green cable tunnel construction at castle peak

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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



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



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.



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



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



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



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:



- 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



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]



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



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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Figure 16 – Water Treatment

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green cable tunnel construction at castle peak

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