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Fig. 29: Retaining wall with "arch" form

Schematic plans (from top to bottom)

- simple "arches"

- a greater "rise" also improves the load-carrying capacity more materials and "deny" the flow of the forces, but are usually easier - and hence cheaper - to construct.

Structures with an active cross-section also help to stabilise excavations, an aspect that is always relevant below a certain depth. If the area of the excavation is only small, it can be secured with a (welded) ring of walings. If the corner-to-corner distance is too great, the walings themselves must be braced. This can be done with ground anchors provided there are no adjacent buildings or underground services in the way. The walings can be omitted by increasing the number of anchors. But the reverse is also true: the anchors can be omitted if the building under construction is called upon to help stabilise the excavation. Christian Kerez's competition entry for the extension to the Freudenberg Canton School in Zurich-Enge demonstrates a very obvious concept - and one which applies generally to building underground. Initially, the plan layout seems to be rather random, but upon closer inspection we realise that this is the maximum usable area between existing structures and trees. The outline includes cranks and curved segments which appear to be elaborate and expensive. But the proposed wall of contiguous bored piles means that the geometry of the building is irrelevant because the connections between the piles always remain the same regardless of any change of direction. In other words, whether the wall is straight or curved is irrelevant to its construction.

Furthermore, walls of contiguous bored piles can carry vertical loads (in contrast to sheet piling), which means they can secure the sides of the excavation and also act as external walls in the finished structure. Kerez exploits this property and uses the main floor slab, carried by the piles, to brace the piles and thus eliminate the need for any ground anchors.

Fig. 30: A seemingly random form, but it reflects the trees and adjoining buildings above ground

Christian Kerez: Freudenberg Canton School project, Zurich (CH), 2002

Fig. 30: A seemingly random form, but it reflects the trees and adjoining buildings above ground

Christian Kerez: Freudenberg Canton School project, Zurich (CH), 2002

are aesthetic factors relevant, which have an influence on the three-dimensional manifestation of every project that develops from inside to outside. For there is no external form that has to be "attractive". Despite this great design freedom, the majority of contemporary subterranean structures are simply "boxes", and only forced to deviate from this by infrastructure (services), plot boundaries and geological conditions because economic parameters generally call for simple shapes. Projections and re-entrant corners only enlarge the building envelope and involve elaborate details. Merely in cross-section, where storey-high set-backs render a terraced excavation possible, the sides of which need not be secured against slippage (e.g. timbering, ground anchors), are such forms economic.

The term "informal concept" is an expression covering all those structures whose properties are due neither to geological nor technical/constructional parameters, but rather reflect the fact that we cannot see them. Compact boxes, rambling interiors (internal forces) and partly "distorted" containers (external forces) fall into this category. Frequently, the lack of rules is the sole rule - at least the absence of such rules that can be derived from building below ground.

Fig. 31: Wall of contiguous bored piles

Every second pile Is Installed first and the Intermediate spaces filled with concrete afterwards; the soil provides the formwork.

Fig. 31: Wall of contiguous bored piles

Every second pile Is Installed first and the Intermediate spaces filled with concrete afterwards; the soil provides the formwork.

The rambling interior layout unites a wide range of the most adverse conditions. Sometimes it is the result of optimum space and/or operational requirements; sometimes it is an unavoidable consequence of a regular need for additional space which has to be met by underground means owing to restrictions above ground, or in other cases when a scarcity of space becomes evident even at the planning stage but the provision of another basement storey is seen as disproportionate to the requirements. The additional underground rooms are added where they are required or wherever seems most suitable, for whatever reason. So the rambling interior layout would seem to represent an "anything goes" pragmatism but also a precisely controlled arrangement. Informal, i.e., not governed by rules, also means that responses to external forces, like the underground services or changing geological conditions mentioned above, depend on each individual situation.

Fig. 32: "Vaulted" walls - as a loadbearlng structure with an active form - to resist earth pressure

J0rn Utzon: museum project, Silkeborg (DK), 1963

### Conclusion

J0rn Utzon's Silkeborg Museum project (1963) is a good example of how to unite a number of the themes dealt with above. These result in a more or less expansive interior layout with a series or interlacing of "room containers". The onion-shaped shells brace each other; as structures with an active form, their dimensions and the degree of curvature - on plan and in section - reflect the flow of the forces at work. The changes in the cross-sections can be seen clearly at the openings. Together with the overhead l ighting and the physical experience of immersion (the route through the museum), both of which - as already explained - are not necessarily linked exclusively with building underground, the Silkeborg Museum, had it been built, would have embodied the "underworld" in conceptional and spatial terms unmistakably and without any romantic transfiguration.

Fig. 32: "Vaulted" walls - as a loadbearlng structure with an active form - to resist earth pressure

J0rn Utzon: museum project, Silkeborg (DK), 1963

- Pierre Zoelly: Terratektur, Basel, 1989.

- Henri Stierlin/Pierre Zoelly: "Unterirdisches Bauen", in: Werk, 10, 1975

- Gerhard Auer (ed.): "Sous Terrain", in: Daidalos, 48, 1993.

- Georg Gerster: Kirchen im Fels, Zurich, 1972.

- Vincenzo Albertini, Antonio Baldi, Clemente Esposito: Naples, the Rediscovered City, Naples, 2000.

- Bernard Rudofsky: Architecture without architects, New York, 1964.

- Werner Blaser: Courtyard House In China, Basel, Boston, Berlin, 1979, pp. 111-20.