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Technical visit to the DSD Stanley Sewage Treatment Works in caverns
By Mr Timothy LAM

The captioned visit was organised by the CV Division on 23 July 2016. A total of 15 participants attended. The Stanley Sewage Treatment Works (STW) was commissioned in 1995, and was the first sewage treatment works in Hong Kong built in caverns. It is a secondary STW with a daily capacity of 11,600 cu m.

In addition to seeing the site, the participants also learned how the STW integrated with the surrounding community and environment, and the benefits of locating a STW inside a cavern.

Mr Brian Chan and Ms Angela Tong, engineers of the Drainage Services Department (DSD), gave a presentation on the background of the Stanley STW as well as the latest information on the relocation of the Shatin STW (STSTW) to caverns. This gave participants an overview of the benefits of the relocation project, the potential impact and the corresponding solutions.

A physical model and an animation of the relocated STSTW were shown to the participants. The proposed Shatin cavern site was carefully selected, with due consideration to various aspects including environmental impact, logistics, constructability and sustainability. Upon completion, only the top of the ventilation shaft will be visible, while the surrounding rural area will be green.

During the visit, Ms Vicky Yuen, representative of the DSD, explained the treatment process including the screening and de-gritting, biological treatment, final sedimentation, disinfection and sludge dewatering. In particular, she introduced the compact technologies adopted in this plant, namely the integrated fixed-film activated sludge process in the bioreactors and the lamella settlers in the final sedimentation tanks. The ventilation system, the air monitoring system and the vertical greening system inside the caverns were also demonstrated.

We would like to thank the DSD for their support and effort in making this visit a success.

Ir William Luk (left 2nd) presenting souvenir to the representatives of the Drainage Services Department

Technical visit to the MTR Shek O immersed tube tunnel casting yard
By Ms Cynthia SHUM

A technical visit to the MTR Shek O immersed tube tunnel (IMT) casting yard was organised by the CV Division on 17 September 2016. Around 20 members participated.

Representatives from the MTR Corporation Ltd (MTRC) introduced the different structural types of IMT and the history of IMT in Hong Kong. Shek O was chosen to be the casting yard because it has been serving as an IMT casting basin for many IMT projects like the Lantau Airport Railway and it was far from main public areas. The site formation of the yard includes temporary dock gates for pumping seawater away, a barging point for transporting construction materials and a concrete barging plant for the pre-cast IMT units. The construction method for the IMT was explained in detail as mainly comprising trench dredging in Victoria Harbour, pre-fabrication of the IMT at the casting yard, towing of IMT units to the location and finally backfilling.

After the presentation, the MTRC representatives led the participants to a site walk. Questions regarding the construction methods, logistic arrangements and environmental issues were raised by the participants. One of the challenges discussed was the casting of the steel frames into the ends of the tunnel units, closing tolerances and locating the tunnel immersion joint seals before putting them into the sea as it would be difficult for the tunnel units to reach a certain depth due to their buoyancy.

The MTRC representatives explained that large ballast tanks would be constructed within the elements. Once the casting basin was flooded and the tunnel units were towed by floating barges and ships to the designated pre-dredged trench, valves in the end bulkheads of the units would be opened to allow the ballast tanks to fill with water until stability could eventually be reached.

During the visit, the background of contract no SCL1121 and the basic concepts of IMT construction were also introduced which aroused participants' interest. On behalf of the CV Division, we would like to convey our sincere gratitude to the representatives from MTRC.

Participants in front of one of the immersed tube tunnel units

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