational facilities (sports hall, swimming pool,
exhibition gallery, adventure park, restaurant, retail, car park), storage facilities (archives, food, wine, oil and gas),
commercial and industrial facilities (data centre, office, logistics/warehousing, light manufacturing, maintenance
workshop, underground quarrying) and special facilities (testing laboratory, shooting range, fire-fighting training,
civil defence). The versatility of rock cavern development is well demonstrated by these notable examples.
In the Hong Kong context where land shortage forms the key driver, the benefits of cavern development are
manifold. Relocation of suitable existing government facilities to rock caverns could release surface sites for other
beneficial uses and could remove incompatible land uses by placing unpopular facilities in caverns. Rock caverns
could also provide space to accommodate suitable new public and private sector facilities where there is shortage of
available surface land, thereby reducing further land take. Introducing innovative cavern usage would help
accommodate those facilities that have difficulty in finding suitable surface sites (e.g. maintenance depots and
columbaria) and those that could benefit from the stable and secure environment underground (e.g. archives, wine
storage, laboratories and data centres). Rocks arising from cavern construction could also be used as aggregates to
support the local construction industry. In view of high land and property values in Hong Kong, cavern
development could be an economically viable solution in comparison with other above ground options. Some of the
benefits to the community regarding placing facilities in rock caverns are intangible, e.g. removal of incompatible
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land uses from the local community, provision of additional open space and reduced traffic congestion by relocating
urban facilities to the urban fringes, and these may help improve government/community relationships.
Figure 3. Opportunities for wider application of cavern development from overseas examples
2.2 Strategic studies
In March 2010, the CEDD undertook a scoping study on “Enhanced Use of Underground Space in Hong Kong -
Feasibility Study” (Cavern Feasibility Study) to explore the opportunities to enhance the effective use of land
resources through the planned development of rock caverns. The study, completed in March 2011, demonstrated the
broad viability of cavern development for a range of facility types and in different geographical areas. The study
also identified key issues that would need to be addressed in developing caverns in a planned manner. The
Executive Summary of the Cavern Feasibility Study (Arup, 2011) can be found on the website of the CEDD.
To follow up the findings of the Cavern Feasibility Study, the CEDD instigated a detailed study on “Long-term
Strategy for Cavern Development - Feasibility Study” (Strategic Cavern Study) in September 2012 for completion
by 2016. The study is aimed at developing a holistic approach in planning and implementing cavern development,
so as to render it a sustainable and innovative means for expanding land resources. An outline of the scope and
intended outcomes of the study is summarized by Ng et al (2013). Ross et al (2014) also discussed the feasibility of
developing a number of non-NIMBY cavern facilities that have been examined under the study. This paper would
focus on the initiatives that are being formulated in order to take forward the long-term strategy for cavern
development in Hong Kong. These initiatives are targeted at making a breakthrough and creating sustained impetus
for promoting and facilitating wider application of cavern development in different dimensions. An overview of the
initiatives is given below.
2.2.1 Cavern Master Plan
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A cavern suitability map has previously been developed using a Geographical Information System (GIS) based
approach under the Cavern Feasibility Study (Wallace et al, 2014). The map provides a useful reference to indicate
potential areas of good rock that are suitable for cavern development. Nevertheless, such information alone is not
sufficient to facilitate effective planning of cavern projects since other factors such as accessibility, infrastructural
support and nearby land uses are of equal importance. In view of the above consideration, a Cavern Master Plan
(CMP) has been prepared under the Strategic Cavern Study with the aim of providing a broad strategic planning
framework to guide and facilitate territory-wide cavern development in Hong Kong (Figure 4). It delineates
Strategic Cavern Areas (SCVAs) that are well placed for developing rock caverns to meet the existing or future
needs of the adjoining districts. All SCVAs identified are located in close proximity of existing urban areas with
good supporting infrastructure network and are sufficiently large to enable accommodation of multiple cavern
facilities. These areas offer an additional source of solution space that is versatile for a wide variety of land uses,
e.g. housing new commercial, industrial, community or municipal facilities, subject to the need of the society.
Figure 4. A territory-wide Cavern Master Plan for Hong Kong
The CMP is non-statutory, serving as user guidelines for cavern development. It is accompanied by an
Explanatory Statement (ES) and a set of Information Notes (IN). The ES sets out the objectives of the CMP,
outlines the rationale and methodology of delineating SCVAs, and highlights key issues for implementation. For
each SCVA, an IN is provided to describe the characteristics, development potential as well as constraints of the
area. This includes details of the geological, planning, environment and traffic characteristics and other key
issues/constraints on cavern development. It also outlines the potential land uses and the extent of potential portal
locations. A reference drawing is appended to each IN to illustrate the spatial context of the information provided.
The CMP, ES and INs collectively serve to fulfil the objectives of facilitating territory-wide cavern development
and promulgating essential information that could enable government departments and private sector organizations
to identify suitable cavern sites for their development projects. Future proposals for cavern development in SCVAs,
including the sizes and locations, would be subject to the need of individual projects and detailed technical
feasibility studies, and would be assessed on individual merits. The CMP will be incorporated into the relevant
chapter of the Hong Kong Planning Standards and Guidelines published by the Planning Department (PlanD), and
will be made available through the websites of the CEDD and PlanD.
SCVAs are valuable land resources. This alternative source of long-term land supply should be managed in a
judicious manner. Hence, a suitable mechanism will be implemented to optimize the use of SCVAs by managing
cavern and other subsurface developments in SCVAs, without compromising beneficial surface land use and
developments. Synergy effect on cavern development in SCVAs can be achieved through better integration with the
surface and subsurface developments within the SCVAs.
2.2.2 Relocation of suitable government facilities
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Drawing up relocation plans for suitable government facilities can gradually release surface sites to increase
long-term land supply for other beneficial uses including housing development. It can also in the long run upkeep
the impetus for cavern development. A recent prominent example is the relocation of Sha Tin Sewage Treatment
Works to caverns, which will release a surface land of 28 hectares at the riverfront for other beneficial land uses
(Figure 5). There are also three cavern relocation projects under feasibility studies, namely Sai Kung Sewage
Treatment Works, Sham Tseng Sewage Treatment Works, and Diamond Hill Fresh Water and Salt Water Service
Reservoirs.
Figure 5. Pilot cavern relocation project for Sha Tin Sewage Treatment Works
(currently under investigation and design stage)
Under the Strategic Cavern Study, thirty existing government facilities, including archives, material laboratory,
vehicle pounds, warehouse, service reservoirs, refuse transfer stations, water treatment works, and sewage treatment
works, have been shortlisted for broad planning and technical assessment for relocation to caverns. The selection
process adopted a systematic approach of stocktaking, rationalization, screening, ranking and appraisal to come up
with the shortlisted facilities. Some of them are found to be suitable for early implementation in view of
considerable land gains for residential, commercial or industrial development in meeting the needs of the
community. Further work will be carried out to formulate a list of suitable government facilities for relocating to
caverns, taking into account various factors including financial viability, relocation requirements and any earmarked
schedule for releasing the land.
2.2.3 Guidelines for suitable new facilities
To facilitate proactive consideration of cavern option for suitable new government facilities, a number of
enhancement measures have been proposed for implementation. They include (i) early recognition of potential
cavern sites that may be used as an alternative to surface land sites for projects involving land uses with potential for
rock cavern development; (ii) mandatory option assessment to compare cavern and non-cavern options for selected
types of facilities, viz. refuse transfer station, sewage treatment works and service reservoir, unless there are strong
justifications (e.g. without a suitable cavern site in the serving region); and (iii) strengthening the existing planning
and engineering guidelines for cavern development, including planning, geotechnical, environmental and fire safety
aspects.
For option assessment, an essential component is economic analysis, which aim to compare cavern and surface
options on an equitable basis, taking account of factors such as the value or development potential of the surface
land so released, cost of land formation, value of excavated materials for re-use, and other intangible benefits.
2.2.4 Land use zoning of Selected Cavern Areas
Planning application or zoning amendment is usually required before a hillside can be considered for cavern
development, which would generally take two to three years to complete the entire process. Such application has to
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be supported by a comprehensive study of proposed land uses with respect to the planning and engineering context
of the site. This long lead-time will inevitably have time and cost implications for a cavern project.
Proactively launching detailed studies to facilitate zoning of selected cavern areas (e.g. SCVAs) for suitable land
uses can significantly reduce the project implementation time. The subsurface land can then be disposed of for
private sector development or to enable the Government to earmark sites for cavern development for specific uses,
either for relocation of existing facilities or for new facilities, thus realizing cavern space as part of the long-term
land supply strategy.
2.2.5 Integrated approach to area-based land use studies
For area-based land use studies where there are suitable SCVAs nearby, an integrated approach will be adopted
to facilitate a holistic consideration of the use of both cavern and surface spaces at the early planning stage. This
can help capitalize on the strategic benefits and synergy effect of integrating cavern development with surface
development. For example, some essential supporting and recreational facilities (service reservoir, sewage
treatment works, sports centre, etc.) can be planned for accommodation in rock caverns, thereby reducing surface
land take.
2.2.6 Underground quarrying-cum-cavern development schemes
There are successful examples of underground quarries in other countries, e.g. Switzerland and USA (Figure 6).
The business case for underground quarrying in these countries is reported to be economically viable through the
sale of rock products only, without taking into account the potential additional revenue generated by using the
cavern space created. In Missouri, USA, the city government encourages the private sector to use cavern space
created from underground quarrying for a wide range of business activities, such as cold storage, data centers,
logistics, warehousing and archives (Figure 7). The land bank created by underground quarrying over the past few
decades gives the city government a stable source of land supply. Overall, the development model has proved to be
sustainable and is of reference value to Hong Kong.
A preliminary review on the prospect of underground quarrying in Hong Kong has been carried out under the
Strategic Cavern Study. It suggests that there could be a viable business case for the development and operation
of an underground quarry, provided that the quarry includes business enhancement ancillary operations, such as
concrete batching and rock processing. With proper planning and design of the underground quarries, usable cavern
space could also be formed for accommodating suitable and compatible public or private sector facilities in the long
term.
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Figure 6. Examples of underground quarries in other countries
Figure 7. Different usage of cavern space formed by underground quarrying in Missouri, USA
To further pursue the initiative of underground quarrying-cum-cavern development, it is necessary to examine in
detail if the economic viability of underground quarrying can be established with regard to the associated technical
constraints and opportunities, as well as the potential future use of the cavern space. The technical feasibility of
housing the ancillary operations, e.g. rock processing, concrete batching, in caverns should also be ascertained.
On this basis, the CEDD plans to launch a detailed technical study to establish the technical feasibility, regulatory
and administrative requirements, business case, implementation mechanism and procurement arrangement, so as to
pave the way for future implementation of underground quarrying-cum-cavern development schemes. Subject to
the outcome of this study, the CEDD will consider launching pilot schemes at suitable sites after resolving all the
planning and technical issues and consulting the relevant stakeholders. If underground quarrying-cum-cavern
development is found to be technically feasible and economically viable in Hong Kong, it could supplement surface
quarrying in supplying rock products whilst providing space for rock-related processing services, and bring about
benefit in enhancing the long-term land supply by creating a cavern land bank.
3. Underground Space Development in the Urban Areas
In Hong Kong, underground space developments are commonly found in dense urban areas where surface
solutions to create space and enhance connectivity are precluded due to physical or environmental constraints.
Basements are built to optimize the use of space of individual buildings with development constraints (e.g. building
height restriction). MTR stations typically form the nodes of connections to nearby developments, whilst direct
underground connections between individual developments are relatively rare due to management and liability
issues.
There are notable examples of extensive underground space developments around the world (Figure 8),
illustrating that when underground space usage is planned in a well-thought-out manner, it can open doors to a new
spectrum of sustainable solutions to improve the built environment of densely developed areas, particularly in
respect of space creation, connectivity enhancement and infrastructure reorganization.
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Figure 8. Notable examples of underground space developments
It is clear that enhancing the use of underground space can offer opportunities for significant planning and
development gains. To this end, the CEDD has launched two studies with different focuses with a view to
enhancing the use of underground space resources in a systematic and holistic manner, particularly for creating
urban space for commercial and other community uses, enhancing urban connectivity, and improving the congested
urban environment at ground level. An overview of the studies is given below.
3.1 Scoping Study
The scoping study on “Territory-wide Study on Underground Space Development in the Urban Areas of Hong
Kong”, which commenced in late 2013 for completion by 2016, is aimed at identifying the key issues, opportunities
and constraints of implementing underground space development in the urban context of Hong Kong.
Some existing urban areas, which spread across different districts in Hong Kong, have been selected for the
purpose of examining the applicability and viability of underground space development option for meeting various
objectives and identifying constraints/issues in implementation. They collectively represent most typical urban
settings that would have higher potential for developing underground space to yield the benefits of space creation
and connectivity enhancement, such as locating in the proximity of existing/planned MTR stations, having sizeable
parks or open space, and/or belonging to the major nodes of commercial, social or tourism activities. This territory-
wide review generally shows that underground space development is technically feasible in the urban setting of
Hong Kong and can help to alleviate the problems of limited land resources and poor connectivity. It offers a new
dimension of solutions to unlock the development potential of the existing urban areas and to address the current
district problems. In the long run, it also provides an added source of solution space for a wide range of land uses in
the congested urban environment.
Considering the district demand and interest, there is flexibility in accommodating different land uses (e.g.
community, cultural, entertainment) within an underground space development, thereby enhancing its vibrancy and
attractiveness.
Whilst the versatility of underground space is broadly established, the development potential of individual areas
may be constrained by the existing urban context and some of the proposed land uses may not be fully justified at
the present moment due to insufficient demand or incompatible setting of adjoining areas. For the old districts, the
timing of redevelopment or revitalization would be a triggering factor for making underground space development
option viable and justifiable, which would bring a synergy effect through potential integration with new surface
development, thereby maximizing the overall development potential of the entire area.
Avoid generating excessive traffic and pedestrian flows to the already over-crowded urban areas is identified to
be one of the key issues that need to be addressed when developing underground space in urban areas. It is also
necessary to attend to traffic and social impacts (e.g. disturbance to above-ground facilities) during the construction
stage, practically through implementing a comprehensive traffic management scheme and adopting a well-planned
construction sequence. Physical obstruction (e.g. utilities, tunnels) or legal obstruction (e.g. private lot boundary)
can also greatly constrain the development potential of underground space. These obstructions highlight the need
for a holistic underground master planning for new development or redevelopment areas to cater for long-term
underground space development.
The potential use of underground space for accommodating public transport interchange (PTI), refuse collection
point (RCP) and electric sub-station (ESS) have also been reviewed. Relocating PTI and RCP are found to be
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technically feasible, but the land intake due to the provision of large vehicular ramp and ventilation structure would
basically offset the land gain. Relocating ESS underground is also found to encounter technical difficulties (e.g.
reorganising complex cable network) and potential risk (e.g. flooding, over-heating due to ventilation system
failure). However, the prospect of locating new infrastructure facilities underground is much better through
integrated surface and underground space planning.
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3.2 Sites-specific Study
The sites-specific study on “Pilot Study on Underground Space Development in Selected Strategic Urban Areas”
was launched in June 2015 for completion by 2017, with particular focus on the evaluation of the overall
opportunities and challenges of underground space development in four selected Strategic Urban Areas, namely
Tsim Sha Tsui West, Causeway Bay, Happy Valley and Admiralty/Wan Chai (Figure 9). Strategies for developing
underground space will be formulated with a view to improving the living environment, enhancing district
connectivity and creating new usable space to address the developmental needs in these areas.
The four study areas are selected as they are commercial, leisure and recreational hot spots and of high
developmental potential. They are located along existing or planned MTR lines, which serve as transportation hubs
to nearby areas. However, the limited land resource has hindered the sustainable development in these areas, and
the road space is insufficient to accommodate the large crowd of people and traffic flow introduced by the economic
transformation. The new and planned development/redevelopment projects as well as the sizeable parks or leisure
space within these areas may provide suitable conditions and opportunities for underground space development.
The study aims to establish the overall merits of adopting underground space development in these areas,
formulate an Underground Master Plan for each area and identify potential priority projects. Broad technical
assessment will also be carried out for each priority project to facilitate further implementation where suitable.
Figure 9. Four study areas under the Pilot Study on Underground Space Development
4. Conclusion
The urban areas in Hong Kong have been densely developed with limited land for further developments. The
complex urban fabric evolved over the past decades has imposed many constraints on improving the built
environment and limited the growth potential of the urban areas. Hong Kong’s topographical setting with steep
natural hillsides also restricts the extent for further urban expansion. Rock caverns and underground space are
viable sources of long-term land supply to meet the challenge of expanding land resources to sustain the population
and economic growth of Hong Kong. Based on overseas experience, the applicability can be open to a broad variety
of beneficial uses that are of reference value to the Hong Kong context. In view of the high land and property
values, rock cavern and underground space development could be an economically viable solution in comparison
with other above ground options. By adopting a proactive and holistic approach in planning and implementation, it
is intended to enable rock cavern and underground space development to become a sustainable means of increasing
land supply and optimize the use of these ‘hidden’ land resources in a more effective and efficient manner.
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Acknowledgements
This paper is published with the permission of the Head of the Geotechnical Engineering Office and the Director
of Civil Engineering and Development, the Government of the Hong Kong Special Administrative Region.
Unless otherwise indicated, the photographs found in this paper are subject to copyright owned by the CEDD.
Prior written consent is required for a third party who intends to reproduce, distribute, display or otherwise use such
photographs in any way or for any purpose. Such requests for consent shall be addressed to the CEDD via email at
undergroundspace@cedd.gov.hk. More information of the studies mentioned in this paper can be found on the
project website (www.undergroundspace.gov.hk).
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