The EIA(DE) Report had identified a range of measures to mitigate potential impacts on cultural heritage resources within the Study Area, as a result of construction and operation activities. The assessment has identified eight historical buildings or features and one known archaeological site (Shui Lau Tin) within the Study Area.
Mitigation measures required to ameliorate the potential impacts to acceptable levels have been recommended, as shown below:
12.1.1 Archaeological investigation
After land resumption but before the construction work takes place for the proposed Shui Tsan Tin pumping station (P4), Ng Ka Tsuen pumping station (P5) and Pang Ka Tsuen pumping station (P6) commence, a period of 6 months for each of the sites should be allowed for an archaeological investigation, the method to be agreed with the AMO. It is recommended that the survey be divided into two phases. The first phase is to undertake 6 test pits with dimensions 1 m x 1.5 m (2 test pits for each site) to assess if archaeological deposits exist and their possible extent. If no archaeological interest is identified, no further action is required. If deposit of archaeological interest is found, a rescue excavation should be undertaken if the deposits are found to be common and not significant, so that preservation by record is achieved. However, if a significant archaeological deposit is identified, the design of the P4, P5 and P6 should be reviewed to integrate the preservation in situ of the archaeological materials or, if feasible, an alternative site for P4, P5 and P6 should be sought. Rescue excavation of highly significant archaeological deposits should be regarded as the last resort. AMO should be consulted on the result of the test pit investigation and any proposed excavation or presentation mitigation measures. The archaeological investigation would be laid down as a condition in the Environment Permit.
It is recommended that any archaeological work (i.e. watching brief and site investigations) undertaken by the Contractor should be conducted by a qualified archaeologist, who is licensed by the Antiquities Authority under the Antiquities and Monuments Ordinance (Cap. 53). The Contractor should inform AMO at least four weeks prior to the commencement of the archaeological evaluation, of the expected start date and programming of the works. This is to allow AMO the opportunity of arranging on-site monitoring of the works.
The licensing procedure is a statutory requirement stated in Sections 12 and 13 in the Antiquities and Monuments Ordinance (Cap.53).
As no direct impact on the identified historical buildings and features is expected, therefore, no mitigation is necessary.
12.1.2 Vibration Impact and Monitoring
Vibration Testing and Monitoring
Based on the proposed construction works, potential vibration impacts are envisaged at Site E, Ng Ka Tsuen historic compound. Site activities comprising of bored piling and vibration compaction within 7.5 m and vibropiling within 25m of the Site E will likely exceed the 4mm/s ppv limit. It is therefore recommended to set back the pumping station so that these separation distances from the Ng Ka Tsuen historic compound can be achieved.
For construction phase, the Contractor will be required to
avoid encroachment of the safe margin of 7.5 m from the Ng Ka Tsuen historic
compound and pay particular attention to remove underlying obstructions such as
buried foundations, floors etc before continuing with bored piling and sheet
piling operations. All sheet
piling operations must be set back 25 m from the Ng Ka Tsuen historic
compound. If demolition within 7.5
m of the Ng Ka Tsuen historic compound is required, preference should be given
to hand methods and use of mini excavator (Bobcat type plant) to remove
materials.
Careful selection of
pile driving equipment by the Contractor to meet the criteria is
recommended. For sheet piling, a
resonant free vibrator is recommended. It is a type of pile driver designed so
that the rotating system of weights within the driver can be accelerated to
operating speed with the rotating masses in balance and, once at operating
frequency and above resonant mode, the weights are rearranged to be out of
phase and hence generated the vibration required to drive the pile.
It is also recommended
that a trial of sheet piling using the proposed plant be undertaken by the
Contractor and the resulting vibration monitored by the Contractor and Engineer
prior to proceeding with works. The monitoring should be taken at the closest
accessible point to the historic building
to enable the Engineer to assess the potential risk from the expected
levels of vibration at Ng Ka Tseun.
Structural Survey
In addition,
and prior to the commencement of site operations it is recommended that a
structural survey of Ng Ka Tsuen historical compound (Site E) be undertaken by
specialist surveyor approved by AMO to record the condition of the historic
buildings. The survey should be
conducted four weeks prior to the commencement of the construction works. This survey shall focus on the quality
of construction and finishes and the general condition of the structure and its
finishes. A schedule of defects (including cracks) is to be prepared for
submission to the Engineer and AMO and shall be supported by photographic
record. Upon approval to proceed with the works, vibration monitoring is
recommended during the site preparation, earthworks and foundation and
reinstatement stages.
Upon completion
of the structural survey, it is recommended that vibration monitoring be
carried out,
firstly on a trial basis to ascertain locations of highest impact within the
building relative to an appropriate ground level location on the worksite
boundary; the setting of these levels shall be subject to AMO approval and
accessibility. Thereafter
monitoring shall be undertaken for the duration of the works and, where
exceedance of the limit is noted, the works shall be stopped and the Contractor
shall be required to prepare a plan for the approval of the Engineer and AMO to
ensure that the works will continue without exceedance of the vibration
criteria. If at any time the
Engineer should note any damage to the decoration and structure of Ng Ka Tsuen
historical compound, the Contractor shall stop works and prepare a method
statement outlining provisions to ensure no further deterioration of the
building. Such measures may
comprise alternative selection of plant, method of construction, use of a cut
off trench located close to the source of the vibration.