10.1.1.1 The landfill gas hazard assessment undertaken in the EIA identified the hazards that are likely to be generated from the Pillar Point Valley (PPV) Landfill, during the construction and operation phases of this Project and evaluate the associated risk. The EIA Report recommended that some precautionary measures are required to protect the proposed Tuen Mun – Chek Lap Kok Link (TMCLKL) toll plaza from the landfill gas risk due to the PPV Landfill. Use of “semi active” or “enhanced passive” gas control protection system referenced from the EPD’s Landfill Gas Hazard Assessment Guidance Note were recommended. Regular monitoring during construction and operation phases was also recommended.
10.2 Monitoring and Measurement of Landfill Gas
10.2.1.1 During construction, a Safety Officer should be appointed to carry out the monitoring works. The monitoring frequency and areas to be monitored should be set down prior to commencement of ground-works either by the Safety Officer or an approved and appropriated qualified person. The routine monitoring should be carried out in all excavations, manholes, chambers, relocation of monitoring wells and any other confined spaces that may have been created. All measurements in excavations should be made with the extended monitoring tube located not more than 10 mm from the exposed ground surface. Monitoring should be performed properly to make sure that the area is free of landfill gas before any man enters in the area.
10.2.1.2 For excavations deeper than 1m, measurements should be carried out:
· at the ground surface before excavation commences;
· immediately before any worker enters the excavation;
· at the beginning of each working day for the entire period the excavation remains open; and
· periodically through the working day whilst workers are in the excavation.
10.2.1.3 For excavations between 300mm and 1m deep, measurements should be carried out:
· directly after the excavation has been completed; and
· periodically whilst the excavation remains open
10.2.1.4 For excavations less than 300mm deep, monitoring may be omitted, at the discretion of the Safety Officer or other appropriately qualified person.
10.2.1.5 Depending on the results of the measurements, actions required will vary and should be set down by the Safety Officer or other appropriately qualified person. As a minimum these should encompass those actions specified as follow:
Table 10.1 Actions in the Event of Landfill Gas being Detected in Excavation / Confined Area
|
Parameter |
Measurement |
Action |
|
Oxygen |
< 19% |
- Ventilate to restore oxygen to > 19% |
|
|
< 18% |
- Stop work - Evacuate personnel / prohibit entry - Increase ventilation to restore to > 19% |
|
Methane |
> 10% LEL (> 0.5% v/v) |
- Prohibit hot work - Ventilate to restore methane to < 10% LEL |
|
|
> 20% LEL (>1% v/v) |
- Stop work - Evacuate personnel / prohibit entry - Increase ventilation to restore to < 10% |
|
Carbon Dioxide |
> 0.5% |
- Ventilate to restore oxygen to < 0.5% |
|
|
> 1.5% |
- Stop work - Evacuate personnel / prohibit entry - Increase ventilation to restore to < 0.5% |
10.2.1.6 During operation, regular monitoring of landfill gas should be done at the tunnel, subway, and any other underground structures within the landfill consultation zone. Monitoring is required to verify the effectiveness and to ensure the continued performance of the implemented protection measures.
10.2.1.7 Inspection and LFG monitoring should be carried out at buildings and enclosures (eg. toll control building, toll booths, tunnel, subway, service manholes, etc) prior to the operation to ensure the design measures and functioning properly. The monitoring should be continued through the operation of the Project. In particular for the first year of operation, monthly monitoring is recommended, and quarterly (or at a frequency agreed by EPD) for second year on. Should the monitoring reveal the presence of landfill gas within the tunnel, subway, or other confined area, the seal of the joints shall be inspected and consideration shall be given to seal the cracks. Action level can refer to Table 10.1, and should abnormality is observed, it should be reported to EPD and the PPV Landfill operator.
10.2.1.8 In addition, if any construction is required for the maintenance work during operation stage, the responsible party should follow the protective measures and monitoring works as recommended in Clause 10.2.1.1~10.2.1.5 of this report.
10.2.1.9 The monitoring programme and detailed actions should be included in the detailed assessment (to be carried out in the designed design stage) and submitted to EPD for approval.