There is a pressing need to optimise the supply of land for various uses by sustainable and innovative approaches to support social and economic development. It is the established policy of the government to adopt a multi-pronged approach to expand land resources. One practicable approach is rock cavern development, which is a viable source of long-term land supply. In 2011, the Civil Engineering and Development Department completed a study on “Enhanced Use of Underground Space in Hong Kong”. Amongst other findings, the study has broadly demonstrated that relocation of the existing Sha Tin Sewage Treatment Works (STSTW) to caverns is technically feasible and financially viable.

Releasing of about 28 hectare of land after relocating the existing STSTW to caverns brings multifold benefits to the communities of Sha Tin and Ma On Shan Districts as a whole. On one hand, the environment of the existing STSTW site and its surroundings will be greatly improved. Comparing to the existing open-plant arrangement, the odour management of the proposed relocated STSTW in caverns, with caverns as natural barrier, can be efficiently enhanced so as to minimise the odour impact on the surrounding communities. On the other hand, developing the vacated site for residential and other beneficial uses will benefit the community by meeting the public’s needs.

Existing Sha Tin Sewage Treatment Works

STSTW is the largest secondary sewage treatment works in Hong Kong. It occupies 28 hectares of land and serves a population of about 630,000 in Sha Tin and Ma On Shan Districts, which produce 250,000 cubic metres of sewage per day.

STSTW was commissioned in 1982. To meet the continuous population growth in Sha Tin and Ma On Shan, the STSTW extension works were progressively. The treatment capacity of the STSTW was increased to 340,000 cubic metres per day after the extension works.

There are six sewage pumping stations for transferring the sewage collected via the existing underground sewerage network within the six sewage catchment areas in Sha Tin district to the STSTW for centralised treatment.

At present, the treated effluent from the STSTW is pumped to the effluent export tunnel starting at Nui Po Shan of A Kung Kok, and then transported to Kai Tak River in Wong Tai Sin for discharge. This arrangement can make use of the flushing effect by the treated, clean effluent from the STSTW, and hence improve the water quality and environment of the Kai Tak River.

Proposed Caverns

When conducting the review of the proposed relocation site, the feasibility study consultant has identified a nearby site suitable for cavern development assessment for the existing STSTW, including Nui Po Shan of A Kung Kok, Shek Mun, Ma On Shan, Kau To Shan South and Kau To Shan North.

The feasibility study has confirmed that Nui Po Shan of A Kung Kok is the best site for the relocation of the STSTW. The geology of this area, belonging to hard granite with no obvious weak zones and faults, suits for construction of large caverns most.

In the proximity of the existing STSTW and effluent export tunnel (which transports the treated effluent from the STSTW to Kai Tak River in Wong Tai Sin for discharge), relocating the STSTW to this area will minimize the impact on the upstream sewerage system and the downstream disposal system, thereby minimizing the disturbance to the whole Sha Tin District, reducing the construction and operation costs and shortening the construction period.

Scope of Project

The future cavern complex for the relocated STSTW will be the largest of its type ever built in Hong Kong. It needs to be implemented in stages. Tentatively, the project will be implemented in 5 stages, namely site preparation and access tunnel construction (The Stage 1 Works); main caverns construction; sewage treatment facilities installation; modification and construction of upstream sewerage and pumping stations; and decommission and demolition of existing STSTW.

Site Preparation and Access Tunnel Construction (The Stage 1 Works)

The Stage 1 works has commenced in February 2019. It comprises site preparation works at the main access tunnel portal area, including construction of the relevant retaining structures; construction of about 350 metres of the main access tunnel leading to the proposed cavern complex at Nui Po Shan of about 500 metres of the access road leading to the proposed ventilation shaft and ancillary works.

The contractor adopts Building Information Modeling, 4D Computer Assisted Virtual Environment and Geographic Information Systems that incorporated rich information and data. These technologies integrate rich information and data in 3D models to facilitate monitoring of construction progress on a cloud platform.

Besides, the team adopted innovative construction methodology, Design for Manufacture and Assembly and Building Information Modelling for the erection of temporary vehicular bridge works, which was reduced from six nights to two nights. It not only reduced the noise but also alleviated the traffic burden of A Kung Kok Street.

As of August 2020, construction of Community Liaison Centre and temporary vehicular bridge have been completed, whereas construction of main access tunnel, temporary noise barrier along A Kung Kok Street, the access road adjoining A Kung Kok Shan Road and ventilation shaft are in progress. The access tunnel blasting works at Nui Po Shan was commenced since 4 September 2020.

Drill and Blast Method

The drill and blast method is the most commonly used method for hard rock excavation, both in Hong Kong and overseas. It is very safe and effective. The method has been successfully utilised for the construction of numerous projects like tunnels, shafts, rock caverns, and many underground MTR stations located in rock, including Quarry Bay, Tai Koo, Fortress Hill and North Point.

Blasting works for the caverns will be carried out at significant depths of 80 to 300 metres beneath Nui Po Shan, and will be enclosed by blast doors to ensure safety as well as mitigate noise. The tunnel ventilation system will be provided with built-in filters for dust removal. Blasting analysis indicates that noise and vibration at the nearby residential areas and other development due to the blasting will be minimal, and that the daily activities of the public will not be affected.

During the course of construction, Drainage Services Department, site supervisory staff and contractors will maintain close liaison with the nearby residents and stakeholders. Through stringent control measures, we will ensure effectively that the works will not affect the structural integrity of nearby buildings, and minimise any effects of the works on the neighbourhood.

The project involves the excavation of some 5 million tonnes of rock. To ensure that construction will progress efficiently, a number of works fronts will need to be deployed concurrently. To match this, a temporary magazine will have to be erected in the vicinity of the Project site close to the position of the ventilation shaft.

The magazine and its siting will have to be approved and licensed by the Mines Division of Civil Engineering and Development Department. The contractor will closely scrutinize the storage of explosives and other operations to ensure compliance with statutory requirements, established standards and international best practices. The temporary magazine will only store a small amount of explosives for a few days’ use. It will be removed as soon as blasting works are completed.

Odour Control Measures

General odour control measures include dosing of chemicals at upstream sewage pumping stations to suppress odour generation from sewage; covering up of the source of odour, e.g. inlet works, primary sedimentation tanks and sludge treatment facilities to suppress odour emission; installation of deodourising units to clean up the collected foul air; dosing of chemicals at sludge treatment facilities to suppress odour generation from sludge.

Caverns as natural barriers are highly conducive to effective odour control. All odour sources will be enclosed and de-odourisation facilities will be provided inside the caverns. The location of the cavern ventilation shaft has also been further optimized and shifted to a more remote spot.

Through the multi-barrier control measures, detailed EIA analysis indicates that the arrangement will not cause any perceivable odour. The maximum predicted odour level at any of the nearby residential areas / development (Chevalier Garden, Kam Tai Court, Mui Tsz Lam Village, A Kung Kok Fishermen Village, Tai Shui Hang Village, Shing Mun Springs, the Castello, Sha Tin Hospital, etc) will be less than one Odour Unit. This is far lower than the five Odour Unit limit stipulated in the Environmental Impact Assessment Ordinance.

The outlet of the ventilation shaft will be inconspicuous and landscaped to blend in with the surroundings of Nui Po Shan.

Conclusion

Cavern construction is an established technology. A number of cavern schemes for various uses have been successfully adopted around the world. In fact, there are successful local examples of accommodating facilities in rock caverns, including the Stanley Sewage Treatment Works completed in 1995, as well as Island West Refuse Transfer Station and Kau Shat Wan Explosives Depot both completed in 1997. Also, in 2009, the University of Hong Kong reprovisioned the Western Salt-water Service Reservoirs in rock caverns to release the site for its Centennial Campus development.

It is anticipated that the construction period of the project would last for more than 10 years. The Stage 1 works are scheduled to be completed by the end of 2022, whereas tender invitation of Stage 2 works is being prepared.

References:
Drainage Services Department
Official website of “Relocation of Sha Tin Sewage Treatment Works to Caverns”