12                                        Heritage Resources

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.

 

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