14.1
A section of the
alignment will be located underneath the Ngau Tam Mei Landfill (NTML) and the Barging
Point in Kwai Chung (Rambler Channel) and the Nursery Site in Siu Lang Shui
will be located close to the Gin Drinkers Bay Landfill (GDBL) and Siu Lang Shui
Landfill (SLSL), respectively (see Figure
Nos. NOL/ERL/300/C/XRL/ENS/M61/101 to /103).
14.2
The section of
Project alignment underneath the NTML will be constructed at about -16 mPD ([1]) comprises twin parallel bored tunnels. There will be no works above ground level in
the vicinity of the NTML.
14.3
The Barging Point in Kwai
Chung will be used for unloading of plant and equipment and construction
materials from barges to trucks for onward transport to the Project sites
during the construction phase. There
will be no excavation works or underground structure within the Barging Point
site. There will be a container-type
site office which will be raised above ground.
The Barging Point will only be used during the construction phase of the
Project.
14.4
The Nursery Site in Siu Lang Shui will be used for
transplanting trees affected by the Project during the construction phase. Shallow excavation by mechanical equipment
will be required for placing the trees. Towards the end of
the construction period, the trees within the Nursery Site will be removed and
transferred back to the area previously affected by the Project construction
works. The Nursery Site will not be used
during the operational phase.
14.5
There are potential risks associated with developments close
to a landfill site due to sub-surface migration of landfill gas. This Section describes the methodology and
presents the findings of a qualitative landfill gas hazard assessments of the
Project.
Environmental Legislation, Standards and Guidelines
14.6
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 and two associated work sites fall within the Consultation Zone of
NTML, GDBL and SLSL, respectively, a Qualitative Landfill Gas Hazards
Assessment (QLFGHA) is required to assess the potential risk due to landfill
gas migration from these landfills to the Project Site.
14.7
A Practice Note for
Professional Person (ProPECC PN 3/96)
([2])
and Guidance Note([3])
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.
Assessment
Criteria and Methodology
14.8
In accordance with the Guidance
Note on Landfill Gas Hazard Assessment, the risk due to landfill gas may be
evaluated based upon the following three criteria:
·
Source - the rate and
concentration of gas generation by the landfill;
·
Pathway - the nature of and
length of potential pathways through which landfill gas can migrate and
leachate flow, such as geological strata, utility services; and
·
Target - the level of
vulnerability of various elements of the development to landfill gas.
14.9
Each of these criteria is further described in the
sub-sections below.
14.10
The classification of the Source (ie the landfill) is
determined as follows:
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.
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.
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 6 months, is less than 5% (v/v).
Pathway
14.11
Generally, three types of pathway are considered for the
transmission of landfill gas. They are:
·
Man-made
pathways, eg utility connections,
stormwater channels, etc,
·
Natural
pathways such as rock jointing planes, fissures and other naturally occurring
phenomena which may promote or give rise to the transmission of gas over
distances; and
·
A combination of the previous two
categories. An example of the latter may
be, for instance, where a specific geological feature promotes gas transmission
but which stops short of directly linking the landfill and target. A man made connection, however may also
co-exist near the edge of the geological feature, which in combination with the
former, may act to link the two sites.
In this instance, careful assessment of the likelihood of the mechanism
acting to link the two pathways needs to be undertaken before assigning an
appropriate pathway classification.
14.12
The broad classification of a Pathway is 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 to 100m for unsaturated permeable soil or fissured rock or 100
to 250 m for man-made conduits |
|
Long/indirect |
Path
length of 100 to 250m for unsaturated permeable soils and fissured rock |
14.13
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:
·
a broad assessment of
the specific permeability of the soil;
·
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 groundwater flow patterns.
Target
14.14
Different levels of vulnerability or sensitivity of
potential targets for landfill gas have been 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 · 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. |
14.15
The above examples of different categories within each
criterion are to be used as a general guide only and specific aspects of a
development may render it more or less sensitive than indicated. Account needs to be taken of any particular
circumstances when assigning a target to one of the three indicated categories.
Assessment of Risk Criteria
14.16
Following the determination of the categories for the
source, pathway and target in which the landfill, pathway and development fall,
a qualitative assessment of the overall risk may be made by reference to Table 14.1 which is extracted from the EPD’s
Guidance Note on Landfill Gas Hazard Assessment. The potential implications associated with
the various qualitative risk categories are summarised in Table 14.2. It should be
noted that the different levels of risk determine the likely extent of the
protection measures required to ensure the safety of a development, but with
the possible exception of the very high risk category, development is not
precluded for any of the assessed levels of risk.
Table 14.1 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 14.2 Summary of
General Categorisations of Risk
|
Level of Risk |
Implication |
|
Very high |
At the
very least, extensive engineering measures and alarm systems are likely to be
required. An emergency actions plan
should also be developed so that appropriate actions may be immediately taken
in the event of high gas concentrations being detected within the
development. |
|
High |
Significant engineering
measures will be required to protect the planned development. |
|
Medium |
Engineering measures
required to protect the development. |
|
Low |
Some precautionary
measures will be required to ensure that the planned development is safe. |
|
Very low |
No protection or
precautionary measures are required. |
The Source
Ngau Tam Mei Landfill
14.17
NTML is a 2 hectare landfill site located to the north-east
of Yuen Long and to the east of the
14.18
The landfill was operated between 1973 and 1975, although
uncontrolled dumping occurred prior to 1973.
The amount of waste deposited was estimated to be around 90,000 m3,
mainly industrial and domestic waste.
14.19
The landfill was formed in a sloping valley with bow-shaped
hills flanking the site along the western, northern and eastern
boundaries. The profile of the landfill
comprise two platforms separated by a gentle slope. The base of the landfill was not lined. A rock bund was installed at the
south-western toe of the landfill for stability purposes and this is likely to
facilitate dissipation and venting of landfill gas.
14.20
The landfill was restored between 1999 and 2000 under Contract No. EP/SP/30/95 North West New
Territories Landfills and Gin Drinkers Bay Landfill Restoration let by the
EPD. As part of the restoration works,
an impermeable cap was installed on the platforms. Horizontal gas collectors and passive gas
vents were installed.
14.21
A number of gas monitoring wells have been installed around
the site to monitor the potential landfill gas migration. The location of these monitoring wells is
shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/104.
The monitoring data from October 2006 to September 2008 are presented in
Appendix 14.1 and are summarised in Table
14.3.
Table 14.3 Landfill Gas Monitoring Results in NTML (October 2006 to September 2008)
|
Borehole |
Methane (%v/v) |
Carbon Dioxide (%v/v) |
||
|
|
Range |
Average |
Range |
Average |
|
A451 |
<0.1 |
<0.1 |
0.7
– 8.1 |
6.2 |
|
A452 |
<0.1
– 0.5 |
0.1 |
2.4
– 19.7 |
11.7 |
|
A453 |
<0.1 |
<0.1 |
0.6
– 4.2 |
2.6 |
|
A454 |
<0.1 |
<0.1 |
22.7
– 16.9 |
11.6 |
|
A455 |
<0.1
– 3.8 |
1.0 |
0.3
– 12.0 |
3.3 |
|
A456 |
<0.1 |
<0.1 |
2.5
– 3.7 |
3.7 |
|
A457 |
<0.1 |
<0.1 |
0.1
– 3.4 |
1.8 |
|
A457A |
<0.1 |
<0.1 |
<0.1 |
<0.1 |
|
A458 |
<0.1
– 4.0 |
1.6 |
0.2 – 9.7 |
2.3 |
|
A459 |
<0.1 |
<0.1 |
1.6 – 12.9 |
9.7 |
|
A460 |
<0.1 |
<0.1 |
1.3 – 10.8 |
7.2 |
|
A461 |
<0.1 |
<0.1 |
0.7 – 4.4 |
2.7 |
|
DH403 |
<0.1 |
<0.1 |
0.9 – 9.5 |
4.4 |
|
DH405 |
<0.1 |
<0.1 |
0.3 – 7.9 |
4.5 |
|
DH407 |
<0.1 |
<0.1 |
0.1 – 8.6 |
1.6 |
|
DH408 |
<0.1 |
<0.1 |
<0.1 – 7.9 |
1.4 |
14.22
As shown in Table
14.3, the methane concentrations in most of the perimeter gas monitoring
wells are less than 0.1%. There have
been elevated concentrations of methane in well nos. A455 and A458. These elevated concentrations were reported to
be due to the decomposition
of localised deposit of vegetation/waste materials in the area outside the
landfill. The restoration contractor had
carried out investigation by setting up gas probes at various location along
the boundary of NTML and did not record any gas migration from the
landfill.
14.23
The
carbon dioxide concentration fluctuates between <0.1% and 19.7% 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.
14.24
The
quality of groundwater monitored around the NTML was also reviewed. The location of the
groundwater monitoring wells is shown in Drawing
NOL/ERL/300/C/XRL/ENS/M61/104. The
monitoring results are presented in Appendix 14.1 and summarized in Table 14.4. In general, the organic content of the
groundwater is low. Hence, the chance of
methane generated from groundwater will be low.
Table 14.4 Groundwater Quality Monitoring Results
in NTML (October 2006 to July 2008)
|
Borehole |
Chemical Oxygen Demand (mg/L) |
Total Organic Carbon
(mg/L) |
||
|
|
Range |
Average |
Range |
Average |
|
GW1 |
<2 - 16 |
8 |
<1 - 1 |
1 |
|
A458 |
6 - 28 |
14 |
2 - 5 |
4 |
|
DH403 |
2 - 28 |
8 |
1 - 3 |
2 |
|
DH404 |
10 - 70 |
27 |
1 - 6 |
3 |
|
DH405 |
3 - 87 |
17 |
1 - 3 |
2 |
|
DH407 |
4 - 52 |
15 |
<1 - 9 |
6 |
14.25
Given the
small size and age of the landfill, the passive nature of landfill gas control
measures and the recent landfill gas and groundwater monitoring results, it would
be reasonable to classify the NTML as a “medium” source.
14.26
14.27
As part of the
restoration works of the GDBL under Contract
No. EP/SP/30/95 North West New Territories Landfills and Gin Drinkers Bay
Landfill Restoration let by the EPD, a number of active gas extraction
wells were installed in the waste. In
addition, a horizontal perimeter trench with an active gas collection system
was constructed around the entire site to extract and flare or utilise the
landfill gas.
14.28
The portion where
MTRC's Lantau and Airport Railway cut through the waste have been capped with a
polyethylene membrane and restoration soil.
The intention was to control the infiltration of rainwater and the
release of landfill gas.
14.29
A number of gas monitoring wells have been installed around
the site to monitor the potential off-site landfill gas migration. The location of the monitoring wells closed
to the Barging Point is shown in Drawing
NOL/ERL/300/C/XRL/ENS/M61/105. The monitoring data from October 2006 to September
2008 are presented in Appendix 14.1 and are summarised in Table 14.5.
Table 14.5 Landfill Gas Monitoring Results in GDBL
(October 2006 to September 2008)
|
Borehole |
Methane (%v/v) |
Carbon Dioxide (%v/v) |
||
|
|
Range |
Average |
Range |
Average |
|
GDBGW5 |
<0.1 |
<0.1 |
<0.1 – 3.1 |
0.6 |
|
GW1 |
<0.1 |
<0.1 |
<0.1 – 3.2 |
1.3 |
|
GW2 |
<0.1 |
<0.1 |
0.2 – 12.1 |
5.5 |
|
LF55 |
<0.1 |
<0.1 |
<0.1 – 0.5 |
0.2 |
|
LF56 |
<0.1 |
<0.1 |
0.2 – 0.6 |
0.4 |
|
SVB4 |
<0.1 |
<0.1 |
<0.1 – 0.2 |
0.2 |
|
SVB5 |
<0.1 |
<0.1 |
<0.1 |
<0.1 |
14.30
As shown in Table
14.5, the methane concentrations in all monitoring wells are less than
0.1%. The carbon dioxide concentrations
are generally well below 5% except for GW2.
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.
14.31
The
quality of groundwater monitored around the GDBL was also reviewed. The location of the
groundwater monitoring wells is shown in Drawing
NOL/ERL/300/C/XRL/ENS/M61/105. The
monitoring results are presented in Appendix 14.1 and summarized in Table 14.6. In general, the organic content of the
groundwater is low. Hence, the chance of
methane generated from groundwater will be low.
Table 14.6 Groundwater Quality Monitoring Results
in GDBL (December 2006 to September 2008)
|
Borehole |
Chemical Oxygen Demand (mg/L) |
Total Organic Carbon
(mg/L) |
||
|
|
Range |
Average |
Range |
Average |
|
GW1 |
<25 - 100 |
61.4 |
4 - 18 |
12.5 |
|
GDBGW5 |
<25 - 193 |
118 |
8 - 20 |
13 |
|
LF56 |
122 - 172 |
147 |
<1 |
<1 |
14.32
Given the
large size of the landfill and the landfill is still actively generating
landfill gas, the active nature of landfill gas control measures and the recent
landfill gas and groundwater monitoring results, it is conservatively
classified the GDBL as a “medium” source.
Siu Lang Shui Landfill
14.33
Siu Lang Sui Landfill
(SLSL) occupies an area of 12ha and is located south of the
14.34
The landfill was
formed in a valley and infilling abutted the steep valley sides. The landfill was initially infilled as two
separate areas – firstly to the east and subsequently to the west. Two streams that drained the hill slopes were
diverted in stages via a box culvert beginning at the north edge of the site,
and infilling then continues over the box culvert. The average depth of the landfill is about
10m, although it reaches a maximum of 40m in some places.
14.35
The site was
operational from November 1978 to December 1983 and approximately 1.2 million
tonnes of mostly domestic and industrial wastes, as well as incinerator ash and
green wastes, were deposited during this period ([4]). After closure of SLSL, a soil covering layer of
compacted gravely silty sand varying between 2m and 6m in thickness was placed
directly onto the deposited wastes, and trees were planted.
14.36
The landfill gas
management system comprised vertical and horizontal passive gas vents, which
were installed at regular intervals in the waste and extended as tipping
progressed. A gravel layer was placed at
the top of each waste platform with interconnecting gas vent pipes prior to
placement of the cover soils.
Restoration of the SLSL took place in 1999 to 2000 under Contract No. EP/SP/30/95 North West New
Territories Landfills and Gin Drinkers Bay Landfill Restoration led by
EPD. The landfill now relies solely on
the vertical and horizontal passive gas vents for venting the landfill gas.
14.37
A number of gas monitoring wells have been installed around
the site to monitor the potential off-site landfill gas migration. The location of monitoring wells is shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/106.
The monitoring data from October 2006 to September 2008 are presented in
Appendix 14.1 and are summarised in Table
14.7.
Table 14.7 Landfill Gas Monitoring Results in SLSL
(October 2006 to September 2008)
|
Borehole |
Methane (%v/v) |
Carbon Dioxide (%v/v) |
||
|
|
Range |
Average |
Range |
Average |
|
DH201 |
<0.1 |
<0.1 |
0.7 – 7.9 |
5.3 |
|
DH203A |
<0.1 |
<0.1 |
0.8 – 11.7 |
5.7 |
|
DH204 |
<0.1 |
<0.1 |
0.1 – 11.7 |
1.4 |
|
DP220 |
<0.1 |
<0.1 |
<0.1 – 0.1 |
<0.1 |
|
DP221 |
<0.1 |
<0.1 |
1.1 – 11.4 |
6.5 |
|
DP223 |
<0.1 |
<0.1 |
2.4 – 15.1 |
8.3 |
|
DP224 |
<0.1 |
<0.1 |
2.0 – 12.4 |
8.3 |
14.38
As shown in Table
14.5, the methane concentrations in all monitoring wells are less than
0.1%. The
carbon dioxide concentration fluctuates between <0.1% and 11.4% across the
wells. 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.
14.39
The
quality of groundwater monitored around the SLSL was also reviewed. The location of the
groundwater monitoring wells is shown in Drawing
NOL/ERL/300/C/XRL/ENS/M61/106. The
monitoring results are presented in Appendix 14.1 and summarized in Table 14.8. In general, the organic content of the
groundwater is low. Hence, the chance of
methane generated from groundwater will be low.
Table 14.8 Groundwater Quality Monitoring Results
in SLSL (October 2006 to October 2008)
|
Borehole |
Chemical Oxygen Demand (mg/L) |
Total Organic Carbon
(mg/L) |
||
|
|
Range |
Average |
Range |
Average |
|
GW1 |
<2 - 3 |
3 |
<1 - 1 |
1 |
|
GW2 |
<2 - 8 |
4 |
<1 - 2 |
2 |
|
GW3 |
<2 - 4 |
3 |
<1 |
<1 |
|
DH201 |
16 - 74 |
34 |
<1 - 26 |
11 |
|
DH203A |
22 - 47 |
32 |
<1 - 8 |
6 |
|
DH204 |
12 - 38 |
23 |
5 |
5 |
14.40
Given the small size
and age of the landfill, the passive nature of landfill gas control measures
and the recent landfill gas and groundwater monitoring results, it would be reasonable
to classify the SLSL as a “medium” source.
The Pathway
General
14.41
The potential
pathways through which landfill gas may enter a Project site are three fold,
namely:
·
Through transmission
along natural pathways such as fissures or joints in rock;
·
Man-made pathways
such as through permeable backfill in utilities trenches; or
·
A combination of both.
NTML to Tunnels
14.42 Parts of the NTML site are underlain by sedimentary and volcanic rocks consisting of coarse ash crystal tuff that is slightly metamorphosed. On the lowland to the south and to the north there are debris flow deposits, consisting of silt and sand, gravel and clay with cobbles and boulders. The piezometric levels indicate a steep hydraulic gradient from north-east to south-west. There is no fault or fissures presence below the landfill. The geological map of the area is shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/107
14.43 Recent groundwater monitoring data has been obtained. The data is contained in Appendix 14.1. The location of the groundwater monitoring wells is shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/104 and the monitored groundwater level are summarised in Table 14.9. The minimum groundwater level ranged from 4.74 mPD to 16.06 mPD across the landfill. The XRL tunnels are located at around -16 mPD, which will be around 21 m to 32 m below the groundwater table as shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/114.
Table 14.9 Groundwater Level in NTML (August 2003
to August 2008)
|
Borehole |
Groundwater Level
(mPD) |
||
|
|
Range |
Lowest |
Average |
|
DH407 |
10.51 – 17.54 |
10.51 |
12.78 |
|
DH408 |
16.06 - 35.07 |
16.06 |
20.97 |
|
GW1 |
11.05 – 27.44 |
11.05 |
21.91 |
|
DH403 |
5.05 – 14.43 |
5.05 |
7.64 |
|
DH404 |
5.80 – 15.66 |
5.80 |
8.72 |
|
A458 |
4.74 – 14.96 |
4.74 |
8.68 |
|
DH405 |
5.18 – 11.04 |
5.18 |
7.45 |
14.44
As the solubility of
methane in water is low, the presence of groundwater generally provide a
barrier to the migration of landfill gas.
Based on the geological setting and groundwater table of the NTML area, it
is expected that the potential for landfill gas migration via natural pathway
is low. As discussed in Section 15,
groundwater flow into the tunnels will be effectively precluded based on the
chosen method of construction and the designed tunnel lining system. Hence, with no groundwater flowing to the
tunnels, it is expected that the presence of the tunnels will not alter the
existing hydrogeologic regime of the area.
In addition, as methane is lighter than air, any methane in the soil
would tend to migrate sideway or upward instead of downward. The potential of migration of methane from
the landfill to the proposed tunnels is considered very low.
14.45 Utility check indicates that there are no underground utilities below the landfill. There will not be any utility connecting the tunnels and the NTML or in the vicinity of the landfill.
14.46
Based on the vertical
profile between the NTML and the tunnels, and the characteristics of the
geological and hydrogeological setting, the pathway between the NTML and the
tunnels are filled with groundwater, which is a good barrier to landfill gas
migration. Hence, although the vertical
separation distance between the NTML and the tunnels is about 36m, the pathway
for potential landfill gas migration from NTML to the tunnels is classified as
“long / indirect”.
GDBL to Barging Point
14.47
The proposed Barging
Point for transportation of plant and equipment and construction material is
located at approximately 120m from the GDBL.
No surface work will be carried out at the Barging Point. The ground separating the Barging Point and
the GDBL is paved and comprises the Kwai Chung Incinerator (decommissioned and
being demolished),
14.48 While groundwater data at the Barging Point is not available, recent groundwater monitoring data at GDBL has been obtained to identify the groundwater level in the area. The data is contained in Appendix 14.1. The location of the groundwater monitoring wells closest to the Barging Point is shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/105 and the monitored groundwater level are summarised in Table 14.10. The average groundwater level ranged from 2.67m to 4.40m below the well head across the landfill. While information pertaining the well head level is not available, it is conservatively assumed that the unsaturated zone for potential landfill gas could range from 2.67m to 4.40m.
Table 14.10 Groundwater Level in GDBL (December 2006
to September 2008)
|
Borehole |
Groundwater Level (m
below well head) |
|
|
|
Range |
Average |
|
GDBGW5 |
2.96 – 4.37 |
3.66 |
|
GW1 |
3.13 – 4.04 |
3.77 |
|
GW2 |
0.60 – 3.85 |
2.67 |
|
LF56 |
4.05 – 4.74 |
4.40 |
14.49
The underground
utilities between the GDBL and the Barging Point is shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/109. Underground utilities are presence along
14.50 Based on the distance between the GDBL and the Barging Point, the characteristics of the geological setting and the groundwater level, and the presence of underground utilities, the pathway for potential landfill gas migration from GDBL to the Baring Point is classified as “long / indirect”.
SLSL to Nursery Site
14.51
The Nursery Site is
located at approximately 110 m from the SLSL.
Part of the SLSL and the Nursery Site are underlain by volcanic rocks
consisting of granite, basalt and quartzphyric rhyolite. No faults or fissures are presence between
the SLSL and the Nursery Site. The
geological map is shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/110.
14.52 While groundwater data at the Nursery Site is not available, recent groundwater monitoring data at SLSL has been obtained to identify the groundwater level in the area. The data is contained in Appendix 14.1 and the monitored groundwater level are summarised in Table 14.11. The average groundwater level ranged from 3.99m to 18.89m below the well head across the landfill. While information pertaining the well head level is not available, it is conservatively assumed that the unsaturated zone for potential landfill gas could range from 3.99m to 18.89m.
Table 14.11 Groundwater Level in SLSL (November 2006
to November 2008)
|
Borehole |
Groundwater Level (m
below well head) |
|
|
|
Range |
Average |
|
GW1 |
8.55 – 14.50 |
12.34 |
|
GW2 |
4.09 – 7.59 |
6.65 |
|
GW3 |
15.46 – 22.96 |
18.89 |
|
DH203A |
5.67 – 6.87 |
6.50 |
|
DH204 |
3.63 – 4.69 |
3.99 |
|
DH201 |
4.03 – 7.20 |
6.51 |
14.53
The underground utilities
between the SLSL and the Nursery Site are shown in Drawing NOL/ERL/300/C/XRL/ENS/M61/111. Underground utilities are presence along
14.54 Based on the distance between the SLSL and the Nursery Site, and the characteristics of the geological setting and groundwater table, the pathway for potential landfill gas migration from SLS to the Nursery Site is classified as “long / indirect”.
The Target
Tunnels
Construction
Phase
14.55
About 750m of the
alignment will run within the Consultation Zone of NTML and underneath the
lower platform of the NTML (see Drawing
NOL/ERL/300/C/XRL/ENS/M61/101). The section of the alignment underneath the
NTML comprises twin parallel 8.15m ID TBM bored tunnels and a 400mm thick
pre-cast concrete segmental lining is proposed.
The tunnels will be constructed using a 9.325m diameter TBM. The tunnels will be constructed at around
-23.5 mPD by tunnel boring method. Drawing NOL/ERL/300/C/XRL/ENS/M61/112 presents the
preliminary design of the typical bored tunnel section which may be subject to
revision in the detailed design stage.
14.56
The design of the
permanent tunnel and cavern linings follows the requirements of MTRC’s New Works
Design Standard Manual, which has been followed in other MTRC’s railway line in
14.57
Apart from the tunnel
alignment, there are no other structure or construction works located within
the Consultation Zone of the NTML.
14.58
Since the tunnels are
directly underneath the NTML and the tunnel construction works involve workers
working in below ground void and contain sources of ignition, the sensitivity
of this target is high
Operational
Phase
14.59
During the
operational phase, the tunnels will be used by the XRL trains. There will be no unauthorised access to the
tunnels. It is expected that human entry
into the tunnels will only be required in case of maintenance and in emergency
situation.
14.60
Adequate ventilation will be maintained in the tunnels
during their operation by virtue of the mechanical ventilation provided as well
as the piston effect of trains to disperse any gas that may accumulate during
operational hours. Mechanical
ventilation will also be provided during time of maintenance works.
14.61
Since the tunnels are
below ground void directly underneath the NTML and contain sources of ignition,
the sensitivity of this target is classified as high.
Barging Point
14.62
The Barging Point is
about 1 ha and will be used for unloading plant and equipment and construction
material from barges to truck for onward transport to the Project
worksites. The Site is currently vacant
and is paved with concrete. The proposed
works within the Barging Point Site include fixing the barge loading ramp along
the seawall. A typical section drawing
of the barge loading ramp is shown in Drawing
NOL/ERL/300/C/XRL/ENS/M61/113. There will be no
excavation works and no underground structure will be installed. There will be a site office (in container)
within the site, which will be raised above ground.
14.63
As the works will not
involve excavation and the activities within the Barging Point will essentially
be outdoor and the site office will be raised 500mm above ground, the
sensitivity of this target is low.
14.64
The Barging Point
will not be used during the operational phase.
Nursery Site
14.65
The Nursery Site is
about 1. ha and will be used as a nursery site for transplanting trees affected
by the Project. The site is currently
vacant and is paved with concrete. The
proposed works within the Nursery Site include removing the paving concrete to
expose the soil underneath and to excavate the soil (around 1m deep) for
placing the transplanted trees. The
minor excavation process will be done by machinery and no workers will enter
the excavated area. Towards the end of
the construction period, the trees within the Nursery Site will be removed and
transferred back to the area previously affected by the construction
works. No buildings or structures will
be constructed within the Nursery Site.
14.66
As the works will
only involve shallow excavation and the construction activities will
essentially be outdoor, the sensitivity of this target is low.
14.67
The Nursery Site will
not be used during the operational phase.
Source-Pathway-Target Analysis
14.68
On the basis of the
source, pathways and targets identified above, a source-pathway-target analysis
has been undertaken and is presented in Table
14.12 according to EPD’s assessment framework.
Table
14.12 Qualitative Assessment of Landfill Gas Hazard Associated
with the Project.
|
Source |
Pathway |
Target |
Qualitative Risk |
|
Construction Phase |
|||
|
Ngau Tam Mei Landfill – passive landfill gas control
system, comprehensive landfill gas monitoring programme (Category: medium) |
Pathway saturated with groundwater,
no fault/fissures, no direct man-made pathways (Category: long / indirect) |
Twin bored tunnels – tunnel construction works involve workers working in below
ground void and contain sources of ignition (Category: high) |
Medium |
|
(Category: medium) |
Distance to waste around 120m,
no fault/fissures, large unsaturated zone in fill material, no direct
man-made pathways (Category: long / indirect) |
Barging Point – no excavation, outdoor activities, site
office raised above ground (Category: low) |
Very Low |
|
Siu Lang Shui Landfill - passive
landfill gas control measures, comprehensive landfill gas monitoring
programme (Category:
medium) |
Distance
to waste around 110m, no fault/fissures, unsaturated zone in rock, no direct
man-made pathways (Category: long / indirect) |
Nursery Site – minor excavation,
outdoor construction activities (Category:
low) |
Very Low |
|
Operational
Phase |
|||
|
Ngau Tam Mei Landfill – passive
landfill gas control measures, comprehensive landfill gas monitoring
programme (Category:
medium) |
Distance
to waste less than 50m, no fault/fissures, target below groundwater table, no
direct man-made pathways (Category: long/ indirect) |
Twin bored tunnels – below ground void directly underneath the NTML and contain
sources of ignition (Category:
high) |
Medium |
14.69
The Qualitative
Landfill Gas Risk Assessment has indicated the risk of landfill gas ingress
into the tunnels is medium while the risk of landfill gas ingress to the
Barging Point and the Nursery Site is very low.
According to EPD’s Guidance Note,
engineering measures will be required to protect the tunnels and no additional
precautionary measures will be required to ensure that the works in the Nursery
Site and the Barging Point are safe.
14.70
This section of the report provides general advice and
recommendations for the avoidance of landfill gas risks during the construction
and operation of the Project.
14.71
In general, the
measures taken for the restoration and aftercare of the NTML, GDBL and and SLSL
to control of landfill gas should not be relied upon to ensure the safety of
adjoining developments. However, it must
also be acknowledged that the restoration works undertaken will have the effect
of lowering the potential for an incident to occur off-site when compared to
the historical situation. Allowance for
this has been made in the qualitative assessment undertaken.
General Hazards Related to Landfill Gas
Landfill Gas
14.1
All contractors participating in the works and operational
staff should be aware that potential of methane and carbon dioxide 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.
·
Methane is odourless and
colourless, although in landfill gas it is typically associated with numerous
highly odoriferous compounds which gives 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.
·
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.
·
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.
·
Gas density. Methane is lighter than air whereas carbon
dioxide is heavier than air. Typical
mixtures of landfill gas are likely to have a density close to or equal to that
of air. However, site conditions may
result in a ratio of methane to carbon dioxide which may make the gas mixture
lighter or heavier than air. As a
result, landfill gas may accumulate in either the base or top of any voids or
confined spaces.
General Recommended Precautionary and Protection Measures for Tunnels
Construction Phase
14.2
The construction works to be undertaken at the site present
construction workers and others with risks resulting from contact with landfill
gas and leachate. Whilst the risks are
not expected to be significant, owing to the use of powered mechanical
equipment to undertake most of the excavation works, there may still be
instances where human exposure may be inevitable. Precautionary measures to be adopted during
construction are outlined in Paragraphs 8.3 to 8.49 of EPD’s Guidance Note (a copy of Chapter 8 of
the Guidance Note is enclosed in
Appendix 14.2 for reference). The
following recommendations referenced from the Guidance Note should be adopted when the construction of the
tunnels is within the Consultation Zone of the NTML:
Safety Measures
· All personnel who work on 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 should be posted warning of the potential hazards.
· Those staff who work in, or have responsibility for “at risk” areas, including 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.
·
During all works,
safety procedures will be implemented to minimise the risks of fires and
explosions and asphyxiation of workers (especially in confined space).
·
Safety officers,
specifically trained with regard to landfill gas related hazards and the
appropriate actions to take in adverse circumstances, will be present on all
worksites throughout the works.
·
Smoking and naked
flames will be prohibited within confined spaces. 'No Smoking' and 'No Naked Flame' notices in
Chinese and English will be posted prominently around the construction
site. Safety notices should be posted
warning of the potential hazards.
·
Welding,
flame-cutting or other hot works may only be carried out in confined spaces
when controlled by a 'permit to work' procedure, properly authorised by the
Safety Officer. The permit to work
procedure will set down clearly the requirements for continuous monitoring of
methane, carbon dioxide and oxygen throughout the period during which the hot
works are in progress. The procedure
will also require the presence of an appropriately qualified person who shall
be responsible for reviewing the gas measurements as they are made, and who
shall 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 will be permitted to carry out
hot works in confined areas.
·
A mechanical
ventilation system must be in use at all times during which personnel are
engaged in works inside the tunnel or excavation and be evacuated in the event
of power outages. Work must not be
carried out in the absence of mechanical ventilation and supervision of
adequately trained safety personnel. In exceptional case where
work is carried out under non-ventilated condition, any electrical equipment
used, such as motors and extension cords, should be intrinsically safe.
· Adequate fire extinguishing equipment, fire-resistant clothing and breathing apparatus sets should be made available on site.
Monitoring
·
Monitoring of
methane, carbon dioxide and oxygen must be carried out using appropriately
calibrated portable gas detection equipment.
The actions detailed in Table
14.13 must be carried out in the event of gas trigger levels being
breached.
Table 14.13 Actions
in the Event of Gas Being Detected
|
Parameter |
Measurement |
Action |
|
O2 |
< 19% v/v |
Increase underground
ventilation to restore O2 to >19% v/v |
|
|
< 18% v/v |
Stop works Evacuate all personnel Increase ventilation
further to restore O2 to >19% v/v |
|
CH4 |
> 10% LEL |
Prohibit hot works Increase ventilation to
restore CH4 to <10% LEL |
|
|
>20% LEL |
Stop works Evacuate all personnel Increase ventilation
further to restore CH4 to <10% LEL |
|
CO2 |
>0.5% v/v |
Increase
ventilation to restore CO2
to <0.5% v/v |
|
|
> 1.5% v/v |
Stop works Evacuate all personnel Increase ventilation
further to restore CO2 to <0.5% |
·
The exact frequency
of monitoring should be determined prior to the commencement of works, but
should be at lease 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 14.13 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.
·
All measurements
taken are to include at a minimum the areas where personnel are operating as
well as the highest and lowest elevations within the tunnel. Measurements are to be carried out with the
sensor located not greater than 10mm from the exposed rock surface and at
locations where landfill gas has the highest potential to enter the tunnel such
as highly fissured areas, dykes, identified faults ([5]),
etc.
14.3
In addition,
construction works to be undertaken in confined space should follow the
relevant Regulations under Chapter 59 Factories and Industrial Undertakings
Ordinance and Chapter 509 Occupational Health and Safety Ordinance.
14.4
A tunnel walkover
survey to test for presence of flammable gas at all joints and cracks, if
identified, shall be conducted upon completion of the tunnel work within the
NTML Consultation Zone. Rectifications,
such as sealing of cracks and inspection of tunnel seals, shall be carried out
for any signs of the presence of flammable gas.
The survey should be conducted under non-ventilated condition and before
starting the work of the day.
14.5
Weekly monitoring of
methane, carbon dioxide and oxygen in the form of a walkover survey at 20m
intervals for section of tunnels under NTML and 50m interval within the NTML
Consultation Zone should be conducted after completion of the tunnel
construction works and not less than 3 months before commencement of
operation. The survey shall be conducted
under non-ventilated condition and before starting the work of the day. Should methane concentration be higher than
10% LEL, mechanical ventilation is required to be switched on 30 minutes before
the starting the work of the day and throughout the working day. In addition, the monitoring frequency should
be increased to daily. If the situation
persists in three consecutive occasions, inspection of tunnel seals should be
carried out to identify the location of potential gas ingress. Rectifications, such as sealing of cracks,
shall be carried out. The daily monitoring
requirement can be resumed to weekly if the concentration of methane is consistently
below 10% LEL in 7 consecutive days and the ventilation requirement can be
resumed to normal.
14.6
A summary of the
monitoring results shall be submitted to EPD for record before the commencement
of operational phase. The results shall
be reviewed and agreed with EPD before the commencement of operation to
determine the monitoring requirements during the operational phase.
Design Phase
14.7
The risk level associated with potential landfill gas hazard
at the tunnels is medium. According to
the EPD’s Guidance Note, this
requires the use of “semi active’ or enhanced passive gas controls and
detection systems in some situation. As
described in para 14.56, the design
of the tunnel will incorporate a dense and low permeability precast
concrete. Appropriate sealant will be
applied to joints to prevent the ingress of groundwater, which will also form a
low permeability gas barrier. Hence, the
tunnel structure itself will provide a good barrier to landfill gas ingress (ie
passive gas control). Good workmanship
and adequate construction supervision will be required to ensure the actual
works are implemented as per the design requirements. This will be implemented by MTRC’s Material
and Workmanship Specification.
14.8
Adequate ventilation will be needed as part of the tunnel
design to act as an active gas control when needed. The current design of the mechanical
ventilation system can provide at least 4 air change per hour. Taking account of the low potential of
landfill gas migration into the tunnels, this air change will be adequate to
prevent accumulation of methane gas to the LEL.
14.9
Upon completion of the landfill gas protection measures, a
report on the implemented landfill gas protection measures with relevant
as-built drawings and other detailed information showing that the design
measures mentioned in this assessment to protect the tunnels from landfill gas
hazard have been properly incorporated shall be submitted to EPD.
Operational Phase
14.10
Adequate ventilation will be maintained in the tunnels during
their operation by virtue of the piston effect of trains to disperse any gas
that may accumulate during operational hours.
However, during non-operational hours and particularly at the time when
the first train travels through the tunnels each day, there is potential for
accumulated gas to be ignited by an electrical spark caused by the train. To reduce the likelihood of this occurrence,
the measures outlined below should be adopted in the section of tunnels underneath
the NTML and within the NTML Consultation Zone:
·
Ventilation of the
tunnels should be switched on for half an hour before the first train is
expected([6])([7])
,.
·
All maintenance
personnel and station staff working within the tunnels should be educated in
the dangers of landfill gas and the signs and symptoms of asphyxia.
·
Smoking within the
tunnels should be prohibited at all times.
·
An assumed presence
of landfill gas should be adopted at all times by maintenance workers. Gas monitoring before the maintenance work
to confirm the absence of landfill gas, communication with maintenance staff,
enforcement of the no smoking order should be implemented.
14.11
The monitoring
requirement during the operational phase shall be discussed with EPD before the
commencement of operation. Weekly
monitoring of methane, carbon dioxide and oxygen in the form of a walkover
survey at 20m intervals for section of tunnels under NTML and 50m interval
within the NTML Consultation Zone is tentatively proposed ([8]). The survey should be conducted under
non-ventilated condition and before the first train operates and start-up of
ventilation, if applicable. Should
methane concentration be higher than 10% LEL, mechanical ventilation is
required to be switched on at least 30 minutes before the first train operates
to sufficiently dilute the concentration of methane to well below the lower
explosive limit to ensure that no explosion will occur when the trains pass
through the tunnel until the end of the daily operation. The monitoring frequency should be increased
to daily. If the situation persists in
three consecutive occasions, inspection of tunnel seals should be carried out
to identify the location of potential gas ingress. Rectifications, such as sealing of cracks,
shall be carried out. The daily
monitoring requirement can be resumed to weekly if the concentration of methane
is consistently below 10% LEL in 7 consecutive days and the mechanical
ventilation requirement can be resumed back to normal condition.
14.12
A summary of the
monitoring results should be submitted to EPD for record at the end of the
monitoring period.
14.13 It is recommended that an annual walkover survey in the tunnels within the Consultation Zone of the NTML should be conducted to test for the presence of flammable gas at joints and cracks, if identified. Rectifications, such as sealing of cracks and inspection of tunnel seals, shall be carried out for any signs of presence of flammable gas. The survey should be conducted under non-ventilated condition and before the first train operates and start-up of ventilation, if applicable.
General Recommended Precautionary and Protection Measures for Barging Point
14.14
The following
precautionary measures should be implemented at the Barging Point:
·
Site Office should be
raised 500mm above ground.
·
Utilities services connected
to the site office and the annulus around these service entry points should be
properly sealed by means of sealant, collaras or puddle flanges as appropriate.
·
Smoking should be
prohibited at the Barging Point.
·
All personnel who
work on site and all visitors to site should be made aware of the potential
hazards of the landfill gas by posting suitable warning notices of the
potential hazards of landfill gas as well as “no smoking” sign.
General Recommended Precautionary and Protection Measures for Nursery Site
14.15
The following
precautionary measures should be implemented at the Nursery Site:
·
Smoking should be
prohibited within the Site.
·
All personnel who
work on site and all visitors to site should be made aware of the potential hazards
of the landfill gas by posting suitable warning notices of the potential
hazards of landfill gas as well as “no smoking” sign.
Environmental Monitoring and Audit
14.16
Monitoring is not required at the Barging Point and the
Nursery Site. For the construction and
operation at the tunnels underneath the NTML and within the Consultation Zone
of NTML, the monitoring requirement specified in Para 14.73 to 14.77 and
14.17
This section has provided a qualitative assessment on
potential hazards associated with landfill gas migration from NTML to Project
tunnels, GDBL to the Barging Point and SLSL to the Nursery Site. All three landfills are considered as a
“medium” source of gas migration. The
source-pathway-target analysis shows that landfill gas risk posed by NTML to
the Project tunnels is medium while the landfill gas risk posed by the GDBL and
SLS to the Barging Point and the Nursery Site, respectively, is very low.
14.18
Landfill gas protection measures
and monitoring requirements have been recommended for the construction and
operation of the Project. It is expected
that with the proposed landfill gas protection measures in place, the potential
risk of landfill gas migration to the Project will be minimal.