Underground space development has become an attractive option to supplement the limited surface land resource of Singapore. With a high population density and rapidly developing economy, underground space development has become an economic imperative. In October 2007, the government, under the Ministry of National Development, set up an inter-agency Underground Master Planning Task Force that aims to map out the long-term development of the underground space, bringing the underground space development to a strategic level. In 2010, the Economic Strategies Committee made developing underground space part of the government’s long-term economic strategy.
The first feasibility study of rock cavern construction started in 1990, and invariably led to the development of the first underground ammunition storage facility, opening a new frontier for space creation in land-scarce Singapore. This was followed by the currently on-going Jurong rock cavern project for hydrocarbon storage being constructed under the seabed. With the government push for a concerted long-term development, rock cavern development in Singapore is positioned to accelerate.
This paper gives a review of the history of underground rock cavern space development in Singapore, highlights the potential utilizations in Singapore, and discusses the various planning issues. A comprehensive list of references in relation to underground rock cavern development in Singapore is included.
Singapore consists of one larger and several smaller islands, and lies at the southern end of the Malay Peninsula with a roughly diamond shaped area between latitude 1o09’N and 1o28’N and longitude 103o38’E and 104o06’E. With a total land area of approximately 700 km2 and a population of 5.0 million, the average population density stands at a very high 7,500/km2.
With a high population density and rapidly developing economy, Singapore faces a severe lack of land space for development. The increasing population, coupled with the government plan for more green space for the population, means more competition for land. Traditionally, Singapore has relied on two solutions for this land scarcity: high-rise buildings and land reclamation. In Singapore, 22% of surface land is reclaimed. High-rise housing, both private and public, is the norm for the Singapore population. However, land reclamation has become increasingly more difficult due to deeper water, lack of sand supply, and environmental and even territorial issues.
Like many other cities in the world, Singapore’s initial and main use of underground space has been in the area of transport systems and some basement construction for shopping and parking purposes in the city areas [ITA, 2004]. One notable exception is the Deep Tunnel Sewage System [THE STRAITS TIMES, 2000] which has an island-wide network of tunnels connected to two major waster water treatment to the east and west. Due to the geology of Singapore, almost all these underground projects have been constructed in soft ground. The relatively thick overburden of the Singapore geology means any rock cavern construction has to go rather deep underground.
The possibility of using deep rock caverns was first raised in 1989 [BROMS, 1989]. This was followed by several feasibility studies for cavern construction in the Bukit Timah Granite, which eventually led to the development of the underground ammunition facility (UAF) by the Ministry of Defence [THE STRAIGTS TIMES, 2008]. With a land savings of more 300 ha, the UAF demonstrated the significant benefits of using rock caverns to create space and improve safety, and served as a catalyst and case study for many later studies and thinking on the use of rock cavern space.
2. Singapore Geology and Rock Conditions
Singapore is of moderately low relief. Most of land areas range 10-30 m in elevations. The area of highest relief is at the Northern Central of Singapore, Bukit Timah area, where the highest hill rises to 163 m above mean sea level.
In regional geology, Singapore is of a southerly projection of the Geology of the Malay Peninsula, which is the southeastern extremity of the Eurasian tectonic plate [Pitts, 1984].
2.1 Main Geological Formations
The Singapore rocks consist mainly of, in order of geologic age from old to young, four geologic formations [PWD, 1976; DSTA, 2009]: (1) Bukit Timah granite, (2) Sajahat Formation of metamorphic quartz sandstone and mudstone, (3) Gombak norite, and (4) Jurong Formation sedimentary rocks.
The Bukit Timah granite (BT) forms the major basement rock of Singapore and covers about one-third of the Singapore island, with distribution at central main island, Pulau Ubin island, and the surrounding sea area. The term granite is used in a general sense for the entire suite of acid rocks including granite, adamellite, granodiorite, and the acid and intermediate hybrids (mainly of granodioritic and dioritic composition) which resulted from the assimilation of basic rock within the granite.
The Sajahat Formation (S) is probably the eldest rock formation in Singapore. It is variably metamorphosed sedimentary rocks comprising quartzite, sandstones, and argillite. The Sajahat Formation (S) mainly distributes at Pulau Sajahat, Sajahat Kechilkong, along north coast of Pulau Tekong and as far to the east as Tanjong Renggam.
The Gombak Norite (GN) is a body of noritic and gabbroic rock with exposure on the western side of the Bukit Timah Granite on Singapore Island. The unit is named after Bukit Gombak where noritic and gabbroic rocks are well exposed in a number of quarries. The noritic and gabbroic rocks are coarse-grained and plagioclase-rich with varying amounts of clino- and orthopyroxene minerals appearing as interstitial grains giving an intergranular texture to the rock.
The Jurong Formation (JF) sedimentary rock is the youngest rock formation in Singapore [Fontaine & Lee, 1993]. It overlies the above basement rock formations, and extensively covers one-third of the main island area of Singapore. It distributes at south and southwest Singapore. Extensive areas in the Jurong Formation have been affected by dynamic metamorphism resulting from tectonic activity. The grade of metamorphism is low, and it is still possible to determine the facies that has been affected. Overlying the above basement rocks, shallow overburden deposits include residual soils, the Kallang Formation, the Fort Canning Boulder [Han et al., 1993] and the Old Alluvium.
The Old Alluvium Formation is dominantly terrestrial deposit of early Pleistocene age and distributes at eastern part of Singapore, occupying one-third of Singapore main island area [Burton, 1964; Ong et al., 2007]. It comprises mainly medium dense to very dense, semi-indurated, and clayey quartzo-feldspathic coarse sand and five gravels.
The Kallang Formation includes both marine and terrestrial sediments laid down from late Pleistocene to the present day. These deposits cover much of the coastal plain, immediate offshore zone and the deeply incised river valleys. The Marine Sedimentary Unit is found in the eastern part of Singapore. It occurs over an area covering one quarter of Singapore Island.
2.2 Soil Overburden
The tropical climate is conducive to the erosion and weathering process of Singapore rocks. In the Jurong sedimentary formation, the weathering depth extends to a depth of 45m and is generally deeper in the faulted areas and in the mudstone region, whereas the thickness of the weathered zone for the Bukit Timah granitic formation reportedly varies from 10m to 40m [TAN et al, 2001] and 20 m to 50 m [SHARMA et al, 1999]. Residual soils occupy approximately two thirds of the land area of Singapore. The residual soils derived from the Jurong sedimentary and the Bukit Timah granitic formations are found in the western and central regions of Singapore, respectively. The relatively flat terrain and thickness soil overburden in the Singapore geology makes it a challenge in finding good access for rock cavern construction.
3. A Brief History of Rock Cavern Space Development
The development rock cavern space in Singapore can be divided into five phase, as summarized in Table 1. Figure 1 shows the locations of the major site investigations for cavern feasibility studies on a simplified geology map of Singapore.
The first feasibility study of rock cavern construction started in 1990, and led to the development of the first underground ammunition storage facility, opening up a new frontier for space creation in land-scarce Singapore. Several other possible uses were also studied in this phase, including defence and civil defence, pumped storage, and warehouses [Broms & Zhao, 1993; WALLACE et al, 1993 & 1995; ZHAO et al., 1994 & 1995; WONG, 1993; Bye et al., 2003].
The successful development of the UAF was significant for Singapore from a land use point of view. It demonstrated the feasibility and many benefits of rock cavern use. Land savings from 300 ha to 900 ha were reported in the media. It should be noted a 900 ha land saving represents more than 1% of the total land area of Singapore. The UAF also represented a major breakthrough in overcoming a psychological barrier for many other government agencies that had been reluctant to move the first move for various reasons, and provided many good learning opportunities.
The Jurong Rock Cavern project was preceded with several feasibility studies by the Nanyang Technological University and Jurong Town Corporation [ZHAO LEE et al, 1999; ZHAO LIU et al, 1999]. While land use was a major consideration in the JRC project, the decision for the JRC project was primarily based business viability.
Prior to the development of the UAF and JRC project, the majority of the studies was limited to specific geological formations and led by individual agencies based on their need. In October 2007, the government, under the Ministry of National Development, set up an inter-agency Underground Master Planning Task Force (UMPTF) that aims to map out the long-term development of the underground space, bringing the underground space development to a strategic level. UMPTF includes two Sub-groups. Sub-group 1 is focused on relatively shallow basement types of underground space, while Sun-group 2 for deep rock cavern space.
The government is doing a detailed feasibility study of an underground science city [THE BUSINESS TIMES, 2008 and 2009] and a warehouse and has commissioned rock cavern usage feasibility studies for several other applications [THE BUSINESS TIMES, 2009c]. Other potential applications being studied include: Power station and electrical substations, Incineration plants, Water reclamation plants, Data centre, Warehousing and logistics, airport logistics, R&D facilities, Landfills, and Reservoirs.
Most of these usages are land-consuming industries which have been predicted to have a significant need for more space. The preliminary conceptual design has been developed for each specific usage, and capital and life cycle costs for the URC developments were also studied and compared to the costs for corresponding aboveground developments.
In 2010, the Economic Strategies Committee [ESC, 2010] made developing underground space a part of the government’s long-term economic strategy. A report of the ESC recommended that the government acts early to catalyze the development of underground space by:
• Developing an underground master plan,
• Creating basement spaces in conjunction with
new underground infrastructural developments,
• Establishing a national geology office to collate
• Developing a subterranean land rights and
valuation framework to facilitate underground
• Investing in underground development R&D
and directly investing in cavern level test-beds.
Period Major Development
1990-1994 • Feasibility Study of Rock Cavern Construction in the Bukit Timah Granite
1995-1998 • Feasibility Study of Rock Cavern Construction in the Jurong Formation
• First Tasks Force on promoting use of underground space was set up and led by the Urban Redevelopment
Authority. The report from this Tasks Recommends that MINDEF take the lead
• Feasibility study of the underground ammunition facility by MINDEF
1997-2000 • Feasibility study of rock cavern construction in the Jurong Formation [Zhao Lee et al., 1999]
• Feasibility Study of Underground Science City (USC) at Kent Ridge [ZHAO et al, 2001]
• Construction of the UAF started in 1999 [THE STRAITS TIMES, 1999]
2001-2007 • Feasibility Study, detailed feasibility & design of hydrocarbon storage caverns at Juromg Island [ZHAO LEE
et al, 1999; ZHAO LIU et al, 1999]
2007-present • MINDEF commissioned the Underground ammunition facility in 2008 [THE STRAITS TIMES, 2008]
• Government set up inter-agency Underground Master Planning Task Force (UMPTF)
• Jurong Rock Cavern project started construction in 2009 [THE BUSINESS TIME, 2009b]
• Feasibility study on Underground Science City at Kent Ridge [ZHAO et al, 2001; ZHAO & CAI 2009]
• Feasibility study on Underground Warehouse at Tanjong Kling
Table 1 Summary of Major Developments in Rock Cavern Space Use in Singapore
4. Planning for Underground Rock Cavern Space
Singapore, as a modern city, has well established an optimized surface land/open space master plan to achieve a balance catering the various needs for municipal & industrial developments. Following the same principle, underground rock cavern space also needs to be well planned. The need for master planning was highlighted several years ago when the DTSS tunnel and a metro tunnel met at almost the same elevation. Eventually, the DTSS had the upper hand because it was designed for gravity flow.
Demands for land space for industrial development and demands for living places are always a conflict. The master plan shall be made based on survey on industries demands and population increment. More importantly, it shall be based on the predication of long-term development trend along with the global economic tracks, while providing a framework for reforming urban areas into desirable and effective environment in which to live and work by people. Accordingly, how many industrial usages shall go underground can be projected with years in order to reserve more surface lands to population.
Optimized planning for underground rock cavern development shall be an essential precursor to the specific developments of major underground caverns. Unfortunately, it is may already be too late for the near-surface zones near the city centre. The tangled web of utilities and transit system commonly found is due to a lack of coordination and planning. However, it is till at right timing for the far-surface zones - rock caverns to have an optimized plan.
4.1 The Need for Master Planning and its Benefits
From a broader perspective, the need for master planning and its benefits can be discussed from the following considerations.
(a) The underground is now considered a natural resource in many cities in the form of minerals, ground water, geothermal energy, and underground space. As a natural (and strategic) resource, the use of underground space must be planned so as to derive the maximum benefits to society in a sustainable manner.
(b) Underground space are three dimensional, and cannot be easily seen. An advantage of this 3D property is that different layers of underground space can be planned and zoned based on their functions and technical requirements.
(c) Unlike surface buildings, underground structures cannot be “taken down” once developed. Conversion of rock cavern space to other uses requires provision for the necessary supporting infrastructure. A more effective plan is required.
(d) Good space, be it aboveground or underground, is always in demand. As such there will be competition for use of underground space. The example mentioned earlier is a good example. Another example is the relatively high hills where rock mass quality is generally better and at a shallower depth.
(e) There are limited surface access and exit points for underground space. These are significant from both the points of view of construction and safety considerations (fire exits, emergency exit).The issue of surface access is especially important for Singapore due to the flat terrain and thick soil overburden. From a national point of view, it might be good to have planned access points for future construction.
(f) The geologic and hydro-geologic conditions greatly affect the types, sizes and costs of rock caverns that can be constructed. Master planning allows us to match geologic conditions and rock mass qualities to the type of rock cavern uses to achieve a more efficient use of this natural resource. For example, it is logical to reserve the good rock for applications requiring large spans.
(g) Finally, master planning makes it possible for the integration and linkages of various applications for strategic reasons.
4.2 Planning Issues
Legal Framework and Policies
Legal and administrative restrictions may act as significant barriers to the use if underground spaces. The protection of the rights and ownership of existing surface or underground users, the business model, pricing mechanism, the administrative control of national reserves, and the provision of personal safety and environment protection are issues that must be resolved in all cities. And this is no less critical for Singapore.
One particular interesting issue is whether the government, releasing the potential value of cavern space, should charge a space premium to government agencies or private developers for creating the cavern space. Such a premium would be effectively a penalty. On the other hand, the government could give incentives to encourage the creation of rock cavern space.
Disposal of the excavated rock from cavern construction is another tricky issue in Singapore. Good quality rock can be used construction material such as aggregates. In many countries, the excavated rock can be sold to offset the cost of construction. In Singapore, however, if the rock is disposed of as waste, it is considered a cost to the contract and there is no issue. However, if it is sold, the government charges a royalty and revenue from the sale of the rock goes back to the government. This situation gives no incentive for the optimized use of the excavated rock.
Safety Standards for Rock Caverns
Singapore has a high standard of fire safety. However, for rock cavern facilities, the only fire safety standards that can be used and accepted by the authority are those for basement construction. This is always a contentious issue between the developer and the
Another interesting example is the safety standards related to construction. Excessive safety requirements artificially increase the cost of construction. For rock blasting, Singapore does not even have a national standard for allowable blasting vibration limits. As a result, the concerned agencies typically look for the most conservative allowable limit from other countries and divide it by half as the Singapore limit.
Geological Data and Investigations for Bedrock
For planning rock space development, it is important to have reliable geological data, especially concerning the bed rock. In this aspect, Singapore faces some challenges because of the lack of a central office for geological information for many years.
Coordination among Government Agencies
Coordination among the various government agencies is important for the planning and safeguarding of supporting surface land sites, and integration of aboveground and underground facilities, and different types of underground facilities. Dual-use or multi¬purpose facilities can help reduce cost of construction but cannot be planned without the full participation of the stakeholders.
Investment Time Frame
It is well known that many underground facilities, especially concerning transport systems, were built out of necessity – when there is no other choice but to go underground. Very often, the cost of such construction is very high due to the congestion and extensive existing buildings. On the other hand, the cost of construction can be much lower in areas where there is relatively little development. This takes a very long-term view on the part of the planner and decision maker – to go underground when the do not have to.
Cost Benefit Analysis
One of the foremost challenges for the use of underground space is the perceived high cost of construction. Large underground development may require massive investments with relatively high risks of construction problems, delays and cost overruns.
While many tangible and intangible benefits have been discussed, decisions on development options are often made entirely on the basis of construction costs. It is therefore important to develop tools that allow for effective and fair evaluation the cost and benefits. Such tools can help government agencies to make decisions on a systems view, taking into consideration many other intangible benefits, including those with high strategic significance.
In one feasibility study for an underground reservoir, the author suggested that the cost of cavern construction be compared to that of water desalination or re-cycled water because with an average annual rainfall of 2.5 m, Singapore does not lack water, but storage space. Otherwise, the cost comparison would immediately rule out the rock cavern option.
Specific Uses and Planned Space Creation
In the Singapore, land reclamation can be done without any specific use identified for the land. On the other hand, almost all decisions on cavern development are based on identified specific uses. While such facilities have very good fit-for-purpose value, they often come with very high price tags because of lack of scale and a high percentage of preliminary work and basic support infrastructure (access, power supply, etc). On the other hand, if large underground cavern space can be developed, the cost of construction per unit space can be significantly lowered due to the economy of scale and shared common infrastructure. In areas where the rock quality is good, this can be combined with underground aggregate mining, which can further lower the cost of cavern construction. In other countries, underground aggregate mining by itself is a viable business, and the space created as a by-product is essentially free.
Capability Development and R&D Support
To quote Mr Quek Tong Boon, Chief Defence Scientist of the Ministry of Defence Singapore, “rocks underground are in abundance, but rock engineers are a rare breed.” Large-scale underground space development will require the necessary in-country capability and R&D support. R&D support can help develop new technologies that facilitate planning and design, allow for more efficient, economic and safer construction, and improves the performance of the facility and reduces costs of maintenance and repairs during construction.
Public Perception and Receptivity One major challenge to the planners and decision makers is public perception. Traditionally, the underground has been associated with dark and dangerous dungeons. While the use of basement
spacefor parking and shopping malls has been accepted by the general public, deep underground rock cavern space still creates some form unease. Another perception is that underground space is very expensive. This is a challenge that’s difficult to overcome if we only look at construction costs. A system engineering thinking with the ability to consider the other opportunities is necessary to change this perception.
4.3 ITA Recommendations
Corresponding to the concerns and challenges related to effective planning of the underground, the International Tunnelling Association [ITA, 1990] has recommended a direction for subsurface planning, as follows:
The subsurface is a resource for future development similar to surface land or recoverable minerals. Once an underground opening is created, the subsurface can never be restored to its original condition and the presence of this opening can affect all future uses of the surface and the subsurface in its vicinity. These factors require responsible planning for all uses of the underground to ensure that resource is not damaged or usurped by uncoordinated first uses.
The awareness of the underground option among planners, developers and financiers should be increased so that subsurface planning issues are properly addressed. Subsurface planning should be an integral part of the normal land use planning process.
National, regional and local polices should be prepared to provide guidelines, criteria and classifications for assessing appropriate use of underground space, identifying geologic conditions, defining priority uses and resolving potential utilisation conflicts. Site reservation policies should be established for important future uses and for especially favourable geologic conditions.
The rapid economic development in Singapore, coupled with a high population density, has meant increasing competition for land. As such, the use of underground space has become an economic imperative. Rock cavern development has become a new frontier for space creation in land-scarce Singapore. With the government push for a concerted long-term development of the underground, underground space development in Singapore is positioned to accelerate.
This paper has presented a brief history of rock cavern space development in Singapore and discussed the need for and benefits of master planning and the various planning issues that need to be addressed before the use of underground space can reach its true potential. The development of the underground ammunition facility and the Jurong Rock Cavern project are good examples of planned rock cavern space use to address specific application and land use issues. However, many planning and technical issues related to the use of underground space may present challenges and even obstacles to a rock cavern development and must be addressed in a systematic manner, treating the underground as a strategic resource whose use must be properly planned and coordinated.
BURTON, C.K. 1964, The Older Alluvium of Johore and Singapore, Journal of Tropical Geography, 18:p30-42.
BYE, T.R., BROCH, E., BIAN, H.Y., & ZHAO J.,2003, Feasibility of developing water service reservoirs in rock caverns in Singapore, Proceedings ofUnderground Singapore 2003, p17-22.
BROMS, B.B., 1989, Singapore – A City ofOpportunities and Challenges, Proceedings of the Seminar on Rock Cavern – Hong Kong, MALONE,A.W. & WHITESIDE, P.G.D., (eds), The Institution of Mining and Metallurgy, p131-138.
BROMS, B.B. & ZHAO, J., 1993, Potential Use of Underground Caverns in Singapore, Proceedings of Rock Caverns for Underground Space Utilization, Nanyang Technological University, Singapore, p11-21.
DSTA, 2009, Geology of Singapore (2nd Edition),
Defence Science and Technology Agency, Singapore. ESC, 2010, Report of the Economic Strategies Committee - ESC Subcommittee on Maximising Value from Land as a Scarce Resource, Economic Strategies Committee, Government of Singapore.
FONTAINE, H., & LEE, K.W., 1993, A Triassic Limestone (Pandan Limestone) Discovered by Drilling at Singapore, CCOP Newsletter, 18(3).
HAN, K.K., WONG, K.S., BROMS, B.B. & YAP, L.P., 1993, The Origin and properties of the Bouldery Clay in Singapore, Geotechnical Engineering, Southeast Asian Geotechnical Society, 24(2), p151-166.
HEFNY, A.M., & ZHAO, J., 1999, Hydraulic fracturing in situ stresses measurements in the Bukit Timah granite of Singapore. International Journal of Rock Mechanics & Mining Science. ITA, 2004, Focus on Singapore, The Tribune, ITA
News Letter, 28.
ONG, J.C.W., MOE, S., FLANAGAN, R.F., TANG, S.K. & CAI, J.G., 2007, Buried GRANITE Ridge in Old Alluvium, Proceedings of Underground Singapore 2007.
PAKIANATHAN, L.J., PEART, M., CAI, J.G., GOH, S.M., ROSSER, H. & GANESHAN, V., 2008, Excavation in the Jurong Formation – Potential Hazards and Possible Preventive Measures, Proceedings of Underground Singapore 2008. PITTS, J. 1984, A Review of Geology and Engineering Geology in Singapore, Quart. Jour. Engineering Geology, 17: p93-101.
PWD, 1976, Geology of the Republic of Singapore, Public Works Department, Government of Singapore. SHARMA, J.S., CHU, J., & ZHAO, J., 1999, Geological and Geotechnical Features of Singapore: an Overview, Tunneling and Underground Space Technology, 14(4): p419-431.
TAN, K.H., ONG, H.L. & CHEN, S.G., 2001, Design Issues for Rock Caverns at Mandai, Proceedings of Underground Singapore 2001, p169-179.
THE BUSINESS TIMES, 2008, Underground “City” to Free Up Space, The Business Times, 13/5/2008. THE BUSINESS TIMES, 2009a, Underground Science City to Take Shape, The Business Times, 22/7/2009.
THE BUSINESS TIMES, 2009b, 17 April 2009. Hyundai Clinches $890m Jurong Rock Cavern Contract, The Business Times, 17/4/2009. THE BUSINESS TIMES, 2009c, Warehousing, Too, May Go Underground – Study to Examine Feasibility of Centre at Tanjong Kling, The Business Times, 3/11/2009.
THE STRAITS TIMES, 1999, Mindef goes underground, The Straits Times, 12/8/1999.
THE STRAITS TIMES, 2000, Deep Tunnel Sewage System, The Straits Times, 8/7/2000.
THE STRAITS TIMES, 2008, Singapore’s Ammo Stored Safely – Underground, The Straits Times, 8/3/2008.
TOR, Y.K, ZHU, Q., ZHAO, J, & ZHOU, Y., 2005, A Study of a Prototype 3D Geological Information System for Rock Engineering and Underground Infrastructure Planning, Proceedings of Underground Singapore 2005. Tunneling and Underground Construction Society Singapore.
WALLACE, J.C., HO, C.E. & BERGH-CHRISTENSEN, J. 1993, Geotechnical Feasibility of Rock Cavern Construction in the Bukit Timah Granite of Singapore, Proceedings of Rock Caverns for Underground Space Utilization, Nanyang Technological University, Singapore, p53-66.
WALLACE, J.C., HO, C.E., BERGH-CHRISTENSEN, J., ZHAO, J., ZHOU, Y.X. & CHOA, V., 1995, A Proposed Warehouse-shelter Cavern Scheme in Singapore Granite, Tunnelling and Underground Space Technology, 10(2): p163-167.
WONG, I.H., 1993, Underground Pumped Storage Scheme in the Bukit Timah Granite of Singapore. Proceedings of Rock Caverns for Underground Space Utilisation, Nanyang Technological University, Singapore, p67-74.
ZHAO, J., 1994, Potential use of abandoned quarries for underground space development in Singapore. Proceedings of 7th International IAEG Congress, p4409-4414.
ZHAO, J., 1996a, Construction and Utilization of Rock Caverns in Singapore, Part A: Bedrock Resource of the Bukit Timah Granite, Tunnelling and Underground Space Technology, 11(1): p65-72. ZHAO, J., 1996b, Cost Comparison Between Underground Caverns and Aboveground Facilities in Singapore, Proceedings of North American Tunneling 96, p855-860.
ZHAO, J. & BERGH-CHRISTENSEN, J., 1996, Construction and Utilization of Rock Caverns in Singapore, Part D: Two Proposed Cavern Schemes, Tunnelling and Underground Space Technology, 11(1): p85-91.
ZHAO, J. & BERGH-CHRISTENSEN J., 2000, Two Cavern Schemes in the Jurong Formations of Singapore, Proceedings of the International Conference on Tunnels and Underground Structures, A.A. Balkema, p363-370.
ZHAO, J., BROMS, B. B., ZHOU, Y., & CHOA, V., 1994a, A Study of the Weathering of the Bukit Timah Granite, Part A, Bulletin of the International Association of Engineering Geology, 49: p97-105.
ZHAO, J., BROMS, B. B., ZHOU, Y., & CHOA, V., 1994b, A Study of the Weathering of the Bukit Timah Granite, Part B, Bulletin of the International Association of Engineering Geology,50: p105-111.
ZHAO, J. & CAI, J.G., 2009, Planning and Architectural Considerations of Underground Science City in Singapore, Proceedings of ACUUS2009, p127-132.
ZHAO, J, CAI, J.G. & HEFNY, A.M., 2001, Creation of the Underground Science City in Rock Caverns below the Kent Ridge Park in Singapore, Geotechnical Engineering Monograph 4, Geotechnical Research Center, Nanayang Technological University, Singapore. ZHAO, J, CHEN, C.N. & CAI J.G., 2002, A Hydrogeological Study of the Sembawang Hot Spring in Singapore, Bull. Eng. Geol Env, 61: p59-71. ZHAO, J., CHOA, V., & BROMS, B.B., 1996, Construction and Utilization of Rock Caverns in Singapore, Part B: Development Costs and Utilization. Tunnelling and Underground Space Technology 11(1): p73-79.
ZHAO, J., HUDSON, J.A. & HO, C.E, 1994, Assessing the Rock Mass Quality of the Bukit Timah Granite in Singapore for Cavern Construction, Proceedings of International Symposium on Underground Openings for Public Use, Gjovik, Norway, p263-272.
ZHAO, J. & LEE, K.W., 1996, Construction and Utilization of Rock Caverns in Singapore, Part C: Planning and Site Selection, Tunnelling and Underground Space Technology, 11(1): p81-84.
ZHAO, J., LEE, K.W. & CHOA, V. 1995, Construction and Utilization of Rock Caverns in the Bukit Timah Granite of Singapore, Geotechnical Engineering Monograph 1, Geotechnical Research Centre, Nanayang Technological University, Singapore.
ZHAO, J., LEE, K.W., CHOA, V, LIU, Q. & CAI, J.G., 1999, Underground Cavern Development in the Jurong Formation of Sedimentary Rocks, Geotechnical Engineering Monograph 2, NTU-PWD Geotechnical Research Centre, Nanyang Technological University, Singapore.
ZHAO, J., LIU, Q., LEE, K.W., CHOA, V. & THE, C.I., 1999. Underground cavern development in the Jurong Sedimentary Rock Formation. Tunnelling and Underground Space Technology, 14(4): p449-459.
ZHAO, J., NG, W.L., CAI, J.G., & ZHANG, X.H., 2003, Feasibility of Underground Hydrocarbon Storage Caverns at the Jurong Island, Proceedings of Underground Singapore 2003, p62-71. ZHAO, J., SONG, H.W., BIAN, H.Y., ZHOU, Y., SEAH, C.C. & CAI, J.G., 2003, An Evaluation of Constructing the World’s Larges Cavern for Public Use in Singapore, Proceedings of Underground Singapore 2003, p38-43.
ZHAO, J., ZHOU, Y. & CHOA, V., 1994, Utilization of Rock Caverns in the Bukit Timah Granite for Civil Defense Purposes, Journal of the Institution of Engineers Singapore, 134: p72-76.
ZHAO, J., ZHOU, Y.X., HEFNY, A.M., CAIL, J.G., CHEN, S.G., LI, H.B., LIU, J.F., JAIN, M., FOO, S.T. & SEAH, C.C., 1999, Rock Dynamics Research Related to Cavern Development for Ammunition Storage, Tunnelling and Underground Space Technology, 14(4): p513-526.
ZHAO, J., ZHOU, Y., SUN, J., LOW, B.K. & CHOA, V., 1995, Engineering geology of the Bukit Timah Granite for Underground Cavern Construction, Quarterly Journal of Engineering Geology, 28(2): p153-162.
ZHOU Y., 2001, Engineering Geology and Rock Mass Properties of the Bukit Timah Granite, Proceedings of Underground Singapore 2001, Tunneling and Underground Construction Society Singapore, p308-314.
ZHOU, Y., 2002, Lessons from Planning and Construction of Large Tunnel and Caverns in Hard Rock, Proceedings of the ITA World Tunnel Congress 2002, Sydney.
ZHOU, Y., 2003, Rock Engineering for an Underground Storage Facility in Singapore, Proceedings of Norway-Singapore Workshop on Protection, Oslo, Norway.
ZHOU, Y., CHOW, K.S, ZHAO, J., SONG, H.W., 2004, Construction and In-situ Monitoring of Large-Span Rock Caverns under Favourable Stress Conditions, International Journal of Rock Mechanics and Mining Sciences, 41: p541.
ZHOU, Y., SEAH, C.C., GUAH, E.H., FOO, S.T., WU, Y.K. & ONG, P.F., 2000, Considerations for Ground Vibrations in Underground Blasting, Proceedings of the International Conference on Tunnels and Underground Construction, Singapore, AA Balkema. ZHOU, Y., ZHAO, J., CAI, J.G., & ZHANG, X.H., 2003, Behaviour of Large-span Rock Tunnels and Caverns under Favourable Horizontal Stress Conditions, Proceedings of ISRM 2003 Symposium – Technology Roadmap for Rock Mechanics, South African Institute of Mining and Metallurgy.