Building underground

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Introduction

Alois Diethelm

Fig. 2: From outside there is no hint of the existence of a subterranean space - the vault as a structurally ideal form

Swedish-style potato storehouse

Fig. 2: From outside there is no hint of the existence of a subterranean space - the vault as a structurally ideal form

Swedish-style potato storehouse

Fig. 1: A secret underground alternative world

Film set in "James Bond 007 - You Only Live Twice", 1967

Fig. 1: A secret underground alternative world

Film set in "James Bond 007 - You Only Live Twice", 1967

Subterranean structures are all around us yet we hardly notice them- a situation that, depending on the circumstances, we find fascinating, matter-of-course or even objectionable. Because it is invisible, complete or partial lack of knowledge about an underground structure leads to suppositions about the actual conditions. We speculate about the city beneath the city as a living organism with the most diverse infrastructures, or in the form of traces of bygone times (e.g. Rome, as the result of destruction and reconstruction), and hope that "secret" structures such as fortifications and bunkers lie behind unassuming doors and hatches. At the same time, modern underground building work in Europe - and in Switzerland specifically - is an expression of a spatial expansion that attempts to preserve our familiar urban landscape. So in existing structures, whose architectural value is to be found not least in the interaction between the building and its external spaces, new space requirements are fulfilled with "invisible", i.e., subterranean, interventions. The same fate awaits those structures that are regarded by the general public as a "necessary evil", concessions to a modern way of life, e.g. basement garages.

What I shall try to do here is to assign the characteristics of underground structures to various categories: on the one hand, in terms of their relationship with the topography, and, on the other, according to the applied principles of creating enclosed spaces. I shall deal with the specific conditions, options and restrictions that accompany building underground. I shall repeatedly pose the following questions: "How do we experience the subterranean world?" "Which concepts are intrinsic to this?" "Where are additional measures required?"

The substructure in the superstructure

Today, in our latitudes every building activity, even those "purely" above ground, starts with an excavation. What we mean by doing this is to found the building on a frost-resistant material capable of carrying the weight of the construction. The easiest way of achieving this - and one which is linked with the advantage of creating additional space - is to provide a basement or a cellar. We dig out the ground to form a large pit and, in a first step, enable the construction of subterranean space according to the principles of building above ground. Effecting the design is the following distinction, whether the building fills the excavation completely, which means that of the sides of the excavation must be appropriately secured (e.g. timbering), or whether the building - even after completion - is positioned as an autonomous edifice

Fig. 4: Underground building In superstructure style

Top (from left to right): the excavation that remains open - with the spoil used to form an enclosing wall - excavation backfilled

Bottom: possible relationships between substructure and superstructure

Fig. 4: Underground building In superstructure style

Top (from left to right): the excavation that remains open - with the spoil used to form an enclosing wall - excavation backfilled

Bottom: possible relationships between substructure and superstructure detached from the sides of the excavation, and so the subsoil exerts no pressure on the walls. The latter approach enables an identical form of construction to be used for both substructure and superstructure, and simultaneously simplifies natural ventilation and day-lighting issues. The substructure component in the superstructure still poses the question of the relationship between the parts above and below ground. And this concerns not only the vertical component, which manifests itself in the number of storeys above and/or below ground, but also the horizontal expansion. In other words, we have a structure that, depending on the "depth of penetration", exhibits more or fewer basement storeys, but also a basement extending over a larger area than the storeys above. What we see at ground level is therefore frequently only a fraction of the entire structure - as if it were a submarine at anchor with only the conning-tower protruding above

Fig. 5: Trees indicating the extent of the extension below ground

Tadao Ando: Vitra Seminar House, Weil (D), 1993

Fig. 5: Trees indicating the extent of the extension below ground

Tadao Ando: Vitra Seminar House, Weil (D), 1993

Fig. 6: Horizontal extensions

Relationships (from top to bottom): inside—outside - outside-inside - centred

Fig. 6: Horizontal extensions

Relationships (from top to bottom): inside—outside - outside-inside - centred

the water. We can therefore assume that the majority of flat roofs are not be found on buildings but rather over apparently firm soil in the form of roads, plazas and gardens and in this way remain "invisible".

The relationship with the "overworld"

Subterranean space quickly reminds us of damp grottoes with gloomy lighting conditions. But are such images still relevant today when we consider modern methods of construction and contemporary architectural briefs? Only a few forms of use that are met with underground really have to take place underground. The possible reasons for going below ground level were mentioned in the introduction; mostly, they reflect the external perception desired (streetscape/landscape). In such cases the interior gains nothing extra for being underground. On the contrary, the reduced options for admitting daylight are regarded as a disadvantage. As a result, the type of I ighting and the degree of contact with the outside world, or rather the world above ground, the "overworld", becomes a decisive criterion for contemporary subterranean structures.

Here, we see the contrast between overhead I i ght-ing through openings in the ceiling/f loor above and lateral I ighting through perforated walls. Interior spaces of any size may be positioned in front of these perforations - openings or walls completely "missing". The spectrum ranges from lightwells with minimum dimensions to larger external spaces that frequently are also accessible. The relationship between these external spaces and the "over-world" fluctuates between a mere visual link and a physically usable space continuum. Points of reference such as buildings, trees and people situated within the field of vision help us to grasp the subterranean external space for what it is, whereas the physically usable connection between "overworld" and "underworld" generates an interweaving of spaces - either with the aim of bringing the surroundings down below ground, or taking the subterranean use upwards into the streetscape or landscape. In contrast to lightwells, which - as their name suggests -merely serve to admit daylight, patio-type external spaces also bring the weather below ground and counteract the feeling of confinement often associated with underground buildings. We therefore question another aspect of our experience of underground spaces: the isolation - when an interior space is perceived as being unaffected by the weather, the seasons or other events. A good example is a military bunker, whose independence is further emphasised by having its own power supply. Recording studios and rehearsal facilities that have to be cut off from the outside world acoustically, or wine cellars in which a constant climate is vital, provide further examples. The consequences of excluding the outside world are mechanical ventilation and artificial light; the latter - like the provision of rooflights - can also be regarded as intrinsic to the nature of subterranean spaces. But this applies to enclosed spaces above ground too and, generally, to all introverted spaces, something that Pierre Zoelly demonstrates impressively with his modified sectional drawing of the Pantheon, where he continues the terrain up to the oculus. So do we need traces of incoming water on the walls in order to experience the space below ground as subterranean?

Fig. 8: A ramp links the underground entrance with the surrounding street level.

Renzo Piano, Richard Rogers: Centre Pompidou, Paris (F), 1977

Fig. 8: A ramp links the underground entrance with the surrounding street level.

Renzo Piano, Richard Rogers: Centre Pompidou, Paris (F), 1977

Topographical concepts

Detaching ourselves from aesthetic or, indeed, even ideological aspects, building underground - like any other form of building - has its origins in mankind's need for shelter and protection. Protection from the vagaries of the weather (sunshine, rain, wind, etc.) or other people or animals. Starting with the actual relevance of these dangers and taking into account the given topographical and geological conditions, the possibilities range from caves (natural, reworked or man-made) to depressions to soil-covered elevations.

Caves - the solid prehistoric huts Natural caves or crags were shelters for humans that did not require any special skills to r ender them habitable. The spatial experience of the solid construction was therefore a solution that was associated with the need for shelter and protection long before humans had learned to use tools to work stone. Closures made from animal skins and woven twigs and branches, frequently reusable furnishings among nomadic peoples, were additions whose technologies (e.g. woodworking) gradually evolved to become significant components of simple construction methods. If caves had to be hollowed out first, the builders chose geological situations that promised easy working, although these usually involved materials with a lower strength. Even today then in constructing galleries and in some cases caverns we still use methods in which timber or steel assemblies are inserted or slid forward in line with progress underground to support the remaining subsoil. In the simplest case this involves strengthening the surface to prevent collapse. However, in the case of loose or soft materials this can even become a temporary or permanent primary supporting structure which is

replaced by or encased in a loadbearing concrete I ining. Depending on the thickness of the material that separates the subterranean spaces from ground level, it is only a small step to open-cut or cut-and-cover working, in which a loadbearing structure is covered with soil only after being completed.

Basically, the cave represents that form of underground building for which the topography is only important in terms of access and, possibly, daylighting. It is frequently a by-product, e.g. in the extraction of natural resources, or is chosen because of climatic or acoustic conditions that are found only at a certain depth.

Depressions - a daylighting concept Depressions can have connections to other spaces or form their own space. These latter spaces are those topographical depressions suitable for use as, for example, sleeping-places in the open air shielded from the wind - the most primitive form of human shelter. Amphitheatres, like the one in Stratos, exploit the natural, pitlike topography in order to create terracing for spectators with a minimum of reworking, with the floor of the "pit" becoming the stage, the arena. Man-made depressions

Fig. 10: Courtyards and stairs (1st floor)

Typical village, Xi-an region, China

Fig. 11: Securing underground galleries with timber, enlarged upon provision of the lining

Tunnel cross-sections through Albula railway line

Fig. 11: Securing underground galleries with timber, enlarged upon provision of the lining

Tunnel cross-sections through Albula railway line time. It is therefore conceivable that rooms are initially excavated on just two sides, with the other two sides being used only when the need for more space or a growing family makes this necessary.

Viewed from above, Bernard Zehrfuss' extension to the UNESCO complex in Paris is nothing other than one of these aforementioned Chinese villages. Looked at more closely, however, we can see that the principles he has employed follow different functional, structural and urban planning concepts. Whereas in China the depressions mark the start of the building process, in the Zehrfuss concept they are merely undeveloped "leftovers". The UNESCO complex was built using conventional superstructure methods in a cut-and-cover procedure. If underground building is necessary for climatic reasons in some cases, in others it is the surrounding built environment that forces an "invisible" extension.

Fig. 10: Courtyards and stairs (1st floor)

Typical village, Xi-an region, China represent another concept for introducing light and air into adjoining subterranean interior spaces, in some cases also providing access to these. The settlements in the Xi-an region of China with their sunken courtyards are an ideal example of the multiple use of depressions: they form an entrance courtyard for the adjoining chambers, provide these with daylight and also serve as communal areas or living quarters. These generously sized, normally square depressions are, like galleries, the starting point for horizontal space development which, through further excavation, enables the creation of further rooms at any

Fig. 12: Top: UNESCO main building, originally with an open plaza; bottom: later underground extension with courtyards

Marcel Breuer, Bernard Zehrfuss, Pier Luigi Nervi: UNESCO, Paris (F), 1958

Fig. 12: Top: UNESCO main building, originally with an open plaza; bottom: later underground extension with courtyards

Marcel Breuer, Bernard Zehrfuss, Pier Luigi Nervi: UNESCO, Paris (F), 1958

Fig. 13: Spoil used to form an embankment, as noise barrier and to provide access to upper floor

Fritz Haller: Bellach School, Solothurn (CH), 1959

Fig. 13: Spoil used to form an embankment, as noise barrier and to provide access to upper floor

Fritz Haller: Bellach School, Solothurn (CH), 1959

Fig. 14: Timber frame covered with peat

Long house, Iceland

Fig. 14: Timber frame covered with peat

Long house, Iceland

The main building, which Zehrfuss designed in 1958 with Breuer and Nervi, takes on a particular position within the urban environment: to the north it embraces the Place de Fontenay in highly contextual fashion, whereas to the east and west - adhering to the principles of Modernism - it leaves large open areas, the buildings on which form a sporadic, small-scale, random composition. The underground extension managed to preserve the volumetric relationships; however, the character of the external spaces underwent a major transformation. It is therefore wrong to say that subterranean interventions always allow the urban constellation to remain intact.

Elevations - man-made topography In the examples up to now underground space was created by removing material: directly in the case of the cave, indirectly in the case of structures built in open excavations. Elevations, on the other hand, require the addition of material - in the ideal case spoil (excavated material) that is not removed from the site but instead retained for shaping the land.

Fritz Haller's Bellach School at Solothurn (1959-60) shows us the potential inherent in excavated material, not in the sense of underground building directly but rather in the form of a concept that can be applied to this. Alongside the school an embankment has been built which protects against noise and provides access to the upper floor of this building (which has no internal staircases).

A given topographical situation often invites the creation of subterranean spaces above ground level: an additional hill is added to an undulating landscape, or an existing elevation is raised. Military hospitals or reservoirs function in this way. In doing so, the reservoir, for instance, benefits from the elevated position (pressure), is less exposed to climate-related temperature fluctuations (owing to the enclosing earth embankment), and is less of a "disruption" in the surrounding rural or urban landscape. In both cases - military hospitals and reservoirs - a gently rolling meadow blurs the underlying geometry.

Besides the strategy of incoherence between inside and outside as a traditional form of camouflage, an alternating effect is desired in other cases: interior and exterior

Fig. 15: The building becomes the topography.

Bearth & Deplazes: Carmenna chair- lift, Arosa (CH) 2000

appearance have an impact on each other. This is very evident at the valley station of the Carmenna chair-lift in Arosa (Bearth & Deplazes). The gently undulating topography has been transformed into a folded roof form. On the entrance side the folds appear to mirror the outline of the mountain peaks in the distance. However, the longer the distance between the folds on the roof, the less distinctive is the separation between the man-made and the natural topography. The soil covering changes the folds into vaults, and on three sides the roof surfaces blend with the rising and falling terrain. On the mountain side the chair-lift itself and the opening through which it enters the interior of the "hill" are the only evidence of this artificial topography.

Fig. 17: The choice of materials and form allow the building to match the landscape.

Skogar open air museum, Icelanc

Fig. 15: The building becomes the topography.

Bearth & Deplazes: Carmenna chair- lift, Arosa (CH) 2000

Fig. 16: Silhouette of folded roof against outline of mountains in background

Bearth & Deplazes: Carmenna chair-lift, Arosa (CH), 2000

Fig. 17: The choice of materials and form allow the building to match the landscape.

Skogar open air museum, Icelanc

While in Arosa the fusion with the landscape was a key element in the designer's intentions, it is almost a byproduct in the grass-covered peat buildings of Iceland. Owing to the lack of suitable clay for the production of roof tiles, roofs have been covered with peat since Iceland's settlement in the 9th century. Grass grows on the peat roofs and the ensuing dense network of roots forms an interwoven, water-repellent layer, which is adequate waterproofing in areas with low rainfall (approx. 500 mm p.a.). However, the durability of the waterproofing function is directly dependent on the pitch of the roof. If it is too steep, the rainwater drains too quickly, which means the peat dries out and develops cracks during periods of little rainfall. On the other hand, if the pitch is too shallow, the water seeps through. The peat also regulates the moisture level and assumes various storage functions. A simple timber roof structure (cf. steel rrame to valley station in Arosa) serves as a supporting framework for the peat, which is prevented from sliding down the roof slope by the solid external walls. These "green" roofs among the gently undulating landscape look like knolls, whereas the moss-covered brown peat walls recall a geological fault. So the integration is not due to the fact that grass has been laid like a carpet over the structure, but rather through the adaptation of given conditions - the texture of the landscape as well as its rhythm. Examples can be seen in the villages in the valleys of Engadine or

Fig. 19: The natural rockface was reworked here to create what appears to be a man-made block.

Abu Simbel Temple, Egypt

Fig. 19: The natural rockface was reworked here to create what appears to be a man-made block.

Abu Simbel Temple, Egypt

Ticino, where the houses are built exclusively of stone. It is a local stone and forms, as monolithic rockface or loose boulders, the backdrop for the houses and retaining walls made from the very same stone; the transitions are fluid. The situation is very similar with Baiao House by Eduardo Souto de Moura, where the rubble stone facades on either side seem to become retaining walls for the neighbouring hillside, and the transition between roof and terrain is unnoticeable.

Fig. 18: Interlacing of building and topography with (retaining) walls

Eduardo Souto de Moura: Baiao House, Baiao (P), 1991-93

Fig. 18: Interlacing of building and topography with (retaining) walls

Eduardo Souto de Moura: Baiao House, Baiao (P), 1991-93

If what we have here is the naturalness of man-made constructions, then it is the reverse in constructions like the Abu Simbel Temple, where at the entrance stand four figures 20 m high which were carved out of the rock, i.e., the artificiality of the natural.

Concepts for creating spaces

In the foregoing the actual construction process for subterranean structures was mentioned only as an aside. In the following I shall look at the principles for creating space - from the properties of the single room right up to the three-dimensional development of internal layouts - that arise owing to the special conditions and possibilities that building below ground level open up for us.

Geological concepts

The geological relationships influence the formation of space on various levels. For instance, the dissimilar properties of adjacent rock strata can steer the space develop-

Fig. 20: Vaults in tuff stone, Naples

Trapezoidal (left) and elliptical (right) cross-sections as structurally ideal forms

Fig. 20: Vaults in tuff stone, Naples

Trapezoidal (left) and elliptical (right) cross-sections as structurally ideal forms ment in such a way that the chosen stratum is the one that can be worked more easily (e.g. soft sandstone instead of limestone). Consequently, the actual position of a space or a sequence of spaces can be defined by the economic aspects of the geology. In this case a change in the stratum may in the end form the boundary to our underground expansion; depending on the structure of the adjoining rock, however, the load-carrying capacity and the associated unsupported spans can also limit the dimensions of our underground rooms. In the simplest case we remove only that amount of the "soft" rock necessary to leave walls or pillars supporting the overlying, more or less horizontal rock strata exclusively in compression without any additional structural means. If the vertical distance between the hard strata is insufficient, we are forced to work the overlying rock into structurally beneficial shapes such as arch-shaped, trapezium-shaped or elliptical vaults or domes in order to create larger spans. Faced with the reverse situation (strata too far apart), the spatial development is subject only to the conditions of one type of rock. Of course, here again - within homogeneous geological conditions - larger spans are achieved by raising the roof.

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Fig. 21: Creating open space in solid rock

Left: soft rock - short spans with arches Centre: hard rock - short spans without arches Right: hard rock - large span with arch

So the architectural vocabulary can reflect the structural options, on the one hand, but can also, on the other, attest to the construction process. That might be drilled holes for jemmies, or rounded corners due to the circular movements of the human arm when removing material with a pickaxe.

The spread of "geological concepts" during the pre-industrial age was linked directly with rock properties such as ease of working and high strength. From that viewpoint, loess (a marlaceous sand) is ideal; indeed, it gave rise to a tradition of underground building in the Stone Age that is still found today, primarily in China (Henan valley). Other examples of this can be found in the Matmata region of Tunisia and in Gaudix (Granada province), Spain.

On the other hand, the creation of interior spaces within harder rock formations has only been possible with reasonable effort since the introduction of dynamite (1867) and mechanical mining methods. Admittedly, the Egyptians were constructing extensive rock tombs in the Valley of the Kings as long ago as about 1500 BC, and in the Middle Ages a number of churches were hewn completely out of rock in Ethiopia. This latter example extends

Fig. 23: A Lalibela Church, Ethiopia, a 1400

The rock has been worked on all sides to create monolithic walls and columns

Fig. 23: A Lalibela Church, Ethiopia, a 1400

The rock has been worked on all sides to create monolithic walls and columns

Fig. 22: Classification of Ethiopian rock-hewn churches

From left to right: built-up cave church, rock-hewn cave church, rock-hewn monolithic church

Fig. 22: Classification of Ethiopian rock-hewn churches

From left to right: built-up cave church, rock-hewn cave church, rock-hewn monolithic church from hollowing out the interior to exposing the church on all sides, where the removal of material leaves monolithic walls standing which in turn support the overlying rock forming the roof. Protected by the enclosing rock formations, these churches are difficult to find, but nevertheless exhibit the sort of facades we would expect to see on free-standing churches.

Today, the working of coherent masses of rock is mainly carried out to extract the rock itself, to provide access to deposits of natural resources (e.g. coal, salt, etc.), or to remove obstacles (e.g. tunnel-building or conventional mining). Contemporary examples in which the specific properties of the rock are used directly are much rarer. One of these properties is the high storage capacity of rock; in combination with the underground location and hence the independence from the influences of daily and seasonal climatic variations this property offers temperature conditions that can be created and maintained with a minimum of technology.

This fact is exploited, for example, in the Great Midwest Underground (Kansas City, Missouri) - a subterranean cold store, warehouse and production facilities, with a r loor area totalling nearly 300 000 m2. This example is mainly interesting because, in addition to the storage characteristics of the rock, its good load-carrying capacity was also exploited to the full. As with the aforementioned rock churches, the hollowing-out process produces a monolithic structure (a regular grid of pillars) that need no further strengthening.

Constructional concepts

One decisive factor - and herein lies a considerable difference to building above ground - is the earth pressure that acts on a substructure permanently and from several sides. In this context we can distinguish between two types of construction: autonomous systems, which can simply withstand the pressure, and complementary systems, which function only in the presence of external forces. This latter effect can be seen at the tombs in Monte Alban in south-eastern Mexico, where the slabs of rock forming the roof are not sufficiently stable without the load and the resistance of the overlying soil.

Fig. 25: The courtyard is a central element and can have almost any number of chambers on all sides.

Left: Luoyang, Henan valley (China); right: Matmata (Tunisia;

Fig. 25: The courtyard is a central element and can have almost any number of chambers on all sides.

Left: Luoyang, Henan valley (China); right: Matmata (Tunisia;

We can divide autonomous systems further into those where the loadbearing elements have an active cross-section or active form. If the size of a component is such that it - obeying the laws of gravity - is itself stable and the horizontal forces present can be carried within its cross-section, we speak of an active cross-section. On the other hand, we can build a more slender structure when the shape of the loaded component corresponds to the flow of the internal forces (element with active form). From this point of view, vaults (cf. tunnels) are ideal structures, the principle of which can be turned through 90° to form an "arched" r etaining wall. Like the wall to the tank compound at the aluminium works in Chippis, the plasticity of a series of curved shells allows us to deduce the forces that are at work. However, a shallow curvature

Fig. 26: Solid rock hollowed out to create a cold store and warehouse

Great Midwest Underground, started in and continually expanded since 1940, Kansas City (Missouri, USA)

Fig. 26: Solid rock hollowed out to create a cold store and warehouse

Great Midwest Underground, started in and continually expanded since 1940, Kansas City (Missouri, USA)

Introduction guarantees only their buckling resistance, not their stability. That would require additional ribs, an increase in the "rise" or a whole ring of shells. Structures with an active form are generally more labour-intensive, but require less material and render visible the forces within the structure, while structures with an active cross-section consume

Fig. 27: Large-format stone slabs leaning against each other and wedged into the soil

Tombs (plan and section) in Monte Alban, Mexico

Fig. 27: Large-format stone slabs leaning against each other and wedged into the soil

Tombs (plan and section) in Monte Alban, Mexico

Informal concepts

Actually, building underground allows us to create "uncontrolled", additive, rambling i nterior layouts because there is no visible external face. By this we mean the provision of rooms and spaces without the effects of the customary external "forces". There is no urban planning context, which as a parameter influencing the form predefines a certain building shape to fit a certain plot, nor

Fig. 28: Retaining wall with "arch" form to resist earth pressure

Tank compound, aluminium works, Chippis (CH)

Fig. 28: Retaining wall with "arch" form to resist earth pressure

Tank compound, aluminium works, Chippis (CH)

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