Hong Kong Water Supply Shek Pik Reservoir – Part 2 Reservoir Construction
Tymon Mellor: With continued water shortages on Hong Kong Island and Kowloon, pressure was on the Shek Pik construction team to complete the reservoir as fast as possible. The situation was so bad that during a particularly bad drought in the summer of 1963, the Government hired 13 tankers to ferry water from the Pearl River to a new terminal at Tsuen Wan. With the clearance of the two major villages in the Shek Pik valley, construction of the dam and wider scheme could commence in earnest.
With the successful completion of the grouting trials reported in May 1958, Soil Mechanics and Soletanche were instructed to continue the installation of the grout curtain to provide the necessary water cut-off for the dam. Following a world-wide tender, in July 1959 Societe Francaise d’Entreprises de Dragages et de Travaux Public were awarded the main contract for the construction of the Shek Pik dam for completion in November 1963 for $32.7 million.
The grouting trial had demonstrated that with suitable grouting methods, an effective cut-off could be achieved. This approach was adopted in the areas of alluvium. In areas of completely weathered rock, a grout injection method was adopted to open up and seal the joints. The work was undertaken once a minimum of 6m of fill had been placed in order to avoid blowouts. In areas of good rock, drill holes at 3m centres were cored to a depth of up to 70m for the grouting.
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Concurrent with the grouting operation, work commenced on the earth moving operation. The dam was designed to use fill material available within 1.6km (1 mile) of the dam site. The residual soil would form the core of the dam, and the down-stream shoulder would be formed by material excavated from the hills within or adjacent to the reservoir basin. Material excavated from the valley floor would be used for the free draining upstream face[i]. With a height of 55m and a crest length of 717m, the dam would require the placement of 4.7 million m3 of fill material.
The fill material was to be sourced from local borrow pits, but when excavation commenced it became apparent that there was significant variability in the material and much of it had to be rejected. A number of alternative options were explored, but 0.6 million m3 of imported sand was selected to replace the unsuitable material as it could be compacted in the rainy season.
The bulk of the dam was constructed within a 21-month period between 1st October 1961 and 1 July 1963. A total of 4.2 million m3 or 87% of the total fill was placed in that period. With over 1,000 men on site, the team operated motor scrapers, facing shovels and dump trucks operating two 10 hour shifts each day to meet the construction programme.
The final dam was constructed of earth fill up to 55m high and required around 4.8 million m3 of fill. The upstream slope is protected from wave action by rock rip-rap and the downstream slope was protected from rain erosion by turf.
During the later stages of construction of the embankment, cracks were identified in the dam core. Following investigations, it was concluded these were caused by settlement of the foundations of the dam. The cracks were grouted and monitoring indicated negligible water leakage.
Impounding and filling of the reservoir began in June 1963 and by July there was sufficient water to be drawn off, ahead of the original programme. However, parts of the supply system were not complete, including the new pumping station at Sandy Bay. With the provision of temporary pumps, water from Shek Pik entered the distribution system in November 1963. By the wet season of 1964, the reservoir was full and all the works were complete.
The reservoir was formally opened by the Governor, Sir Robert Black on the 28th November, 1963 unveiling a plaque for the project. He noted, “The Shek Pik Scheme will go a long way towards reducing the risk of a repetition of the crisis through which Hongkong has been living since May”. A reference to the severe water shortage affecting the territory.
The reservoir had a final capacity of 5.5 million gallons and cost around HK$33 million (at 1964 prices) with the whole water supply scheme costing $235 million.
Catchwaters & Their Tunnels
The direct catchment of the reservoir was only 777 hectares but with the construction of the catchwaters, this was increased by 4,532 hectares, equivalent to 40% of Lantau Island[ii]. The scheme initially included 17.5km of catchwater, but this was extended to nearly 19km in preparation for the impounding of water.
The catchwater on the north site of Lantau Island utilised pipelines and a number of tunnels to supply water to the reservoir on the south side of the island. A similar approach was taken for catchwaters to the west of the reservoir.
With completion of the reservoir, it became apparent that the reservoir catchment could be increased with the provision of a new water tunnel to capture the streams in the Tung Chung area. In late 1963, Paul Y Construction was awarded a contract for the construction of the 5.1m diameter 7.2km long tunnel for a cost of $23m[iii] to intercept three streams. After 20 months of tunnelling requiring a workforce of around 600 men, the tunnel broke through in August 1965 and following installation of the concrete lining, it came into operation in April 1966.
To avoid water overtopping the dam, overflow water is directed in to a bellmouth spillway connected to a 450m long driven tunnel discharging directly into the sea. This tunnel was completed early to be utilised as a river bypass route during the construction of the dam. The tunnel was lined down with concrete to 5.1m diameter and connected to the 24m wide bellmouth with a 50m high shaft. The tunnel excavation also includes a 0.91m scour pipe, used to empty the reservoir for maintenance.
From the valve tower, located to the east of the Shek Pik dam, the water could be drawn-off at five different levels, depending on the reservoir level and quality of the water. From the tower, the water enters the supply tunnel which is 2.2m in diameter in good rock and unlined, and 1.5m diameter where concrete lining was required for support. The tunnel, 7.8km long includes access shafts at Tong Fuk and Cheung Sha and connects to a 1km long 1.2m diameter pipe to convey the water to a pumping station at Pui O. The pumping station connects to an 800m long 1.2m diameter pipeline connecting to a second tunnel 1.4km long and terminating at the Silvermine Bay treatment works.
The contract for the supply tunnel was tendered in August 1959 and awarded in December 1959 to Paul Y Construction for just over $11 million.
Silvermine Bay Water Treatment
On the hillside above Mui Wo, the water treatment works for the Shek Pik scheme was constructed. The facility was designed to treat 26 million gallons a day (with a maximum capacity of 35 million gallons) for supply to Hong Kong Island through two 0.76m diameter submarine pipelines[iv]. The facility included sand filter beds and a covered 5 million gallon service reservoir along with a three-storey building containing 24 flats for the operational staff.
The design of the facility provided for the raw water to pass through three cleaning dishes where the water would remain for four hours while chemicals were added to remove sediment, before passing into service reservoir. The facility was constructed by Paul Y Construction Co. Ltd.
Submarine Pipe Line
From the treatment plant, two 13km long submarine pipe lines conveys the treated water to Sandy Bay on Hong Kong Island. Design and installation of the 0.76m diameter pipes was outside the local experience, and thus, international contractors were sought to undertake the works. In August 1960, the works were awarded to American firm, Healy Tibbetts Construction Co Ltd, in conjunction with J. L. Kier & Co Ltd, of London and Paul Y Construction of Hong Kong.
The design of the pipeline provided for supplies to Peng Chau and Hei Ling Chau. As the pipes pass Chau Kung Island, a cross-connection chamber was constructed within the sea to house the necessary valves and to provide the new water supply to the adjacent islands.
The pipeline was designed to be laid within a trench to support a cover fill depth of between 2.1m to 4.2m, depending on the risk of currents and dragging anchors[v]. The pipes were fabricated from 12mm thick mild steel rolled and butt welded into 8.2m lengths by a fabricator in Singapore before being shipped to a preparation yard at Hung Hom. Here, the pipes were welded into 24.6m lengths before protective coatings and a 75mm thick concrete surround were applied. The pipes were then taken out to an old molasses tanker converted into a pipelaying barge.
To accommodate the two pipes at 3m centres, a trench 12m wide, at sea bed level was excavated with a clamshell bucket, and where boulders were encountered, a 2-ton chisel was used to break the blocks. On the pipelaying barge, the 24.6m pipe lengths were welded together before being lowered through the sea into the prepared trench.
Pipe laying commenced in May 1961 and was completed by February 1962 with little disruption due to inclement weather. Installation of the pipeline commenced from Silvermine Bay and progressed towards the cross-connection chamber. The next section placed was across the Lamma Channel to Sandy Bay. For the deepest section at 36m, the pipes were pulled into place avoiding the need to operate in the busy shipping channel. Pulling winches were secured at Sandy Bay to pull the pipes across. The remaining section of pipeline was completed using the laying barge. With the pipe in place, the trench was backfilled.
The cross-connection chamber was constructed from four 8.7m diameter precast concrete rings, 0.3m thick, sufficient to withstand a 100 ton impact force. Following the installation of concrete piles, the first ring section was placed and 2.4m of concrete was placed inside to provide a hydraulic cut-off and a base for the pipeline. With the completion of the pipeline, the remaining rings were placed allowing the water to be pumped out and also connection of the pipe fittings.
Even before construction of the Shek Pik Scheme had commenced, the Hong Kong Government recognised that this new supply would not solve the water supply problems. By 1959, consultants were looking at developing storage reservoirs at Plover Cover and Hebe Haven. The Government was also looking at producing fresh water from sea water using “Atomic power”[vi] and there was pressure to import water from the new reservoir under construction in near by Sham Chun, in the Mainland. The eventual solution would be an integrated solution to provide capacity and flexibility to deal with future water supply problems.
Reservoir Construction Ina.fr
[i] Shek Pik Dam, William J Carlyle, ICE Paper 1965
[ii] Shek Pik Dam, William J Carlyle, ICE Paper 1965
[iii] Tunnel to Shek Pik Completed, SCMP 24 Aug 1965
[iv] Treatment Works at Silver Mine Bay, SCMP 27 Oct 1962
[v] Shek Pik Submarine Pipeline, Hong Kong Water Supply, A P Goudy ICE Paper April 1965
[vi] HK Legislative Council Meeting 25 March 1959
This article was first posted on 23rd May 2021.
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