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Regarding media enquiries on the changes of some of the seawall structure of the Hong Kong-Zhuhai-Macao Bridge (HZMB) Hong Kong Boundary Crossing Facilities (HKBCF) project from steel cellular seawalls to rubble mound seawalls, the Highways Department (HyD) today (September 25) responded as follows:
The HyD had applied, based on the whole 6.1 km long seawall to be constructed by traditional fully-dredged method, and was granted an Environmental Permit (EP) for HZMB HKBCF project by the Environmental Protection Department (EPD) in 2009. The HyD has along attached importance to environmental protection and required the consultant company to conduct research on the feasibility of adopting "non-dredged" method for construction of seawalls, even the EP was granted, in order to further minimise the environmental impact of the HZMB project.
The HyD decided to introduce the "non-dredged" method to construct the seawalls of the HKBCF artificial island, which is the first of its kind in Hong Kong, after the consultant confirmed the feasibility of using this method for constructing seawalls. Before carrying out the construction works for the HKBCF artificial island, the HyD submitted an application to the EPD in 2010 for the variation of the EP with a view to changing the traditional fully-dredged method to "non-dredged" method for further minimising the environmental impact of the HKBCF project. After the amendment, the seawalls of HKBCF artificial island were to be formed by a 5.1 km long steel cellular seawalls and a 1 km long rubble mound seawalls. While no dredging of marine mud layer underneath the seawalls is involved in the construction of these two kinds of seawalls, both involve stone columns to be built and rubble mound to be placed on the surface. The reclamation works started in end 2011.
Comparing the traditional method of construction of seawalls in which the dredging of the marine deposit is needed, the "non-dredged" method does not involve the dredging of sea bed, disposal of marine deposit and backfill at the base of the seawalls. Adopting the "non-dredged" method will avoid the dredging and disposal of about 22 million cubic metres of marine deposit. Meanwhile, the amount of backfilling materials to be used will be reduced approximately by half, the construction traffic in sea can be reduced approximately by half during construction period, and the suspended solids in the sea water can be reduced by approximately 70%. By and large, the "non-dredged" method for the construction of seawalls will not only minimise the impact of the works to the environment, but also corresponds to the principles of sustainable development.
Comparing the two "non-dredge" methods, the works procedures for construction of steel cellular seawalls are more complicated with more large equipment involved when compared with that for the construction of rubble mound seawalls. Due to the height restriction in the vicinity of the airport, large equipment (e.g. large lifting appliances) used in constructing steel cellular seawalls could not access to some of the seawall locations which were closer to the airport on the HKBCF artificial island until the height restriction is lifted during the closure of south/north runway. Therefore the construction schedule for part of the steel cellular seawalls had to tie in with the timetable of the south/north runway closures at the airport. The timetable of runway closures had been changed since the reclamation works started. The construction of steel cellular seawalls was therefore further constrained. In order to minimise the impact on the progress of seawall construction caused by the above factors, the contractor suggested amendments on works details to change part of the steel cellular seawalls in the western and southern sides of the artificial island to non-dredged rubble mound seawalls in 2013. The consultant scrutinised the suggestions according to the contract provisions. After taking into account the fact that the construction time would not be extended, the construction cost would not be increased and it was also an environmental friendly "non-dredged" construction method, the HyD agreed to accept the contractorˇ¦s proposal according to the contract. After the amendments, the 5.1 km long steel cellular seawalls and the 1km long rubble mound seawalls were changed to 3.6 km long and 2.5 km long respectively.
The HyD had consulted the EPD on the aforementioned amendments on works details for using non-dredged method to construct the seawalls. In view that both construction methods belong to non-dredged method which involves less impact on the environment, the EPD considered that the concerned amendments on works details involved no change to the environmental impact assessment (EIA) report and no variation to the EP would be required. Therefore, the EPD would only be required to be notified about the concerned amendments according to the EP conditions.
Since the surcharge period started in 2014, various amounts of movement have been recorded on the HKBCF artificial island. In designing the non-dredged reclamation works, the consultant had considered and anticipated that some settlement and lateral movement of the reclaimed land would occur during the construction stage, and lateral movement on the top of the steel cells was also anticipated during construction. According to records, the movement of the reclaimed land during construction was generally normal and the steel cells moved only laterally at their top, but not at their bottom. Of the steel cells, larger movement of up to about 6 to 7 metres was observed at two adjoining steel cells. The contractor had already carried out remedial measures at his own cost to strengthen the marine mud around these moved steel cells to ensure that both the reclaimed land and the seawalls were structurally safe, and the function of the seawalls in compliance with the design requirements.
The HyD will endeavor to overcome the challenges and difficulties encountered for completing the works at the earliest.
Ends/Friday, September 25, 2015
Issued at HKT 23:25
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