11.1.1
This section presents the risk assessment of landfill gas (LFG) hazards
arising from the construction and operational phases of the “Tseung Kwan O – Lam Tin
Tunnel and Association Works” (the Project).
11.1.2
The outlined
scope of the Project under this Assignment is to provide a highway connecting
TKO at Po Shun Road in the east and Trunk Road T2 in the west with the associated interchange. It comprises the following:
·
a dual two-lane
highway approximately 4.8 km long.
About 2.6 km of the
highway is in the form of
tunnel;
·
slip roads,
depressed roads, viaducts, TKO Interchange, ventilation building, tunnel portal
facilities and around
3 ha reclamation
on TKO side;
·
slip roads,
branch Tunnels,
viaducts, Lam Tin Interchange, tunnel portal facilities, ventilation and administration buildings on
Kowloon side;
·
the associated
building, civil, structural, marine, electrical and mechanical, traffic control
and surveillance system (TCSS), landscaping, and environmental protection and
mitigation works.
11.1.3
In particular,
the tunnels, roads and associated facilities at the Lam Tin Interchange fall
within the 250m Consultation Zone of the Sai Tso Wan Landfill (Figure 11.1).
11.1.4
Sai Tso Wan
Landfill was operated between 1978 and 1981. It covers about 14 ha and is estimated
to have received about 1.6 million tones of domestic and industrial waste. Waste is believed to be up to 50m deep. Prior to waste deposition, the site was underlain by
French drains for the collection of groundwater and leachate. On completion of waste deposition, the landfill was
capped with a final cover layer of silty sand containing a trace of clay. The final cover comprises depths
averaging 7.5m on the +80mPD afteruse platform and 3.5m on the side slopes.
11.2
Environmental
Legislation, Standards and Guidelines
11.2.1
Relevant
legislation and associated guidance notes applicable to the assessment of the
LFG hazards include:
·
Section 1.1(f) in
Annex 7 of the Technical Memorandum on EIAO (EIAO-TM);
·
Section 3.3 in
Annex 19 of the EIAO-TM;
·
Landfill Gas
Hazard Assessment for Development Adjacent to Landfills (ProPECC PN 3/96); and
·
Landfill Gas
Hazard Assessment Guidance Note (1997) (EPD/TR8/97, Guidance Note).
11.2.2
Under Annex 7 of
the EIAO-TM, an evaluation of the potential risk posed by landfill gas is
required for any development which is proposed within 250m of the edge of waste
of a landfill site, known as Landfill Consultation Zone. Since a section of the tunnels, roads
and associated facilities at the Lam Tin Interchange fall within the
Consultation Zone of Sai Tso Wan Landfill, a Qualitative Landfill Gas Hazards
Assessment (QLFGHA) is required to assess the potential risk due to landfill
gas migration from Sai Tso Wan Landfill to the Project Site and recommend the
mitigation measures to be undertaken for the Project.
11.2.3
ProPECC PN 3/96 and Guidance
Note for the assessment of the hazards which landfill gas may present to
developments close to landfills have been issued by the EPD. These documents provide an assessment
framework to be followed when evaluating the risks related to developments
described under Section 6.5, Chapter 9 of the Hong Kong Planning Standards and Guidelines. The ProPECC
PN 3/96 and Guidance Note apply
to all developments proposed within the Landfill Consultation Zone.
11.3
Assessment
Criteria and Methodology
Criteria
11.3.1
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.
Source
11.3.2
The
classification of the Source (i.e. Sai Tso Wan 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 (e.g. 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. |
11.3.3
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.
11.3.4
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 (e.g., 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 (e.g., 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 (e.g., size of
site and age of waste).
11.3.5
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.
11.3.6
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.
Pathway
11.3.7
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 |
11.3.8
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.
11.3.9
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.
11.3.10 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.
Target
11.3.11
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 authorized, 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. |
11.3.12 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 11.1. The potential implications associated
with the various qualitative risk categories are summarized in Table 11.2.
Table 11.1 Classification of Risk Category
|
Source |
Pathway |
Target Sensitivity |
Risk Category |
|
|
|
High |
Very
High |
|
|
Very
short / direct |
Medium |
High |
|
|
|
Low |
Medium |
|
|
|
High |
High |
|
Major |
Moderately
short / direct |
Medium |
Medium |
|
|
|
Low |
Low |
|
|
|
High |
High |
|
|
Long
/ indirect |
Medium |
Medium |
|
|
|
Low |
Low |
|
|
|
High |
High |
|
|
Very
short / direct |
Medium |
Medium |
|
|
|
Low |
Low |
|
|
|
High |
High |
|
Medium |
Moderately
short / direct |
Medium |
Medium |
|
|
|
Low |
Low |
|
|
|
High |
Medium |
|
|
Long
/ indirect |
Medium |
Low |
|
|
|
Low |
Very
Low |
|
|
|
High |
High |
|
|
Very
short / direct |
Medium |
Medium |
|
|
|
Low |
Low |
|
|
|
High |
Medium |
|
Minor |
Moderately
short / direct |
Medium |
Low |
|
|
|
Low |
Very
Low |
|
|
|
High |
Medium |
|
|
Long
/ indirect |
Medium |
Low |
|
|
|
Low |
Very
Low |
Table 11.2 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. |
11.3.13 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 11.3. The terms used in Table 11.3 are defined in Table
11.4.
Table 11.3 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 11.4 Definition of Control Terms
|
Term |
Definition |
|
Active Control |
Control of gas
by mechanical means e.g. 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 e.g. 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 System |
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. |
11.4
Assessment of
Potential Risk
Source
11.4.1
Sai Tso Wan
Landfill was operated between 1978 and 1981. The
restoration works under the Urban Landfills Restoration Contract on the Sai Tso
Wan Landfill commenced in April 1997 and were completed in May 1998. The Aftercare Period started in May 1998 for 20
years. The restoration works at Sai
Tso Wan Landfill has created a recreation ground which was opened for public
use in April 2004. An
active landfill gas management system has been constructed by the landfill restoration
contractor. The system comprises a
network of extraction pipes and wells, which deliver the gas to an on-site
landfill gas flare facility. The
final cover depths average 7.5m on the +80mPD afteruse platform and 3.5m on the
side slopes.
11.4.2
A number of gas
monitoring wells have been installed around Sai Tso Wan Landfill to monitor the
potential landfill gas migration.
The location of these monitoring wells is shown in Figure 11.2. The
monitoring data from November 2009 to October 2010 are summarised in Table 11.5 and
the details are presented in Appendix 11.1.
Table 11.5 Landfill Gas Monitoring Results in Sai Tso Wan Landfill (November 2009 to October 2010)
|
Well/
Probe |
Methane (%) |
Carbon
Dioxide (%) |
|||
|
Range |
Average |
Range |
Average |
||
|
DPR2 |
|
0.0 |
0.0 |
0.0-0.8 |
0.3 |
|
DPR3 |
|
0.0 |
0.0 |
0.0-1.2 |
0.3 |
|
DPR5 |
|
0.0 |
0.0 |
0.0-1.1 |
0.4 |
|
DPR11 |
|
0.0 |
0.0 |
0.0-0.2 |
0.0 |
|
DRP12 |
|
0.0 |
0.0 |
0.0 |
0.0 |
|
DRP13 |
|
0.0 |
0.0 |
0.0-0.1 |
0.0 |
|
DRP16 |
|
0.0 |
0.0 |
0.0-0.4 |
0.1 |
|
DRP18 |
|
0.0 |
0.0 |
0.0-0.7 |
0.2 |
|
DRP19 |
|
0.0 |
0.0 |
0.0-0.7 |
0.2 |
|
G1 |
A |
0.0 |
0.0 |
0.0-0.2 |
0.0 |
|
|
B |
0.0 |
0.0 |
0.0-1.0 |
0.2 |
|
|
C |
0.0 |
0.0 |
0.0-1.2 |
0.1 |
|
G2 |
A |
0.0 |
0.0 |
0.0-0.2 |
0.1 |
|
|
B |
0.0 |
0.0 |
0.0-2.0 |
0.4 |
|
|
C |
0.0 |
0.0 |
0.0-1.3 |
0.2 |
|
G3 |
A |
0.0 |
0.0 |
0.0-0.4 |
0.1 |
|
|
B |
0.0 |
0.0 |
0.0-0.4 |
0.1 |
|
|
C |
0.0 |
0.0 |
0.0-0.2 |
0.0 |
|
G4 |
A |
0.0 |
0.0 |
0.0-0.4 |
0.2 |
|
|
B |
0.0 |
0.0 |
0.0-0.4 |
0.2 |
|
|
C |
0.0 |
0.0 |
0.0-0.2 |
0.1 |
|
GW1 |
|
0.0 |
0.0 |
0.0-0.2 |
0.0 |
|
GW2 |
|
0.0 |
0.0 |
0.0-0.5 |
0.1 |
|
GW3 |
|
0.0 |
0.0 |
0.0-0.6 |
0.1 |
|
GW4 |
|
0.0 |
0.0 |
0.0-0.6 |
0.2 |
|
GW6 |
|
0.0 |
0.0 |
0.0-1.5 |
0.2 |
|
GW7 |
|
0.0 |
0.0 |
0.0-0.7 |
0.2 |
|
GW8 |
|
0.0 |
0.0 |
0.0-0.4 |
0.0 |
|
GW9 |
|
0.0 |
0.0 |
0.0-0.8 |
0.3 |
11.4.3
As shown in Table 11.5, the methane concentrations
in all perimeter gas monitoring wells are not detectable.
11.4.4
The carbon
dioxide concentration fluctuates between undetectable (0.0%) and 1.5% across
the monitoring wells. Elevated
carbon dioxide concentrations could be due to background variation or methane
being oxidized. In accordance with
the EPD’s Guidance Note, levels of CO2 that exceed 5% above
background would be considered “significant” migration. In the absence of any background CO2
information for reference, it is conservatively assumed that the potential for
off-site migration of landfill gas cannot be eliminated.
11.4.5
The quality of
groundwater monitored around Sai Tso Wan Landfill was also reviewed. The location of the groundwater
monitoring wells is shown in Figure 11.3. The monitoring results are presented in Appendix 11.1 and summarized in Table 11.6. In general, the organic content of the
groundwater is low. Hence, the
chance of methane generated from groundwater will be low.
Table 11.6 Groundwater Quality Monitoring Results in Sai Tso Wan Landfill (March 2010 and September 2010)
|
Well |
Chemical
Oxygen Demand (mg/L) |
Total Organic
Carbon (mg/L) |
||
|
Range |
Average |
Range |
Average |
|
|
GW1 |
<2-51 |
27 |
1-3 |
2 |
|
GW2 |
4-38 |
21 |
<1-3 |
2 |
|
GW3 |
<2 |
<2 |
<1 |
<1 |
|
GW4 |
3-30 |
17 |
1-3 |
2 |
|
GW6 |
4-23 |
14 |
<1-4 |
3 |
|
GW7 |
3-5 |
4 |
<1-1 |
1 |
|
GW8 |
5-7 |
6 |
<1-1 |
1 |
|
GW9 |
7-13 |
10 |
<1 |
<1 |
11.4.6
Given the small
size, age and current use as recreation ground of the landfill, the recent
landfill gas and groundwater monitoring results, the active landfill gas
management system, and based on previously approved EIA reports, namely the EIA
for further development of TKO, it would be reasonable to classify Sai Tso Wan
Landfill as a “Medium” source.
Pathway
11.4.7
According to Hong
Kong Geological Survey Map, the geological formation beneath Sai Tso Wan
Landfill comprises natural earth and waste. The geological formation outside the
boundary of Sai Tso Wan Landfill comprises fine to medium grained granite. It is noted that a Northwest to
Southeast trending fault line cuts across the Sai Tso Wan Landfill.
11.4.8
While groundwater
data at the Project sites is not available, recent groundwater monitoring data
has been obtained. The data is
contained in Appendix 11.1. The location of the groundwater
monitoring wells is shown in Figure 11.3
and the monitored groundwater level are summarised in Table 11.7.
Table 11.7 Groundwater Level in Sai Tso Wan Landfill (March 2010 and September 2010)
|
Borehole |
Groundwater
Level (mPD) |
||
|
Range |
Lowest |
Average |
|
|
GW1 |
11.11-12.97 |
11.11 |
12.04 |
|
GW2 |
44.68-49.18 |
44.68 |
46.93 |
|
GW3 |
50.94-51.57 |
50.94 |
51.26 |
|
GW4 |
54.64-54.67 |
54.64 |
54.66 |
|
GW6 |
42.80-49.34 |
42.80 |
46.07 |
|
GW7 |
13.00-15.71 |
13.00 |
14.36 |
|
GW8 |
17.99-18.70 |
17.99 |
18.35 |
|
GW9 |
27.72-27.73 |
27.72 |
27.73 |
11.4.9
The minimum
groundwater level ranged from +11.11 mPD to +54.64 mPD across the
landfill. The tunnels and
facilities are located at around -15 mPD to +5mPD, which will be around 6 m to 70
m below the groundwater table of Sai Tso Wan Landfill. As the solubility of
methane in water is low, the presence of groundwater generally provides a
barrier to the migration of landfill gas.
11.4.10 In general, the path length between Sai Tso Wan Landfill
and the construction area of project site is less than 50m to 250m, and
therefore the pathway in general is categorised as Very Short/Direct. It
should be noted that the final cover of the Sai Tso Wan Landfill is at
approximately +80mPD elevation.
11.4.11 The ventilation building of TKO-LT Tunnel is
approximately 50m south of the landfill at an elevation of approximately -16mPD,
and therefore the pathway is categorised as Moderately Short/Direct.
11.4.12 The TKO-LT Tunnel that connects to Tseung Kwan O is
approximately 60m south of the landfill, and therefore such pathway is
categorised as Moderately Short/Direct.
The slip road tunnel S2 is approximate 65m south of the landfill, and therefore
such pathway is also categorised as Moderately
Short/Direct. While the tunnel that connects to Trunk Road T2 is
approximately 195m southwest from the landfill, and therefore the pathway is
categorised as Long /Indirect.
11.4.13 The administration building, training ground,
detention area, fuel filling station, electric vehicle charging station, sewage
pumping station and car parking are located at a platform approximately 120m
southwest to the Sai Tso Wan Landfill at an elevation of approximately +5mPD,
and therefore the pathway is categorised as Long /Indirect.
11.4.14 The recovery vehicle bases are located at least 120m
south of the landfill at an elevation of approximately -13mPD, and therefore
the pathway is categorised as Long/Indirect.
11.4.15 The drainage pumping station is located at least 143m
south of the landfill at an elevation of approximately -22mPD, and therefore
the pathway is categorised as Long/Indirect.
Target
Construction Phase
11.4.16 During construction, the project site would be
occupied by construction workers who are well trained and with proper and safe
construction methodology to be followed.
Also, the construction would be mainly carried out in an outdoor
environment. Therefore, in general
the group is considered as Low
Sensitivity targets except for specific targets as below.
11.4.17 For deep excavations and other construction activities
in indoor or enclosed areas, such as construction of tunnels, administration
building, ventilation building, plant rooms, etc, the works would be carried
out by trained construction workers with proper construction methodologies and
safety procedures to be followed.
Therefore, this group is considered as Medium Sensitivity targets.
Operational Phase
11.4.18 As shown in Figure
11.1, part of the TKO-LT Tunnel, particularly the tunnels, roads,
buildings, tunnel facilities of the Lam Tin Interchange, falls within the 250m
Consultation Zone of the Sai Tso Wan Landfill.
11.4.19 The buildings, training ground, car parking and other tunnel
facilities are located approximately 50m to 250m from the landfill at elevation
of +5mPD to -22mPD. These building
would be occupied by operational staff, who will have been briefed on the
potential hazards relating to the landfill gas. The target sensitivity of these groups
are categorised as Medium Sensitivity.
11.4.20 For maintenance activities within service voids,
manholes, and other confined spaces, if any, the works should be performed by
authorized and 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
group is categorized as Medium Sensitivity.
11.4.21 Both the tunnels connecting to Trunk Road T2 and to
TKO are longer than 230m and therefore mechanical ventilation is required. These mechanically ventilated tunnels
would be accessed by vehicles from the general public who might not be aware of
the potential landfill gas hazard.
However, as the tunnels are mechanically ventilated and the area is less
likely to be susceptible to accumulation of landfill gas, and therefore this
group is categorized as Medium
Sensitivity.
Source-Pathway-Target Analysis
11.4.22 Based on the information above, qualitative
source-pathway-target analysis has been undertaken and is summarized in Table 11.8 according to EPD’s
assessment framework.
Table 11.8 Qualitative Risk Assessment Matrix
|
Source |
Pathway |
Target Sensitivity |
Risk |
|
From
Sai Tso Wan Landfill - active landfill gas management system (Medium
Source) |
During
Construction |
||
|
Path length of less than 50m for unsaturated
permeable soil or fissured rock (Very Short/ Direct Pathway) |
Construction workers in general outdoor
construction area (Low Sensitivity Target) |
Low |
|
|
|
Construction workers for deep excavation and
other works at indoor/enclosed area (construction of buildings, plant rooms,
tunnels, etc.) (Medium
Sensitivity Target) |
Medium |
|
|
|
During
Operation |
||
|
Path
length of 50m-100m for unsaturated permeable soil or fissured rock (Moderately Short/ Direct Pathway) |
Staffs and maintenance workers working at
indoor area: Administration Building, maintenance vehicle
garage/workshop, Ventilation Building (Medium Sensitivity Target) |
Medium |
|
|
Vehicles inside mechanically ventilated
tunnel connects to TKO (Medium Sensitivity Target) |
Medium |
||
|
Path
length of 100-250m for unsaturated permeable soil or fissured rock (Long / Indirect Pathway) |
Staffs and maintenance workers working at
indoor area of Tunnel Facilities (Medium Sensitivity Target) |
Low |
|
|
|
Vehicles inside mechanically ventilated
tunnel connects to Trunk Road T2 (Medium Sensitivity Target) |
Low |
|
11.4.23 The Overall
Risk Levels for both construction phase and operational phases associated
with the Sai Tso Wan Landfill are Medium,
based on the highest level of risk for the potential impacts identified.
According to EPD’s Guidance Note,
engineering measures will be required to protect the project site.
11.5.1
This section
provides general advice and recommendations to minimize the landfill gas risks
during the construction and operation of the Project.
General Hazards Related to
Landfill Gas
11.5.2
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.
11.5.3
All contractors
participating in the works and operational staff should be aware of that
methane and carbon dioxide may present in the soil and all works should be
undertaken on the basis of an "assumed presence of landfill
gas". In addition the
following properties of landfill gas should be noted.
11.5.4
Methane is odorless
and colorless, although in landfill gas it is typically associated with
numerous highly odoriferous compounds which give some warning of its
presence. However, the absence of odour
should not be taken to mean that there is no methane. Methane levels can only be reliably
confirmed by using appropriately calibrated portable methane detectors.
11.5.5
Landfill gas
migration may be a hazard because of the combustibility and in some cases explosive
nature of methane. Methane is a flammable gas and will burn when mixed with air
between approximately 5 and 15% (v/v) (the Lower Explosive Limit (LEL) and
Upper Explosive Limit (UEL) respectively).
If a mixture of methane and air with a composition between these two
values is ignited in a confined space, the resulting combustion may give rise
to an explosion. Methane is also an
asphyxiant.
11.5.6
Carbon dioxide,
the other major component of landfill gas is an asphyxiating gas and causes
adverse health effects at relatively low concentrations. The long-term Occupational Exposure
Limit (OEL) is 0.5% (v/v). Like
methane, it is odourless and colourless and its presence (or absence) can only
be confirmed by using appropriately calibrated portable detectors.
11.5.7
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 exposed 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.
Recommended Precautionary
Measures
11.5.8
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 control. Detection
systems in some situations”. The
recommended preventive measures are presented below.
Construction Phase
Appointment
of Safety Officer
11.5.9
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 provided 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
0-100% v/v |
|
Carbon dioxide |
0-100% |
|
Oxygen |
0-21% |
Safety Measures
11.5.10 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.
11.5.11 An excavation procedure or code of practice to
minimize landfill gas related risk should be devised and carried out.
11.5.12 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.
11.5.13 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.
11.5.14 Welding, flame-cutting or other hot works should be
confined to open areas at least 15m from any trench or excavation.
11.5.15 Welding, flame-cutting or other hot works may 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).
11.5.16 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.
11.5.17 Where there are any temporary site offices, or any
other buildings located within the Sai Tso Wan 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.
11.5.18 Any electrical equipment, such as motors and extension
cords, should be intrinsically safe.
11.5.19 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.
11.5.20 During construction, adequate fire extinguishing
equipment, fire-resistant clothing and breathing apparatus (BA) sets should be
made available on site.
11.5.21 Fire drills should be organized at not less than six
monthly intervals.
11.5.22 The contractor should formulate a health and safety
policy, standards and instructions for site personnel to follow.
11.5.23 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.
11.5.24 Service runs within the Consultation Zone should be
designated as “special routes”; 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).
11.5.25 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
11.5.26 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.
11.5.27 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
throughout the working day whilst workers are in the excavation.
11.5.28 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.
11.5.29 For excavations less
than 300mm deep, monitoring may be omitted, at the discretion of the Safety
Officer or other appropriately qualified person.
11.5.30 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 11.9.
Table 11.9 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% |
11.5.31 The exact frequency of monitoring should be determined
prior to the commencement of works, but should be at least once per day, and be
carried out by a suitably qualified or qualified person before starting the
work of the day. Measurements shall
be recorded and kept as a record of safe working conditions with copies of the
site diary and submitted to the Engineer for approval. The Contractor may elect to carry out
monitoring via an automated monitoring system. In this event, the gas levels specified
in Table 11.8 shall be so programmed
to automate the actions in the table and in the event of the trigger levels
being breached, to activate suitable audible and visual warning devices.
11.5.32 The hazards from landfill gas during the construction
stage within the Sai Tso Wan 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 11.2. In the operational 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.
Design Phase
11.5.33 According to Table
11.2, engineering measures are required to protect the tunnel, interchange
and its facilities from the landfill gas risk due to the Sai Tso Wan
Landfill. These preventive measures
required include the use of ‘semi active’ or enhanced passive gas controls, as
well as detection system in some situations. However, as the details of the Project
are not yet available, only generic preventive measures can be
recommended. At the subsequent
detailed design stage, this assessment shall be reviewed and the detailed
design (drawings and specification) of the precautionary measures as well as
the requirement for maintenance and monitoring should be prepared by a
competent person and should be submitted to EPD for vetting.
Building
Protection Design Measures
11.5.34 Engineering measures for building structures with
ground level or below ground rooms / voids (such as buildings, plant rooms,
workshops) including the following should be adopted in the detailed design:
·
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. Typical details of the gas resistant
membrane is shown in Figure 11.4;
·
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 (Figure 11.5 refers);
·
Synthetic
composite geotextiles which provide a free-venting cellular structure and
provide preferential pathways for release of gas;
·
Provision of
mechanical ventilation to ensure sufficient air change at all time.
Design
Measures for Sub-Surface Building Services
11.5.35 As shown in Figures
11.6 to 11.10, 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;
·
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; and
·
For building
services penetrating through the ground, collar seal should be adopted to
prevent gas ingress into the room where the service pipes/cables enter the
building.
Design
Measures for Tunnels within Consultation Zone
11.5.36 The recommendations given below are designed to reduce
the risk of gas ingress to the tunnels sections that falls within the Sai Tso
Wan Landfill Consultation Zone.
·
A low
permeability gas membrane to be incorporated into the lining of the tunnel
sections that fall within the landfill Consultation Zone.
·
Adopt a
conservative lapping and sealing method for the membrane lining and ensure
rigorous protection/inspection measures are enabled during the
placement/sealing and joining of the gas membrane during construction.
·
Consider the
cost-effectiveness of the use of additional internal join/surface
sealants/liners/finishes, etc, with the proposed construction joint design or
pursue superior designs at the tunnel interface specifically to combat cracking
and aid watertightness over the lifetime of the tunnels.
·
Provision of
mechanical ventilation to ensure sufficient air change at all time.
Design
of LFG Precautionary Measures
11.5.37 When the detailed design of the Project is available,
the detailed design consultant / contractor is required to undertake review on
this assessment taking into account of the more readily available detailed
information to finalize the design of the landfill gas precautionary measures
recommended in this report. The
detailed design of gas precautionary measures and a landfill gas monitoring
programme should be submitted to EPD for vetting.
11.5.38 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 precautionary measures. The design should also be checked and
certified by a qualified independent consultant. The contractor should ensure that the required
precautionary measures are implemented and constructed in accordance with the design
and maintenance and monitoring programme should be established as a
precautionary measure.
11.5.39 Along with the detailed precautionary measure designs,
the monitoring programme and detailed actions should be submitted to EPD for
approval in the detailed design stage.
Operational Phase
Guidance
for Entry into Service Rooms / Voids, Manholes and Chambers
11.5.40 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 Code of Practice
for Safety and Health at Work in Confined Space should be followed to
ensure compliance with the Regulation.
11.5.41 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.
11.5.42 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.
Other
Safety Measures and LFG Monitoring During Operation
11.5.43 Operational staff and maintenance workers should be
informed of the potential LFG hazards, and appropriate safety procedures (such
as guidance for entering confined area as indicated above) should be followed.
11.5.44 During operation, regular monitoring of methane,
carbon dioxide and oxygen should be done at the tunnel, subway, and any other
underground structures within the landfill consultation zone a precautionary
measure.
11.5.45 Inspection and LFG monitoring should be carried out at
buildings and enclosures (e.g. administration building, ventilation building,
workshop, tunnel etc) prior to the operation as preventive measures. The monitoring should be continued
through the operation of the Project.
In particular for the first year of operation, monthly monitoring is
recommended. Should the monitoring
reveal the presence of landfill gas within the tunnel, buildings or other
confined area, the seal of the joints shall be inspected and consideration
shall be given to seal the cracks.
11.5.46 In addition, if any construction is required for the
maintenance work during operational stage, the responsible party should follow
the precautionary measures and monitoring works as recommend in Sections 11.5.9 to 11.5.32 of this report.
11.6
Environmental
Monitoring and Audit
11.6.1
For the
construction and operation within the Consultation Zone of Sai Tso Wan Landfill,
the monitoring requirement specified in Sections 11.5.26 to 11.5.31 and Sections 11.5.43 to 11.5.46 shall be followed.
11.7.1
The landfill gas
hazard assessment shows that the overall level of landfill gas risk posed by
the Sai Tso Wan Landfill onto the Lam Tin Interchange section and related
tunnel facilities of the Project would be categorised as “Medium”. Appropriate precautionary
measures have been proposed to minimize the landfill gas risk for the proposed
project site during construction and operational phases. In particular, it is noted that landfill
gas membrane should be installed at the tunnels, underground structures, and
basement & ground floor of the buildings within the Consultation Zone of
the Sai Tso Wan Landfill. Routine
monitoring is recommended as a precautionary measure.
11.7.2
As the detailed
information on the Project (e.g. Design) is not yet available, this assessment
is based on best available information to date. A review on this report should be
conducted to finalize the protection design based on the finalized project
information at detailed design stage.
The detailed design of the LFG precautionary measures and monitoring
programmes shall be submitted to EPD for vetting. The requirement of submitting the precautionary
measures designs, detailed LFG hazard assessment, and a detailed monitoring
programme should be included in the scope of the subsequent detailed design
study.
11.7.3
Provided that all
the recommended precautionary measures are implemented properly, the safety of
all personnel and general public (i.e. passengers of vehicles using the TKO-LT Tunnel)
presence at the proposed Project site would be safeguarded and there would be
no adverse impact anticipated on the feasibility of the proposed Project.
11.7.4
The landfill gas hazard assessment has
been conducted in accordance to and compiled with the section 1.1(f) in Annex 7
and section 3.3 in Annex 19 of the EIAO-TM and “Landfill Gas Hazard Assessment
Guidance Note” (1997) issued by EPD.