6.1.2
In view of the exceedence of
the recommended maximum “safe” rate (10 L m-2
per day) of methane emission (derived
from Waste Management Paper No. 26A) under the worst case scenario of 100%
biodegradable total organic carbon (TOC) and the identification of ‘at risk’
rooms at the proposed developments at the western and eastern corners of the
typhoon shelter reclamation, it
is recommended that monitoring of gas emission rates should be undertaken
following the completion of the reclamation area at the proposed
developments. The review of the gas
monitoring results would determine the need for gas protection measures to be
incorporated in the building design to prevent the ingress and / or
accumulation of any methane gas emissions to potentially dangerous
concentrations. Typical gas protection
measures are described in Section 6.5 for both the ground level and below ground
structures at the developments.
6.1.3
With the incorporation of
the recommended gas protection measures in the design of the developments, if
found to be necessary, together with the implementation of the other
recommended precautionary measures, the risk to people and property due to
biogas emissions from the WDII reclamation is considered to be low. The proposed mitigation measures, if
necessary, should be examined further at the detailed design stage with regard to the specific design details of individual
buildings and the confirmed land use.
6.2.1
Monitoring should be
undertaken via purposely installed monitoring wells within boreholes drilled
into the fill material. The boreholes
should be drilled down to the level of the groundwater (mean sea water level) and
standard landfill gas-type monitoring wells installed. These should be fitted with a removable cap
and gas monitoring valve so that gas concentrations may be measured as well as
flow rates from the open well. During
the drilling of boreholes, the safety and working procedures described in the
Landfill Gas Hazard Assessment Guidance Note (EPD, 1997) should be followed.
6.2.2
Concentrations of methane
gas should be measured using portable gas monitoring instruments, as described
in Section 6.4.1. Fluxes should also be
measured if the emission velocities are not too low. It is recommended that monitoring should be
undertaken monthly for a period of at least one year prior to the commencement
of construction works on the reclamation.
It is also recommended that gas flow rates from the wells should be
monitored under different meteorological conditions and to include some
occasions when atmospheric pressure is falling quite quickly (for example,
immediately preceding a typhoon).
6.3.1
The proposed locations for
borehole monitoring are shown in Figure 6.1.
BH1 is located at the western corner of the existing Causeway Bay
Typhoon Shelter where a 5-star hotel is planned. BH2 is located at the proposed open space
area at the west of the Causeway Bay Typhoon Shelter where continuous
monitoring throughout the construction period is possible. BH3 is located at the eastern corner of the
existing Causeway Bay Typhoon Shelter where an entertainment complex is
planned.
6.3.2
Should more intensive
monitoring are required for detailed analysis, it is recommended that further
monitoring wells should be located in areas designated for open space as it may
be possible to continue monitoring at these locations throughout the
construction period. Monitoring wells
should be located away from the areas dredged for the permanent stormwater
culvert (as shown in Figure 4.2) and the temporary channel / culvert
construction.
6.4.1
Monitoring shall be carried
out using intrinsically safe, portable gas monitoring instruments. The gas
monitoring instrument shall:
·
be capable of continuous
monitoring of methane;
·
be capable of continuous
barometric pressure and gas pressure measurements;
·
be capable of monitoring
temperature of the gas;
·
where possible, comply with
BS6020 and be approved by BASEEFA as intrinsically safe, suitable for use in a
Zone 2 area to BS5345;
·
normally operate in
diffusion mode unless required for spot sampling, when it should be capable of
operating by means of an aspirator or pump;
·
display any parameters
monitored by clear unambiguous readings given on an alpha numeric display LCD
screen with wide angle viewing;
·
have low battery, fault and
over range indication incorporated;
·
store monitoring date, and
shall be capable of being down-loaded directly to a personal computer;
·
measure in the following
ranges:
- methane 0 -
100% Lower Exposure Limit and 0 - 100% v/v
- barometric pressure mBar (absolute)
- gas pressure (relative to
atmospheric) Pascals atmospheric
- temperature 0 – 100
°C
·
have removable and rechargeable batteries with more than 12 hours
continuous operating life;
·
have back-up batteries; and
·
have an oxygen sensor with a
life of not less than twelve months and other sensors shall have a life of more
than two years before deterioration in performance of the sensor.
6.4.2
To measure the gas flow
rates from the open monitoring wells, very sensitive techniques (such as
micro-anemometer) will need to be used to measure the anticipated very low flow
rates.
6.4.3
The gas monitoring equipment
shall be calibrated and maintained in accordance with the manufacturer’s
recommendations for calibration and maintenance.
6.5.1
The mitigation measures
recommended in the EIA Report are described below, and comprise precautionary
gas protection measures to be incorporated in the design of the developments,
precautions to be taken during construction works on the reclamation, and
measures to be taken prior to entry of any below ground services or confined
space within the reclamation site. The
implementation schedule of the recommended mitigation measures is presented in
Appendix A.
Precautionary
Gas Protection Measures
General Guidelines
6.5.2
At this stage it is
difficult to formulate specific guidelines on what measures would be required
for the measured rates of gas emission as this would depend on the detailed
design of the individual buildings to be constructed. The following criteria may be used as general
guidelines. The maximum “safe” rate of
methane gas emission of 10 L m-2 per day derived from Waste
Management Paper No. 26A on Landfill Completion is proposed to be adopted as
the trigger value.
Scenario 1
6.5.3
If rates of methane emission
are consistently much less than the trigger value (10 L m-2 per
day), including monitoring occasions when atmospheric pressure is falling
rapidly, then it is considered that the buildings will not require gas
protection measures.
6.5.4
The trigger value is an area
emission rate (that is, rate at which gas is emitted per unit area of the
reclamation). In order to convert this
into an emission rate from a borehole, it is necessary to make an assumption
about the "area of influence" of a freely venting borehole that
depends on a number of factors. A key
factor is the ease by which gas can escape from the surface of the site. For a site with cover in the form of low
permeability paving or concrete, it would be expected that a borehole would
have a much greater area of influence than if the site had soft landscaping.
6.5.5
To be conservative, it is
proposed to adopt an area of influence of 20 m2 (radius of 2.5m)(),
which would give:
·
Trigger value of 10 L m-2
per day x 20 m2 = 200 L per day emitted from the borehole
6.5.6
The criterion for “safe”
flow rate from a free venting borehole becomes:
·
Flow rate of methane (in
terms of litre per day) < 200 L per day or
·
(Gas flow rate in terms of
litre per day) x (concentration of methane in gas (in % gas)) < 200 L per
day
Scenario 2
6.5.7
If the rate of methane
emission frequently exceeds the trigger value or shows a rising trend such that
future emission rates are likely to exceed the trigger value, then any
buildings to be constructed on that part of the site will require some form of
gas protection measures, that is,
·
(Gas flow rate in terms of
litre per day) x (concentration of methane in gas (in % gas)) > 200 L per day.
6.5.8
The type of gas protection
measures would be dependent on the design and use of the particular
building. A possible measure is the
incorporation of a low gas permeability membrane in the floor slab of the
building. Further investigation may be
required to determine the area of land that is affected by gas emissions. The analysis and assessment of the results
and design of any gas protection measures should be undertaken by suitably
qualified and experienced professionals who are familiar with the properties of
biogas and building protection design measures.
Scenario 3
6.5.9
If there are occasional
exceedances of the trigger value for methane emission rate from a borehole or
if there is a significant fluctuation of the monitoring results with some
readings coming close to the trigger value, then any trends in the results will
need to be assessed to determine their significance and the need for any
building protection measures. It may be
necessary to undertake further monitoring by extending the monitoring period,
for example, if a spuriously high reading is noted towards the end of the
monitoring period or if it seems likely that future emission rates may exceed
the trigger value. The analysis and
assessment of the monitoring results and design of any gas protection measures
should be undertaken by suitably qualified and experienced professionals who
are familiar with the properties of biogas and building protection design
measures.
Scenario 4
6.5.10 If the rate of methane emission from any borehole frequently exceeds the
upper UK guidance value of 432 L m-2 per day (that is, Carpenter’s
guidance level at which it is recommended that development should not take
place), or shows a rising trend such that future emission rates are likely to
exceed this value, then no buildings should be constructed on that part of the
site. That is when:
·
Upper UK guidance value of
432 L m-2 per day x 20 m2
= 8,640 L per day emitted from
the borehole; or
·
(Gas flow rate in terms of
litre per day) x (concentration of methane in gas (in % gas)) > 8,640 L per day.
6.5.11 Depending on the monitoring results, it may be necessary to incorporate
a number of gas protection measures into the design of the proposed
development. Specific details cannot be
provided until the results of the monitoring are available, and the proposed
landuse and building design are known and confirmed. A combination of different measures may be
used for protecting both the ground level and underground structures at the
development against possible risks due to biogas emissions. Discussions would need to be held with the
developer and architects to determine the protection measures which are the
most appropriate and feasible. Typical
gas protection measures that may be adopted are described below.
Measures to Prevent Ingress of Gas into ‘At Risk’ Rooms
6.5.12 To prevent the ingress of methane gas into a building, a low gas
permeability membrane may be incorporated in the design of the floor and any
below ground walls of identified ‘at risk’ rooms (for examples, rooms housing
electrical equipment, pumps or switchgear).
In addition, measures should be taken to avoid or seal any openings in
the floor (for example, at services entry points). Such techniques are commonly used where there
is a risk of landfill gas entering a building and have been employed on a
number of developments in Hong Kong.
6.5.13 There are various proprietary products available in the market and the
specific details of their application will depend on the detailed design of the
‘at risk’ rooms. Possible measures
include gas-resistant polymeric membranes that can be incorporated into the
floor or wall construction as a continuous sealed layer. Membranes should be able to demonstrate low
gas permeability and resistance to possible chemical attack. Other building materials such as dense
well-compacted concrete or steel shuttering also enhance resistance to gas
permeation. In all cases, extreme care
is needed during the installation of the membrane and subsequent construction
works to avoid damage to the membrane.
Ventilation within ‘At Risk’ Rooms
6.5.14 As an additional measure for the protection of specific ‘at risk’ rooms,
mechanical ventilation may be provided to ensure that, if any gas enters the
room, it is dispersed and cannot accumulate to potentially dangerous
concentrations. For particularly
sensitive rooms, such as below ground confined spaces which contain sources of
ignition, forced ventilation may be used in addition to the use of a low gas
permeability membrane.
6.5.15 The basement car park proposed at the development would be susceptible
to ingress and accumulation of any biogas emissions from the reclamation. The basement car park ventilation system will
be designed to ensure that the car park air quality guidelines given in ProPECC
PN 2/96 Control of Air Pollution in Car
Parks are achieved. The minimum ventilation rate for a basement
car park is 5 to 6 air changes per hour in order to comply with the EPD
requirement on carbon monoxide concentrations within car parks.
6.5.16 It is recommended that several ventilation systems should be installed
and evenly distributed within the basement car park. Therefore, even during equipment failure, it
is unlikely that the entire exhaust system would break down. To cater for the situation of power failure,
it is recommended that a back-up power supply shall be provided for the
ventilation system so that certain designated exhaust systems would still
operate. Under normal conditions, the
power failure should be rectified within a few hours.
Protection of Utilities or Below Ground Services
6.5.17 Below ground ducts or trenches for the installation of utilities or
services (for example, telecommunications, gas, water, electricity supply or
drainage connections) would be particularly prone to the ingress and
accumulation of any biogas emissions. It
is therefore important to prevent such ducts and trenches acting as routes by
which gas may enter buildings by avoiding, as far as possible, the penetration
of floor slabs by such services. In
addition, any unavoidable penetrations should be carefully sealed using puddle
flanges, low permeability sealant and / or membrane.
Precautions
during Construction Works
6.5.18 Special care must be taken during the first two years of construction
activities on the reclamation.
Sub-surface excavations into the mud layers might encounter gas
occasionally, but not at levels likely to be dangerous provided that the gas
vents freely to atmosphere. Emission
rates are unlikely to be sufficient to sustain a flame. These gas bubbles will only occur for short
periods, and therefore, as a precaution, smoking and naked flames in the
vicinity of drilling activities and excavations of 1 m depth or more should be
prohibited.
6.5.19 Precautions may be required to ensure that there is no risk due to the
accumulation of gas within any temporary structures, such as site offices,
during construction works on the reclamation area. It may be necessary, for example, to raise
such structures slightly off the ground so that any gas emitted from the ground
beneath the structure may disperse to atmosphere rather than entering the
structure. A minimum clear separation
distance of 500 mm, as measured from the highest point on the ground surface to
the underside of the lowest floor joist, is recommended in the Landfill Gas Hazard Assessment Guidance Note,
EPD (1997).
Precautions
Prior to Entry of Below Ground Services
6.5.20 Following construction, accumulation of gas within any below ground
services can pose a risk to the staff of the utility companies. As a good working practice, prior to entry
into any confined space within the reclamation site (such as manholes,
underground culverts and utility casings), the gas atmosphere within the
confined space should be monitored for oxygen, methane and carbon dioxide. Personnel should be made aware of the
potential dangers and advised to take appropriate precautions.
6.5.21 The working practices should follow the Landfill Gas Hazard Assessment Guidance Note, EPD (1997) guidelines
as follows:
·
Any chamber, manhole or
culvert that is large enough to permit access to personnel should be subject to
entry safety procedures. Such work in
confined spaces is controlled by the Factories and Industrial Undertakings
(Confined Spaces) Regulations of the Factories and Industrial Undertakings
Ordinance. Following the Safety Guide to Working in Confined Spaces
ensures compliance with the above regulations.
·
The entry or access point
should be clearly marked with a warning notice (in English and Chinese) which
states that there is the possibility of flammable and asphyxiating gases
accumulated within.
·
The warning notice should
also give the telephone number of an appropriate competent person who can
advise on the safety precautions to be followed before entry and during
occupation of the manhole.
·
Personnel should be made
aware of the dangers of entering confined spaces potentially containing
hazardous gases and, where appropriate, should be trained in the use of gas
detection equipment.
·
Prior to entry, the
atmosphere within the chamber should be checked for oxygen, methane and carbon
dioxide concentrations. The chamber may
then only be entered if oxygen is greater than 18% by volume, methane is less
than 10% of the Lower Explosive Limit, which is equivalent to 0.5% by volume
(approximately), and carbon dioxide is less than 0.5% by volume.
·
If either carbon dioxide or
methane is higher, or oxygen lower than the values given above, then entry to
the chamber should be prohibited and expert advice sought.
·
Even if conditions are safe
for entry, no worker should be permitted to enter the chamber without having another
worker present at the surface. The
worker who enters the chamber should wear an appropriate safety / recovery
harness and, preferably, should carry a portable methane, carbon dioxide and
oxygen meter.
6.5.22 In general, when work is being undertaken in confined spaces sufficient
approved resuscitation equipment, breathing apparatus and safety torches should
be available. Persons involved in or
supervising such work should be trained and practised 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
consistently employed.