10
Landfill Gas Hazard
10.1
Introduction
10.1.1.1
This section identifies the hazards that are likely to
be generated from the WENT Landfill and the proposed WENT Landfill Extension,
during the construction and operation phases of this Project and evaluates the
associated risk. Mitigation measures and good site practices are
recommended with reference to the EPD guidelines.
10.2
Environmental Legislation, Policies, Plans, Standards
and Criteria
10.2.1.1
Environmental Protection Department (EPD) has issued
two guidance notes regarding landfill gas hazard assessment, namely ProPECC PN 3/96 - Landfill Gas Hazard Assessment for
Development Adjacent to Landfill and EPD/TR8/97 - Landfill Gas Hazard
Assessment Guidance Note. These two guidance notes set out the conditions
under which a landfill gas hazard assessment should be carried out and provide
guidance on undertaking a landfill gas hazard assessment. The guidance
notes recommended that in general, assessment of landfill gas hazard is
required for proposed developments that lie within the 250m Consultation Zone around a landfill. Figure 10.1 shows the details of
the planned facilities of STF that will be located within the consultation zone
of the existing WENT Landfill.
10.2.1.2
Since the Project Site in the eastern part of the Ash
Lagoon falls within the 250m Consultation Zone
of WENT Landfill, a qualitative landfill gas hazard assessment addressing the
landfill gas hazards and recommending the mitigation measures is undertaken for
the Project.
10.3
History and General Description of the WENT Landfill
10.3.1.1
WENT Landfill is an 110ha Landfill, which was
commissioned in 1993, receiving waste from the West
East New
Territories by road as well as West
Kowloon, North Lantau
and Island East Transfer Stations, Island West Transfer Station, Outlying
Island Transfer Stations by barge. Its operation life is about 25 years
and its aftercare period after the completion of operation is 30 years.
10.3.1.2
The WENT landfill is designed in a “Confined and
Contained” approach. Comprehensive landfill gas and leachate
collection systems were installed and operated. Also, there is a
protective lining system to stop any migration of landfill gas and leachate which forms a barrier on the pathway to nearby
development. Periodic monitoring exercise is being conducted to ensure
that landfill gas and leachate are properly collected
and treated.
10.3.1.3
There is a proposed project on extensions of the
existing WENT Landfill. As shown in Figure 10.2, the proposed extension work of WENT
Landfill will be divided into 6 phases covering total tipping volume of 81Mm3.
The time line of the proposed extension works is shown below.
|
Work
|
Construction Year
|
Operation Year
|
|
Phase
1
|
2016
|
2018
|
|
Phase
2
|
2017
|
2019
|
|
Phase
3
|
2018
|
2020
|
|
Phase
4
|
2020
|
2022
|
|
Phase
5
|
2021
|
2023
|
|
Phase
6
|
2022
|
2024
|
10.4
Potential Landfill Gas Hazards
10.4.1.1
The typical composition of landfill gas is about 60%
volume of methane and 40% volume of carbon dioxide, although these percentages
can vary widely depending on the site conditions. Also present are trace
quantities of hydrogen sulphide, nitrogen and gaseous
hydrocarbons such as hexane, octane and heptane.
10.4.1.2
Landfill gas migration can be a dangerous hazard
because of its combustible and in some cases explosive nature of methane; and
the asphyxiant nature of carbon dioxide.
10.4.1.3
Landfill gas has the potential to cause fire, explosion
or asphyxiation if it migrates into and accumulates in confined space such as
building basements, underground car parks, lift shafts, pumping stations, and
maintenance chambers. For the same reasons, temporary structures such as
site huts and any other unventilated enclosures erected during construction
stage are also exposed to landfill gas hazards. Underground services,
such as sewer drains, storm drains and service ducts, may also be susceptible
to the potential hazards as they act as pathways for landfill gas.
Besides, any faults present in geological formation also act as pathways for
landfill gas.
10.4.1.4
As shown in Figure 10.1, part of the STF site
(including site of the administration building & laboratory, maintenance
workshop, desalination plant, seawater pumping station & storage tank)
falls within the 250m Consultation Zone of
existing WENT Landfill. Further, as illustrated in Figure 10.2, all parts of the STF
site would fall within the 250m Consultation Zone of the proposed WENT Landfill
extension and will be affected by the proposed WENT Landfill extension.
The overall risk level of landfill gas hazard posed by WENT Landfill and its
proposed extension to the proposed STF is assessed and demonstrated below as
recommended in EPD/TR8/97 - Landfill Gas Hazard Assessment Guidance Note.
10.5
Landfill Gas Assessment Criteria and Methodology
10.5.1
Landfill Gas Assessment Criteria
10.5.1.1
In accordance with the Landfill Gas Hazard Assessment
Guidance Note, the risk due to landfill gas may be evaluated based upon the
following three criteria:
·
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; and
·
Target – elements of the development that are sensitive
to the effects of landfill gas.
10.5.2.1
The classification of the Source (ie the landfill) should be undertaken as follows:
|
Minor
|
Landfill sites at which gas controls have
been installed and proven to be effective by comprehensive monitoring which
has demonstrated that there is no migration of gas beyond the landfill
boundary (or any specific control measures) and at which control of
gas does not rely solely on an active gas extraction system or any other
single control measure which is vulnerable to failure; or
Old landfill sites where the maximum
concentration of methane within the waste, as measured at several locations
across the landfill and on at least four occasions over a period of at least
3 months (preferably longer), is less than 5% by volume (v/v).
|
|
Medium
|
Landfill site at which some form of gas
control has been installed (eg lined site or one
where vents or barriers have been retrospectively installed) but where there
are only limited monitoring data to demonstrate its efficacy to prevent
migration of gas; or
Landfill site where comprehensive
monitoring has demonstrated that there is no migration of gas beyond the
landfill boundary but where the control of gas relies solely on an active gas
extraction system or any other single control system which is vulnerable to
failure.
|
|
Major
|
Recently filled landfill site at which
there is little or no control to prevent migration of gas or at which the efficacy
of the gas control measures has not been assessed; or
Any landfill site at which monitoring has
demonstrated that there is significant migration of gas beyond the site
boundary.
|
10.5.2.2
The 'significance' of migration should be assessed by
reference to the concentration, frequency and location at which gas is
detected. For guidance, it should be assumed that any
concentration of methane or carbon dioxide greater than 5% v/v above background
levels in any monitoring well outside the landfill's boundary indicates
significant migration. Lower concentrations may still be 'significant' if
they are observed in more than one monitoring well, on several occasions or in
monitoring wells located some distance from the site boundary. In
general, concentrations of greater than 1% v/v methane or 1.5% v/v carbon
dioxide (above background levels in each case) indicate less than adequate
control of the gas at source.
10.5.2.3
In classifying the source term, account needs to be
taken of the likelihood and probable effect of a failure of the gas controls.
Thus, if it has been demonstrated that there is no migration of gas and
there is little danger of the gas controls failing (eg
if these comprise solely of passive measures such as a liner) it can be assumed
that the site represents a "Minor" Source. Where there is no
gas migration but this may be as a result of a single, "vulnerable"
control measure (eg. an active extraction system with
no warning of failure), the site should be regarded as
a "Medium" or even a "Major" Source depending on the other
factors (eg size of site and age of waste).
10.5.2.4
Where the effectiveness of the gas controls has not
been proven by off-site monitoring or if there is some doubt as to the adequacy
of the monitoring, this should be taken into account when considering the
impact of the control measures on the Source term. Assessments should
always err on the side of caution and, in general, if the effectiveness cannot
be demonstrated, the assessment should be undertaken on the same basis as if
the controls were not in place.
10.5.2.5
The reliability of the monitoring, for determining the
efficacy of the gas controls, needs to take account of the design, number and
location of the monitoring points together with the frequency and duration over
which monitoring has been undertaken. Monitoring should have been
undertaken under different weather conditions including, in particular, periods
of low or falling atmospheric pressure.
10.5.3.1
The broad classification of the Pathway should be
undertaken as follows:
|
Very short /
direct
|
Path length of less than 50m for
unsaturated permeable strata and fissured rock or less than 100m for man-made
conduits
|
|
Moderately
short / direct
|
Path length of 50-100m for unsaturated
permeable soil or fissured rock or 100-250m for man-made conduits
|
|
Long /
indirect
|
Path length of 100-250m for unsaturated
permeable soils and fissured rock
|
10.5.3.2
In classifying the pathway, however, adjustment to the
above general guidelines will often be required to take account of other
factors which will affect the extent of gas migration including the following:
·
particular permeability of the soils;
·
spacing, tightness and direction of the fissures/joints;
·
topography;
·
depth
and thickness of the medium through which the gas may migrate (which may be
affected by groundwater level);
·
the
nature of the strata over the potential pathway;
·
the
number of different media involved; and
·
depth
to groundwater table and flow patterns.
10.5.3.3
Thus, although there may be permeable soil between the
landfill site and a proposed development say 80m from
the edge of the site, if the soil layer is very shallow and thin with its upper
surface exposed to the atmosphere, then it will be appropriate to consider this
as a long/indirect pathway. This could of course alter if the land
between the landfill site and the development was paved over or altered in some
other way which reduced the potential for gas release. Similarly, if the
land is flat, the surface may be prone to waterlogging
which will also effectively seal it at times of heavy rain. In general, a
conservative approach should be adopted and it should be assumed that any such
permeable surface soils may become less permeable in the future.
10.5.3.4
If it is known that a conduit (man-made or natural
feature such as a fault plane) leads directly from the landfill to the
development area, it should be regarded as a "direct/short" pathway
even if it is longer than 100m.
10.5.4.1
Different types of target may be broadly classified as
follows:
|
High
sensitivity
|
Buildings and structures with ground level
or below ground rooms/voids or into which services enter directly from the
ground and to which members of the general public have unrestricted
access or which contain sources of ignition.
This would include any developments where
there is a possibility of additional structures being erected directly on the
ground on an ad hoc basis and thereby without due regard to the potential
risks.
|
|
Medium
sensitivity
|
Other buildings, structures or service voids
where there is access only by authorised, well
trained personnel, such as the staff of utility companies, who have been
briefed on the potential hazards relating to landfill gas and the specific
safety procedures to be followed.
Deep excavations.
|
|
Low
sensitivity
|
Buildings/structures which are less prone
to gas ingress by virtue of their design (such as those with a raised floor
slab).
Shallow excavations.
Developments which involve essentially
outdoor activities but where evolution of gas could pose potential problems.
|
10.5.4.2
The classification of the above LFG sources, pathway
and target are categorized. Having determined into which categories of
source, pathway and target the combination of landfill and development fall, a
preliminary assessment of the overall risk may be made by reference to Table
10.1. The potential implications associated with the various
qualitative risk categories are summarized in Table 10.2.
Table 10.1
Classification of Risk Category
|
Source
|
Pathway
|
Target
Sensitivity
|
Risk
Category
|
|
Major
|
Very short/direct
|
High
|
Very high
|
|
Medium
|
high
|
|
Low
|
Medium
|
|
Moderately short/direct
|
High
|
High
|
|
Medium
|
Medium
|
|
Low
|
Low
|
|
Long/indirect
|
High
|
High
|
|
Medium
|
Medium
|
|
Low
|
Low
|
|
Medium
|
Very short/direct
|
High
|
High
|
|
Medium
|
Medium
|
|
Low
|
Low
|
|
Moderately short/direct
|
High
|
High
|
|
Medium
|
Medium
|
|
Low
|
Low
|
|
Long/indirect
|
High
|
Medium
|
|
Medium
|
Low
|
|
Low
|
Very low
|
|
Minor
|
Very short/direct
|
High
|
High
|
|
Medium
|
Medium
|
|
Low
|
Low
|
|
Moderately short/direct
|
High
|
Medium
|
|
Medium
|
Low
|
|
Low
|
Very low
|
|
Long/indirect
|
High
|
Medium
|
|
Medium
|
Low
|
|
Low
|
Very low
|
Table 10.2
Summary of General Categorization of Risk
|
Category
|
Level
of Risk
|
Implication
|
|
A
|
Very high
|
The type of development being proposed is very
undesirable and a less sensitive form of development should be considered. At
the very least, extensive engineering measures, alarm systems and emergency
action plans are likely to be required.
|
|
B
|
High
|
Significant engineering measures will be required to
protect the planned development.
|
|
C
|
Medium
|
Engineering measures will be required to protect the
proposed development.
|
|
D
|
Low
|
Some precautionary measures will be required to ensure
that the planned development is safe.
|
|
E
|
Very low (insignificant)
|
The risk is so low that no precautionary measures
are required.
|
10.5.4.3
Five generic forms of protection will be used in mitigating
the hazards to development. These generic forms corresponding to the five
risk levels are set out in Table 10.3. The terms used in Table
10.3 are defined in Table 10.4.
Table 10.3
Generic Protection Measures for Planning Stage Categorization
|
Category
|
Generic
Protection Measures
|
|
A
|
For the planned development
active control of gas, supported by barriers and detection systems.
Another, less sensitive form of development should also be considered.
|
|
B
|
Active control of gas,
including barriers and detection systems (1).
|
|
C
|
Use of “semi active” or
enhanced passive controls. Detection systems in some situations.
|
|
D
|
Passive control of gas only.
|
|
E
|
No precautionary measures
required.
|
Note:
(1) The
gas protection measures required to allow the safe development of a Category A risk development will need to be more extensive than those
for a Category B risk development.
Table 10.4
Definition of Control Terms
|
Term
|
Definition
|
|
Active control
|
Control of gas by mechanical means eg ventilation of spaces with air to dilute gas, or
extraction of gas from the development site using fans or blowers.
|
|
'Semi active'
control
|
Use of wind driven cowls and other devices which
assist in the ventilation of gas but do not rely on electrically powered
fans.
|
|
Passive control
|
Provision of barriers to the movement of gas eg membranes in floors or walls, or in trenches, coupled
with high permeability vents such as no-fines gravel in trenches or
voids/permeable layers below structures.
|
|
Detection systems
|
Electronic systems based upon, for example,
catalytic oxidation or infra-red measurement principles, which can detect low
concentrations of gas in the atmosphere and can be linked to alarms and/or
telemetry systems.
|
10.6
Assessment of Potential Risk
10.6.1.1
The existing WENT Landfill is a large and operating
landfill (i.e. a significant source of LFG) and it relies on an active
extraction system for LFG management. The gas control system has been
proven to be effective by comprehensive monitoring which has demonstrated that
there is no migration of gas beyond the landfill boundary with reference to the
recent monitoring data (Appendix
10.1). The Source of landfill gas at WENT Landfill is therefore
categorized as MEDIUM.
10.6.1.2
The proposed future WENT Landfill Extensions is a
source to generate LFG during the operation and aftercare phases. With
the LFG control measures and utilization facilities in the future WENT Landfill
Extensions, it is anticipated that the source of LFG will be properly
controlled within the site similar to the operation of the existing WENT
Landfill. The source of LFG at the future landfill extension is therefore
categorized as MEDIUM.
10.6.2.1
The buildings which fall within the existing WENT
Landfill consultation zone are located 100-250m
from the landfill. Besides, the presence of the nullah
and the sloping seawall (built with a lot of rubble and aggregate) facing the nullah act as a passive venting system to stop LFG from
further migrating to the STF site. Although the upstream area of the nullah bed (south of the STF site, Figure 10.1 refers) could be
exposed at the lowest tide, the majority of the nullah
is still covered with water at most of the time. In particular, the
intertidal area (east of the STF site) has high water level. Thus, this nullah will provide a form of protection to the STF site.
10.6.2.2
According to the Hong Kong Geological Survey Map (Scale
1:20,000) on the solid and superficial geology of Tsing
Shan (Castle Peak), the superficial geology of
the STF was mainly “marine sand” with part of “undivided, mainly marine
mud”. The subsoil underneath the STF facilities would mainly be ash,
which is very compact with low porosity. Consider the nullah
in between STF and WENT Landfill, the compact and low porosity subsoil, and a
relatively high watertable, it is anticipated that
the chance for LFG migrating to the STF site would be very remote.
10.6.2.3
In view of more than 100m distance from existing WENT
Landfill, the Pathway is classified as LONG/INDIRECT.
10.6.2.4
For the proposed future WENT Landfill Extensions, the
separation distance with the STF is anticipated to be shorter than 50m.
The Pathway of LFG at the future landfill extensions is classified as VERY
SHORT/DIRECT.
10.6.3.1
As shown in Figure 10.1, Administration Building
& Laboratory, Maintenance Workshop, Desalination Plant, Seawater Pumping
Station & Storage Tank would fall within the 250m Consultation Zone of the existing WENT Landfill. It is
anticipated that deep excavations for the construction of the building
foundation and the construction of underground utilities would be involved
during the construction phase of the proposed Project. The risk level of
deep excavations for Target Sensitivity is categorized as MEDIUM.
10.6.3.2
As shown in Figure 10.3A, underground structures such as culvert and basement of the
Seawater Pumping Station are anticipated. During operation phase, when
the gas at these locations accumulate and mix with air, the resultant mixture
may explode in the presence of an ignition source. There is a risk of asphyxiation
to persons using poorly ventilated enclosed spaces, where gas is accumulated.
However, access to the underground service voids will only allow
authorized or well-trained personnel who have been briefed on the potential
hazards relating to landfill gas and the specific safety procedures to be
followed. As such, the risk level for this Target Sensitivity is
categorized as MEDIUM.
10.6.3.3
Administrative Building & Laboratory and
Maintenance Workshop are generally restricted to authorized personnel.
The risk level for this Target Sensitivity is therefore categorized as MEDIUM.
10.6.3.4
The earliest operation phase of the proposed future WENT
Landfill Extensions will commence in Year 2016 while the construction of STF
would be completed by Year 2012. As such, the future WENT Landfill
Extensions will not affect the construction of STF.
10.6.3.5
However, all parts of the STF such as the incineration
system would fall within the consultation zone of the future WENT Landfill
Extensions. As shown in Figure
10.3B, underground structures of the incineration system such as the
sludge hopper and boiler water system are anticipated. In view that
access to the service voids will only allow authorized or well-trained
personnel who have been briefed on the potential hazards relating to landfill
gas and the specific safety procedures to be followed, the risk level for this
Target Sensitivity is categorized as MEDIUM.
10.6.4
Summary of Qualitative Source-Pathway-Target Analysis
10.6.4.1
Based on the information above, source-pathway-target
analysis have been undertaken and the Overall Risk Level for both
construction and operation phases of the proposed Project associated with the
existing WENT Landfill is Low while the risk level for the operation
phase of the project associated with the proposed WENT Landfill Extension is Medium. Source-Pathway-Target analysis have
been undertaken and are summarized in Table 10.5.
Table 10.5
Qualitative Risk Assessment Matrix
|
Source
|
Pathway
|
Sensitivity Target
|
Risk
|
|
From Existing
WENT Landfill:
(Medium Source)
|
More than 100m from the existing WENT
Landfill
Nullah in between acting as passive barrier
(Long/Indirect Pathway)
|
Deep excavation during construction phase
(Medium Sensitivity Target)
|
Low
|
|
Operation & Maintenance of Seawater
Pumping Station during operation phase
(Medium Sensitivity Target)
|
Low
|
|
Administration Building & Laboratory during operation phase
(Medium Sensitivity Target)
|
Low
|
|
From Proposed
Future WENT Landfill Extensions
(Medium Source)
|
Less than 50m from the proposed future
WENT Landfill Extensions:
(Very Short/ Direct Pathway)
|
Operation & Maintenance of
Incineration System and Seawater Pumping Station during operation phases
(Medium Sensitivity Target)
|
Medium
|
|
Administration Building & Laboratory during operation phase
(Medium Sensitivity Target)
|
Medium
|
10.7
Recommended Protection Measures
10.7.1
Introduction
10.7.1.1
According to Table 10.2, some precautionary
measures are required to protect the proposed STF from the landfill gas risk
due to the existing WENT Landfill and engineering measures will be required to
protect the proposed STF from the proposed WENT Landfill extension.
According to Table 4.2 of the EPD’s Landfill Gas
Hazard Assessment Guidance Note, the generic protection measures required
include “use of ‘semi active’ or enhanced passive gas controls protection
system in some situations”. The recommended preventive measures are
presented in the following sections.
10.7.2
During
Construction Phase
Appointment of Safety
Officer
10.7.2.1
A safety officer, trained in the use of gas detection
equipment and landfill gas-related hazards, should be present on site
throughout the groundworks phase. The Safety
Officer should be provide with an intrinsically safe portable instrument, which
is appropriately calibrated and able to measure the following gases in the
ranges indicated below:
Methane
0-100% LEL and 1-100%
v/v
Carbon dioxide 0-100%
Oxygen
0-21%
Safety Measures
10.7.2.2
For staff who work in, or have responsibility for “at
risk” area, such as all excavation workers, supervisors and engineers working
within the Consultation Zone, should receive appropriate training on working in
areas susceptible to landfill gas, fire and explosion hazards.
10.7.2.3
An excavation procedure or code of practice to minimize
landfill gas related risk should be devised and carried out.
10.7.2.4
No worker should be allowed to work alone at any time
in or near to any excavation. At least one other worker should be
available to assist with a rescue if needed.
10.7.2.5
Smoking, naked flames and all other sources of ignition
should be prohibited within 15m of any excavation or ground-level confined
space. “No smoking” and “No naked flame” notices should be posted
prominently on the construction site and, if necessary, special areas should be
designed for smoking.
10.7.2.6
Welding, flame-cutting or other hot works should be
confined to open areas at least 15m from any trench or excavation.
10.7.2.7
Welding, flame-cutting or other hot works may be only
be carried out in trenches or confined spaces when controlled by a “permit to
work” procedure, properly authorized by the Safety Officer (or, in the case of
small developments, other appropriately qualified person).
10.7.2.8
The permit to work procedure should set down clearly
the requirements for continuous monitoring for methane, carbon dioxide and
oxygen throughout the period during which the hot works are in progress.
The procedure should also require the presence of an appropriately
qualified person, in attendance outside the 'confined area', who should be
responsible for reviewing the gas measurements as they are made, and who should
have executive responsibility for suspending the work in the event of
unacceptable or hazardous conditions. Only those workers who are
appropriately trained and fully aware of the potentially hazardous conditions
which may arise should be permitted to carry out hot works in confined areas.
10.7.2.9
Where there are any temporary site offices, or any
other buildings located within the WENT Landfill Consultation Zone which have
enclosed spaces with the capacity to accumulate landfill gas, then they should
either be located in an area which has been proven to be free of landfill gas
(by survey using portable gas detectors); or be raised clear of the ground by a
minimum of 500mm. This aims to create a clear void under the structure
which is ventilated by natural air movement such that emission of gas from the
ground are mixed and diluted by air.
10.7.2.10
Any
electrical equipment, such as motors and extension cords, should be
intrinsically safe.
10.7.2.11
During
piping assembly or conduiting construction, all
valves/seals should be closed immediately after installation. As
construction progresses, all valves/seals should be closed to prevent the
migration of gases through the pipeline/conduit. All piping/conduiting should be capped at the end of each working day.
10.7.2.12
During construction, adequate fire extinguishing
equipment, fire-resistant clothing and breathing apparatus (BA) sets should be
made available on site as shown in follow:-
·
Fire drills should be organized at not less than six
monthly intervals; and
·
The
contractor should formulate a health and safety policy, standards and
instructions for site personnel to follow.
10.7.2.13
All
personnel who work on the site and all visitors to the site should be made
aware of the possibility of ignition of gas in the vicinity of
excavations. Safety notices (in Chinese and English) should be posted at
prominent position around the site warning danger of the potential hazards.
10.7.2.14
For
staff who work in, or have responsibility for ‘at risk’ areas, such as all
excavation workers, supervisors and engineers working within the WENT Landfill
Consultation Zone should receive appropriate training on working in areas
susceptible to landfill gas, fire and explosion hazards.
10.7.2.15
Service runs within the Consultation Zone should be
designated as “special routes” as shown in Figure 10.4A; utilities companies should be informed of this and precautionary
measures should be implemented. Precautionary measures should include ensuring
that staff members are aware of the potential hazards of working in confined
spaces such as manholes and service chambers, and that appropriate monitoring
procedures are in place to prevent hazards due to asphyxiating atmospheres in
confined spaces. Detailed guidance on entry into confined spaces is given
in Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong).
10.7.2.16
Periodically during ground-works construction within the 250m Consultation Zone,
the works area should be monitored for methane, carbon dioxide and oxygen using
appropriately calibrated portable gas detection equipment. 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 appropriately
qualified person.
Monitoring
10.7.2.17
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 into the area.
10.7.2.18
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 out the working day whilst workers are in the excavation.
10.7.2.19
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
10.7.2.20
For
excavations less than 300mm and 1m deep, monitoring may be omitted, at the
discretion of the Safety Officer or other appropriately qualified person.
10.7.2.21
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 in Table 10.6.
Table 10.6
Actions in the Event of Landfill Gas Being Detected in Excavations/Confined
Areas
|
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%
|
10.7.2.22
The
hazards from landfill gas during the construction stage within the existing
WENT Landfill Consultation Zone should be minimized by suitable precautionary
measures recommended in Chapter 8 of the Landfill Gas Hazard Assessment
Guidance Note. A copy of which is enclosed in Appendix 10.2. In the
operation phase, if it is necessary to carry out construction works, landfill
gas precautionary measures same as those recommended for the construction stage
above should be followed.
10.7.3
Design of Landfill Gas Protection Measures and other
Protection Measures During Operation Phase
Gas Barrier
10.7.3.1
The
most common way of preventing gas from entering an area of ground is to set a
“gas barrier” into the ground which is either keyed into low permeability
strata or extends at least 1m below the lowest groundwater level.
10.7.3.2
The presence of a gas barrier to the movement of gas
may lead to a gradual build up of gas on the landfill side of the barrier if
the gas migration pathway is covered by low permeability materials. To
relieve the potential build up of gas, it may be necessary to install
additional measures for venting the gas such as trenches filled with no-fines,
granular material, e.g. gravel, connected to venting pipes which will provide a
preferential pathway for the release of gas to atmosphere. An outline of
a landfill gas cut-off barrier is shown in Figure 10.4B and a more detailed
typical design of it is shown in Figure
10.4C.
10.7.3.3
To cut off any landfill gas migration from the existing
WENT landfill to STF, it is proposed that a landfill gas cut-off trench barrier
be built along the boundary of the STF facility from the bank of the nullah to the boundary of the existing WENT Landfill
consultation zone as shown in Figure
10.5. In addition, in the proposed WENT Landfill extension project,
a cut-off trench barrier will be built along the boundary between the STF and
the proposed WENT Landfill Extension under the WENT Extension project.
This will cut off any gas migration to the STF from the WENT Landfill
Extension. The indicative location of the proposed cut-off trench barrier
is shown in Figure 10.5.
10.7.3.4
It is also recommended that several landfill gas
monitoring wells be installed into the ground on the development side of the
gas barrier. These are used to measure the concentrations of methane and
carbon dioxide within the ground and hence determine the effectiveness of the
measures in preventing LFG migration.
Building Protection Design
Measures
10.7.3.5
Passive control measures for buildings structures with
ground level or below ground rooms / voids including the following could be
considered in the detailed design if necessary:-
·
Gas-resistant polymeric membranes which can be
incorporated into the floor or wall construction as a continuous sealed layer. Membranes should be able to demonstrate low gas permeability
and resistant to possible chemical attack and may incorporate aluminum wafers
to improve performance;
·
Other
building materials, e.g. dense well-compacted concrete or steel shuttering
which provide a measure of resistance to gas permeation;
·
Creation of a clear void under the structure which is
ventilated by natural structure and provides preferential pathways for release
of gas;
·
Synthetic composite geotextile
which provide a free-venting cellular structure and provide preferential
pathways for release of gas; and
·
Passive control measures may be used in low and medium
risk situations where gas emissions are expected to be at relatively low rates
and concentrations and venting to atmosphere is unlikely to cause a hazard or
nuisance due to the low concentration or high dilution which will occur.
Design Measures for Sub-Surface Building Services
10.7.3.6
As shown in Figure 10.4B to Figure 10.4C and Figure 10.6A to Figure 10.6C, generic protection measures for the
sub-surface building services including the following are recommended:-
·
A gas
barrier used to prevent movement of gas through services may form part of a
more extensive barrier to prevent general mitigation towards the development.
The gas barrier may be made of clay (or clay-rich soils), bentonite or polymeric membranes (e.g. HDPE). In the
case of water pipes and sewers which are not always fully filled, water traps
e.g. U-bends, should be provided to effectively seal off the conduit and
prevent gas-phase transport; and
·
Vent pipes or gridded manhole covers may be used to
avoid build-up of gas in underground utilities manholes. Venting stacks
may be built into inspection chambers or connected to collection pipes in high
permeability drainage layers adjacent to gas barriers. Under all
circumstances, care should be taken when accessing any manhole chambers
especially those which are not fitted with vents and necessary safety
procedures must be followed.
Guidance for Entry into
Service Rooms / Voids, Manholes and Chambers
10.7.3.7
During the operation phase, any service voids, manholes,
chambers or culvert within the proposed site, which is large enough to permit
access to personnel should be subject to entry safety procedures. Works
in confined spaces are controlled by the Factories and Industrial
Undertakings (Confined Spaces) Regulation of the Factories and
Industrial Undertakings Ordinance and the Safety Guide to Working in
Confined Spaces should be followed to ensure compliance with the
Regulation.
10.7.3.8
In general, when work is being undertaken in confined
spaces, sufficient approved resuscitation equipment, breathing apparatus and
safety torches should be made available. Persons involved in or
supervising such work should be trained and practiced in the use of such
equipment. A permit-to-work system for entry into confined spaces should
be developed by an appropriately qualified person and the system should be
consistently employed. The safety measures
recommended in Chapter 8 of the Landfill Gas Hazard Assessment Guidance Note
should also be strictly followed.
10.7.3.9
All the access to confined spaces should be restricted
only to authorized personnel who are aware of the landfill gas hazard. No
general public should be permitted or allowed to access the service voids,
manholes, chambers or wells.
10.7.3.10
Regular Monitoring of landfill gas should be done at
the monitoring wells mentioned in Section 10.7.3.4 as well as at the
underground service voids and manholes by the STF contractor.
Monitoring is required to verify the effectiveness and to ensure the
continued performance of the implemented protection measures.
10.7.3.11
As
this Project is at the Preliminary Design Stage, a detailed design is not
available yet. When the detailed design of the Project is available, the
Design-Build-Operate (DBO) contractor of the STF is required to undertake
further landfill gas hazard assessment to take account of the more readily
available detailed information to finalize the design of the landfill gas
protection measures recommended in this report. During the detailed
design stage, a review of this preliminary qualitative risk assessment should
be carried out and a detailed qualitative landfill gas risk assessment should
be prepared. The detailed qualitative landfill gas risk assessment
together with the detailed design of gas protection measures should be
submitted to EPD for vetting.
10.7.3.12
The
design of the landfill gas precautionary measures to be adopted on-site should
be performed by a competent professional person who has knowledge on LFG
protection measures appointed by the DBO contractor of the STF. The
design should also be checked and certified by a qualified independent
consultant. The DBO contractor should ensure that the required protection
measures are implemented and constructed in accordance with the design and a maintenance and monitoring programme
should be established to ensure the continued performance of the implemented
protection measures. The above requirements should be included in the
tender documents of the STF.
10.8
Conclusions
10.8.1.1
The landfill gas hazard assessment shows that the
overall level of landfill gas risk posed by the existing WENT Landfill to the
Project is low while the risk posed by the proposed WENT Landfill extension is
medium. Appropriate protective measures have been proposed to minimize
the landfill gas risk for the proposed project site during construction phase
and operation phase. In particular, it is
proposed that landfill gas barriers be built at the site boundary of this
Project to cut off any landfill gas migration from WENT Landfill and its
proposed extensions, and monitoring wells be installed into the ground for verifying
the effectiveness of the above measures.
10.8.1.2
Provided that all the recommended protection measures
are implemented properly, the safety of the site workers and all personnel presence
at the proposed Project site would be safeguarded and there would be no adverse
impact anticipated on the feasibility of the proposed Project.