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Pleasant valley someday

Concrete

One of the world's largest pumping stations is nearing completion in an artificial lake 800km south of Cairo.

Dave Parker reports from Egypt.

Only 5% of Egyptian land is habitable.

This small percentage supports nearly 70M people and the population is booming. Illegal building is eating into the remaining agricultural land around the Nile and in the Delta.

Creating new croplands out of Egypt's deserts is the obvious solution to the question of how the growing number of hungry mouths should be fed. But only in Egypt could the government expect to see such a vision become reality within the foreseeable future.

Within 15 years the South Valley Project (SVP) should increase Egypt's inhabitable land fivefold. Some 16M people are expected to migrate southwards towards the border with Sudan, mainly to work in agriculture. Mineral and alternative energy production are other long term possibilities.

The key is irrigation. Some of the essential water will be extracted from underground aquifers, but the bulk will come from Egypt's greatest treasure, the 150bn. m of fresh water in Lake Nasser, held back by the Aswan High Dam.

Crops like grapes and melons already flourish in the desert at the Kadco research farm near the famous Abu Simbel temples .

But on the ground the first crucial stage in the SVP is nearing completion at Toshka, on the shores of Lake Nasser, 240km south west of the High Dam. Here the Mubarak pumping station will soon feed up to 5bn. m3 a year of Nile water into the El-Sheik Zayed Canal. This water, distributed through a 250km network of sub-canals, will eventually irrigate more than 230,000Ha and support 3M new residents.

Joint venture design consultant Hamza Associates chairman Dr Mahmoud Hamza says the indicative design for the pumping station, which was to be set into the lake's shore, would have posed some major geotechnical challenges: The client originally envisaged a row of 21 pumps set behind a 350m long wall.

'Water levels in Lake Nasser vary by up to 35m, ' notes Hamza. Allowing for a 15m head of water needed by the pumps for efficient operation, the wall would have had up to 50m depth of water on one side, exerting huge overturning forces. 'Making this wall stable would have been an expensive exercise'.

Ground conditions were an unpromising mixture of friable sandstones, claystones and gravels. Design alternatives that minimised the need for complex foundations had to be sought as a matter of urgency.

Another early proposal Hamza disliked was the concept of long intake tunnels or pipelines stretching kilometres out into the depths of Lake Nasser. 'These would also have been very expensive, and there would have been long term maintenance problems, ' he says.

'A dredged intake channel was a much more appropriate solution - but this would have meant dredging 50m deep, and everyone said this couldn't be done.'

However, Hamza remembered seeing a Krupp machine dredging up coal from below 60m of water in Yugoslavia.

With this technology available, the advisors to the Ministry of Irrigation approved the open cut approach.

A radical revision of the pumping station concept was the next breakthrough. Hamza's solution was to design the station as a giant 67m deep and 140m long by 40m wide concrete box. This would be situated in the middle of an artificial intake lake almost 2km back from the lake shore. The box would be far more efficient and stable than the wall originally proposed, with the pumps lined up on the longer sides and all hydrostatic forces balanced out - 'although with its weight only slightly greater than its buoyancy there was a theoretical risk of overturning during earthquakes', Hamza adds.

'The answer was minipiles around the base of the box, which would act in tension to restrain any overturning forces.'

Each 8.5m high vertical centrifugal pump has a capacity of 16.7m 3/sec and delivers water into the canal through massive concrete ducts. Internal dimensions are 2.7m by 2.4m, and the ducts not only act as the main access bridge to the pumping station but their mass helps resist the station's inherent tendency to float away.

Another advantage of the island concept is that the pumping station could be constructed inside a dry, dewatered excavation up to 50m deep before the canal linking the new lake to Lake Nasser was complete.

However, pouring such masses of concrete - the walls are 2m thick, the raft foundation more than 3m - in temperatures which can vary between 0infinityC and 55infinityC - was never going to be easy. Making sure the structure was watertight was another challenge.

After much experimentation a mix with a cementitous content made up of 35% Egyptian ordinary Portland cement, 35% ground granulated blastfurnace slag and 30% pulverised fuel ash was chosen for all concrete in contact with water.

Such a mix may have a slow rate of strength gain, but its low heat of hydration was considered essential to minimise the risk of cracking.

In all, some 300,000m 3of concrete were needed.

For the first 3km or so of its length the El-Sheikh Zayed canal is actually above existing ground level, formed from twin parallel concrete troughs supported on backfill.

This layout was selected, says Hamza, so that one trough could be closed for maintenance while the other kept the water flowing. One leg is already complete and construction of the second well advanced.

Once the canal can be formed by straightforward excavation a single trapezoidal cross section is adopted. Width at the top is 54m, at the bottom 30m, water depth is 6m.

Around 70km has been constructed to date. This main artery will eventually stretch for more than 300km, with four main branches up to 70km long extending north and west.

Project team Design and build joint venture contractor: Arab International Company for Construction/ Skanska Cementation International/ Hitachi Corporation Design joint venture: Hamza Associates/Lahmeyer International

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