Cavern development in Hong Kong
By the Development Bureau, the Civil Engineering and Development Department, the Drainage Services Department and the Water Supplies Department with the coordination of the CV Division
Overall view of cavern development
Hong Kong’s geology – the intrinsic advantage
Hong Kong, positioned as the world city of Asia, is renowned for its modernised developments. Like many urbanised cities worldwide, Hong Kong has a long-standing problem in land shortage. Despite this, the hilly terrain with strong rock in Hong Kong is highly suitable for developing rock caverns, particularly on the urban fringe.
This resource advantage was exploited in as early as the 1980s when the Mass Transit Railway (MTR) constructed its Tai Koo Station in the form of a rock cavern. In 1990, the Geotechnical Engineering Office (GEO) of what was then the Civil Engineering Department fully ascertained the potential of cavern development under its Study of the Potential Use of Underground Space (SPUN). Since then, the Government completed more cavern projects such as Stanley Sewage Treatment Works, Kau Shat Wan Government Explosives Depot and the Island West Transfer Station. In the 2000s, The University of Hong Kong (HKU) relocated two saltwater service reservoirs to its adjoining hillside to release the occupied land for Centennial Campus development. It was a model case for sustainable development by releasing or creating valuable surface land in the built-up areas for more beneficial uses.
Distinctive benefits of rock caverns
Relocation of suitable facilities to rock caverns could release surface sites for other beneficial uses (for example, residential and commercial developments) and remove incompatible land uses. Placing facilities of the “not in my backyard” (“Nimby”) type in caverns will minimise their nuisance to the communities while enhancing the development potential of the released land and its surrounding areas.
Rock caverns can also provide precious space for new facilities where there is a shortage of available surface land, thereby alleviating the pressure of land supply shortage. In addition, with the attributes of a stable environment and excellent security, rock caverns are particularly suitable for certain facilities. Despite these conceivable benefits, this valuable land resource was only capitalised in a discrete manner in the early days. The real benefits of cavern development had not been fully exploited until the promulgation of the long-term cavern development strategy in 2017.
Housing suitable public facilities in rock caverns helps release precious surface land for other beneficial uses, avoid incompatible land uses and minimise disturbance to the environment.
Long-term cavern development strategy
Cavern Master Plan
With a view to developing a holistic cavern development strategy from planning to implementation, 48 Strategic Cavern Areas (SCVAs) sizeable enough for accommodating multiple facilities with favourable geological and topographic settings as well as easy access to surrounding infrastructure networks have been delineated on a territory-wide Cavern Master Plan (CMP) (Figure 1) published jointly by the Civil Engineering and Development Department (CEDD) and the Planning Department (PlanD).
A Cavern Master Plan was published with a view to developing a holistic cavern development strategy from planning to implementation.
Figure 1: Cavern Master Plan
Under a holistic management approach, the Government also expanded the list of facilities suitable to be housed in caverns in the updated Hong Kong Planning Standards and Guidelines (HKPSG) and implemented a suite of enabling measures to advocate the cavern development initiative. These measures are discussed in the sub-sections below.
Accommodation of government facilities in caverns
The use of cavern option for sewage treatment works, refuse transfer stations and service reservoirs has proven to be not only viable but also highly beneficial, particularly in minimising adverse impacts on the environment and alleviating land use incompatibility. For sewage treatment works and refuse transfer stations, the enclosed cavern environment is conducive to controlling any dusty, odorous and gaseous emissions generated and also minimising the impact on the surrounding land uses. The construction of service reservoirs in caverns could avoid taking up urban surface land and also minimise the potential need for extensive site formation on the hillside, thereby minimising the disturbance to the environment.
Therefore, the Government has formulated a priority list of existing government facilities suitable for relocating to caverns based on considerations such as the existing site conditions, potential land uses, development constraints, relocation timeframe, merits and constraints of relocating these facilities, etc. The facilities on the list will be systematically relocated to caverns to release their corresponding surface land gradually. Moreover, in handling site searches for government projects involving the potential uses listed in the CMP, potential cavern sites as an alternative to surface land will first be identified and considered in the initial project planning stage.
On the other hand, for new government projects involving sewage treatment works, refuse transfer stations and service reservoirs, cavern option assessments with thorough deliberation of all tangible and intangible factors would be carried out in the early planning stage. Furthermore, in large-scale planning and land development studies, due consideration would be given to accommodating new suitable facilities to maximise the synergy effect of surface and subsurface development.
Underground quarry-cum-cavern development
A continual supply of construction materials and formation of space are crucial to the sustainable development of Hong Kong. The synergy between land supply through an underground quarry and the formation of cavern space should be considered for more efficient delivery of cavern projects while providing a local source of rock material supply. With thoughtful planning and strategic site selection, the business case of underground quarrying with the associated operations, including concrete batching and asphalt production, can be established in the short to medium term while the cavern space so created can be fully utilised for strategic after-uses in the long term.
Updating technical guidelines
The 2nd edition of Geoguide 4 – Guide to Cavern Engineering was published in 2018. It provides a standard of good local practices for cavern design in respect of civil and geotechnical engineering. However, the fire safety issue is another crucial area for the successful implementation of cavern projects. Therefore, the GEO, in collaboration with the Buildings Department and the Fire Services Department, is reviewing and updating the Guide to Fire Safety Design for Caverns 1994, with a view to providing more pragmatic guidance on the formulation of safe and cost-effective designs in caverns.
The above comprehensive suite of enabling measures, covering planning issues, technical developments and implementation strategies, is effective in pressing ahead with various cavern initiatives and facilitating the long-term development of cavern projects. The initiatives have led to the recent rise of government cavern studies and projects, leaving their footprints on nearly one-third of the 48 SCVAs to date. Some of these projects are discussed below.
Relocation of Sha Tin Sewage Treatment Works to caverns
The Stanley Sewage Treatment Works is one of the early pioneering cavern facilities built in the 1990s, demonstrating the key considerations for housing sewage treatment works in caverns. Currently, the Drainage Services Department (DSD) is relocating the Sha Tin Sewage Treatment Works (STSTW) to caverns at Nui Po Shan in SCVA No. 20 - A Kung Kok (Figure 2).
Figure 2: Relocation of Sha Tin Sewage Treatment Works to caverns
The existing STSTW has been in operation since 1982. It is the largest secondary sewage treatment works in Hong Kong, currently serving a population of about 690,000 in Sha Tin and Ma On Shan Districts and treating about 280,000 m3 of sewage per day. Facing Shing Mun River and Tolo Harbour, it occupies a prime location of some 28 hectares, which are planned mainly for innovation and technology development after the relocation of STSTW.
Inside caverns, the odorous and gaseous emissions generated during the sewage treatment process can be confined for effective treatment by appropriate environmental and odour control measures, including deodourisation facilities that minimise the environmental impacts on the surrounding communities. The outlet of the ventilation shaft will be inconspicuous and landscaped to blend in with the surroundings of Nui Po Shan. Through the multi-barrier control measures, it is anticipated that the arrangement will not cause any perceivable odour to residents in the area.
Here are some highlights of the cavern design:
Modernised treatment facilities
Sewage treatment processes using new technologies, including lamella settler for primary treatment, moving bed biofilm reactor (MBBR) for biological treatment, dissolved air flotation (DAF) for solid-liquid separation and ultra-violet (UV) for disinfection, will be adopted. The new technologies are more compact and efficient, which enable the future facility in caverns to have the same treatment performance while occupying only half of the existing STSTW measured 28 hectares.
The conventional sewage sludge treatment, which produces flammable biogas as a by-product, cannot be adopted in the confined cavern environment from a fire safety perspective. Therefore, with comprehensive pilot trials to ascertain the robustness, treatment performance and operational reliability, direct dewatering of the sludge to 30% dry solid content will be adopted in the future facility before disposal at T·Park in Tsang Tsui, Tuen Mun.
Advanced fire safety design
Because of the unique nature and massive scale of this cavern development, neither the prevailing Codes of Practice related to fire safety and the minimum fire service installations developed for typical building types, nor the current version of the Guide to Fire Safety Design for Caverns 1994, would be fully suitable. Alternatively, DSD adopts the fire engineering approach for the cavern facility by devising a comprehensive fire safety strategy in terms of fire-resisting construction, smoke control strategy and fire services installations, which best suit the nature and functional requirements of achieving the equivalent level of safety as a code-compliant design.
Innovative cavern construction technologies
In this signature cavern project, the proposed cavern halls will span up to 32 m and have a height of up to 32 m, making them the largest caverns of their type ever built in Hong Kong (Figure 3). One of the innovative engineering applications during the construction stage is the Measurement-While-Drilling (MWD) system deployed for the drill-and-blast method for the excavation of the caverns. Electro-hydraulic drilling jumbos equipped with drilling booms with sets of integrated sensors are used for the real-time monitoring of the drilling performance of blast holes and the collection of engineering data for further interpretation.
Figure 3: Cavern construction of STSTW
The MWD system is able to improve the accuracy of drilling works for a more precise excavation profile by blasting. It also provides a tool to capture the changes in ground conditions and engineering parameters for rock mass characterisation along the longitudinal cavern drive like never before. By combining the rock parameters with the survey data of cavern faces captured by Light Detection and Ranging (LiDAR) scanning, the data collected can form part of the cavern ground condition mapping, which is a three-dimensional record of geological features and geotechnical parameters across the cavern face, crown and wall in the as-built record. The MWD system not only enhances the quality control and safety supervision of drilling operations, but also opens up a gateway for engineers to acquire essential knowledge and data for geotechnical design and asset management planning of underground space development in the future.
In addition to the MWD system, another application of innovative technology is the use of HoloLens, a pair of mixed reality head-mounted display, to enhance supervision in the drill-and-blast excavation works, including the verification of the layout of blast hole pre-drilled by the drilling Jumbo. Prior to the explosive charging procedure of blasting works, the layout of hundreds of drilled blast holes should be verified by site supervision staff based on the approved blast design. Through the mixed reality environment of the HoloLens (Figure 4), such verification procedure could be significantly enhanced in terms of accuracy, duration and comprehensiveness with the aid of an image of blast design projected on the HoloLens head-mounted display unit.
Figure 4: Mixed reality display for blast holes drilling
Smart robotic applications
Innovative smart safety measures have also been developed and adopted during the excavation and construction of access tunnels and caverns of STSTW. One of the many smart safety measures used is the Resident Site Robotic Supervisor (RSRS) 「岩洞探哥」, which was designed to reduce construction risk and enhance hazard management in the cavern/tunnel blasting works. The RSRS comprises an unmanned robotic car that acts as a multi-functional mobile platform for various applications to enhance site safety, including air quality monitoring and surveying of the excavated face with LiDAR device.
Joint Cavern Development at Anderson Road Quarry Site
The Public Works Central Laboratory (PWCL) under the GEO, CEDD is currently accommodated in a four-storey building at Cheung Yip Street, Kowloon Bay, with a harbour front site of about 3,850 m2 in area. The Government is planning to free up the occupied site for housing development by relocating the laboratory to caverns in SCVA No. 28 – Tai Sheung Tok. Besides, the Government Records Service (GRS) provides archival facilities for public records, namely records appraised as having historical value for long-term preservation. The storage capacity of its existing facilities is close to saturation, and there is an imminent need to build a new Archive Centre (AC) to meet future storage demands. The chosen site of the new AC adjoins the cavern site in SCVA No. 28, which has been designated for the relocation of PWCL. This enables these two facilities to be synergistically developed together under a Joint Cavern Development (Figure 5) project tasked to the GEO, CEDD, following the “single site, multiple use” principle.
Figure 5: An artist’s impression of Joint Cavern Development at Anderson Road Quarry Site
Ideal cavern environment
Given the keen competition for surface land in the urban area with other land uses such as residential development, forming caverns to house suitable facilities can release the surface land so occupied or earmarked for other more suitable uses. Besides, the stable environment (for example, shelter from shock and vibration, constant humidity, and steady temperature) in rock caverns is ideal for laboratory testing operations, storage of testing specimens as well as archival holdings. The stable humidity and lower temperature in caverns allow the facilities to be operated in a more energy-efficient manner, making their operations more environmentally friendly and cost-effective in the long run.
Rock reinforcement approach
In Hong Kong, it has long been a common practice to adopt cast-in-situ concrete lining as the permanent support of tunnels or caverns, and to neglect the structural reinforcement contributed by the first pass, sprayed concrete lining primarily due to the perceptions of its techniques as immature and the lack of internationally recognised standards and specifications. However, following the improvement of sprayed concrete technique in recent years, high-quality sprayed concrete lining has been applied worldwide in tunnel or cavern projects as the permanent support. The project team of the Joint Cavern Development will enhance the application of the rock reinforcement approach, which uses pattern rock bolts and sprayed concrete as permanent works instead of cast-in-situ concrete lining, thereby significantly reducing the overall lining thickness and the total volume of rock excavation required to produce a cost-effective cavern lining design.
Exploration of new material
One of the key concerns in the past about the first pass lining was the potential damage it could cause by blasting vibration with its application. To ensure the structural integrity of the first pass lining, GEO has initiated a study on the development and application of Vibration Resistant Sprayed Concrete (VRSC), which is a novel material that enhances the durability of the sprayed concrete lining against blasting vibration. While the development of VRSC is in progress, its trial use and full application as sprayed concrete lining in the Joint Cavern Development project are probable as the results of the study materialise.
Other cavern projects in the pipeline
Relocation of multiple service reservoirs to caverns
Accommodating service reservoirs in caverns was demonstrated to be highly beneficial with the Western Salt Water Service Reservoirs, a project in which the reservoirs were relocated in 2011 to caverns behind HKU for campus development. There are many service reservoirs located in the heart of the city and relocation of them would yield considerable land benefits. The Water Supplies Department (WSD) is taking forward a number of projects involving the relocation of service reservoirs to caverns, including Diamond Hill Fresh Water and Salt Water Service Reservoirs (DHSRs).
Relocation of service reservoirs to caverns requires unique and careful planning from water supply arrangement and hydraulic points of view, in addition to geotechnical considerations. In order to ensure an adequate and reliable water supply to the territory, waterworks installations need sufficient capacities and safety margins to cope with variations in water consumption. In regard to service reservoirs, the design storage capacity is governed by demand fluctuations during the day, the need for fire-fighting, and the requirement to maintain water supply during maintenance. The rock cavern should also be at an appropriate altitude to enable provisions of the required water pressure for consumers at the pick-up points. The invert and top water levels of the relocated service reservoirs are designed with a view to maintaining hydraulic performance. The relocated service reservoirs should also be in the vicinity of the existing service reservoirs so as to facilitate the connection of new water mains with the existing water supply network at suitable locations.
Among the relocation projects that WSD is taking forward, the project for relocating DHSRs to caverns (Figure 6) in SCVA No. 26 - Lion Rock West is in the most advanced stage, with construction works planned to commence by the end of 2022. With the advancement in construction technology, there are a number of enhancements in the design of the DHSRs project, including the adoption of the more cost-effective hybrid of rock reinforcement approach and the conventional reinforced concrete lining, and the provision of a smoke extraction system and a pressurised emergency safe passage to improve the fire safety of the confined cavern space with limited evacuation access. Moreover, in order to expedite the construction programme, the DHSRs project adopts Design for Manufacture and Assembly (DfMA) for parts of the internal structure of the access tunnel and valve chambers.
Figure 6: Relocation of DHSRs to caverns (portal area)
Tai Sheung Tok Transfer Station in caverns
Being a successful precedent project of housing a refuse transfer station in caverns in Hong Kong, the Island West Transfer Station was built in a cavern complex inside the mountain of Mount Davis at Kennedy Town in 1997. As with sewage treatment works, the major plants and equipment are operated in caverns isolated from the surrounding environment such that waste handling operations are contained. Thus, the cavern setting provides environmental benefits in terms of the containment of noise and odour and the reduction of adverse visual impact, making the setting far superior to its surface counterpart. As part of the waste management strategy, the Environmental Protection Department (EPD) has started planning the development of Tai Sheung Tok Transfer Station (TSTTS) in SCVA No. 28 - Tai Sheung Tok in an environmentally friendly and cost-effective way that addresses the burden of the overloaded refuse transfer station network. The municipal solid waste received will be compacted and containerised in purpose-built containers. All major operations, including waste tipping and compaction, will only be conducted inside caverns, thereby avoiding nuisances to the public. Innovative construction and operation approaches will be considered to reduce carbon footprint through adopting low-carbon materials and design, enhancing the energy efficiency of the facility and using renewable and new energy. TSTTS is designed to become a modern state-of-the-art carbon-neutral refuse transfer station and will be a role model for future waste management facilities.
Engaging the private sector
With the long-term cavern development strategy pointing the direction, the cavern development opportunities have been fully embraced in the public sector. Looking forward, one can see that many other private facilities such as data centres that are proven suitable and even more environmentally friendly to be placed in caverns. There are many successful overseas examples of cavern data centres showing that rock caverns can provide an environment with stable temperatures for energy-efficient and decarbonisation operation as well as sizable space with excellent security for data storage and processing. It is understandable that the private sector may not be familiar with cavern development, but a clear message has to be conveyed to private stakeholders that there are distinct benefits of placing data centres in caverns, and that engineers in Hong Kong are also well equipped and ready to materialise the innovative concept. To this end, a Stakeholder Forum on Developing Data Centres in Caverns was organised jointly by the Development Bureau (DEVB), the Innovation, Technology and Industry Bureau (ITIB), the Office of Government Chief Information Officer (OGCIO) and GEO of CEDD in September 2022 (Figure 7). The forum was a success and the stakeholders supported the view that the Government should formulate a facilitative strategy to promote private cavern data centre development. It signified an important milestone in engaging the private sector to make use of the rock cavern opportunities as an alternative solution to the ongoing developmental need of data centres in Hong Kong.
Rock caverns provide a stable and protected environment which is particularly advantageous for housing data centres in terms of energy saving and security.
Figure 7: Stakeholder Forum on Developing Data Centres in Caverns
The recent rise of cavern projects marks an important milestone in cavern development in Hong Kong. It showcases the use of rock caverns to unlock precious land resources in congested urban areas. The various innovative cavern engineering technologies also enable a more rational and economical design concept for future cavern projects. In the new era, the CMP and the suite of enabling measures will keep abreast with the needs and advancement of Hong Kong, steering the broader application of cavern development in the territory. By adopting a proactive and holistic approach, Hong Kong will continue to create cavern space in a smart, balanced and effective manner for the sustainable development of this Asia’s World City.