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UK, says Poland's geotechnical sector is largely underdeveloped. Most geotechnical and foundation design work, he says, is carried out by structural engineers who are "geotechnical by experience". A few university professors ran consultancy practices as a sideline and they are deemed custodians of the nation's geotechnical knowledge.

For Arap, it has been something of an uphill struggle to develop a market for top-end consultancy based around geotechnical expertise within a commercial private business. But through projects such as Zlota 44, Amp's geotechnical capability is beginning to be recognised among Poland's wider engineering community and, more importantly, the development and investment community. Amp's foundation design has taken practical Polish geotechnical engineering to new heights of sophistication, as well as saving the client money. Developer Oreo Group's gleaming new 54-storey edifice will occupy a central Warsaw location across the road from the imposing Palace of Culture and Science, an architectural show­case from the communist era.

Oreo's site was previously occupied by a seven-storey office development constmcted in 1989, with two basement levels all founded on a raft. When built, it was the first application of diaphragm walls in the city. A major challenge is the fact that the site is constrained on all sides, with neighbouring buildings just 600 mm from the basement wall. The original intention to provide four or five levels of underground car parking would have meant not only a major demolition job to remove the existing thick base­ment raft, but also a phenomenal amount of monitoring and a very costly legal undertaking — even assuming constmction went exactly to plan.

Armed with this knowledge, Oreo rethought and redesigned the building's lower floors, making use of just two basement levels and allowing a combination of commercial and car parking to extend up to the eighth level. Structural loads at foundation level were of the order of 600KPa-800KPa, but rather than providing a deep-piled solution Amp investigated whether it could be designed as a piled raft system.

These would be designed similar to those used in Frankfurt, where piles are designed purely to limit settlement, rather than to carry loads. As a local twist to this approach, Amp proposed using

single diaphragm wall panels as barrettes. From a constmction point of view this meant the original diaphragm wall could be left in place, the old raft shaved off, and a new raft cast on top of it. The new raft would be 2 m thick below the tower area, reducing to a thickness of 1.5 m elsewhere.

This approach would produce a much less expensive founda­tion. Leszczynski realised this approach required investigation using 3D finite element analysis to determine whether it would work as a true piled raft and also to determine how the connection between the barrettes and the raft affected the behaviour of the foundation. An accurate determination of the length of the barrettes was also needed.

Leszczynski's hunch.

For Leszczynski, it was vital that the analysis accounted for soil-stmcture interaction and that a more sophisticated soil model than Mohr Coulomb was used. The approach meant Amp had to commission a much more comprehensive ground investigation than is typical in Poland, on the basis that there would be little point in doing a complex finite element analysis if it were not confident it had correctly identified the ground conditions.

The investigation included eight boreholes and eight cone penetration tests, with shear-wave velocity measurements to determine strain ratios. By taking samples and reconstituting these at their insitu density, Amp was able to correlate the ground stiffness to its grading and density. This confirmed Leszczynski's hunch supported by observed settlements in existing buildings in Warsaw that the ground was much stiffer than allowed for in conventional analysis.

In profile, the ground at the site comprised the thick and ubiquitous blanket of made ground, present throughout Warsaw as an uncomfortable reminder of the war. Below this, the natural ground is made up of two layers of boulder clay, the result of two glaciations.

This clay, says Leszczynski, is "a very good material, very stiff, not susceptible to swelling and has a high content of gravel and boulders". The boulder clay horizons are underlain by a very dense interglacial gravel and sand and, below this, a stiff Tertiary Clay, similar to London Clay at about 40 m. Amp used MIDAS