David Wilson, construction manager for Arab ContractorsBalfour Beatty, takes a handful of cash from his pocket and stacks them on his desk. 'The best way to describe this building is to take a pile of coins and tilt them over at 8¦, ' he says, pushing the coins with his finger. 'Then slice the top off at 16¦.'
Only this pile of coins is 160m in diameter and 10 storeys high.
The Alexandria Library's unusual form is a mixture of ancient and modern symbols.
'We wanted to use a circle, ' explains Christoph Kapeller of Norwegian architect Snohetta.
'Its universal shape symbolises the myth of the original library which was said to contain all the knowledge of the ancient world. Then I saw a photograph of a 486 computer chip in a magazine.'
Kapeller used the circular chip as the model for the library roof in order to create the 21st century image he wanted. The next key decision was to tilt the disc shape, creating the distinctive roof slope.
'That was to create a building frozen in motion but connected in time, ' he says.
'Having it dip underground means it is dipping down into the past with the future on top in the form of the computer chip roof.'
The building's cladding, 6000m 2of grey granite transported 1500km from a quarry in the Aswan region, looks like geological strata and continues the rock of ages theme. Closer inspection of the stone reveals hieroglyphics and all the known alphabets of the world.
Geometry and proportions are defined by the library's contents. 'We multiplied the dimensions of the book racks to get the 9.6m by 14.4m structural module, ' explains Kapeller. 'And the 16.08¦ slope of the roof was reached by making each floor slab meet roof beams at the top of the columns.'
Orientation is dictated by the crucial need to minimise the amount of direct sunlight entering the building. 'It was most important that the skylights face directly north in order to ensure minimum exposure to direct sunlight, ' says Kapeller's colleague Kjetil Thorsen. 'The vertical skylights are on the diagonal of each roof panel, so we rotated the whole building until the diagonals faced north. We then added a sun shade to further remove direct sunlight and prevent heat gain.'
Client for the project, Egypt's ministry of education, put forward six consulting engineers as candidates to work with Snohetta to carry out all engineering design.
But the architect rejected them all in favour of Hamza Associates.
'Dr Hamza is the best geotechnical engineer in the country, ' explains Christoph Kapeller. 'We knew the underground works were extremely complex so we chose Dr Hamza, whose company had the capacity necessary for completion of the design and construction supervision.'
The Snohetta/Hamza consortium signed the US$13M (£9M) project design and supervision contract on 24 October 1993.
When Hamza sat down to consider the library's foundations he found he was faced with three principal issues.
The first was the huge asymmetric loading caused by the wedge shape of the building. With ten floors at the back and only one at the front, the structure is constantly trying to tilt backwards. This lop-sided loading will increase over its first few years as the number of books builds up to a planned total of seven million.
The severe eccentric gravity loads have to be transmitted to the foundation. The eccentricity of the loads increases when analysing the building under load combinations which include wind, earthquake and uplift.
The foundation raft was designed to transmit the loads from all structural components of the library superstructure to the piles. The load varied from high compression loads to high tension loads. It is supported on compression piles on the heavier south part of the library and tied down by tension piles on the lighter north part. In the centre piles capable of handling compression and tension piles are provided.
The engineer also had to consider large uplift forces resulting from the fact that the library's lowest basement level lies 12m below the water table.
This combination of high uplift forces and strongly asymmetrical dead and live loads inevitably meant complex and expensive foundations.
Ground conditions virtually dictated founding the piles in the lower band of sandstone, some 27m below fourth (lowest) basement level. Hamza briefly considered constructing the basement within a circular sheet piled cofferdam but its sheer size - at least 160m diameter - ruled out internal propping, and Egyptian law would have banned the use of ground anchors under the neighbouring buildings which crowd round the site.
So a bold decision was taken.
A circular diaphragm wall, believed to be the largest ever attempted, would be constructed through the 'middle sand' and limestone down to the lower sandstone, a total depth of 35m. It would be continuously reinforced in the horizontal direction, a technique Hamza believes had never been tried outside the Far East. Piling to support the foundation raft would take place inside the circular wall.
In conventional diaphragm wall designs, the reinforcement is inserted as a series of discrete elements with no connections between adjacent panels. When the soil inside an unpropped, unanchored diaphragm wall box is excavated, the concrete in effect acts as a series of individual cantilevers. Unless these are heavily reinforced, there is a risk of differential movement, cracking and water ingress; a risk the designers of the library were not prepared to accept.
However, if a circular wall has continuity of horizontal reinforcement, it acts as a homogeneous cylinder, resisting earth pressure through the development of hoop stress.
It is also a much more efficient structure, significantly cutting the weight of the reinforcement. In this case the fact that the cylinder was obliquely truncated was a further problem for Hamza to resolve.
The third issue that faced Hamza Associates was the extremely aggressive ground conditions resulting from the building's proximity to the sea.
The consultant's head of quality control and material testing, Dr Sharobim points out that 'the Mediterranean is only 30m away across the Corniche. As a result the groundwater is highly saline and aggressive due to the presence of high chloride and sulphate ions.
'This situation was further complicated by the fact that the client requires a life span for the building in excess of 100 years, ' he says. 'As a consequence extraordinary precautions were necessary in order to ensure its long-term integrity.'
Hamza took extraordinary precautions to ensure long structural life for the foundations. 'Life span is a function of the rate that chlorides penetrate the concrete, ' he says. Both wall and piles have 150mm cover to the reinforcement, and Portland high slag blast furnace cement is specified for all the underground structural concrete, which had a water/cement ratio below 0.30 in the interests of low permeability and long-term durability.
Sharobim says: 'It is well known that high slag blast furnace cement has higher resistance to chloride ion penetration than other types of cement. Also, when this cement contains more than 74% slag the Tricalcium Aluminate content in this cement is significantly reduced, thereby increasing the sulphate resistance of the slag cement.
'The specification also required that the fine and coarse aggregates were washed to reduce the combined chloride content to less than 0.04%, ' he adds.
The specification called for a minimum cement content of 400kg/m 3because of the aggressive environment, 45Mpa strength and a slump between 180mm and 220mm for workability. Two principal concrete mixes were used on the construction of the library.
C45 concrete with an OPC cement content of 450kg/m 3and water/cement ratio of 0.299 was used on the frame. Where the concrete comes into contact with the groundwater, high slag blast furnace cement content of 420kg/m 3and a water/cement ratio of 0.337 is used.
Within the retaining wall, Hamza has designed a two-part raft structurally separated from the diaphragm wall, supported by piles designed to operate in both tension and compression.
Half the tension piles have an extra reinforcement cage inside the standard cage. 'We wanted to make sure if the main cage corroded there would still be enough reinforcement for the job, ' says Hamza. 'Our first thought was to use an I-beam of equivalent cross section, but there was the risk of long-term galvanic action if the beam and bars were of slightly different steels.'
Cathodic protection was specified originally, but eventually abandoned - 'With regret. We thought it would solve all our corrosion problems, ' says Hamza. 'But in practice the CP industry could not supply what we needed.'
Two corrosion monitoring systems were used for measuring chloride penetration depth and predicting the rate of corrosion of the reinforcement.
All reinforcement steel was made electrically connected for CP impressed current when needed in the future.
Sharobim says: 'The technical specifications were carefully prepared to cover all activities of the work and include the required properties of each material used for construction and applicable standard specifications and codes of practice. Similarly the requirements for the inspection, testing, testing frequency, tolerances, storage and handling of each material were precisely specified.'
Piling was carried out within very tight limits. Casings were driven down to the limestone before boring began. A feature of the piling operation was the underreaming tool, which was claimed to be the first to feature hydraulic operation and in-cab continuous read out of depth and diameter. The single or double underreams it produced were checked for shape by the Japanesedeveloped Koden ultrasonic profiler, which also gave a quick read-out of the alignment and cross section of the main shaft before the complex reinforcement cage was lowered in.
A complex grouting operation began a few hours after the pile was placed.
Tubes Ó manchette were used to give a combination of base and shaft grouting, making it possible to eliminate the proposed horizontal grout blanket at this level, saving £4M of the £36M foundation costs.