Tung Chung New Town Extension – Tung Chung East Reclamation

By the Civil Engineering and Development Department and AECOM with the coordination of the HKIE Civil Division

The Civil Engineering and Development Department (CEDD) of the HKSAR Government is putting forward the Tung Chung New Town Extension (TCNTE) project to further develop Tung Chung into a comprehensively planned new town to meet local housing, social, economic, environmental and local needs. The extension covers the eastern and western flanks of the existing Tung Chung New Town. The overall area exceeds 200 hectares which includes 130 hectares of reclamation in Tung Chung East (TCE) (Figure 1).

Figure 1: Tung Chung New Town Extension

The TCNTE project will provide about 62,100 housing units development (49,500 and 12,600 new units in TCE and Tung Chung West (TCW) respectively) upon full development for a population of about 184,000. In terms of economic development, TCE will form a commercial hub, providing a total floor area of approximately 827,000 sq m for office and retail uses. Together with some 50,000 sq m of retail floor area in TCW, a total of about 40,000 job opportunities can be created.

To serve local residents and the general public, the open space in TCE comprises mainly Waterfront Promenade, Central Green and Linear Park. The 5-km-long Waterfront Promenade, which extends from the west all along the coast to the east of the newly reclaimed TCE, will integrate with continuous footpath and cycle track networks. At the core part of TCE, where it is characterised by mixed developments of both housing and commercial activities, the Central Green and Linear Parks will provide pedestrian and visual corridors to connect the activity nodes at TCE with the planned TCE Station, the waterfront and the existing Tung Chung Town Centre. In particular, the Central Green will serve as the major focus and activity node of the TCE area.

Comprehensive networks of cycle tracks and pedestrian walkways are planned to promote convenient cycle and pedestrian movements within the TCE area. These networks will be provided along the Waterfront Promenade and the Linear Park system and are intended to connect with the existing town centre area (Figure 2).

Figure 2: Tung Chung Extension Area - Pedestrian and cycling network concept

TCNTE was tasked in the Chief Executive’s Policy Address in 2018 as a pilot trial for the adoption of city concepts that are smart, green and resiliently environmental. The on-going TCE reclamation entails a number of initiatives that showcase sustainable reclamation in Hong Kong.

History of reclamation in Hong Kong

In the last century, there was a rapid and continuous growth in population in Hong Kong, from less than 2 million in the 1950s to over 7.4 million in 2020. The stable growth of population at the same time drove a substantial economic growth and transformed Hong Kong from a re-export port with GDP per capita of less than HK$2,400 in 1961 to an international metropolitan with GDP per capita of over HK$340,000 in 2020. The unremitting growth of economy in Hong Kong would not have been possible without the provision of sufficient land to meet the development needs. Out of the numerous land supply options, land reclamation has been the most tenable supply in the interest of the public. Today, around 27% of Hong Kong people live on reclaimed land formed in the past decades.

In the 20th century, there were different land reclamation projects, for example Kai Tak Airport Extension (1957 - 1974) and new town developments. Formation of land by reclamation for new town development in Hong Kong was initiated in the 1970s, including Tsuen Wan, Shatin, Tai Po, Ma On Shan, Tseung Kwan O and Tung Chung etc. (1973 - 2003). Other notable reclamation projects include the Disneyland development at Penny’s Bay, Hong Kong International Airport at Chek Lap Kok, and artificial island for Hong Kong Port of Hong Kong-Zhuhai-Macao Bridge. Currently, reclamations for the Three Runway System (3RS) at the Hong Kong International Airport (commenced in 2016), the TCNTE and Integrated Waste Management Facilities (IWMF) (both commenced in 2017) projects, with a total reclaimed area of about 796 ha, are in progress.

The reclamation works in TCE commenced in December 2017, seizing a record of less than 6 years from planning to commencement of works for projects of similar scale.

Sustainable reclamation in Tung Chung East

Concerns over impact of land reclamation on its surrounding environment and ecology are not uncommon. However, with proper investigation, planning and mitigation measures, it is possible to work out some environmentally friendly solutions to strike a balance between development and conservation. Over the past few decades, reclamation methods and ground treatment techniques have advanced considerably to meet the technical requirements and social acceptance. In the TCE reclamation project, some sustainable measures, which include the adoption of non-dredged methods for the reclamation, in particular a new technique called the deep cement mixing (DCM) method; eco-shorelines along the proposed seawalls to enhance biodiversity; and the use of construction and demolition (C&D) materials as the major source for reclamation filling material, have been adopted.

“With the use of DCM technology, about 7 hectares of land was formed and delivered for public housing development in just 27 months.”

Non-dredged deep cement mixing method

Most of Hong Kong marine environment is covered by very soft to soft marine sediment of silty to clayey materials. The soft marine sediment is highly compressible and exhibits a large and time-dependent settlement under the weight of reclamation fill. The conventional drained method using prefabricated vertical drains (PVDs) and surcharging has been applied for years but this method takes time to complete. In general, a lengthy surcharging period of up to a year has to be allowed for consolidation, and additional fill material has to be replenished as a result of settlement. In addition, the formation and subsequent removal of surcharge mounds around the site involve intensive earthworks that render high carbon emission and a relatively long construction period.

Considering the imminent need of land supply, TCE reclamation is the first public works project bringing in the DCM method to speed up the reclamation works. The reinforced sediment becomes a composite and the DCM treated soil act like pillars to sustain the majority of the loading to control settlement. DCM method solidifies the ground quickly and can advance the completion of reclamation by about 6 months as for the case in TCE when compared with the conventional drained method. The works contract commenced in end 2017, and is scheduled for general completion by 2023. With the use of DCM technology, the first parcel of land (about 7 hectares) was formed and delivered for public housing development in just 27 months since the commencement of the works contract. As of Q4-2020, four sites with a total area of around 14 hectares (hatched areas in Figure 3) were handed over for housing development. By Q3-2021, over 80% of reclamation filling has been completed and is exposed above the sea level. Apart from programme benefit, the demand for fill material for replenishing settlement can be reduced. In the TCE reclamation, up to around 6 million tonnes of fill material was saved. This not only eased the demand of fill material but also reduced 3,000 vessel trips passing through the north Lantau water channel near Brothers Marine Park. The reduction in vessel trips reduced the noise and air impacts and minimised the disturbances to marine habitats. The reclamation works have been proceeding at a very promising pace with comparatively less impact on the environment. All these attribute to the adaptation of DCM method in the reclamation works.

Figure 3: Aerial photo of Tung Chung East Reclamation (Q3-2021)

Adaptation of deep cement mixing method and overcoming challenges during construction in Tung Chung East Reclamation

Choice of binder for sustainability

Research and development of the DCM method were commenced in the late 1960s, and design manuals have been established in the Mainland, Japan and the U.S. [Ref. 1 to 7]. Since the design standards and specifications for DCM works have yet to be developed in Hong Kong, the project team made reference to the Japanese and American codes with some necessary adjustments to suit the local environment. The choice of suitable binder also augments the sustainability. Ordinary Portland Cement (OPC) is commonly adopted as the binder for modern DCM works but it has an intrinsic drawback for its high carbon emission upon production. As an alternative, the project team has applied Portland Blast Furnace Cement (PBFC) as the binder for DCM works in the TCE reclamation. PBFC is a mixture of Ground Granulated Blast Furnace Slag (GGBS) and OPC at a ratio of 60% : 40%, which is equivalent to Type B Blast Furnace Cement in Japan for DCM works. GGBS is a byproduct of steel industry. Not only is it environmentally friendly for beneficial reuse of this by-product, it also improves overall durability in upholding end-product resistance to alkali-silica, sulphate and chloride reactions. Moreover, a higher strength can be achieved with PBFC and this can reduce the binder consumption by 20% on average in this project. The reduction in greenhouse gas emission, in terms of carbon-dioxide (CO₂), is remarkable. The carbon emission during production for GGBS is only 6 - 7% of that for OPC [Ref. 8]. As such, using PBFC as the binder for DCM works in this project can reduce around 600,000 tonne CO₂ equivalent greenhouse gas emission.

Optimisation of DCM design

With a view to harnessing this novel technology and achieving cost effectiveness in the ground treatment design, the project team has taken the initiative to conduct a detailed study in collaboration with The Hong Kong University of Science and Technology (HKUST) and the Geotechnical Engineering Office (GEO) of the CEDD through a series of laboratory testing, physical centrifuge model tests and 3-dimensional numerical modelling. The study has considered various configurations of DCM columns to optimise different design scenarios. Rigorous finite element analysis coupled with centrifuge modelling have enlightened the project team the ground response and behaviour under a sustained loading, so that a more rational design can be formulated. The collaboration with academy has reinforced the design approach and paved the way for a more cost-effective DCM design.

Overcoming site constraints and airport height restrictions

Construction of robust DCM foundation for the 4.9 km-long seawall is critical to the reclamation, and marine-based DCM method has been adopted. Typical DCM barge is equipped with three sets of mixing shafts (Figure 4), and multi-levels length mixing blades (Figure 5). However, the site is geographically bounded by Tuen Mun-Cheuk Lap Kok Link (TM-CLKL) southern approach viaducts to the east and this renders a big challenge for the DCM works in this area (Figure 6).

Figure 4: Three sets of mixing shafts on DCM barge

Figure 5: Mixing blades configuration

Figure 6: Construction challenges

A portion of the site is situated underneath the TM-CLKL viaducts and the maximum allowable headroom is around 17 m measured from mean sea level, making it impossible for typical DCM barges to be mobilised in this area. To tackle this headroom constraint, the project team has developed a special type of “low headroom” DCM barge with telescopic type design adopted for mixing shafts (Figure 7).

Figure 7: Low headroom DCM barge

With this innovative telescopic mixing shaft, the overall height of DCM barge is reduced to less than 15 m from mean sea level and the barge can pass underneath the viaduct safely. During DCM operation, the mixing shaft can be extended downward by around 13.8 m (Figure 8) and this low headroom DCM barge is able to install DCM columns down to a depth of 22 m below mean sea level. This innovative design is the first of its kind in the world deployed for large scale marine DCM works and all DCM columns have been successfully installed on schedule.

Figure 8 – Extension of DCM mixing shafts

To aid the supervision, all the barges have been equipped with IoT sensors including Highest Altitude Monitoring Unit (HAMU) and Global Navigation Satellite System (GNSS). With wireless transmission to the BIM-based Intelligent Management Platform, a “Geofence” has been established and real-time alert would be given to all responsible supervisory personnel and concerned operators in case any barge is identified to be close to the bridge pier structure. (Figure 9).

Figure 9: Real-time monitoring on Intelligent Management Platform using Digital Twin Technology

Innovative application of deep cement mixing method

DCM method is very effective and can be applied in different ways. The project team has not just applied this in reclamation, but also developed a novel retaining wall system. During land formation, the existing outfalls are extended to the newly constructed seawall. One of the drains is a 4-cell box culvert, but the alignment of this box culvert is surrounded by four housing sites and the working area is a narrow isle. The excavation depth is about 6 to 7 m. If conventional ELS system using sheet piles, closely spaced king posts and struts, is adopted, the working environment would become very congested. Such congested environment could bring about problems such as site safety, difficult RC structure construction, potential interface issues with the adjacent sites, etc. which may delay project delivery. In view of this, the project team hatched a novel ELS scheme to remove these obstacles.

DCM columns can be installed individually or continuously to form a wall-type or even a block-type structure. In applying this flexibility, the project team has designed and formed a big U-shaped retaining structure with DCM columns (Figure 10). The side walls are basically mass gravity retaining wall which can retain the adjacent ground safely so that struts are no longer required. As the DCM columns have been installed continuously, the excavation is nearly water-tight and there is no need for dewatering.

Figure 10: DCM wall for box culvert construction

With this novel design scheme, the project has saved both ost and time. As it is no longer required to erect struts or king posts, the working environment is much better, which helps boost productivity significantly. Furthermore, the revised design has avoided erection and lifting of heavy steelworks, and there is no doubt that this method is much safer as high-risk working activities/procedures are avoided. Even though DCM method has been used, the overall embodied carbon emission can be reduced by about 6,000 tonnes as compared with conventional structural steelwork. This example has showcased how the project team has brought in new technology, further adapted the method and developed a novel application to resolve a complicated engineering problem.

Development of design standards and specifications

The project team of TCE reclamation works has fully assimilated and adapted the DCM method in many aspects, and is determined to pursue the development of local practice on DCM application. One example is related to laboratory testing which is an important step for DCM works as the test results provide essential indicators of the achievable strength, in terms of Uniaxial Compression Test (UCS). Laboratory mixing tests in TCE reclamation were initially carried out based on the test procedures as given in “Practice for Making and Curing Stabilized Soil Specimens without Compaction [Ref. 9]”. Noting the differences between Hong Kong sediment and that in Japan, the project team has worked with GEO to bring in specially-made compaction table for preparing laboratory mixing specimen. With the promising results obtained, the project team has modified the testing procedures, which would be a key update to the local laboratory testing guidance for DCM works. This is a yardstick for the development of the local standard of DCM method; with the collocation of more construction data, it is expected that the good practice in TCE reclamation can be further developed into local design manual and specifications.

Eco-shorelines along the proposed seawalls

Along the TCE new reclamation area, eco-shorelines – the first of their kind in Hong Kong – are being adopted to enhance seabed biodiversity by providing a suitable habitat for marine species through mimicking the physical conditions of natural inter-tidal zones as far as practicable. The eco-shorelines comprise three different types, namely rocky, mangrove and vertical (Figure 11). Rocky eco-shorelines will be provided at locations relatively susceptible to wave actions or with insufficient sunlight, where bio-blocks with varying levels and sizes of cavities will be placed at inter-tidal zones to retain seawater during low-tide conditions, with a view to providing appropriate habitats and shelters for marine species. Mangrove eco-shorelines will be provided at inter-tidal zones along seashores less susceptible to sea waves. As regards the vertical eco-shorelines, they will feature pots, cavities, eco-tiles and similar items to provide uneven surfaces for the easy attachment and growth of tiny marine organisms.

Figure 11: Three different types of eco-shorelines in Tung Chung East

The use of construction and demolition materials

With a view to stepping up the sustainability, TCE reclamation has been maximising the reuse of public fill comprising rocks, concrete, asphalt, rubbles, bricks, stones and earth, as well as mechanical sand as the reclamation fill materials. Through proper sorting, the “waste” generated from construction and demolition projects can be transformed into useful reclamation fill materials, and public fill makes up over 70% of the reclamation fill materials. The remaining is mechanical sand which is, in fact, a by-product of quarrying and it has been a very effective replacement of marine sand for serving as sand blanket. The beneficial reuse of public fill and mechanical sand can satisfy the demand of fill materials in TCE reclamation on one hand and rule out the demand for marine sand which would minimise the environmental impact on the other.

"The new DCM technology is a sustainable reclamation method that expedites land formation without compromising the marine ecology."

Closing remarks

To meet population growth and economic development, the Government has put in enormous efforts into land supply over the past half-century. To expedite land formation, a sustainable reclamation method – the new DCM technology is proven to be effective in strengthening soft marine sediment in-situ without compromising the marine ecology. The use of PBFC as binder has taken a further step for beneficial reuse of industrial by-product and suppressing carbon emission. In deploying the DCM technique, the project team has carried out detailed analysis and optimised the design having due regard to Hong Kong conditions with a view to striving for engineering excellence and sustainability. The project team has assimilated and adopted the DCM method, and has extended its application to serve as an excavation and lateral support system. The application of DCM method as temporary earth retaining structure for box culvert construction has been a novel design in Hong Kong and has offered significant programme benefit and enhance construction safety. Besides, various environmental enhancement initiatives including the adoption of eco-shorelines along the seawall to enhance biodiversity and the use of construction and demolition materials i.e. the reuse of public fill and mechanical sand as the reclamation fill materials have further enhanced the sustainability of the TCE reclamation project. The experience gained in this project will surely be of value in developing sustainable solutions in future reclamation projects.

Acknowledgement

The authors would like to thank the Sustainable Lantau Office of the CEDD, consultants AECOM, contractor Build King-SCT JV and all other stakeholders for their supports. Partnering and mutual trust form a strong foundation to the success of the TCE project.

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