Underground Space Development
Hong Kong has been using underground space for commercial purpose and provision of community and transport facilities for many years. Most existing use of underground space for development purposes is related to individual projects such as basement car parks, shopping arcades, subways, railway stations and tunnels. A holistic planning strategy from a macro and multi-level perspective, including the consideration of underground space creation and connection, is currently lacking.
Good underground planning and effective use of underground space can enhance the connectivity with the surroundings, improve the urban environment at ground level, create space for different commercial and public facilities, and optimise the development potential of scarce land resources in the long term.
- The Civil Engineering and Development Department (CEDD) has joined forces with the PlanD to commission a "Pilot Study on Underground Space Development in Selected Strategic Urban Areas – Feasibility Study". Four strategic urban areas, namely Tsim Sha Tsui West, Causeway Bay, Happy Valley and Admiralty/Wan Chai, have been selected for further study. The project aims at evaluating the overall merits of developing underground spaces and identifying the key issues involved; formulating Underground Master Plans for these areas; and drawing up suitable conceptual schemes.
Benefits of Development
Moving suitable activities to underground space can release valuable above-ground resources.
Where there is shortage of surface land, rock caverns can provide space to accommodate suitable facilities, including some for which it is difficult to find suitable surface sites, such as maintenance depots, sewage treatment works and columbaria. Other facilities that can benefit from the stable and secure underground environment can also be set up, such as archives, warehousing, laboratories and data centres, reducing the amount of land required for them as a result.
In addition, there is potential for integrating underground quarrying with the development of a cavern land bank. With proper planning and design of underground quarries, usable cavern space can be formed to accommodate a variety of public or private sector facilities.
Underground space development can enhance connectivity and improve the above-ground pedestrian environment in congested districts through the formation of underground linkage networks. For instance, public parks and recreational spaces close to MTR stations in urban areas would offer the opportunity to develop the shallow underground spaces beneath them and connect them directly to transport and other facilities. In this way, additional walking space could be created, enhancing connectivity between different areas and helping to alleviate road congestion.
Where public acceptance and individual site situations permit, underground space development can also create additional space to accommodate suitable community, cultural and recreational facilities; provide covered public space where at-grade space is lacking; and offer space for retail and other commercial activities as well as space for other uses, complementing or even enhancing the existing urban context.
- Overseas experience shows that cavern development can be successfully extended to a variety of uses such as (i) community and recreational facilities (e.g. sports centres, swimming complexes); (ii) storage facilities (archives, food/wine cellars, oil and gas storage); (iii) commercial and industrial facilities (data centres, logistics/warehouses, maintenance depots); and (iv) special facilities (columbaria, testing laboratories).
Costs of Development
The development cost of individual cavern projects may vary, depending on a host of factors including topographical and geotechnical conditions, and environmental considerations of the specific site. If the geotechnical conditions of the site are more complicated with greater environmental limitations, construction cost may be higher to allow for strengthening works and environmental mitigation measures. In addition, the height, size and structural layout of rock caverns will depend on the facilities moving into them. This will have direct implication on the structure of the cavern and tunnels, as well as the construction cost of the facilities and building services equipment within the caverns.
Compared with other land development options, cavern development is probably the most expensive in terms of per square metre cost (can be a few times higher than the cost of near-shore reclamation).
- Take the relocation of Sha Tin Sewage Treatment Works as an example, the development cost of the cavern as well as associated infrastructure cost is estimated to be about HK$208,000 per square metre.
- Considering cavern engineering work alone, the development cost is approximately HK$77,000 per square metre (assuming a cavern height ranging from 15 to 25 metres.)
The development cost for underground space is generally high as well. Besides, compared with above-ground structures and facilities, the operation, management and maintenance costs of underground structures and facilities are bound to be higher.
Various technical and implementation issues need to be resolved for underground space development, such as fire safety, land ownership and town planning issues, interfaces with existing underground uses (for example, railway stations), impact on above-ground facilities, and the high development cost.
Challenges and Uncertainties
Development of caverns is a costly and lengthy process. The development cost of individual cavern projects may vary, depending on site situations, geotechnical conditions, environmental considerations and the specific land use.
Rock cavern developments will likely involve statutory procedures (for example, those under the Environmental Impact Assessment Ordinance, Country Parks Ordinance and Town Planning Ordinance). Together with the construction and associated engineering works which may vary with project scale and technical complexity, a cavern development project may take a lead time of 10 to 15 years from conception to realisation. In terms of space creation involving similar levels of time and budget, cavern options are likely to create much less space than other land supply options such as rezoning and reclamation.
Cavern development cannot offer a quick fix to the imminent problem of a shortage of developable land, particularly when the sub-surface land created by caverns is not suitable for residential uses.
Rock caverns are therefore best seen as a source of long-term land supply.
To develop underground spaces, a number of issues have to be properly resolved beforehand. These include land use planning issues, engineering works (including the obstruction of existing underground pipelines and impacts on above-ground facilities), implementation and financial arrangements, and maintenance and management responsibilities. The amount of developable sites and scale of development is usually limited by various constraints arising from the built environment.
Other challenges in developing underground space include the possible disturbance to existing surface facilities during construction of underground space beneath them; the occupation of ground-level space by structures arising from the underground space development; the impact on traffic, pedestrian flow and surrounding environment; and the high development cost.
- In view of the shortage of developable land, hidden land resources such as rock caverns and underground space may offer room to accommodate suitable public or infrastructural facilities, and to support the relocation of some above-ground facilities and reduce the amount of land occupied by them. This will in turn release surface land for housing or other beneficial uses. Rock caverns and underground space can therefore indirectly increase above-ground developable space in the long term.
- Development of caverns and underground space is a costly and lengthy process. The space so created is generally not suitable for residential use and may not be a suitable source of land supply for high-density development.