Lateral restraint is key to Budapest's basement boom reports.
After the debris was cleared from heavy Second World War bombing, the semiderelict sites in the Hungarian capital Budapest were used as open-air car parks. But with the economic upsurge following the fall of the Iron Curtain there has been increasing pressure on bringing these sites back into more productive use.
With sensible foresight, planning regulations dictate that new developments for offices, shopping centres and even houses provide adequate parking. As a result, basement construction in Budapest is big business.
And because these sites are usually surrounded by structurally sensitive buildings, their development often calls for involved and intricate foundation work - which perhaps explains why a growing number of Europe's leading geotechnical contractors, such as SoletancheBachy, Keller, Bauer and Strabag have a Budapest base.
Historically Budapest was two towns, separated by the Danube.
Buda on the west side is mountainous, rising to around 500m above sea level, whereas Pest on the east is flat. The underlying geology is different too, with deep river deposits of sands and gravels up to 13m to16m deep in Pest, but much shallower in Buda.
But according to Lorand Sata of HBM, the Hungarian arm of Soletanche-Bachy founded in 1995, basement depths are determined as much by cost/benefit ratios as geological variations.
The deep foundation team at Hidepito, the Hungarian bridge construction firm which owns 47% of HBM, has been active in the city since the late 1980s.
Sata says that while a threestorey basement can be built using only one level of supports, basements with four or five storeys need two or sometimes three supporting levels.
The usual approach is to construct a perimeter diaphragm wall, and use rotary bored piles to carry internal structural loads. However when it comes to providing lateral support for the retaining walls, far greater flexibility and ingenuity is required.
Increasingly top-down construction provides the only real option - often with a bit of jet grouting for good measure, particularly when the excavations are close to buildings on shallow foundations.
Two recent HBM projects, both on the Pest side of the Danube, demonstrate the point (see boxes).
At the Elisabeth Center, the top concrete slab and a layer of ground anchors provided a single support level for the three-storey basement. But at the deeper basement for Hotel Astron, neighbours refused to allow the use of temporary ground anchors below their properties. Instead, the main contractor had to use heavy internal propping between the partially completed top slab and the front wall of the basement at street level to achieve the four level dig. Both contracts were worth around E1M.
As Sata puts it: 'It seems that the use of top-down is the result of several factors - partly technical, economical and environmental, but it has much to do with the developer's relationship with the neighbours.'