Picture a steeply sloping hillside bisected by a raging 80m 3/second stream.
Not, most would agree, the ideal site on which to build a water treatment works to supply 3M people.
But at Tai Po in Hong Kong a colossal scheme is under construction, ranged on a 30% gradient above the urban sprawl of Kowloon. To compound the challenges posed by the hilly local topography, local Feng Shui practitioners identified part of the site as a 'dragon vein' - a source of spiritual energy - forcing the works onto a tiny 300m by 400m plot.
As treatment works go, Tai Po is exceptional, says Michael Hieatt, technical director at consulting engineer Binnie Black & Veatch responsible for design. It will ultimately treat 1,200M litres/day, or enough for half of Hong Kong's 6.5M population.
Site constraints have called for unusual compactness in design.
and processes have been stacked on top of one another to save space. Technologies such as dissolved air flotation, hitherto rare in Hong Kong, have been deployed to optimise treatment efficiency.
Meanwhile, major engineering work has been required to bring raw water to the works from the Guangdong transfer system 1.4km away, and deliver treated water to a new service reservoir in Butterfly Valley, 12km distant. Two tunnelled aqueducts are being built to link the supply system together through intervening hills.
Construction of the Tai Po plant started in 1997 and has been let by client the Water Supplies Department in three phases.
Joint venture contractor Leighton Kumagai has now all but completed the treated water tunnel, Hong Kong firm Gammon Construction is well advanced with the treatment works and pumping station and Gammon-Kvaerner joint venture is making rapid progress on construction of the service reservoir.
The £283M project is on course for commissioning at the end of next year, Hieatt reports.
Tai Po will initially deliver just 250M litres/day via a single treatment 'stream', but is being designed and built with its ultimate capacity in mind. Future development phases will increase the capacity of the first stream to 330M litres/day initially.
Later, two new streams will be added and all three will be equalised, increasing output to 3x330M litres/day, or 990M litres/ day combined. A final phase will see capacity increased further to 440M litres/day per stream.
Shoe-horning processes onto the site has led Binnie Black & Veatch to sit the dissolved air plant on top of the main settlement tank and treated water reservoir.
Tai Po's main chemical building is stacked four storeys tall.
In-situ cast reinforced concrete is being used for construction of all of Tai Po's structures - the contractor is currently pouring 500m 3a day.
All structures are designed to resist earthquakes, says Hieatt.
Although Hong Kong is not a high-risk zone, no chances can be taken when thousands of cubic metres of water are perched on a hillside, he notes.
'For structures containing this volume of water, seismic competence is considered important.
For example, there are 10 biological filters, and each measuring 10m by 150m by 5m deep - that's 7,500m 3. Quite a substantial volume to be holding at any one time.'
To ensure stability of the site, Binnie Black & Veatch has had to incorporate a 400m long culvert beneath the works. 'This sounds a paltry bit of engineering, ' Hieatt concedes. But the structure has to contend with peak flows of 80m 3/second. And because the level of the 8m wide culvert falls by 73m as it passes through the site, designers have had to employ dam spillway technology to keep the significant hydraulic energy in check.
The culvert incorporates four plunge pools.
'You have to anticipate the energy that will be unleashed in a flash flood. If you didn't have large culvert the stream could wash the platform away, ' explains Hieatt.
The culvert has been excavated into the works' platform - ground is mainly decomposed granite with house-sized boulders of competent rock, broken up with impact hammers or drill and wedge techniques.
The aqueduct tunnels, let as design and build contracts, have been delivered using entirely different techniques.
Leighton Kumagai used drill and blast to excavate the 3.5m diameter, D-shaped raw water aqueduct. However, two 8,000kN Robbins tunnel boring machines were deployed to drive 12km through granite and tuffs for the 3m diameter treated water aqueduct.
An overburden of 600m weighs down on the tunnel, but Hieatt reports little ground stabilisation was needed. Working in two shifts, 20 hours a day, up to 50m progress a day was made.
'With 3.8m diameter through that kind of rock, that's pretty good progress, ' he remarks.
Water ingress has challenged the contractor though.
Up to 500m head of groundwater bears down on the tunnel and geothermal springs rise up through the invert at a rate of 250 litres/second. During construction pipes were laid back to the treatment works portal to drain inflowing water away.
The tunnel lining has been designed to resist the intense pressure of surrounding groundwater - intrusion would lead to contamination of the treated supply. With 42mm thick steel walls, it is installed in 12m long 'cans' welded together insitu. The annular void around the lining is backfilled with concrete.
Internally, a 25mm thick cement mortar layer has been applied to provide protection against corrosion, with a 2mm thick sacrificial steel layer. Overall design life for the treated water aqueduct is 100 years.
Hieatt says that work is progressing well on excavation of the first quadrant of the Butterfly Valley service reservoir. It has involved knocking about 50m off a 'sugar loaf' hill to create an adequate platform for the hexagonal reservoir. But even so, space is tight at 8.6m and the reservoir is over 2m deeper than the Hong Kong norm.
Like the treatment works, the reservoir is to be developed in phases, with the first delivering 40,000m 3holding capacity. It will ultimately contain 200,000m