14.1.1.1 According to the findings of the Northwest New Territories (NWNT) Traffic and Infrastructure Review conducted by the Transport Department, Tuen Mun Road, Ting Kau Bridge, Lantau Link and North Lantau Highway (NLH) will be operating beyond capacity after 2016 due to the increase in cross boundary traffic, developments in the NWNT, and possible developments in North Lantau, including the Airport developments, the Lantau Logistics Park (LLP) and the Hong Kong – Zhuhai – Macao Bridge (HZMB). To cope with the anticipated traffic demand, a new connection between NWNT and North Lantau – Tuen Mun – Chek Lap Kok Link (TM-CLKL) and Tuen Mun Western Bypass (TMWB) is one of the possible options.

14.1.1.2 The proposed TM-CLKL and TMWB will provide a direct route linking NWNT and North Lantau, from north to south, the Kong Sham Western Highway (KSWH), port back-up areas in NWNT, Tuen Mun River Trade Terminal, the existing EcoPark in Tuen Mun Area 38, the Airport, the proposed LLP, HZMB and North Lantau developments. The new connection will significantly reduce the travelling time between the KSWH and the NWNT region at its northern side, and North Lantau at its southern side.

14.1.1.3 In 2005, Highways Department (HyD) commissioned an engineering feasibility study (FS), namely Tuen Mun Chek Lap Kok Link and Tuen Mun Western Bypass – Feasibility Study (Agreement No. CE 28/2005 (HY)), to evaluate the technical feasibility and impacts of the Project. The FS recommended that the TM-CLKL shall be a dual 2-lane road with a total length of about 9 km with about 5 km long submarine tunnel and 4 km long elevated structure.

14.2 The Assignment

14.2.1.1 Maunsell Consultants Asia Ltd. has been appointed by HyD to carry out the Assignment on Tuen Mun – Chek Lap Kok Link - Investigation under Agreement No. CE 52/2007 (HY). The Assignment commenced on 19 May 2008 and shall be completed within 24 months, i.e. by mid-May 2010. The indicative alignment of TM-CLKL given in the Brief is presented in Figure 14.1.

14.2.1.2 The original scope of the Project under this Assignment includes:

a) construction of a dual 2-lane trunk road with a total route length of about 9 km long with about 5 km long submarine tunnel across Urmston Road and 4 km long elevated structures on both sides of the tunnel connecting to the proposed HZMB Hong Kong Link Road (HKLR), the Hong Kong International Airport (HKIA) and North Lantau in the south and the TMWB in the north;

b) reclamation adjacent to Tuen Mun River Trade Terminal and near Tai Mo To for the northern and southern landfalls respectively for the tunnel;

c) construction of a toll plaza at the southern landfall near Tai Mo To (to be reviewed in line with the HZMB HKBCF location);

d) provision of administration building and ancillary buildings, workshops and operator facilities, including control points, recovery vehicle bases, parking areas and provision of toll facilities for the management, operation and maintenance of the tunnel; and

e) construction of associated civil, structural, geotechnical, marine, environmental protection and mitigation works, ventilation buildings, tunnel ventilation system and associated facilities, petrol filling station, landscape and drainage works, street lighting and tunnel lighting, traffic aids including sign gantries, watermains and fire hydrants, fire services system, tunnel operation and control systems, communication systems, toll collection and accounting systems, traffic control and surveillance system (TCSS), security and access control system, supervisory control and data acquisition system, power supply, central monitoring and control system and electrical and mechanical (E&M) works.

14.2.1.3 However, the site selection study for the HZMB HKBCF proposed to integrate the TM-CLKL southern landfall reclamation with the HKBCF reclamation at northeast Chek Lap Kok. This arrangement will provide a cost-effective connection between TM-CLKL and HKBCF.

14.3 Purpose and Structure of Section

14.3.1.1 As the project is at its early stage, detailed information (such as detailed design, cross sections of the building and tunnel, underground services, etc) are not yet available. This assessment is preliminary and based on limited but best available information. At the detailed design stage, the detailed design consultant should review this LFG assessment and conduct a detailed LFG hazard assessment. The finalized design of the LFG protection measures should be completed by a competent professional person with relevant experience, and the design of the protection measures, the detailed LFG hazard assessment and the monitoring programme should be submitted to EPD for vetting. These requirements should also be included in the scope of the detailed design study of the TMCLKL.

14.3.1.2 This preliminary landfill gas hazard assessment identifies the hazards that are likely to be generated from the Pillar Point Valley (PPV) Landfill, during the construction and operation phases of this Project and evaluates the associated risk. Mitigation measures and good site practices are recommended with reference to the EPD guidelines.

14.3.1.3 This Section includes the following sub-sections:

l Section 14.1~3 - this introduction;

l Section 14.4 - summarizes the environmental legislation, policies, plans, standards and criteria relevant to the assessment;

l Section 14.5 - provides the history and general description of the Pillar Point Valley Landfill;

l Section 14.6 - an overview of potential landfill gas hazards;

l Section 14.7 - details the assessment methodology;

l Sections 14.8~11 - presents assessment results;

l Section 14.12 - lists out the recommended mitigation measures;

l Section 14.13 - presents the conclusion.

14.4 Environmental Legislation, Policies, Plans, Standards and Criteria

14.4.1.1 Environmental Protection Department (EPD) has issued two guidance notes regarding landfill gas hazard assessment, namely ProPECC PN 3/96 - Landfill Gas Hazard Assessment for Development Adjacent to Landfill and EPD/TR8/97 - Landfill Gas Hazard Assessment Guidance Note. These two guidance notes set out the conditions under which a landfill gas hazard assessment should be carried out and provide guidance on undertaking a landfill gas hazard assessment. The guidance notes recommended that in general, assessment of landfill gas hazard is required for proposed developments that lie within the 250m Consultation Zone around a landfill. Figure 14.1 shows the layout of the toll plaza of the Tuen Mun Chek Lap Kok Link and the consultation zone of the existing closed Pillar Point Valley Landfill.

14.4.1.2 Since the toll plaza and related facilities fall within the 250m Consultation Zone of PPV Landfill, a preliminary landfill gas hazard assessment addressing the landfill gas hazards and recommending the mitigation measures is undertaken for the Project.

14.5 History and General Description of the Pillar Point Valley Landfill

14.5.1 Principal Characteristics of Pillar Point Valley Landfill

14.5.1.1 PPV Landfill is a 33.79ha landfill, which was commissioned in 1983 and was closed in 1996. Throughout its operation life of 14 years, PPV Landfill had received approximately 11 million tonnes of waste.

14.5.1.2 In the construction of the PPV Landfill, a liner system was installed across the narrow floor of the valley, being 30 metres wide at its narrowest point. The liner system consists of a groundwater collection layer with pipes for discharge together with a leachate collection layer, also with a network of pipes for discharge. These two layers are separated by either a PVC or an HDPE membrane.

14.5.2 Restoration Works

14.5.2.1 After the PPV Landfill was closed, restoration works were commenced in year 2004. The restoration works were completed in 2006 and the principle element or works are as follows:

l High integrity capping systems across the top platform of the landfill to reduce infiltration, reduce leachate generation and control leachate levels;

l Modification to the existing leachate and groundwater collection systems to intercept the flows and ensure that they are conveyed to the proposed leachate treatment works;

l Passive landfill gas system in specific areas to prevent landfill gas migration offsite;

l Active landfill gas extraction system to control and collect landfill gas for use at the leachate treatment works;

l Realignment of the natural stream at the toe of the landfill and the formation of a platform on the eastern left bank of the stream for the construction of the treatment compound;

l A leachate treatment works for the treatment of collected leachate.

14.5.3 Landfill Gas and Ground Water Monitoring

14.5.3.1 Gas monitoring wells have been installed around the site to monitor the potential landfill gas migration (Figure 14.2). Recent monitoring data from January 2008 to December 2008 was obtained from EPD as recorded in Appendix L1, and a summary of the monitoring data is presented as follows:

Table 14.1 Pillar Point Valley Landfill LFG Monitoring Data (Jan 08 ~ Dec 08)

Drillhole	Methane %v/v Average (Range)	Carbon Dioxide %v/v Average (Range)
GM1	0.02 (0 ~ 0.1)	8.9 (5.7 ~ 12.5)
GM2	0.03 (0 ~ 0.1)	7.6 (5.3 ~ 10.5)
GVQ1	0.01 (0 ~ 0.1)	6.5 (3.3 ~ 9.8)
GVQ2	0.02 (0 ~ 0.1)	7.1 (0.4 ~ 14)
GVQ3	0.02 (0 ~ 0.1)	1.2 (0.4 ~ 2.1)
P5	0.06 (0 ~ 0.4)	1.8 (0.3 ~ 5.3)
GM4	0.03 (0 ~ 0.1)	4.5 (2.7 ~ 5.9)
GM5	0 (0 ~ 0)	6.5 (5.0 ~ 7.5)

14.5.3.2 Monitoring data from ground water monitoring wells are also obtained (Figure 14.3 and Appendix L2 refer), and the summary of ground water level is tabulated below:

Table 14.2 Pillar Point Valley Landfill Ground Water Monitoring Data (Jan 08 ~ Dec 08)

Location	Top of Monitoring Well (mPD)	Average Static Water Depth	Average Reduced Water Depth (mPD)
GWQ(A)2	35.3	8.8 m	26.4
GWQ(A)3	39.6	5.1 m	34.4
GWQ(A)4	46.5	5.3 m	41.1
PWQM9	52.7	9.4 m	43.3
PWQM2	83.5	5.1 m	78.4
PWQM3	84.8	4.4 m	80.4
PWQM4	119.0	5.3 m	113.7
GV4	166.7	15.9 m	150.8
GWQ(B)1	143.8	4.3 m	139.6
PWQM5	150.1	8.8 m	141.3
GV5	156.7	6.2 m	150.5
PQWM6	157.1	0.4 m	156.7
GWQ(B)2	156.8	8.2 m	148.5
GWQ(B)3	156.5	1.1 m	155.4
PWQM7	158.8	42.4 m	116.4
GWQ(B)4	151.6	5.9 m	137.6
PWQM8	109.0	22.3 m	86.7
GV2	24.5	6.2 m	18.3

14.5.4 Pillar Point Valley Landfill, Toll Plaza and Works Area of the Project

14.5.4.1 Note that in Figure 14.1, a corner of the toll plaza and the Works Area WA19 seem to have encroached into the site boundary of the PPV Landfill. However, from the communication with EPD, it is understood that those area would have been handover to LandsD prior to commencement of the construction of the Project. For consistency, this preliminary LFG hazard assessment is carried out in accordance to the information on the EPD/TR8/97 – Landfill Gas Hazard Assessment Guidance Note.

14.6 Potential Landfill Gas Hazards

14.6.1.1 The typical composition of landfill gas is about 60% volume of methane and 40% volume of carbon dioxide, although these percentages can vary widely depending on the site conditions. Also present are trace quantities of hydrogen sulphide, nitrogen and gaseous hydrocarbons such as hexane, octane and heptane.

14.6.1.2 Landfill gas migration can be a dangerous hazard because of its combustible and in some cases explosive nature of methane; and the asphyxiant nature of carbon dioxide.

14.6.1.3 Landfill gas has the potential to cause fire, explosion or asphyxiation if it migrates into and accumulates in confined space such as building basements, underground car parks, lift shafts, pumping stations, and maintenance chambers. For the same reasons, temporary structures such as site huts and any other unventilated enclosures erected during construction stage are also exposed to landfill gas hazards. Underground services, such as sewer drains, storm drains and service ducts, may also be susceptible to the potential hazards as they act as pathways for landfill gas. Besides, any faults present in geological formation also act as pathways for landfill gas.

14.6.1.4 As shown in Figure 14.1, the toll plaza and related facilities fall within the 250m Consultation Zone of existing PPV Landfill. The overall risk level of landfill gas hazard posed by PPV Landfill to the proposed TMCLKL toll plaza is assessed and demonstrated below as recommended in EPD/TR8/97 - Landfill Gas Hazard Assessment Guidance Note.

14.7 Landfill Gas Hazard Assessment Criteria and Methodology

14.7.1.1 In accordance with the Landfill Gas Hazard Assessment Guidance Note, the risk due to landfill gas may be evaluated based upon the following three criteria:

l Source – location, nature and likely quantities/ concentrations of landfill gas which has the potential to affect the development;

l Pathway – the ground and groundwater conditions, through which landfill gas must pass in order to reach the development; and

l Target – elements of the development that are sensitive to the effects of landfill gas.

14.8 Source

14.8.1.1 The classification of the Source (ie the landfill) should be undertaken as follows:

Minor

Landfill sites at which gas controls have been installed and proven to be effective by comprehensive monitoring which has demonstrated that there is no migration of gas beyond the landfill boundary (or any specific control measures) and at which control of gas does not rely solely on an active gas extraction system or any other single control measure which is vulnerable to failure; or

Old landfill sites where the maximum concentration of methane within the waste, as measured at several locations across the landfill and on at least four occasions over a period of at least 3 months (preferably longer), is less than 5% by volume (v/v).

Medium

Landfill site at which some form of gas control has been installed (eg lined site or one where vents or barriers have been retrospectively installed) but where there are only limited monitoring data to demonstrate its efficacy to prevent migration of gas; or

Landfill site where comprehensive monitoring has demonstrated that there is no migration of gas beyond the landfill boundary but where the control of gas relies solely on an active gas extraction system or any other single control system which is vulnerable to failure.

Major

Recently filled landfill site at which there is little or no control to prevent migration of gas or at which the efficacy of the gas control measures has not been assessed; or

Any landfill site at which monitoring has demonstrated that there is significant migration of gas beyond the site boundary.

14.8.1.2 The 'significance' of migration should be assessed by reference to the concentration, frequency and location at which gas is detected. For guidance, it should be assumed that any concentration of methane or carbon dioxide greater than 5% v/v above background levels in any monitoring well outside the landfill's boundary indicates significant migration. Lower concentrations may still be 'significant' if they are observed in more than one monitoring well, on several occasions or in monitoring wells located some distance from the site boundary. In general, concentrations of greater than 1% v/v methane or 1.5% v/v carbon dioxide (above background levels in each case) indicate less than adequate control of the gas at source.

14.8.1.3 In classifying the source term, account needs to be taken of the likelihood and probable effect of a failure of the gas controls. Thus, if it has been demonstrated that there is no migration of gas and there is little danger of the gas controls failing (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).

14.8.1.4 Where the effectiveness of the gas controls has not been proven by off-site monitoring or if there is some doubt as to the adequacy of the monitoring, this should be taken into account when considering the impact of the control measures on the Source term. Assessments should always err on the side of caution and, in general, if the effectiveness cannot be demonstrated, the assessment should be undertaken on the same basis as if the controls were not in place.

14.8.1.5 The reliability of the monitoring, for determining the efficacy of the gas controls, needs to take account of the design, number and location of the monitoring points together with the frequency and duration over which monitoring has been undertaken. Monitoring should have been undertaken under different weather conditions including, in particular, periods of low or falling atmospheric pressure.

14.9 Pathway

14.9.1.1 The broad classification of the Pathway should be undertaken as follows:

Very short / direct	Path length of less than 50m for unsaturated permeable strata and fissured rock or less than 100m for man-made conduits
Moderately short / direct	Path length of 50-100m for unsaturated permeable soil or fissured rock or 100-250m for man-made conduits
Long / indirect	Path length of 100-250m for unsaturated permeable soils and fissured rock

14.9.1.2 In classifying the pathway, however, adjustment to the above general guidelines will often be required to take account of other factors which will affect the extent of gas migration including the following:

l Particular permeability of the soils;

l Spacing, tightness and direction of the fissures/joints;

l Topography;

l Depth and thickness of the medium through which the gas may migrate (which may be affected by groundwater level);

l The nature of the strata over the potential pathway;

l The number of different media involved; and

l Depth to groundwater table and flow patterns.

14.9.1.3 Thus, although there may be permeable soil between the landfill site and a proposed development, say 80m from the edge of the site, if the soil layer is very shallow and thin with its upper surface exposed to the atmosphere, then it will be appropriate to consider this as a long/indirect pathway. This could of course alter if the land between the landfill site and the development was paved over or altered in some other way which reduced the potential for gas release. Similarly, if the land is flat, the surface may be prone to waterlogging which will also effectively seal it at times of heavy rain. In general, a conservative approach should be adopted and it should be assumed that any such permeable surface soils may become less permeable in the future.

14.9.1.4 If it is known that a conduit (man-made or natural feature such as a fault plane) leads directly from the landfill to the development area, it should be regarded as a "direct/short" pathway even if it is longer than 100m.

14.10 Target

14.10.1.1 Different types of target may be broadly classified as follows:

High sensitivity

Buildings and structures with ground level or below ground rooms/voids or into which services enter directly from the ground and to which members of the general public have unrestricted access or which contain sources of ignition.

This would include any developments where there is a possibility of additional structures being erected directly on the ground on an ad hoc basis and thereby without due regard to the potential risks.

Medium sensitivity

Other buildings, structures or service voids where there is access only by 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.

14.10.1.2 The classification of the above LFG sources, pathway and target are categorized. Having determined into the 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 14.3. The potential implications associated with the various qualitative risk categories are summarized in Table 14.4.

Table 14.3 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
Medium	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 14.4 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.

14.10.1.3 Five generic forms of protection will be used in mitigating the hazards to development. These generic forms corresponding to the five risk levels are set out in Table 14.5. The terms used in Table 14.5 are defined in Table 14.6.

Table 14.5 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 14.6 Definition of Control Terms

Term	Definition
Active control	Control of gas by mechanical means eg ventilation of spaces with air to dilute gas, or extraction of gas from the development site using fans or blowers.
'Semi active' control	Use of wind driven cowls and other devices which assist in the ventilation of gas but do not rely on electrically powered fans.
Passive control	Provision of barriers to the movement of gas eg membranes in floors or walls, or in trenches, coupled with high permeability vents such as no-fines gravel in trenches or voids/permeable layers below structures.
Detection systems	Electronic systems based upon, for example, catalytic oxidation or infra-red measurement principles, which can detect low concentrations of gas in the atmosphere and can be linked to alarms and/or telemetry systems.

14.11 Assessment of Potential Risk

14.11.1 Source

14.11.1.1 According to the PPV Landfill information and monitoring data provided by EPD, it is understood that the landfill is of 33.79ha and had received approximately 11 million tonnes of waste until it ceased operation in 1996. Restoration works were completed in year 2006, with capping system, monitoring, leachate collection and treatment facility installed. As for LFG control, active extraction and passive venting were incorporated. From the monitoring data, methane content is less than 0.1 %v/v on average. However, the average carbon dioxide content is around 1.2 to 8.9 %v/v. In accordance to the Guidance Note, CO₂ level exceed 5% above the background would be considered “significant migration”. In the absence of any background CO₂ level as a reference, it is conservatively assume potential off-site migration of LFG cannot be completely avoided.

14.11.1.2 In view of the available information, the source of PPV Landfill is therefore categorized as Medium.

14.11.2 Pathway

14.11.2.1 According to the geological map (Figure 14.4), the geology of the toll plaza site locate is mainly fine to medium grained granite. Also from the map, photogeological lineament is observed in south west corner of the site near the roundabout, and mineral veins are also observed in east side of the site near the tunnel portal. However both of these features are only within the consultation zone of the PPV Landfill and do not pass through the landfill.

14.11.2.2 From the groundwater monitoring data of the PPV Landfill, the ground water level ranges from +17.6 to +115.7mPD (Appendix L2 and Table 14.2 refers). According to the vertical profile of the toll plaza and road network (Figure 14.5), the elevation of the site ranges from +6.605 to +22.403mPD.

14.11.2.3 The project covers a large area; and it is noted that some works area during construction phase would be right next to the PPV Landfill. These areas would be used as temporary open storage during construction and would remain as open area during operation. Therefore, the site is categorized as Very short / direct except for the specific locations as below.

14.11.2.4 The toll control building, toll booths and the subway at the toll plaza are located at least 83m away from the site boundary of the landfill, and therefore is categorized as Moderately short / direct.

14.11.2.5 The tunnel east of the toll plaza leading towards the roundabout at Lung Fu Road is around 222m away from the site boundary of the landfill, and therefore is categorized as Long / indirect.

14.11.3 Target

14.11.3.1 During construction, the 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 except for specific targets as below.

14.11.3.2 The specific location of the site office is yet to confirm. Should the site office to be located within the PPV Landfill consultation zone, the construction workers and supporting staffs would be working in an indoor environment with potential LFG hazards. However, they should be well informed of the situation and specific safety procedures should be given to them to follow. Therefore, this group is considered as Medium sensitivity.

14.11.3.3 The construction of the toll control building and the subway would involve excavation. During finishing works and renovation, construction workers would be working in an indoor environment with potential LFG hazards. However, they should be well informed of the situation and specific safety procedures should be given to them to follow, therefore this group is considered as Medium sensitivity.

14.11.3.4 The construction of the tunnel to Lung Fu Road roundabout would involve excavation. During finishing works, construction workers would be working in a confined environment with potential LFG hazards and hot works might be required. However, they should be well informed of the situation and specific safety procedures should be given to them to follow, therefore this group is considered as High sensitivity.

14.11.3.5 During operation, the toll control building would generally occupied by operating staffs. Rooms or voids with services and utilities penetrating from the ground, such as plant rooms, as well as underground structures or basement (if any) should be restricted to the staffs only. Since the staffs would be informed of the potential LFG hazards and safety / emergency procedures would be in place. Therefore, this group is considered as Medium sensitivity.

14.11.3.6 The subway and the toll booths would create a confined environment. Since the staffs accessing the subway and working in the booth would be informed of the potential LFG hazards and safety / emergency procedures would be in place. Therefore, this group is considered as Medium sensitivity.

14.11.3.7 The tunnel to Lung Fu Road roundabout would be use by vehicles of the general public. Since the general public might not be aware of the potential LFG hazards and vehicles can be considered as an ignition source, this group is considered as High sensitivity.

14.11.4 Summary of Qualitative Source-Pathway-Target Analysis

14.11.4.1 Based on the above information, a qualitative source-pathway-target analysis has been undertaken and the overall risk level for both construction and operation phases are summarized below:

Table 14.7 Summary Risk Matrix

Source		Pathway	Targets	Risk
Pillar Point Valley Landfill – Passive control, active extraction, comprehensive monitoring (*Medium*)		During Construction
		Less than 50m (closest point) from PPV Landfill, no fault/fissure, no man-made conduit (*Very short / direct*)	General – Construction workers, well trained and follow specific safety procedures, mainly outdoor works (*Low sensitivity*)	Low
			Site Office¹ – Construction workers and support staffs, well trained and follow specific safety procedures, indoor environment (Medium Sensitivity*)	Medium
		Approximately 80m+ away from PPV Landfill, no fault/fissure, no man-made conduit (*Moderately short / direct*)	Toll Control Building , subway, toll booth – Construction workers, well trained and follow specific safety procedures, involve excavation and indoor construction activities (*Medium sensitivity*)	Medium
		Over 200m away from PPV Landfill no fault/fissure, no man-made conduit (*Long / indirect*)	Tunnel to Lung Fu Rd roundabout – Construction workers, well trained and follow specific safety procedures, involve excavation, hot works and works in confined environment (*High sensitivity*)	Medium
		During Operation
		Approximately 80m+ away from PPV Landfill, no fault/fissure, no man-made conduit (*Moderately short / direct*)	Toll Control Building , subway, toll booth – Operation staffs, well informed of potential LFG hazards and follow specific safety procedures, restricted to staffs only (*Medium sensitivity*)	Medium
		Over 200m away from PPV Landfill no fault/fissure, no man-made conduit (*Long / indirect*)	Tunnel to Lung Fu Rd roundabout – General public in vehicle, combustion engine can be considered as ignition source (*High sensitivity*)	Medium
Note: *1	As the site office location is not yet confirmed, it is conservatively assumed to be very close to the landfill.

14.11.4.2 Therefore, the overall risk during construction phase is Medium, and for operation phase is also Medium.

14.12 Recommended Protection Measures

14.12.1.1 According to Table 14.4, engineering measures are required to protect the proposed toll plaza and its facilities from the landfill gas risk due to the PPV Landfill. According to Table 4.2 of the EPD’s Landfill Gas Hazard Assessment Guidance Note, the generic protection measures required include “use of ‘semi active’ or enhanced passive gas controls. Detection system in some situations”.

14.12.1.2 As the project is still at an early stage and the detailed information are not yet available, detailed and specific protection measures cannot be provided. However, options and generic protective measures will be recommended. The recommended preventive measures are presented in the following sections.

14.12.2 Protection Measures During Construction Phase

Appointment of Safety Officer

14.12.2.1 A safety officer, trained in the use of gas detection equipment and landfill gas-related hazards, should be present on site throughout the groundworks phase. The Safety Officer should be provided with an intrinsically safe portable instrument, which is appropriately calibrated and can measure the following gases in the ranges indicated below:

l Methane 0-100% LEL and 1-100% v/v

l Carbon dioxide 0-100%

l Oxygen 0-21%

Safety Measures

14.12.2.2 For staff who work in, or have responsibility for “at risk” area, such as all excavation workers, supervisors and engineers working within the Consultation Zone, should receive appropriate training on working in areas susceptible to landfill gas, fire and explosion hazards.

14.12.2.3 An excavation procedure or code of practice to minimize landfill gas related risk should be devised and carried out.

14.12.2.4 No worker should be allowed to work alone at any time in or near to any excavation. At least one other worker should be available to assist with rescue if needed.

14.12.2.5 Smoking, naked flames and all other sources of ignition should be prohibited within 15m of any excavation or ground-level confined space. “No smoking” and “No naked flame” notices should be posted prominently on the construction site and, if necessary, special areas should be designed for smoking.

14.12.2.6 Welding, flame-cutting or other hot works should be confined to open areas at least 15m from any trench or excavation.

14.12.2.7 Welding, flame-cutting or other hot works may be only be carried out in trenches or confined spaces when controlled by a “permit to work” procedure, properly authorized by the Safety Officer (or, in the case of small developments, other appropriately qualified person).

14.12.2.8 The permit to work procedure should set down clearly the requirements for continuous monitoring for methane, carbon dioxide and oxygen throughout the period during which the hot works are in progress. The procedure should also require the presence of an appropriately qualified person, in attendance outside the 'confined area', who should be responsible for reviewing the gas measurements as they are made, and who should have executive responsibility for suspending the work in the event of unacceptable or hazardous conditions. Only those workers who are appropriately trained and fully aware of the potentially hazardous conditions which may arise should be permitted to carry out hot works in confined areas.

14.12.2.9 Consideration should be given to provide additional ventilation for confined area such as the toll control building, subway, and the tunnel towards Lung Fu Road roundabout. This measure would reduce the potential of LFG accumulation in those areas at risk.

14.12.2.10 Where there are any temporary site offices, or any other buildings located within the PPV 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.

14.12.2.11 Any electrical equipment, such as motors and extension cords, should be intrinsically safe.

14.12.2.12 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.

14.12.2.13 During construction, adequate fire extinguishing equipment, fire-resistant clothing and breathing apparatus (BA) sets should be made available on site.

l Fire drills should be organized at not less than six monthly intervals.

l The contractor should formulate a health and safety policy, standards and instructions for site personnel to follow

14.12.2.14 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.

14.12.2.15 For staff who work in, or have responsibility for ‘at risk’ areas, such as all excavation workers, supervisors and engineers working within the PPV Landfill Consultation Zone should receive appropriate training on working in areas susceptible to landfill gas, fire and explosion hazards.

14.12.2.16 Service runs within the Consultation Zone should be designated as “special routes” as shown in Figure 14.6; 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).

14.12.2.17 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

14.12.2.18 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 10mm 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.

14.12.2.19 For excavations deeper than 1m, measurements should be carried out:

l at the ground surface before excavation commences;

l immediately before any worker enters the excavation;

l at the beginning of each working day for the entire period the excavation remains open; and

l periodically through out the working day whilst workers are in the excavation.

14.12.2.20 For excavations between 300mm and 1m deep, measurements should be carried out:

l directly after the excavation has been completed; and

l periodically whilst the excavation remains open

14.12.2.21 For excavations less than 300mm deep, monitoring may be omitted, at the discretion of the Safety Officer or other appropriately qualified person.

14.12.2.22 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 14.8.

Table 14.8 Actions in the Event of Landfill Gas being Detected in Excavation / Confined Area

Parameter	Measurement	Action
Oxygen	< 19%	- Ventilate to restore oxygen to > 19%
Oxygen	< 18%	- Stop work - Evacuate personnel / prohibit entry - Increase ventilation to restore to > 19%
Methane	> 10% LEL (> 0.5 %v/v)	- Prohibit hot work - Ventilate to restore methane to < 10% LEL
Methane	> 20% LEL (> 1 %v/v)	- Stop work - Evacuate personnel / prohibit entry - Increase ventilation to restore to < 10% LEL
Carbon Dioxide	> 0.5%	- Ventilate to restore carbon dioxide to < 0.5%
Carbon Dioxide	> 1.5%	- Stop work - Evacuate personnel / prohibit entry - Increase ventilation to restore to < 0.5%

14.12.2.23 The hazards from landfill gas during the construction stage within the existing PPV 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 L3. In the operation phase, if it is necessary to carry out construction works, landfill gas precautionary measures same as those recommended for the construction stage above should be followed.

14.12.3 Design of LFG Protection Measures and Other Protection Measures During Operation Phase

14.12.3.1 According to Table 14.4, engineering measures are required to protect the proposed toll plaza and its facilities from the landfill gas risk due to the PPV Landfill. These protection 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 protective measures can be recommended. At the subsequent detailed design stage, the detailed design consultant should provide a more detailed assessment and finalize the design of these protective measures. The design (drawings and specification) of the protective measures as well as the requirement for maintenance and monitoring should be prepared by a competent professional person and should be submitted to EPD for vetting.

Building Protection Design Measures

14.12.3.2 Engineering measures for buildings structures with ground level or below ground rooms / voids (such as the toll control building, subway, and toll booth) including the following should be adopted in the detailed design:

l 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.

l Other building materials, e.g. dense well-compacted concrete or steel shuttering which provide a measure of resistance to gas permeation. Note that this cannot be a standalone measure, and must be implemented along with other measures.

l Creation of a clear void under the structure which is ventilated by natural structure and provides preferential pathways for release of gas. (Figure 14.7 refers).

l Synthetic composite geotextiles which provide a free-venting cellular structure and provide preferential pathways for release of gas.

l Provision of mechanical ventilation to ensure sufficient air change at all time.

l Underground structures, especially basements, should be avoided whenever possible. Should underground structures cannot be avoided, other than the design measures described above, detection system should be adopted where necessary.

Design Measures for Sub-Surface Building Services

14.12.3.3 As shown in Figures 14.8 – 14.13, generic protection measures for the sub-surface building services including the following are recommended:

l A gas barrier (Figures 14.8 and 14.9) 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 (Figure 14.10) e.g. U-bends, should be provided to effectively seal off the conduit and prevent gas-phase transport.

l Vent pipes or gridded manhole covers may be used to avoid build-up of gas in underground utilities manholes (Figure 14.11). 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.

l For building services penetrating through the ground, collar seal (Figure 14.12) should be adopted to prevent gas ingress into the room where the service pipes/cables enter the building.

l For services routed to enter building above ground level, atmospheric break-leg (Figure 14.13) should be provided to eliminate the risk of gas entry to the building interior.

l For building structure below ground, such as basement, special gas vent should be adopted. That type of venting arrangement, which may be appropriate to multiple service entries, comprises a vented gas interceptor cavity through which service pipes pass, as shown in Figure 14.14. The aim of this protection measure is to locate the barrier component within the building sub-structure in a sealed entry box which is fitted with a vent stack.

l For services passing through the landfill consultation zone should be properly designed to prevent gas migration via the service ducts. Typical design are shown in Figures 14.6.

Design Measures for Tunnel / Subway

14.12.3.4 The recommendations given below are designed to reduce the risk of gas ingress to the tunnel / subway.

l A low permeability gas membrane to be incorporated into the lining of the tunnels.

l 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.

l 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.

l Passive venting (eg. Vent off to atmosphere, Figure 14.8 and 14.9 refer) should be adopted to prevent accumulation of LFG in the underground structures. The cut-off trench barrier, if adopted should be built along the side of the tunnel to provide passive venting before the LFG reach the tunnel.

l For the short tunnel to Lung Fu Road roundabout, mechanical ventilation is not incorporated in the current design. However, in the detailed design stage, ventilation analysis should be conducted and the option of incorporating mechanical ventilation should be reviewed in the detailed LFG hazard assessment at later stage and adopted if found to be necessary.

Guidance for Entry into Service Room / Void, Manhole and Chambers

14.12.3.5 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. Working in confined spaces is controlled by the Factories and Industrial Undertakings (Confined Spaces) Regulation of the Factories and Industrial Undertakings Ordinance and the Safety Guide to Working in Confined Spaces should be followed to ensure compliance with the Regulation.

14.12.3.6 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.

14.12.3.7 All the access to confined spaces should be restricted only to authorized personnel, and should be informed 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 during Operation

14.12.3.8 As the Project would be jointly used by the TM-CLKL and the Tuen Mun Western Bypass, the exact arrangement for operation are not yet finalized. Nonetheless, the protective measures and maintenance works should be carried out by the operators in joint effort or any agreed terms.

14.12.3.9 Operation staffs 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. Communication channels with EPD and the PPV Landfill operators should be established. In case of abnormal situation (eg detection of gas emission, smell) at the Project site, the operation staffs should report the case to EPD / Landfill operators for advice.

14.12.3.10 During operation, regular monitoring of landfill gas should be done at the tunnel, subway, and any other underground structures within the landfill consultation zone. Monitoring is required to verify the effectiveness and to ensure the continued performance of the implemented protection measures.

14.12.3.11 Inspection and LFG monitoring should be carried out at buildings and enclosures (eg. toll control building, toll booths, tunnel, subway, service manholes, etc) prior to the operation to ensure the design measures and functioning properly. The monitoring should be continued through the operation of the Project. In particular for the first year of operation, monthly monitoring is recommended, and quarterly (or at a frequency agreed by EPD) for second year on. Should the monitoring reveal the presence of landfill gas within the tunnel, subway, or other confined area, the seal of the joints shall be inspected and consideration shall be given to seal the cracks. Action level can refer to Table 14.8, and should abnormality is observed, it should be reported to EPD and the PPV Landfill operator.

14.12.3.12 In addition, if any construction is required for the maintenance work during operation stage, the responsible party should follow the protective measures and monitoring works as recommend in Clause 14.12.2 of this report.

14.12.3.13 Along with the detailed protective measure designs, the monitoring programme and detailed actions should be included in the detailed assessment in the designed design stage and submitted to EPD for approval.

14.12.4 Environmental Monitoring and Audit

14.12.4.1 For the construction and operation within the Consultation Zone of PPV Landfill, the monitoring requirement specified in Clause 14.12.2.18 to 14.12.2.23 and Clause 14.12.3.8 to 14.12.3.11 shall be followed.

14.13 Conclusions

14.13.1.1 The preliminary landfill gas hazard assessment shows that the overall level of landfill gas risk posed by the existing PPV Landfill to the Project is Medium. Appropriate protective measures (passive, semi-active or enhanced passive) have been proposed to minimize the landfill gas risk for the proposed project site during construction phase and operation phase.

14.13.1.2 In particular, proper ventilation should be provided for working area in confined environment such as the administration building, subway, and tunnel towards Lung Fu Rd roundabout. Proper design measures, such as provision of gas resistant membrane and adequately ventilated voids under building structure should be adopted for the toll control building and toll booth. Protective membrane and passive venting should be provided for the tunnel and subway.

14.13.1.3 As the detailed information on the Project (eg. Design) are not yet available, this assessment is only preliminary and only based on best available information to date. A detailed landfill gas hazard assessment in compliance to section 1.15 of the Landfill Gas Hazard Assessment Guidance Note should be conducted to provide a more detailed assessment of the LFG hazard at detailed design stage. The detailed landfill gas hazard assessment together with the detailed design of the LFG protection measures and monitoring programme shall be submitted to EPD for vetting. The requirement of submitting the protective measures designs, detailed LFG hazard assessment, and a detailed monitoring programme should be included in the scope of the subsequent detailed design study.

14.13.1.4 Provided that all the recommended protection measures are implemented properly, the safety of the site workers and all personnel presence at the proposed Project site as well as all future users would be safeguarded and there would be no adverse impact anticipated on the feasibility of the proposed Project.