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Void discovery puts Saudi centre on new footing Keen eyes pick out Saudi wadi deposits during museum investigation

SITE INVESTIGATION has prevented a potentially catastrophic mistake being made in the foundation design for a landmark building in Saudi Arabia.

Additional trial pitting showed that the ground underlying the site of the King Abdul Aziz Historical Centre, originally thought to be competent sediment over the Arab Formation limestone, was in fact riddled with innumerable tiny voids. This meant the soil had the potential for catastrophic collapse on contact with water.

The centre, a national museum, will open in time for the centenary celebrations of the founding of Saudi Arabia in December 1998. The £85M project includes refurbishment of existing palace structures and the creation of a new park in Riyadh. Design is by architects Moriyama and Teshima.

Internation- al consultant Buro Happold, working for client Arriyadh Development Authority, is project manager for the contract, working with main contractor Saudi Oger. It has a number of responsibilities including geotechnical aspects.

Happold commissioned Dames & Moore to carry out a site investigation. This consisted of wash boring and rotary boreholes, trial pits and mechanical cone penetration tests. The investigation showed that there was apparently between 7m and 13m of competent sedimentary deposits overlying limestone.

But laboratory testing showed that the soils have the potential for collapse and additional investigation was needed. Trial pitting revealed stratified wadi deposits consisting of a multiple sequence of sandy silts overlying gravels.

'Further examination of these materials identified a myriad of minute voids,' says Happold senior geotechnical engineer Tom Skailes. The majority are caused by a fine root system with a smaller amount of larger voids up to 2mm diameter formed by burrowing insects such as termites.

The main worry was that the soils would become saturated as a result of irrigation of the area and collapse under load. To evaluate the possible effects of the voids, plate bearing tests were carried out at various points across the site. These were taken up to pressures equivalent to the proposed foundation loads and the ground inundated, a pool of water being maintained for between 24 and 72 hours. The soils clearly showed tendency to collapse when saturated.

Although the current water table on site is around 5m below ground level, historic records indicated that the water had been at 3m. As roots do not extend below the water table, it was decided to design the footings to be placed at this depth. To confirm this theory, further plate tests were carried out at the founding depth and showed that there was no potential for collapse below this depth.

The completed museum will provide 35,000m2 of floor space split between curatorial activities and galleries, designed as a walk-through tour, leading visitors through the story of Islam and the dev- elopment of modern day Saudi Arabia.

Every stage is controlled by the tight prog- ramme, not only in design and construct- tion but also sourcing many of the artefacts. Currently, construction is going on 24 hours a day to meet the anniv- ersary dead- line at the end of the year.

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