·
review of background information and studies
related to the nearest landfills involved;
·
identification of the nature and extent of the
sources, the possible pathways through the ground, underground cavities,
utilities or groundwater and the potential targets associated with the proposed
facilities;
·
qualitative assessment on the degree of risk
which the hazardous emissions may pose to the target for each of the
source-pathway-target combinations;
·
design of suitable precautionary measures and
the types of protection measures necessary for the construction, operation and
decommissioning of the proposed developments and associated works; and
·
identification of monitoring requirement for
assessing the adequacy and performance of the implemented protection measures.
·
WCR – the western shoreline of Junk Bay,
Devil’s Peak, Lei Yue Mun Road, Eastern Harbour Crossing ex-casting basin,
Kaolin Mine Site at Cha Kwo Ling and Cha Kwo Ling Road;
·
CBL – Junk Bay, land area south of TKO Area 86
and Wan Po Road;
·
TCS – the newly reclaimed land south of Po Yap
Road, Eastern Drainage Channel and the toe along TKO Stage I Landfill;
·
TKL – the newly reclaimed land to the east of
Ocean Shores (south of Chui Ling Road); and
·
PSK – the three large terraces to the east of
TKO Stage I Landfill and Wan Po Road.
·
A 850 mm thick soil layer comprising completely
decomposed volcanic (CDV) material or completely decomposed granite (CDG); an
additional 650 mm CDV is also provided in the location where trees or shrubs
are provided;
·
A subsoil drainage layer comprising a synthetic
drainage medium, surrounded by suitable geotextile filters.
·
An impermeable layer (anchored in CDV at the
perimeter) comprising a 1mm thick linear low density polyethylene (LLDPE) geomembrane; and
·
The final intermediate layer of well compacted
500 mm thick soil, free from stones or other sharp particles, above the waste.
·
Monthly monitoring of surface emission of
landfill gas;
·
Monthly monitoring of boreholes and piezometers
within and beyond the site boundary;
·
Monitoring of buildings and confined spaces;
·
Monthly monitoring of controlled landfill gas
emissions;
·
Monthly monitoring of condensate; and
·
Quarterly monitoring of landfill flare.
·
Source – location, nature and likely
quantities/ concentrations of landfill gas which has the potential to affect
the development.
·
Pathway – the ground and groundwater
conditions, through which landfill gas must pass in order to reach the
development.
·
Target – elements of the development that are
sensitive to the effects of landfill gas.
·
Excavations for utilities installation during
construction phase.
·
Road works and drainage works during
construction phase.
·
Manholes, inspection chambers or voids of
services/utilities during operational phase.
Table 14.1 Qualitative Source-Pathway-Target Analysis of proposed development in proximity to Tseung Kwan O Stage I Landfill
Source |
Pathway |
Target |
Risk |
The annual landfill gas
production rate by 2004 was predicted to be less than 20 million m3yr-1. A post-restoration
monitoring programme commenced in late 1997 under the restoration contract
for TKO Stage I landfill. The monitoring is conducted on a regular
basis. LFG (including carbon dioxide)
and oxygen are monitored at some monitoring wells installed within and
outside the boundary of the landfill.
Zero level of methane has
been detected at the monitoring wells installed along the boundary of the
landfill in the vicinity of the proposed development. The carbon dioxide levels varied from 0.0%
to 8.6% (with unknown background level) at 9 of the monitoring wells. Taking into account the zero level of
methane and that gas control (such as vents and barriers) to prevent gas
migration has been installed at the landfill, the landfill gas source of TKO
Stage I would be considered Medium. (Medium Source) |
Natural pathways The geological formation beneath Tseung Kwan O Stage I Landfill and
the proposed project site mainly comprises natural earth and waste. There are no fault lines or cracks
existing between Tseung Kwan O Stage I Landfill and the project site. The
path length between TKO Stage I Landfill and the project site will be about
50-100m. (Moderately Short /Direct
Pathway) |
Deep excavations during
construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target) |
Medium |
Maintenance of services during operational phase Manholes or inspection
chambers of utilities or services. (Medium Sensitivity Target) |
Medium |
||
Man-made pathways There are no utilities or
services directly linking between TKO Stage I Landfill and the proposed
road. As such, the man-made pathways
are considered to be Long/Indirect. (Long/ Indirect Pathway) |
Deep excavations during
construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target) |
Low |
|
Maintenance of services during operational phase Manholes or inspection
chambers of utilities or services. (Medium Sensitivity Target) |
Low |
Source |
Pathway |
Target |
Risk |
Assuming moisture content
in the waste material of 25%, the annual landfill gas production rate by 2004
was predicted at about 22 million m3yr-1. A post-restoration monitoring
programme commenced in July 1997 under the restoration contract for TKO Stage
II/III landfill. LFG (including
carbon dioxide) and oxygen are monitored at some monitoring wells installed
within and outside the boundary of the landfill. Zero level of methane has
been detected at the monitoring wells installed along the boundary of the
landfill in the vicinity of the proposed development. The carbon dioxide levels varied from 0.0%
to 8.9% (with unknown background level) at 16 of the monitoring wells. Taking into account the zero level of
methane and the fact that the landfill site has gas control (such as vents
and barriers) to prevent gas migration, the landfill gas source of TKO Stage
II/III would also be considered Medium. (Medium Source) |
Natural
pathways The geological formation beneath Tseung Kwan O Stage II/III Landfill
and the proposed project site mainly comprises natural earth and waste). There are no fault lines or cracks
persisting between Tseung Kwan O Stage II/III Landfill and the project site.
The path length between TKO Stage II/III Landfill and the project site will
be about 100-250m. (Long /Indirect Pathway) |
Deep excavations during
construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target) |
Low |
Maintenance of services during operational phase Manholes or inspection
chambers of utilities or services. (Medium Sensitivity Target) |
Low |
||
Man-made pathways There are no utilities or services
directly linking between TKO Stage II/III Landfill and the proposed
road. As such, the man-made pathways
are considered to be Long/Indirect. (Long/ Indirect Pathway) |
Deep excavations during
construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target) |
Low |
|
Maintenance of services during operational phase Manholes or inspection
chambers of utilities or services. (Medium Sensitivity Target) |
Low |
Source |
Pathway |
Target |
Risk |
According to Urban
Landfills – Monthly Report for Landfill Gas Management System, March 2004 (prepared
by the Hong Kong Landfill Restoration Group), the average landfill gas flow
rate at the flare station was 320 m3/hr. As part of the Restoration
Contract for Urban Landfills, the restoration monitoring programme commenced
in April 1997. LFG (including carbon
dioxide) and oxygen have been monitored monthly at monitoring probes
installed at the landfill (within and outside the site boundary of the
landfill). The concentration of methane
at the gas probes located at the Sai Tso Wan Landfill ranged from 0.0% to
0.8% with maximum concentration recorded at GW1 during January 2004 and GW7
during March 2004 whilst the concentration of carbon dioxide at the gas
probes located at the landfill ranged from 0.0% to 7.0% (background level =
3.1%) with maximum concentration recorded at G2C during March 2004. With reference to Section 3.10 of Landfill
Gas Hazard Assessment Guidance Note (EPD/TR8/97), the source of LFG at Sai
Tso Wan Landfill is categorised as Medium. (Medium Source) |
Natural
pathways The geological formation beneath the proposed project site near Sai
Tso Wan Landfill mainly comprises fine to medium-grained granite of the
Jurassic Cretaceous age. Although,
the path length between Sai Tso Wan Landfill and the project site will be
about 100-250m, there is a fault running between Sai Tso Wan Landfill and the
project site. Therefore, the natural
pathways are categorised as Short/direct.
(Short /Direct Pathway) |
Deep excavations during
construction phase Deep trench excavation for utilities or services. (Medium Sensitivity Target) |
Medium |
Maintenance of services during operational phase Manholes or inspection
chambers of utilities or services. (Medium Sensitivity Target) |
Medium |
||
Man-made pathways There are no utilities or services
directly linking between Sai Tso Wan Landfill and the proposed road. As such, the man-made pathways are
considered to be Long/Indirect. (Long/ Indirect Pathway) |
Deep excavations during construction
phase Deep trench excavation for utilities or services. (Medium Sensitivity Target) |
Low |
|
Maintenance of services during operational phase Manholes or inspection
chambers of utilities or services. (Medium Sensitivity Target) |
Low |
a.
The Contractor should take special precautions
regarding the possible presence of landfill gas. The Contractor should ensure that the site workers are aware of
potential hazards of working in confined spaces such as manholes and service
chambers, and that appropriate monitoring procedures as per the Code of
Practice on Safety and Health at Work in Confined Spaces (Labour
Department, Hong Kong) are in place to prevent hazards due to asphyxiating
atmospheres in confined spaces;
b.
All relevant construction workers should
undergo training about the risks and indications of landfill gas and should be
thoroughly skilled in first aid and emergency and evacuation techniques;
c.
A no smoking policy should be strictly
adopted. The possibility of methane
rich air being taken into diesel engine plant should not be overlooked,
although this is unlikely to occur;
d.
Welding, flame cutting or other hot works
should only be carried out in trenches or confined area when controlled by a
‘permit to work’ procedure, properly authorised by the Safety Officer;
e.
Ground level construction plant should be
fitted with vertical exhausts at least 0.6m above ground level and with spark
arrestors;
f.
During piping assembly or conduiting
construction at underground or at ground level, all valves/seals should be
closed immediately after installation.
As construction progresses, all valves/seals should be closed as
installed to prevent the migration of gases through the pipeline/conduit. All piping/conduiting should be capped at
the end of each working day;
g.
Mobile offices, equipment stores, mess rooms
etc. should be located on an area which has been proven to be gas free and
on-going monitoring should be carried out to ensure that these areas remain gas
free. Alternatively, such buildings
should be raised clear of ground with a minimum clear separation distance (as
measured from the highest point on the ground surface to the underside of the
lowest floor joist) of 500mm. However,
in this case, it is highly recommended that all the site offices, equipment
stores and mess rooms should be located outside the 250m LFG consultation zone;
h.
As a minimum, no work should be undertaken in
the absence of fire extinguishers;
i.
Adequate fire extinguishing equipment,
fire-resistant clothing and breathing apparatus sets should be made available
on site;
j.
Monitoring of methane, carbon dioxide and
oxygen should be undertaken using appropriately calibrated portable gas
detection equipment. Monitoring
equipment should have, as a minimum, the indication ranges set out below:
Methane |
0-100% Lower Explosive Limit (LEL) and 0-100% v/v |
Carbon Dioxide |
0-100% |
Oxygen |
0-21% |
k.
The monitoring frequency and areas to be
monitored should be specified prior to commencement of ground-works, either by
the Safety Officer, or by an appropriately qualified person. The minimum monitoring frequency would be
twice a day (i.e. the morning before site works commence and after the lunch
break) during all excavation works below ground level and should always be
undertaken by appropriately qualified personnel. All measurements should be recorded and included in the site
diary;
l.
For excavations deeper than 1 m, measurements
should be carried out:
-
at
the ground surface before excavation commences;
-
immediately before any worker enters the
excavation;
-
at the beginning of each half working day (i.e.
morning and afternoon) for the entire period the excavation remains open; and
-
periodically through the working day whilst
workers are in the excavation.
m. For excavations between 300 mm and 1 m deep, measurements should be
carried out:
-
directly after the excavation has been
completed; and
- periodically whilst the
excavation remains open.
n. Depending on the results of the measurements, actions required will vary
and should be set down by the Safety Officer of the Contractor or other
appropriately qualified person. As a
minimum these should encompass those actions specified
in Table 14.4.
Table 14.4 Actions in the Event of Gas Being Detected in Excavations
Parameter |
Measurement |
Action |
Oxygen |
< 19 % |
·
Ventilate to restore oxygen to > 19 % |
|
< 18 % |
·
Stop works ·
Evacuate personnel/prohibit entry ·
Increase ventilation to restore oxygen to > 19 % |
Methane |
> 10 % LEL (i.e. > 0.5 % by volume) |
·
Prohibit hot works ·
Ventilate to restore methane to < 10% LEL |
|
> 20 % LEL (i.e. > 1
% by volume) |
·
Stop works ·
Evacuate personnel/prohibit entry ·
Increase ventilation to restore methane to < 10 % LEL |
Carbon Dioxide |
> 0.5 % |
·
Ventilate to restore carbon dioxide to < 0.5% |
|
> 1.5 % |
·
Stop works ·
Evacuate personnel/prohibit entry ·
Increase ventilation to restore carbon dioxide to <0.5% |
o. Utility companies should be advised of the proposed development being in
the vicinity of the TKO Stage I & II/III Landfill and Sai Tso Wan Landfill
for their attention to observe the relevant regulations and requirements in
such regards when inspection or works are being undertaken at the nearby
area.
a.
As this project only includes road works or
drainage works with no involvement of any building structure, the main
precautionary measures during the operational phase will be mainly during the
inspection or maintenance of the utilities services at the area within the 250m
LFG Consultation Zone.
b.
An assumed presence of landfill gas should be
adopted at all times by maintenance workers. All maintenance workers inspecting
any manhole should be fully trained in the issue of landfill gas hazard. Any manhole which is large enough to permit
to access to personnel should be subject to entry safety procedure. Such work in confined spaces is controlled
by the Factories and Industrial Undertakings (Confined Spaces) Regulations of
the Factories and Industrial Undertakings Ordinance. Following the Code of Practice on Safety and Health at Work in
Confined Spaces (Labour Department, Hong Kong) ensures compliance with the
above regulations. A strictly regulated
“work permit procedure” should be implemented and the relevant safety
procedures must be rigidly followed.
Adequate communication with maintenance staff should be maintained with
respect to landfill gas hazard;
c.
The utility companies should undertake a
landfill gas surveillance exercise at the utility manholes/inspection
chambers. The surveillance exercise
shall be:
(i)
Undertaken using an intrinsically safe portable
instrument, appropriately calibrated and capable of measuring the following
gases in the ranges indicated:
Methane |
0-100% Lower Explosive Limit (LEL) and 0-100% v/v |
Carbon Dioxide |
0-100% |
Oxygen |
0-21% |
(ii) Undertaken for the duration of the site occupancy, or until such time
that EPD agree that surveillance is no longer required;
(iii) Depending on the results of the measurements, actions required will vary
and should be set down by appropriately qualified person. As a minimum these shall encompass those
actions specified in Table 14.5.
Table 14.5 Actions in the Event of Gas Being Detected during Operational Phase
Parameter |
Measurement |
Action |
Oxygen |
< 19 % |
·
Ventilate to restore oxygen to > 19 % |
|
< 18 % |
·
Stop works ·
Evacuate personnel/prohibit entry ·
Increase ventilation to restore oxygen to > 19 % |
Methane |
> 10 % LEL (i.e. >
0.5 % by volume) |
·
Prohibit hot works ·
Ventilate to restore methane to < 10% LEL |
|
> 20 % LEL (i.e. > 1 % by volume) |
·
Stop works ·
Evacuate personnel/prohibit entry ·
Increase ventilation to restore methane to < 10 % LEL |
Carbon Dioxide |
> 0.5 % |
·
Ventilate to restore carbon dioxide to < 0.5% |
|
> 1.5 % |
·
Stop works ·
Evacuate personnel/prohibit entry ·
Increase ventilation to restore carbon dioxide to <0.5% |
[1] Hong Kong Geological
Survey map, Sheet 12, Series HGM20, Edition
1 ‑ 1989 (scale 1:20,000) (published
by Geotechnical Control Office in 1989)
[2] Hong Kong Geological
Survey map, Sheet 11, Series HGM20 Edition 1- 1986 (scale 1:20,000) (published by Geotechnical Control Office in 1986)
[3] Scott
Wilson Kirkpatrick (in association with Aspinwall & Company) (Oct 1994) Agreement CE 11/92 Restoration of Tseung
Kwan O Landfills – Final Report.
[4] Urban Landfills –
Monthly Report for Landfill Gas Management System, March 2004 (prepared by the Hong Kong Landfill Restoration Group)