11.1
This section describes the landfill gas monitoring
requirements at the Project tunnels within the Ngau Tam Mei Landfill (NTML)
Consultation Zone during the construction and operation phases.
11.2
The precautionary measures to minimise landfill gas risk are
summarised in the Implementation Schedule.
Monitoring Parameters, Locations and
Frequency
11.3
Monitoring of landfill gas shall be conducted during the
construction of the tunnels and after the completion of tunnel construction
works. Before the commencement of the
operation, the monitoring data gathered should be reviewed with EPD and to
determine and confirm the need for monitoring and if required the monitoring requirements
during the operation of the Project.
11.4
The proposed parameters, locations and frequency of landfill
gas monitoring for various phases of Project are summarised in Table 11.1.
Table 11.1 Parameters,
Locations and Frequency of Landfill Gas Monitoring
|
Phase |
Monitoring Location |
Monitoring Frequency |
Monitoring Parameters
(% v/v or %LEL) |
|
Construction |
Tunnels
within the NTML Consultation Zone (1) |
At
least daily |
· Methane · Carbon Dioxide · Oxygen |
|
After completion of
tunnel construction work and at least 3 months before commencement of
operation |
Walkover
survey at all joints and cracks, if identified, within the tunnels within the
NTML Consultation Zone (2) |
Once |
· Methane · Carbon Dioxide · Oxygen |
|
|
Walkover
survey at 20m intervals for section of tunnels under NTML and 50m interval
within the NTML Consultation Zone (2) |
Weekly |
· Methane · Carbon Dioxide · Oxygen |
|
Operation |
Walkover
survey at all joints and cracks, if identified, within the tunnels within the
NTML Consultation Zone (3) |
Annually |
· Methane · Carbon Dioxide · Oxygen |
|
|
Walkover
survey at 20m intervals for the section of tunnels under NTML and 50m
intervals within the NTML Consultation Zone (3) (4) |
Weekly
(4) |
· Methane · Carbon Dioxide · Oxygen |
Notes:
(1)
The monitoring shall be conducted before starting the work of the day
(2)
The walkover survey shall be conducted under non-ventilated condition and
before starting the work of the day.
(3)
The walkover survey shall be conducted under non-ventilated condition and
before the first train operates and start-up of ventilation, if applicable.
(4)
Tentative schedule. The actual monitoring requirements shall be discussed and
agreed with EPD before the commencement of operation.
11.5
The instrument should be able to measures flammable gas
concentration (calibrated with methane gas) in the range of 1 to 10,000
ppm. The instrument should be
intrinsically safe if used in confined area.
The walkover survey should be conducted at a slow pace with the inlet
tube of the meter probe a few centimetres above ground surface or close the
construction joints / cracks of the tunnels.
Construction Phase
11.6
The trigger levels and actions in the event of gas being
detected are detailed in Table 11.2.
Table 11.2 Actions in the
Event of Gas Being Detected
|
Parameter |
Measurement |
Action |
|
O2 |
< 19% v/v |
Increase underground
ventilation to restore O2 to >19% v/v |
|
|
< 18% v/v |
Stop works Evacuate all personnel Increase ventilation
further to restore O2 to >19% v/v |
|
CH4 |
> 10% LEL |
Prohibit hot works Increase ventilation to
restore CH4 to <10% LEL |
|
|
>20% LEL |
Stop works Evacuate all personnel Increase ventilation
further to restore CH4 to <10% LEL |
|
CO2 |
>0.5% v/v |
Increase
ventilation to restore CO2
to <0.5% v/v |
|
|
> 1.5% v/v |
Stop works Evacuate all personnel Increase ventilation
further to restore CO2 to <0.5% |
After Completion of Tunnel Construction Works and Before Commencement of Operation
11.7
Weekly monitoring
shall be conducted during this period. Should
methane concentration be higher than 10% LEL, mechanical ventilation is
required to be switched on 30 minutes before starting the work of the day and
throughout the working day. In addition,
the monitoring frequency should be increased to daily. If the situation persists in three
consecutive occasions, inspection of tunnel seals/ joints should be carried out
to identify the location of potential gas ingress. Rectifications, such as sealing of cracks,
shall be carried out. The daily
monitoring requirement can be resumed to weekly if the concentration of methane
is consistently below 10% LEL in 7 consecutive days and the ventilation
requirement can be resumed to normal.
Operational Phase
11.8
Weekly monitoring is
tentatively proposed for the operational phase.
Should methane concentration be higher than 10% LEL, mechanical
ventilation is required to be switched on at least 30 minutes before the first
train operates to sufficiently dilute the concentration of methane to well
below the lower explosive limit to ensure that no explosion will occur when the
trains pass through the tunnel until the end of the daily operation. The monitoring frequency should be increased
to daily. If the situation persists in
three consecutive occasions, inspection of tunnel seals/ joints should be
carried out to identify the location of potential gas ingress. Rectifications, such as sealing of cracks,
shall be carried out. The daily
monitoring requirement can be resumed to weekly if the concentration of methane
is consistently below 10% LEL in 7 consecutive days and the mechanical
ventilation requirement can be resumed back to normal condition.