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5/14/2018 Green Cable Tunnel Construction at Castle Peak - slidepdf.com
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1
BSc(Civil), MSc(Civil), MSc(Env), MHKIE(Civil); Technical Manager, Dragages Hong Kong Limited 2BEng, MIMMM, CEng, RPE(G); Construction Manager, Dragages Hong Kong Limited
3 ACGI, BEng, MSc(GIS), MSc(Management), MHKIE(Geo), RPE(G), MIMMM, CEng, CSci, FRSS,
CRP(HK); Tunnel Engineer, CLP Power Hong Kong Limited 4
BSc, MSc, MHKIOA, AFCHKPWS; Director, Wilson Acoustics Limited
GREEN CABLE TUNNEL CONSTRUCTION AT CASTLE PEAK
Ken Kwok1, Andy Raine
2, Adman Chu
3& Wilson Ho
4
Abstract: CLP Power Hong Kong Limited (CLP) has promoted the design and construction of a
cable tunnel at Castle Peak. Castle Peak Cable Tunnel is a 4.5km long, 4.5m internal diameter boredtunnel launched at Lung Fai Street near Castle Peak Power Station and received at an open space area
near Sun Tuen Mun Centre. This project has undergone development and control of various
environmental challenges in order to construct the tunnel on-time. This paper will present the
environmental planning, statutory control and sustainability consideration after awarding a contract to
the Contractor.
Key words: Tunnel, TBM, Groundborne Noise, Mulch, Compost, Recycle, Sustainable
INTRODUCTION
CLP Power Hong Kong Limited (CLP) supplies electricity to more than 2.2 million customers inKowloon, the New Territories, Lantau and most outlying islands of the Hong Kong Special
Administrative Region, with a service area covering about 1,000 square kilometres.
As part of the ongoing upgrading of its electricity supply network, CLP proposed to construct a cable
tunnel (“the Castle Peak Cable Tunnel”) to enhance the future cable outlets from Black Point and
Castle Peak Power Stations, thereby improving the supply security to the existing network in Tuen
Mun, Yuen Long and the airport.
In 2005, the project was awarded as a design-and-build contract to Dragages Hong Kong Limited
(DHK). The scope of the works include the design, construction, testing and commissioning, and for
a period of one year following completion, maintenance of all elements of the works. Due to the
nature of the contract, DHK is able to carry out design, risk assessment and planning in such a mannerthat full consideration is given to public relations, permit application, environmental protection and
safety issues.
The design alignment for the Castle Peak Cable Tunnel is presented in Figure 1. The tunnel,
excavated by a tunnel boring machine (TBM), is 4.5km long with a 4.5m internal diameter from west
to east of the Castle Peak.
Figure 1 - Map of Tunnel Alignment and Works Areas
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There are two main works areas:
Western end - Castle Peak Works Area (see Figure 2), where the main site office is located, serves as
TBM launched and supplied area. Two utilities, which are vital to the neighbouring Castle Peak
Power Station, namely two 132kV cable circuits and a compress natural gas main, cross the works
area and the TBM launching trough.
Eastern end - Tuen Mun Works Area (see Figure 3), where a 40m deep vertical access/ ventilation
shaft is built. The site is a district open space located at about 35m away from a residential complex -
the Sun Tuen Mun Centre (STMC) which contains more than 3,000 residential apartments.
IDENTIFICATION AND MANAGEMENT OF ENVIRONMENTAL
ASPECT
Environmental Permit was approved by Environmental Protection Department (EPD) in December
2005, DHK at the same time signed a contract with CLP to design and build the Castle Peak CableTunnel Project including submission of technical documents for land application. The early
involvement of Contractor in Environmental Management and Assessment to develop the control and
monitoring system could allow the Contractor to have sufficient time to identify and plan to manage
significant environmental aspects during all stages of the project.
Identification of environmental aspect is basically a review of current legislation, the Contract
Document, the approved Environmental Impact Assessment (EIA) report and the method of
construction. The environmental aspect is mitigated either by site installation, training, engineering
approach and management approach.
Essential site installations to mitigate noise, air, water and waste issues, which needed to be
constructed immediately after the site possession before the TBM launching, were identified in
planning stage, :
- Hoarding & car wash facilities
- Site formation and concrete pavement
- Contractor’s shed, including toilet, washing and workshop facilities
- TBM launching shaft
- Receiving shaft and acoustic noise cover
- Chemical and chemical waste storage facilities
- Spoil shed
- Aggregate shed
- Surface drainage around perimeter of site and water treatment system etc.
Figure 2 – Castle Peak Works Areas Figure 3 – Tuen Mun Works Areas
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The above site installations was presented into a graphical means of working drawings with
supporting design, and approved through proper approval process before construction.
Proper and effective site installations can address approximate 80% of the environmental aspects
identified, the other 20% are mitigated, similar to safety management, through training and
engineering and management approach.
TrainingFor a project having 200 staffs and workers at the peak period, training is important to deliver clear
policy and requirement of environmental protection top down. Essential induction training, specific
training and tool box talk are implemented in the project. Personnel working inside TBM generally
come from different expertises and from various countries and the Castle Peak project is no exception
with 10 different nationalities working on site, therefore training materials and notices are provided in
three languages: Chinese, English and Nepalese to ensure clear and understandable messages are
delivered to the staff and workforce of the project.
Management and Engineering ApproachFor every major works, environmental aspects, risks and its mitigation measures identified earlier in
the planning stage as well as the monthly risk management workshops held in project periods arereviewed and recorded in the associated method statement to ensure the appropriated mitigation to be
implemented and compliance of permit’s conditions (if any) during the course of construction.
A Launching Meeting is carried out before the commencement of the job tasks to communicate the
method of the construction and the related risks to all task members including workers and
subcontractors. During the meeting, suggestion to improve the way to work including environmental
protection is used as a way of bottom up communication to the management for continual
improvement.
Besides the general management arrangement, a few engineering arrangements are decided at the
beginning of the project in order to achieve the project objective: pollution and waste control;
resource reservation and provide safe working environment. These arrangements are presented in the
next two Sections of Environmental and Sustainability Consideration in this paper.
ENVIRONMENTAL CONSIDERATION
TBM Groundborne Noise During planning stage of TBM tunneling projects, groundborne noise prediction is necessary for
construction programming. If the predicted noise is within the Noise Control Ordinance (NCO) limits,
tunneling programme can be condensed based on continuous 24-hour TBM operation. Otherwise,
significantly lengthened tunneling programme would be required. However, groundborne noise
prediction is inherited with considerable uncertainty due to complex mechanisms of vibration
generation, transmission and noise re-radiation under various site conditions [Ref: 1]. The uncertaintystandard deviation is generally in the order of 10dB.
TBM Groundborne Noise Prediction Method Among various groundborne noise prediction methods [Ref:1-4], empirical approach with individual
data for vibration generation, transmission and noise re-radiation is more applicable for accurate
prediction. Vibration generation data relates to TBM type, number and type of cutter discs, thrusting
pressure, rotation speed, total power of rotation motors, advancing speed, rock type, etc. Vibration
transmission data relates to geology strata, building foundation and super-structure. Noise re-
radiation data relates to radiation efficiency of the building materials and room acoustic response.
Before installation of Castle Peak Tunnel TBM, nighttime groundborne noise was predicted in
accordance with this approach to fulfill the requirement described in the project EIA report. The
prediction used large amount of empirical data of a previously measured similar TBM at various
geologies and buildings in Hong Kong.
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Construction Noise Control Regulations in Hong KongDaytime (0700 to 1900 hours) construction noise impact is controlled by Environmental Impact
Assessment Ordinance (EIAO) for designated projects, whereas evening and nighttime (1900 to 0700
hours) construction noise impact is controlled by NCO. For most cases, TBM groundborne noise is
relatively minor humming noise at level around 40 to 60dB(A) which would be generally consideredacceptable during daytime according to the requirements in EIAO. Nighttime (2300 to 0700 hours)
TBM groundborne noise is more problematic because it may lead to sleeping disturbance.
Groundborne noise prediction in planning stage is generally less accurate due to insufficient data of
TBM and geology. Planning consultants often use conservative assumptions to fulfill EIAO
requirements for daytime construction noise prediction and leave the relatively problematic nighttime
TBM groundborne noise prediction for the Contractor to handle.
Nighttime TBM Operation and Construction Noise Permit (CNP)In addition to the TBM groundborne noise prediction submitted to EIAO office, CNP applications are
required to submit to Regional Office of Environmental Protection Department (EPD) for evening and
nighttime TBM operations of the Castle Peak Tunnel
Project. The CNP applications include detailpredictions of airborne and groundborne noise impact.
Airborne noise prediction was based on the overall
TBM Sound Power Level (SWL) measured in
accordance with ISO-3746 in the TBM factory in
China during the TBM fabrication and testing. The
SWL was reconfirmed with site measurement at 2 to 3
days before issue of the permit (subject to practice of
Regional Office). This approach significantly
shortened the waiting time (from around 3 weeks to 3
days) between TBM installation completion and
nighttime operation. A similar approach was also used
in a previous project for West Rail tunnel at TsuenWan Shaft [Ref: 5].
Continuous Vibration MonitoringA 2-channel continuous vibration monitoring system was tailor-made for the Castle Peak Tunnel
Project to monitor the vibration levels at a compress natural gas main located within 4m from TBM
tunnel. The system continuously logged the vibration history for every second and provided 2-level
alarm signals at PPV 12mm/s and 25mm/s. This system ensured appropriate actions, perhaps
stopping TBM operation, would be taken whenever vibration was higher than the alarm levels.
Vibration levels variation history during TBM passing-by was also recorded for every second for
future reference.
Nighttime TBM Operation for Entire TunnelThe predicted groundborne noise levels for the entire tunnel alignment were well within the NCO
nighttime limit except at STMC, where the predicted groundborne noise levels was marginally exceed
the NCO nighttime limit incorporating a prediction safety factor of 10dB(A). Such prediction safety
factor was considered necessary for issue of TBM nighttime CNP. For conservative approach, this
location was excluded in the 1st
CNP application for nighttime TBM operation. When the TBM was
operation at tunnel depth similar to the tunnel depth at STMC which is about 40m, numerous ground
vibration measurements were conducted at various distances from TBM cutter head to provide better
prediction of the TBM groundborne noise. The 2nd
CNP application for nighttime TBM operation
was submitted with updated prediction at STMC showing groundborne noise level was 15dB(A)
below the NCO nighttime limit. Then CNP for nighttime TBM operation was obtained for the entire
TBM tunnel, except the last 20m from the retrieval shaft.
Figure 4 – On site noise measurement to re-
confirm TBM sound power level
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The successful of receiving 24 hours CNP can largely reduce the risk of tunneling safety and other
engineering related issues.
Landfill GasElements of surrounding environment which might affect the project – landfill sites. There is one
strategic landfill, the West New Territories (WENT) Landfill, and two closed landfills, Siu Lang ShuiLandfill (SLSL) and Pillar Point Valley Landfill (PPVL), in the Castle Peak area. The tunnel
alignment was specifically developed to avoid the tunnel alignment running directly beneath the
landfill sites so as to avoid any direct or indirect construction or operational effects on these landfills.
The WENT landfill and Siu Lang Shui Landfill are over a kilometre away from the proposed works
and hence will not have an impact on the proposed cable tunnel.
The existing landfill boundary of the closed landfill site, the PPVL, is situated about 225m away from
the tunnel alignment at its closest point. In addition, the original landfill boundary abuts the tunnel
alignment. However, the cable tunnel is expected to be at a depth of about 180m below ground at this
location.
A preliminary landfill gas hazard (LGH) assessment was done during EIA consultation period based
on the EPD Landfill Gas Hazard Assessment Guidance Note. The assessment was undertaken to
determine the potential sources and pathways for landfill gas and leachate that could reach the cable
tunnel and, based on a qualitative risk assessment matrix, to determine the degree of risk anticipated.
Based on the findings of this assessment, impacts from leachate are not expected to result due to the
distance from the landfill. Although the risk of infiltration of landfill gas into tunnel is very low,
consideration to mitigate the risk needed to be considered. The most important issues were
considered and addressed during the procurement phase of the TBM and the tunnel ventilation
system.
Methane is the key component of landfill gas being flammable and which will burn when mixed withair between approximately 5% and 20% by volume, the Lower Explosive Limit (LEL) and the Upper
Explosive Limit (UEL) respectively. In order to prevent dangerous build up of these gases during
excavation and lining works, early detection, ventilation and evacuation are the main means of control
consideration during the procurement phase and the construction phase of the TBM.
Early detectionThe TBM was fitted with extensive onboard gas monitoring equipment which was linked directly to
the onboard data acquisition software and alarmed via the TBM computer supervision system; three
stage methane detection was deployed: first detector was placed in the TBM front shield to detect
methane in the forward excavation area; the second methane detector was placed in the TBM deduster
which removed dust and fumes from the excavation chamber and prevents contamination of thegeneral tunnel environment; and the last detector was placed adjacent to the TBM control cabin to
monitor the general tunnel environment. Oxygen, Carbon Dioxide, Carbon Monoxide and Nitrogen
Dioxide detectors were also placed in the same location.
VentilationThe TBM was provided with a highly efficient double suction type ventilation system. The first
system removed fumes and dust via an onboard deduster before rejecting the cleaned air to the rear of
the TBM capacity 4m3
per second provided by 3 inline Korfmann Gal 6 ventilation fans fitted with on
board silencers to reduce the impact of noise pollution to the TBM enviroment; and the second
suction system removes air from the TBM forward ring building area and transfers directly to the rear
of the TBM capacity 6m3
per second provided by 4 inline Korfmann Gal 6 ventilation fans also fitted
with on board silencers . Fresh air is supplied to the rear of the TBM via external fan and flexible
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duct installed in the crown of the tunnel – this arrangement allows the 235m TBM to be maintained in
fresh air at all times.
In addition the air supply to the TBM is cooled and dehumidified by means of a two stage counter
current cold water heat exchanger which produces cold air at around 17 deg C – this helps to maintain
the TBM working environment at acceptable level during the warmer summer months.
The external ventilation system provides more air
than is used to ventilate the TBM and the excess is
utilized to dilute any noxious gases removed from
the forward area at the rear of the TBM – in our
case the TBM onboard suction ventilation system
circulated 10 m3
per second. It is equivalent to 1
complete air change on the TBM every 6 minutes
and is fully in line with the requirements of the
COP for safety in tunneling BS6164 . The external
system provided approximately 12m3
per second
with a capability to increase to 16m3
per second inemergency case.
MitigationThe first stage of the process is that the project has
a rigorous risk management procedure with
monthly review and dedicated actions for
individual team members, as part of the risk
process the project has developed a comprehensive
Emergency Response plan and the procedure of
landfill gas mitigation is clearly identified.
The Alarm System has three levels of activation:
Level 1 at Methane >5% LEL shall be the Alert
Level. Methane is present and care should be
exercised.
Level 2 at Methane >10% LEL shall be the Alarm
Level. The methane concentration level is
increasing, excavation and hot works shall be
stopped. Ventilation airflow is maintained or
increased to restore the methane level to less than
10% LEL.
Level 3 at Methane >20% LEL shall be the Evacuation Level. Ensure all hot works are stopped. The
methane concentration level is now exceeding an acceptable level. The tunnel shall be evacuated and
ventilation airflow is increased until the at least Level 2 is re-established.
This three stages system allowed us to guarantee the safety of our workers during the excavation of
the tunnel in the areas adjacent to the landfill.
RadonRadon, an inert radioactive gas, is one of the naturally occurring products of uranium. Any rock or
material containing uranium will also contain radon. Traces of uranium are present in many rocks, but
the concentration of uranium is not a guide to the likely concentration of radon. Radon is readily
Figure 5 – General view inside the tunnel
during construction stage, flexible ventilation
duct and conveyor system mounted in the
crown of the tunnel
Figure 6 – Refuse Chamber equipped in TBM
for emergency precaution use
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soluble in groundwater, from which it is released on contact with free air, and can be transported
significant distances through the ground from its source by this means.
The risk to human health from exposure to radon arises mainly from inhalation of its radioactive
decay products, the daughters of radon. The effects of cellular damage, particularly to the lungs,
consequent upon exposure to these substances are not immediately life-threatening but can increase
the risk of cancer developing later in life.
In tunnel and basement construction and excavation, radon is mainly associated with groundwater
ingress; the groundwater carries the radon in solution from the rock mass into the underground
working where the Radon “out gases” and is released into the working environment. The second
method for radon transmission into the environment is via out-gasing directly from the rock surface
itself however this is of lesser magnitude than the groundwater path.
Radon gas is typically associated with granitic areas commonly found in Hong Kong and of particular
concern during basement construction and tunnel construction. It may also be of concern during
operation phases if insufficient ventilation is provided to dilute and remove the radon from the
enclosed environment.
The three main methods which have been employed to control Radon ingress into the underground
works are as follows:-
Firstly to pre-treat any major rock fissures ahead of the tunnel boring machine by drilling ahead of the
TBM and injecting both cement and micro-cement grouts to reduce and /or eliminate ground water
ingress thereby cutting off the main flow path for the Radon.
Secondly a precast concrete lining is installed close to the excavated rock face.
Thirdly ventilation is provided to dilute and purge any Radon from the environment, the same method
is employed for control during the operational phase.
Radon gas is monitored continuously for 24 hours every week, all the means values measured are
below the required standard of 150 Bq/m3
and most of the results are below 50 Bq/m3. The low level
of radon measured in a situation of granitic zone proves that the three methods are effective enough to
avoid accumulation of radon in tunnel.
Precast Elements for temporary and permanent worksIn general timber formwork is used in traditional cast-insitu method and normally 4 times of re-use of
timber formwork will be allowed to achieve the required finishes of concrete. Wastage of concrete
and reinforcement is also a common problem due to difficult to estimate the exact quantity. On the
other hand the use of Precast Elements is a well known method of reducing wastage of concrete and
reinforcement as better control can be achieved in a casting yard than on site. Steel formworks are
used, instead of timber, for sure will reduce the loading of landfill site of Hong Kong. A good
example of this is the custom build system formwork utilized at the insitu concrete works of the
Tunnel Jointing bays ; the formworks have been able to be reused 10 times and have finally been
recycled for steel scrap ; a 100% reduction on landfill burden.
With the consideration of great advantage of using precast element, it was decided in the beginning of
project to use as much as precast element as for it is practical. At the time of writing this paper, all
design stage was completed. It is found that more than 95% of the structures, both permanent and
temporary works, have been or will be constructed using precast elements. It includes:
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- The permanent tunnel concrete lining,
cable trough and walkway cover for the
4.5km long tunnel is precast. Special
design is adopted to from part of the 5
numbers enlarged cable joint bay areas
along the tunnel with the standard precast
tunnel elements.
- The walls for the temporary spoil storage
area and aggregate storage bins was
formed using precast blocks. The walls
are approximately 7m high in order to
ensure that spoil accumulated are fully
screened in order to prevent the finer
material being blown off site and have
sufficient capacity of 4 days storage.
- The walls of the Tunnel Boring Machine
Launching Trough, which are approximately 8m deep, 10m wide and 120m long, are formed
using precast blocks. These walls were designed to resist not only the retained soil loads butalso surface loads from storage areas adjacent to the trough.
- All haul roads around the site were formed using precast concrete panels. The quality of the
road and its ability to withstand heavy traffic loading without damage is enhanced by the use
of precast elements.
Noise cover for noisy activities Tuen Mun Shaft is about 35m away from a residential complex - STMC which contains more than
3,000 units and is the main sensitive receiver. During planning stage, a series of noise mitigation
measures are decided for the works area such as noise barrier for all stationery plants, quiet gantry
crane as lifting appliance for daily use of transportation / lifting materials, tools and men. In small
details such as between rail and footing of the gantry, a neoplane is used to reduce noise produce from
contacting rail of concrete footing during movement of the gantry crane.
The most important issue is the rock breaking
activities. As no blasting is allowed for the
construction of the shaft, the excavation of the shaft
in rock, without many choices, is to break by
mechanical means. The process includes drilling
holes, splitting, breaking, excavation and mucking
out.
The noise cover, sitting on a circular concrete
diaphragm wall, is designed specific for shaft
activities with careful details in openings and man
access. The noise cover is used to mitigate noise in
day-time when splitting, breaking, excavation and
mucking out. During evening-time, drilling hole is
the only operation to carry out as per the condition of construction noise permit applied. No
exceedance has been found from the impact monitoring carried out at monitoring stations of the
residential complex.
An on-site noise measurement was carried out during the operation of rock breaking to find out the
noise reduction achieved by the noise cover. The procedure of noise measurement is as follows:
1. Take background noise measurement with shaft opened.2. Rock breaker operation
Figure 7 – Precast permanent tunnel segment
(front) and precast temporary spoil shed (rear)
Figure 8 – Noise cover of Tuen Mun Shaft
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3. Take noise measurement at 6 locations at 1m above the shaft opening. Leq,30s would be
taken at each location.
4. Close the shaft with the noise insulation cover.
5. Take noise measurement at the 6 locations at 1m above the shaft cover previously measured
as Item 3.
6. Take noise measurement at 6 locations at 1m below the shaft cover
7. Stop the rock breaker and take background measurements again.
It is proved from the measurement that 24 dB(A) noise reduction can be achieved by the use of noise
cover which the insertion loss is found as 22 dB(A).
SUBSTAINABILITY CONSIDERATION
Re-use of top soil and tunnel spoilAt the site formation stage, although it is designed that the site formation works and the subsequent
TBM launching trough and support facilities/infrastructure works at the Castle Peak works area to
achieve as far as possible a balance between “cut” and “fill” volumes, the site clearance works
revealed that a significant amount of topsoil was present in the area. In an initiative to avoiddisposing of the topsoil at the Public Filling Area it was established that the company who had carried
out the site clearance works could re-use it for various ongoing government landscape contracts.
During the site clearance period, 90 m3
of topsoil was sorted from general fill material and delivered
to the Nursery Yard of trees for re-use.
Rock chips and rock fines are produced during the
excavation of the tunnel by the TBM. The spoil in fact
are good to re-use in haul road construction and sub-
base of trenching works. From the beginning of
project, DHK investigated and approached many
companies for seek opportunities of re-cycling the
tunnel spoil. However, it has a practical difficulty to
transport tunnel spoil by truck and it has “financial
practicality” to transport far away the site as the Castle
Peak works area is very close to Tuen Mun Area 38
Fill Bank.
During construction stage, DHK closely coordinated
with CLP and neighbour Castle Peak Power Station
for recycle use of tunnel spoil. Total 21,400 tonnes of tunnel spoil were delivered to the neighbour
construction sites for use without charge after completion of tunneling.
Figure 9 – Loading tunnel spoil inside Spoil
Shed
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Treatment of Fell Trees – Mulching and CompostingThe concept of tree recycling in this tunnel has been first discussed in the paper referenced [Ref: 20].
As stated in p65 of the mentioned paper, “Removing existing trees is sometimes inevitable for civil
engineering projects especially for the works in rural area. The current government policy on this
issue is stated in the Development Bureau Technical Circular No. 03/2006 where tree felling should
only be considered as a last resort if there is no other practical alternative or the concerned trees haveunrecoverable health problem. Problems arising from those trees with low survival rate where felling
is not allowed, but unlikely to survive after transplantation. It is not uncommon that efforts and
resources were spent to preserve those trees but its fate cannot be changed at the end of the day.
Although felling of a tree means the end of its life, it does not represent the end of its contribution to
the environment.”.
Although at the initial stage of feasibility study in
this project, the Project Client – CLP has already
chosen the option with minimal environmental
impacts, hence building a cable tunnel
underground instead of the option like overhead
lines supporting by the pylon towers, unavoidablythere are structures above ground for the
maintenance purposes. To enable the site
formation works for the above structures, hence in
this case the Castle Peak Portal (see Figure 10)
and the vertical shaft at Tuen Mun, inevitably
numbers of existing trees, which were part of the
former power station development, were necessary
either be transplanted or fell. The idea of carrying
out some voluntary works, which go beyond the
basic economic function in a lawful manner (see
Figure 11), was initiated at the earlier stage of the
project to exercise our commitment on social
responsibility. Having considered the founding
principles of social responsibility and
sustainability, a series of environmental initiative
meetings amongst the project team were
conducted, it was decided to convert the felled
trees to useful products instead of the usual
practice to transport those felled trees to landfill
site. A very simple concept of taking from the
nature and using back to the nature was adopted.
The option of tree recycling to convert to
composted and mulched material for plantingpurpose was finally decided and implemented after those meetings. These felled trees were processed
into the products of compost for soil improvement and mulch for weed control. “Composting is a
biological process for converting organic solid wastes into a stable, humus-like product whose chief
use in as a soil conditioner.” [Ref: 8] and “In agriculture and gardening, mulch is a protective cover
placed over the soil, primarily to modify the effects of the local climate.” (Mulch – Wikipedia, the
free encyclopedia (2008)). As the results, the recycled materials were delivered to the Hong Kong
Housing Society (HKHS) and CLP, Generation Business Group (GBG) in July 2007 (see Table 1).
Figure 10 - Area of Tree Felling at Castle
Peak Portal
Figure 11 - Principal elements of social
responsibility and their evolving magnitudes
[Ref: 22]
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Table 1 - Delivery of Recycled Materials
Facility Date of Delivery Quantity of Material
Delivered
Jat Min Chuen, HKHS 9 July 2007 / 10 July 2007 12m3
of mulch
Clauge Garden Estate, HKHS 10 July 2007 6m3
of mulch
Lok Man Sun Chuen, HKHS 11 July 2007 6m
3
of mulchGBG’s Nursery 20 July 2007 2m3
of compost
The outcomes of using mulched and composted materials to HKHS and GBG are illustrated in Figure
12 and Figures 13 to 14 respectively. There were positive feedbacks of these recycled materials from
both organisations with an appreciation letter from HKHS.
As stated in [Ref: 20], p70, “Using the products from the tree recycling has both technical and
economical benefits and the project team of this cable tunnel project appreciates the importance of
tree preservation. It is not the intention of this paper to encourage civil/ geotechnical engineering
practitioners to act indiscriminately on felling trees but to exercise our commitment on socialresponsibility to save our precious landfill sites and on the re-use of natural materials”. Apart from the
cost implication such as reduction on the cost of transportation, landfill, landscape maintenance,
sphagnum peat moss replacement etc., whether we should apply such environmental initiative
depends on how much we appreciate the project sustainability and our responsibility to the society
and environmental as a professional engineer.
Water treatment and recyclingWith the previous experience in Chi Ma Wan Cable Tunnel where
the launching shaft is located in Chi Ma Wan peninsula without any
water supply or mainline drainage facilities. In the project we were
limited to discharge 30m3 /day therefore DHK have incentive to
recycle as much as possible in order to limit the effluent discharge tosea within target and to save the water resource. DHK found this
practice can be adopted widely and therefore the system is applied to
Castle Peak Cable Tunnel Project.
The source of polluted water are mainly come from the surface run
off collected in ground level, inflow water collected from ground
water into U channel of the tunnel then pumped through pipeline to
ground level, and polluted warm water used by TBM pumped
through pipeline to ground level.
In the operation of the water treatment and recycling system, waste
water is collected and treated by a series of Oil interception, primary
de-sanding and Sedimentation Tanks, then via an AquaSed 80
Figure 12 - Application of Mulching at HKHSFigure 13 and 14 - Application of Composting at
GBG
Figure 15 – 3 numbers of
75,000L recycle water storagetanks
Placement of
com osted material
Approximate after
9 month of planting
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Treatment Plant. The treated water is then stored in three 75,000L
storage tanks and ready for use in tunneling works mainly for the
TBM process and cooling water, vehicle washing, suppressing dust
etc. Used water is continuously recycled for the entire project
duration.
The plant employed on the project has a total HRT of over 5 hours
and has an operating capacity of 60m3
per hour.
Varying from 72% to 88% of total water used at site is recycling
water; the variance is depending largely on the water lost to the
tunnel spoil where recycled water is utilized extensively for dust
suppression and cannot be easily recovered back to the treatment
system. It represents 1.9m3
of water used for 1m3
of rock tunneling
which is more than 50% less than the expected quantity. The results
are encouraging not only from a money saving point of view but also
the project shows the effort of use less water resource and discharge
less water to public drainage system.
CONCLUSION
At the time of writing this conclusion, the TBM tunnelling at Castle Peak is completed and concrete
structures at both ends are ready to start. The most risky environmental aspects have been
successfully mitigated. Dragages Hong Kong Limited, as a main project driving party, has worked
closely with CLP Power Hong Kong Limited and Specialist Consultant Wilson Acoustics Limited
striving to build a green tunnel within the time and financial constraint. With always a consideration
of social responsibility at heart, the sustainability ideas thought ahead in planning stage have been
successfully translated into the reality.
ACKNOWLEDGEMENT
The authors would like to express their gratitude to the project team of Castle Peak Tunnel; CLP
Power Hong Kong Limited; Dragages Hong Kong Limited; and Atkins China Ltd..
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