CHALLENGES AND HARMONY WITH SUSTAINABLE DEVELOPMENT IN BUILDING
AND OPERATION OF A FLOOD RELIEF PUMPING STATION IN SHEUNG WAN
Mr. LEUNG Cheuk-lun, Project Engineer (Civil)
Drainage Services Department, Hong Kong SAR Government
Mr. POON Ka-ho, Engineer’s Representative (Civil)
Drainage Services Department, Hong Kong SAR Government
Mr. MOK Hing-man, Engineer’s Representative (E&M)
Drainage Services Department, Hong Kong SAR Government
Mr. LEUNG Man-woon, Operation & Maintenance Engineer (E&M)
Drainage Services Department, Hong Kong SAR Government
Mr. LAM Chun-pan, Project Manager
Penta-Ocean Construction Co. Ltd
Mr. SHI Guang, Project Manager
China National Chemical Engineering Group Corp.
Abstract: The Sheung Wan Stormwater Pumping Station (SWSPS) is located on reclaimed land at the
waterfront of Victoria Harbour. It comprises a pump house and an underground storage tank measuring
45.6m by 43m on plan and 11m deep as temporary stormwater storage, for alleviating the long persisting
flooding problem of the low-lying area at Sheung Wan. The inherent geographical constraints of the Site,
unfavorable underground conditions, interfacing issues arisen amongst the multi-disciplinary trades of
works, and the increased public expectation for timely commissioning of the SWSPS, had posed particular
difficulties to the designer, resident engineer and contractors to integrate and execute various parts of this
project. This paper presents the difficulties and challenges encountered by the designer, resident engineer,
maintenance engineers and contractors during the design, construction and implementation of pumping
station, and the solutions adopted to overcome them in respect of engineering design, risk management and
public interests.
INTRODUCTION
The low-lying areas of Sheung Wan of Hong Kong Island had been persistently suffered from flooding when
heavy rainfall coincided with high tide. The situation would be the worst during extreme high tide, when
the sea level is higher than the ground level at the Sheung Wan low-lying areas. The seawater would flow
back into existing drainage network and overflow from manholes and gullies. In order to relieve the flooding
risk at Sheung Wan low-lying areas, the Drainage Services Department (DSD) of the Government of
HKSAR decided to construct a stormwater pumping station at the waterfront (ex-Gala Point) of Sheung Wan.
It serves to collect stormwater from the existing drainage networks in the low-lying areas, temporarily store
the stormwater in an underground storage tank with storage capacity of approximately 9,000m3 and
discharge the stormwater into the nearby harbour through submersible pumps. At the same time, a penstock
was installed inside a Flow Diversion Chamber (FDC) to prevent the backward flow of seawater into the
drainage system.
Above the underground storage tank, the open space was developed into a public open area with a pet corner,
waterfront promenade and landscaping works, integrating with the on-going waterfront promenade
improvement works at Sheung Wan to form a continuous public corridor along the waterfront from Sheung
Wan to Sai Wan. The construction commenced in June 2006 and completed in September 2009. The total
cost of the project is approximately HK $200 millions.
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Figure 1. Location Plan of Sheung Wan Stormwater Pumping Station
SITE LOCATION AND GROUND CONDITIONS
The proposed SWSPS is located on reclaimed land at Chung Kong Road, Sheung Wan, Hong Kong (Figure
1). The structures of pumping station measured approximately 56m long and 43m wide. The SWSPS is
surrounded by many structures which are sensitive to ground movement. On the northern side, there is a
vertical concrete blockworks seawall founded on a sand-fill and rock-fill formed bund. On the western side
of SWSPS, it is the existing Sheung Wan Salt Water Pumping Station. The salt water pumping station is
found on raft with a deep intake culvert and a basement. At the nearest point, the structure of SWSPS is
only approximately 1.5m away from the culvert. At the southern side boundary, there are several major
utilities pipelines and cables laid along Chung Kong Road. The eastern end is an open space for temporary
material storage and site offices.
Based on the geological survey map published by the Geotechnical Engineering Office, the Site is underlain
by fill material generally and overlies with medium-grained granite. The ground investigation carried out
for this project shows that the superficial deposits which consists of fills, marine deposits and alluvium are
overlying weathered granite before reaching the rockhead of strong moderately to slightly decomposed
granite. A typical geological section across the Site in the north-south direction is presented in Figure. 2.
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Figure 2. Typical Geological Section across The Site in the North-south Direction
ENGINEERING PROPERTIES OF SOILS
The engineering properties of different soil materials encountered in drillholes in the Site were determined
through both in-situ and laboratory tests.
During the ground investigation works done in 2006, in-situ falling head and constant head permeability
tests were conducted in selected drillholes. The results of permeability test are summarized in Table 1.
Type of Soil
Range of Permeability (m/s)
Sand Fill
6.11 x 10-6 to 1.01 x 10-5
Rock Fill
5.07 x 10-6 to 1.34 x 10-1
Marine Deposits
No test
Alluvium
No test
CDG
6.55 x 10-7 to 1.39 x 10-4
Table 1. Permeability of Different Soil Materials
From the test result, the permeability of rock fill is found in the order of 1 x 10-1 m/s. As the Site is close to
the shoreline, it is practically impossible to dewater the excavation for the foundation and underground
storage tank construction, without a groundwater cut-off to a depth below this highly permeable soil stratum
provided.
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DESIGN CHARACTERISTICS OF THE SWSPS
Flow Diversion Chamber
The flooding problem in Sheung Wan low-lying areas results from the coincidence of high tide and heavy
rainfall, when the seawater flows back into the existing drainage network and hinders the discharge of
surface runoff collected from the low-lying areas by gravity. To prevent the backflow of seawater, a Flow
Diversion Chamber (FDC) with installation of penstock is proposed. The FDC connects the existing
drainage network and outfall with a proposed 2100mm diameter precast concrete drainage pipe along Chung
Kong Road (Figure 3). Under normal operation, the penstock would be closed to prevent seawater backflow.
The surface runoff would be diverted into the underground storage tank via the 2100mm diameter drainage
pipe for temporarily storage, and discharge into the harbour by pumping. Under emergency condition such
as failure of submersible pumps and stored water level inside the underground storage tank reaching warning
level, the penstock would be opened remotely by duty officer at the control centre of DSD, to allow
collected surface runoff to discharge into harbour via the original outfall by gravity.
Pump House
Proposed twin 900 rising mains
Flow Diversion
Chamber
Existing stormwater drains
Proposed stormwater drains
Proposed stormwater drains
Proposed twin 900 rising mains
Proposed stormwater manholes
Proposed temporary road diversion
Proposed Pumping Station
Existing stormwater drains
Figure 3. Schematic Diagram of Drainage Arrangement along Chung Kong Road
and Flow Diversion Chamber
Underground Storage Tank
One of the key components of SWSPS is the underground storage tank. It is designed as a water-retaining
structure with storage capacity of 9380m3. The maximum depth of storage tank is approximately 10m below
ground, and it is supported on piled foundation using pre-bored H-piles. Key design parameters of the
underground storage tank are summarized in Table 2.
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Design parameter of the underground storage tank
Value
Plan Area
1580m2
Soffit Level of Tank
+1.30mPD
Depth from Soffit
5.94m
Invert Level
-4.64mPD
Top Storage Level
0.29mPD
Table 2. Key Design Parameters of the Underground Storage Tank
Pump House
The pump house is a 5m height, single storey reinforced concrete building with two basement levels, which
houses all the electrical and mechanical control, monitoring and associated devices such as penstocks, raked
mechanical bar screens, low flow pumps, submersible pumps and control panels. It also has
loading/unloading areas for vehicles to collect screenings/sludge. As the collected screening/sludge might
generate odour and affect the citizens in the adjacent public open areas, a deodorization unit was installed
inside the pump house. For electricity supply to the electrical and mechanical devices, a transformer and
switchgear room is provided. At the roof of pump house, a 400m2 garden equipped with skylights is
constructed to harmonize the pump house with the adjacent landscaping works in Open Space. The roof
garden also helps to lower the temperature inside the pump house. Moreover, the skylight allows sunlight
entering the pump house during daytime, hence the power consumption for indoor lighting and
air-conditioning can be considerably reduced.
For the best utilization of land, the northern portion of pump house is built as an Open Frame structure with
one basement level. All 6 nos. of submersible stormwater pumps (4 duty and 2 standby) with maximum
pumping capacity each of 1 m3/s and rising mains are installed in the basement of the Open Frame structure.
To facilitate the inspection and maintenance of pumps and pipelines, the covers at the ground level of Open
Frame are removable and overhead traveling crane is also installed at Open Frame. During normal
operation, the Open Frame portion of the pump house is served as a portion of the park for public leisure
activity. When maintenance to the equipment in this portion is required, the whole portion will be closed
temporarily for maintenance work.
Conveyance Channel
The top level of the base slab of underground storage tank is -4.40mPD. When the collected surface runoff
enters the underground storage tank through inlet box culvert (invert level is -2.22mPD), the runoff would
fall onto the base slab of underground storage tank by gravity freely. Turbulence is expected to be
generated and affect the performance of submersible pump system. A conveyance channel was proposed
inside the underground storage tank to transport the runoff smoothly to the base slab of underground storage
tank using an overflow weir (Figure 4), aimed to reduce turbulence generation. The Hong Kong
Polytechnic University was appointed by the DSD to carry out physical hydraulic modeling to optimize the
design of conveyance channel to further improve the hydraulic performance of the conveyance channel.
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Submersible
Pumps
Overflow
weir
Guide Wall
Conveyance Channel
Inlet Box Culvert
Legend:
Direction of stormwater flow inside the Underground Storage Tank
Figure 4. Flow Path of Stormwater inside the underground Storage Tank (Plan View)
Energy Efficient Features
Different from traditional stormwater pumping station designed and operated by DSD, there are a number of
energy-efficient features incorporated in the design of SWSPS. For pump operation, the stormwater pumps
are equipped with variable speed drives and harmonic filters. Energy efficiency could be achieved by
operating the stormwater pumps at optimal capacity to cope with the load demand. The variable speed
drive could also help to reduce the motor starting current, both to save energy and to reduce size of the
upstream power supply. In addition, the harmonic filters could maintain a high power quality, reduce the
energy loss and thus enhance the overall energy efficiency. In order to cope with the fluctuation in the
amount of stormwater inflow, combination of pumps of different sizes is used to suit it. The stormwater
pumps are used to handle large stormwater inflow arising from heavy rainfall whereas the smaller low flow
pump (1 duty and 1 standby) is used to cater for light rainfall, as well as keeping the underground storage
tank in dry condition. It helps avoid any remaining water in the tank upon subsiding of the rainstorm from
getting septic and producing odour.
Gas Detection System
A total of 4 sets of gas sampling units extract the sample air from the stormwater tank comprising H2S, O2
and CH4 detectors are connected to a gas sampling unit. i.e. 4 sampling units serve 12 detectors and the 12
gas detectors will detect the gas concentration in 4 zones of the tank and send the signals to Gas Monitor
panel. A combined visual and audible alarm is provided near the gas monitor panel in the switch room.
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CONSTRUCTION OF FOUNDATION AND DRAINAGE WORKS
Excavation & Lateral Support System
For the design of the excavation & lateral support (ELS) system, the complex ground conditions of the Site,
which imposed significant influences on the selection of retaining wall system, major constraints to the
design of ELS were highlighted as follow:
1.
2.
3.
4.
The Works Area was located adjacent to the seawall (minimum distance less than 15m to the Victoria
Harbour) ;
The Works Area was very close to the existing structure and roads which were sensitive to ground
movement (only 1 to 2m to the existing footpath and the boundary wall of WSD salt water pumping
station);
The level difference of pile caps was about 6m maximum and the maximum depth of pile cap base was
12m below ground; and
The ground water level after excavation should be maintained and the excavated surface should be free
from ground water.
The proposed ELS had to be simple and economical, and be able to achieve the following requirements:
1. facilitate a stable excavation with minimal obstruction;
2. provide an efficient groundwater inflow cut-off; and
3. control the ground movement within acceptable limits.
The options of bored pile retaining wall and diaphragm wall had been considered but discarded. Apart from
cost consideration, it was also foreseen that the trenching phase of diaphragm wall construction through the
rubble rockfill of the seawall might induce ground settlement in the order of 60mm. Whilst for the option
of contiguous bored pile retaining wall, the risk of water leakage through the piles would be high. Hence, an
ELS system comprising sheet piles of maximum 24m length was finally selected (Figure 5). In addition, in
order to minimize the effect of settlement to the adjacent structure, toe grouting up to the CDG strata was
applied to prevent soil loss during construction. This arrangement served as a secure basis in line with
temporary pumping to maintain the water level after excavation.
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Figure 5. Section of the Excavation & Lateral Support System
Regarding the sheet pile installation, an innovative installation method called “silent piling” was introduced
in this project. Unlike normal practice, the sheetpiling was installed by hydraulic jack and less working
space was required. Since the sheetpiles were pressed into the ground by hydraulic jack, the noise generated
was greatly reduced as compared to the conventional method. Hence, minimum nuisance to the public
would result. Furthermore, as less vibration would be caused by using this method, ground settlement and
soil loss would also be greatly reduced. It was particularly important as the stability of adjacent sensitive
structure, especially the seawall, should be maintained. Another advantage of this installation was that the
sheetpile stability could be maintained. The sheetpile being pressed-in would fully encompass and
hydraulically grasped by the machine such that the sheetpiles can be kept vertically. Furthermore, all the
sheetpiles must be locked in during installation as the installation would be based by “action-and-reaction”
principle, i.e. the reaction force from the previous pile would be used for the installation of next pile. No
gaps would then be allowed between the sheet piles and the seepage of underground water would be kept to
minimum.
Another important arrangement of the ELS was the temporary pumping system inside the excavation.
Totally 15 sets of submersible pumps with individual capability of 5 m3 per second were used to maintain
the water level. The temporary pumping system was operated in line with the toe grouting previously
applied to the CDG layer in order to prevent soil loss.
It was noted that the pile caps were in different levels. It was not desirable that the deepest pile cap was
constructed first and backfilled to the level of pile caps due to the restricted construction time. From
Figure 5, it was observed that extra sheetpiles were installed between the pile caps and they could be
constructed individually. The said sheetpiles were installed at a shallow depth as compared to that around
the perimeter of Works Area, no toe-grouting was applied and the grading of materials of sheetpile was
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reduced (from Type IV to Type III). It was because water pressure was already resisted by the sheetpiles
installed around the perimeter and this sheetpiles only acted as the structure supports between the pile caps.
Foundation construction
In this project, different types of foundation namely rock-socketted steel H-piles, mini-piles and raft were
adopted for the underground storage tank and the pump house.
Pre-bored rock socketted steel H-piles, which with the configuration of 610mm outer diameter installed with
305 x 305 x 223kg/m Grade 55C H-section and 4 nos. of 40mm diameter high yield deformed steel bars, to
achieve the required pile capacity of 6900kN. Owning to the close proximity of the Site to the Harbour, the
option of pre-bored rock socketted steel H-pile was proposed since gravels, cobbles and boulders were found
in the fill material encountered at the site in particular near the existing seawall. The obstruction could be
overcome by the ODEX drill head and an unsupported face below the casing toe was avoided. It was
particularly important when excavation took place in adjacent to seawall since settlement would occur if
there was unsupported excavation face.
Another advantage of using pre-bored rock-socketted steel H-piles is less working space would be occupied
by the boring machine. During construction, the steel sheet piles for cofferdam were installed
simultaneously on site in order to minimize the effect of ground disturbance to the existing seawall. Since
the Works Area was limited, smaller plants and equipment would reduce the construction time as more
plants can operate simultaneously.
Mini-piles and raft foundation were used as the foundation for the pump house. Compared to the
underground storage tank, the loading of pump house was relatively smaller. In addition, the adjacent WSD
salt water pumping station was sensitive to the foundation works as it was less than about 10m away from
the proposed foundation works. Therefore, it was preferable that mini-piles and raft foundation were
adopted in order to minimize the adverse effect to the adjacent WSD salt water pumping station and existing
seawall.
Excavation and Pile Cap Construction
The excavation works was one of the critical works in this project. Due to the tight works programme, total
25,000 m3 with maximum 12m in depth of materials would be excavated and disposed of off site within 2
months. It was in turn more than 100 truckloads of material to be removed offsite daily. Under this
circumstance, the traffic load on the existing heavily trafficked Chung Kong Road would be greatly
increased. Despite the excavated material transported via sea is uncommon, especially at the Victoria
Harbour, the possibility for disposal by barges was considered. As the Site was adjacent to the Hong Kong
Macau Ferry Terminal, part of the Harbour area which was classified as restricted area and the Harbour area
adjacent to the Site was so busy that the jetfoils would arrive and depart from the Ferry Terminal. With the
cooperation of relevant parties (e.g. Civil Engineering and Development Department and Marine
Department etc), it was agreed that the excavated material could be disposed of to Tuen Mun Area 38 Fill
Bank by barges provided that the barges should depart from the Site at midnight, in order to minimize the
obstruction to the operation of jetfoils. This arrangement was greatly successful as it could minimize the
adverse effect to public and the construction progress could be kept in time.
Besides the disposal of surplus excavated materials, another challenge of the excavation work was to
maintain the bottom of excavated surface free of underground water. As mentioned in previous section, the
minimum distance of the Works Areas to the Harbour was approxmimate15m, and the bottom level of
excavation was approximately 12m below existing ground level. In addition to this, consideration should be
taken into account the adjacent structures which were very close to the site boundary. The design and
construction of the ELS and temporary pumping system as described provided a secure basis to maintain the
excavated surface free from groundwater.
Another challenge encountered after the excavation was the construction of pile caps. The maximum depth
difference for individual pile cap was 6 m and the total area occupied was 46 m x 54 m. In addition, only
one side of Works Area could allow access of the construction plants and equipment for transporting
9
materials as the other three sides were located at the existing seawall, existing WSD salt water pumping
station and the Chung Kong Road. A temporary steel working platform extended to the midway of ELS was
also introduced to provide additional working space for construction (Photo 1 refers). To secure the support
of working platform and to minimize the risk of water seepage of storage tank, the H-section of 4 nos. of
rock socketted steel H-piles were extended to the existing ground level and served as the support of the
temporary steel platform. After concreting of pile cap, the extended H-section was removed and grouting
was applied to fill up the voids.
The concreting arrangement for the underground storage tank was also important. Considerations must be
taken into account the time required and the impacts to public. The volume of pile cap of the underground
storage tank was 1,200m3 and it was impossible to pour the pile cap in one time (about 170nos. of truckloads
each with 7m3 fresh concrete) under the busy traffic in Chung Kong Road. In addition, the delivery of
concrete was essential as cold joint might form if the fresh concrete could not be delivered on time when
there was traffic congestion during weekdays. As the pile cap of the underground storage tank would serve
as its base slab, any water leakage through the weak cold joint would seriously influence the storage
capacity of the underground storage tank and more importantly, causing the corrosion of steel reinforcement
bars. Despite three pouring points could be provided during pile cap construction, the pile caps was
concreted in two times during Sundays such that adverse impacts to public was minimized and the timely
delivery of concrete could be secured.
Photo 1. Concreting of Pile Cap of the Underground Storage Tank
Drainage Works in Chung Kong Road
Besides the construction of pile caps for the underground storage tank and pumping station, the associated
drainage system along Chung Kong Road which mainly included a twin 900mm diameter rising main,
2,100mm diameter stormwater drain and associated manhole and chambers. Since the Chung Kong Road is
the only vehicular access to the Hong Kong - Macau Ferry Terminal, a new temporary road was required to
diverse the existing traffic on Chung Kong Road, and to minimize the impact on existing traffic during the
construction stage (Figure 3).
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The construction of Flow Diversion Chamber (FDC) was the most critical item within the proposed drainage
system. The existing 900mm and 2,100mm diameter stormwater drain at the upstream was designed to
collect the stormwater from the low-lying area in Sheung Wan, whereas the outfall at the downstream was
an existing 2,100mm diameter outfall to the Victoria Harbour. It was necessary that the proper collection
and discharge of stormwater collected from the upstream should be maintained in order to prevent flooding
at the low-lying area during construction. As such, high efficiency submersible pumps were used to
maintain the flow of stormwater. On the other hand, it was also important to prevent inflow of seawater
from the existing outfall to the low-lying areas at upstream. A temporary stoplog was installed at the
outfall to prevent the inflow of seawater to the construction pit at the FDC. Under normal condition, the
stoplog would be closed and stormwater at the upstream would be pumped to the existing outfall via a
temporary pumping system. In case there were heavy rainstorms (e.g. red/black rainstorm signals hoisted)
or typhoon, the stoplog would be uplifted by electric actuator or manually (only be used when the electric
winch was out of function) and stormwater would directly enter the Victoria Harbour via the existing outfall.
Tidal water was another challenge during the construction of the FDC. The bottom level of construction
pit was at -1.0mPD approximately and the distance between the pit and Victoria Harbour was about 10m.
In addition, since the outfall was located adjacent to the Hong Kong - Macau Ferry Terminal, the tidal waves
generated by the jetfoils passed through were critical. The Contractor had made several modifications to
the temporary stoplog to overcome the tidal waves.
Attention to Emergency Situation (Temporary Flooding Relief Works)
As mentioned above, the existing stormwater flow at the upstream should be maintained during construction.
The temporary stoplog was closed normally and pumps were used to maintain the flow. An emergency
team, including DSD’s and Contractors’ staff, was established to be on duty during non-working hours in
case of emergency. The emergency team would consist of inspector of works, works supervisor, foreman,
electrician, plant operator and labours. All the team members would arrive on site within 15 minutes after
receiving the call.
It was noted that the temporary stoplog would be lifted (opened) and pumps be operated by the emergency
team if the rainstorm occurred outside working area and the upstream stormwater would enter the outfall by
gravity. One of the emergency cases was the Typhoon Hagupit occurred on 23rd September 2008. The
maximum recorded tide level at Victoria Harbour was about +3.5mPD, which was about 800mm above the
lowest level in the Sheung Wan low-lying areas. When high tide coincides with heavy rainfall, the rainfall
could not be effectively drained off by gravity via the existing drainage network. With heroic and fearless
efforts, the emergency team had effectively operated the temporary stoplog and pumping system under the
heavy storm and rains, and thus successfully helped to prevent serious flooding in Sheung Wan.
CONSTRUCTION OF PUMP HOUSE SUPERSTRUCTURE
The superstructure contract was the one of pioneer contracts in the DSD combining both the civil
construction and Electrical & Mechanical (“E&M”) installation works, each of which would conventionally
be awarded in separate contracts.
In order to ensure the earliest completion of the Underground Storage Tank (“UST”) together with Pumping
Station, the time for sectional completion of the critical UST and Pump House (“PH”) allowed was 180 days
only, which was indeed abnormally tight compared to other similar common civil projects. To overcome
such time limitation and various site constraints encountered during the course of the works, the construction
works had to be planned and sequenced effectively in order to ensure the smooth running amongst each
stage of construction and achievement of the target completion dates.
Dismantle of the Excavation & Lateral Support (ELS) System
The primary duties at the very beginning of the Contract were to study the drawings provided by the
Engineer on the ELS and review against the contract construction drawings in order that a logical and
realistic programme for the safe dismantling of the ELS system and efficient construction of the UST could
11
be devised. Upon taking over the Site from the Foundation Contractor in January 2008, it was the first task
to verify the as-built arrangement of the ELS and check against its compliance with the original one allowed
in the planned proposal of removing the ELS. Some discrepancies of as-constructed walings and sheetpiles
were noted at several locations of the Site which not only increased the difficulty of the construction of the
underground structures, but also had even forbidden the erection of formwork for construction of the
substructure walls at certain locations. Having studied the conflicts between the ELS and the permanent
structures, and with support by design checking and verification by Independent Checking Engineer, it was
the next step to devise the comprehensive sequencing for dismantling of the ELS so as to allow the staged
construction of the complete UST safely and to avoid any dummy procedures nor unrecoverable damages to
the permanent structures due to any incorrect method or sequence of dismantling the ELS. Despite the
various differences between the as-built ELS and the original design leading to necessary change of
construction method and sequence, through close coordination with the Engineer, the temporary works
designer and the Independent Checking Engineer, a feasible and cost effective solution was devised quickly
so that the dismantling of the ELS and construction of structural works could be proceeded with right after
the taking over of the Site from the Foundation Contractor.
In order to safely and effectively dismantle the strutting and waling members while maintaining the progress
of the construction, it was unavoidable that some strutting member would be left in the reinforced concrete
structures. To mitigate the potential water seepage problem incurred by the left-in pieces of steel members,
hydrophilic waterstop was installed around the steel member (Photo 2).
Photo 2.
Hydrophilic Waterstop Installed on the Steel Member
Construction of the Underground Storage Tank and Pump House
There were several constraints encountered during the construction of the UST:
Insufficient working space for construction of the RC structures
Along the peripheral wall of the UST, the as-built clearance between the proposed wall of the underground
storage tank and the as-built sheetpiling wall had been found to be far from sufficient, i.e. less than 400mm.
Under such narrow space, no entry of workers was allowed and thus traditional double-sided formwork was
not possible. To effectively overcome such site constraint, a brickwork wall was built against the sheetpile
wall which served as permanent formwork for casting of the RC wall. The other side of formwork for the
RC wall was supported by external steel frame which eventually supported by as-constructed column head
(Figure 6).
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Figure. 6 Formwork Arrangement for the Construction of Peripheral Wall of the
Underground Storage Tank
Control of thermal crack for large size of RC members
The member sizes of the RC structures for the UST were comparatively large, e.g., 1.5m thick wall,
700mm thick slab. In order to minimize the effect of concrete cracking due to thermal contraction of
concrete of large member size, Shrinkage Reducing Agent had been used as additive to the concrete in
order to reduce the shrinkage of the concrete during its curing. To further effectively minimize the
concrete crack incurred by thermal and/or shrinkage effect, the RC walls had been concreted in alternate
pour of less than 20m each (Photo 3). It was also particularly arranged to leave a 2m gap uncast until
28 days of the last pour of concrete such that the overall shrinkage of the peripheral wall had been
minimized.
Photo 3.
Construction of the 1.5m Thick Wall of the Underground Storage Rank
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Source of filling materials for backfilling
Above the UST was an open space to be constructed into a landscaping garden. A 1.5~2.0m thick of
soil, in volume of more than 3,000m3, was required to be placed above the top slab of the UST. In
order to follow the environmental and fill management policy of Hong Kong Government to reduce the
burden of the public fill reception facilities, surplus excavation material from various adjacent Sites had
been received as the source of fill material supply for general filling works above the UST. Close
monitoring had been kept with respective project sites in order to ensure the continuous supply of filling
materials and thus the subsequent superstructure construction works would not be affected.
Congested Site Condition and Plant Accessibility
The construction site, i.e., a cofferdam at initial stage, was bounded by the existing Chung Kong Road,
the existing WSD salt water pumping station and the existing seawall at close distances. The use of
heavy plants to facilitate the site works had been limited, particularly for lifting and movement of
construction materials within the Site. To alleviate the site constraints, special crane truck with long
and rotatable boom (Photos 4 and 5) was deployed for lifting and transportation of construction materials
into the area where it would not be accessible by normal lifting plant.
Photo 4. Special Crane Truck with Long and Rotatory Boom
Photo 5. Lifting of Construction Materials by Special Crane Truck with Long and Rotatable Boom
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Installation of E&M works
During the beginning of the Contract, it was identified that the material delivery for various essential E&M
equipment would be critical to the overall completion of the Project. It was the main task within the first 3
months of the project to get the approval from the Engineer on various essential equipment, e.g., pumps,
penstocks, mechanical bar screen and control switchboard etc. To avoid late delivery of the essential E&M
equipment, orders were placed immediately after approval was received from the Engineer.
To streamline the interface between the E&M installation works and builder’s works, both works had been
carried out concurrently in order to shorten the overall construction period. For example, the skylight of
the pumping station was not constructed to facilitate the lifting and installation of the penstocks and
mechanical bar screens while the steelwork associated with the Open Frame area was only constructed after
the pumps were installed into the barrels. All these sequencing and planning had been carried out in a daily
manner in order to ensure the smooth and coordination of works amongst various trades of subcontractors.
Testing and Commissioning of the Pumping Station
The commissioning test for the SWSPS was conducted in the second week of March 2009, i.e. starting on 9
March 2009.
A schedule of the works for the SWSPS and the commissioning test had been prepared for the project team
to closely monitoring and ensure the commission test to be successfully conducted as targeted (Figure 7).
Jan-09
12/29 1/5
1/4
Works for SWSPS
Feb-09
1/12 1/19 1/26 2/2 2/9
1/11 1/18 1/25 2/1
Mar-09
2/16 2/23 3/2 3/9
2/8 2/15 2/22 3/1
3/16 3/23
3/8 3/15 3/22 3/29
Handover Inspection on SWSPS
Commissioning Test
Intake of Rainwater
Intake of Seawater
Figure 7. Schedule of Handover Inspection and Commissioning Test.
DISCUSSION AND CONCLUSION
The construction works of the project commenced in June 2006 with completion in September 2009. The
underground storage tank, the pumping station and majority of E&M works were completed in March 2009
and the operation of the pumping station by DSD’s maintenance division commenced on 18 March 2009.
The remaining and landscaping works were completed in September 2009 and the Open Areas were then
handed over to Leisure and Cultural Services Department for public enjoyment.
During the wet season in 2009, the Hong Kong Observatory issued 20 times of amber rainfall warning
signals, 2 times of red rainfall warning signals and 3 times of typhoon signals No. 8. The pumping station
was proved to handle the runoff collected successfully and protected the low lying areas in Sheung Wan
against flooding.
ACKNOWLEDGEMENTS
The authors express their gratitude to the Drainage Services Department of HKSAR for their kind
permission to use the data for the preparation of this paper. Assistance from the Main Contractor for
foundation and drainage works (i.e. China National Chemical Engineering Group Corp.), the Main
Contractor for the pumping station main structure, E&M and landscaping works (i.e. Penta-Ocean
Construction Co. Ltd.) and the appointed E&M sub-contractor (i.e. REC Engineering Ltd., formerly
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“Ryoden Engineering Ltd.”) are highly appreciated. The authors would also like to thank all those who
helped in reviewing this paper. The views expressed in this paper are those from the authors and do not
represent the opinions of the Drainage Services Department or the Contractors.
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