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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.

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.

Existing stormwater drains

Proposed stormwater drains

Proposed twin 900 rising mains

Pump House

Proposed Pumping Station

Proposed stormwater manholes

Proposed temporary road diversion

Proposed twin 900 rising mains

Proposed stormwater drains

Existing stormwater drains

Flow Diversion Chamber

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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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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.

Conveyance Channel Inlet Box Culvert

Guide Wall

Submersible Pumps

Overflow weir

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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. The Works Area was located adjacent to the seawall (minimum distance less than 15m to the Victoria

Harbour) ; 2. 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);

3. The level difference of pile caps was about 6m maximum and the maximum depth of pile cap base was 12m below ground; and

4. 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

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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

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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).

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

    Jan-09 Feb-09 Mar-09

    12/29 1/5 1/12 1/19 1/26 2/2 2/9 2/16 2/23 3/2 3/9 3/16 3/23

1/4 1/11 1/18 1/25 2/1 2/8 2/15 2/22 3/1 3/8 3/15 3/22 3/29

Works for SWSPS  

Handover Inspection on SWSPS

Commissioning Test

  Intake of Rainwater

  Intake of Seawater

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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.