In the deserts of South-Central Australia is a most remarkable town called Coober Pedy. Famous as the greatest center for gem opal production, opal mines riddle its desert moonscape like the tunnels of some enormous ant colony. In recent years, however, Coober Pedy has become just as notable for the very unique lifestyle of its residents. Unlike the typical mining communities of the US where a single giant corporation owns everything and the residents live almost like serfs, the mines of Coober Pedy, as well as the several other opal mining towns of the region, are mostly family owned and operated claims, each one a veritable homestead akin to a family farm. This has led to a very unique mode of residential architecture. The sandstone of the region offering a stable and dry structure by itself, many residents of Coober Pedy have turned the tunnels of their mines into their permanent homes, sculpting them into suitable forms and retrofitting them with all the modern conveniences. Locally referred to as 'dugouts' some have even featured such novelties as indoor swimming pools! These homes were first employed for the practical reason of a lack of building materials when miners first moved to the remote and desolate desert region, but the virtues of such homes are many and their use has persisted to the present and expanded to many kinds of community buildings including shops and a church. Most recently, this unique form of architecture has itself turned into both a tourist attraction and a product, many hotels based on this construction and some local developers now making these excavated homes for anyone who wants to live in the town and share in its eclectic lifestyle -even if they have no interest in mining.

a roadheader excavator

The Australian approach to this kind of construction is straightforward and relies on a particular machine known as a roadheader or boom miner. The roadheader was invented in the United Kingdom about 30-40 years ago as a multipurpose mining and construction tool. It consists of a crawler base similar to a mini-excavator which hosts an articulated boom with one or more rotating drums on the end. These drums are mounted in line or perpendicular to the boom and feature an array of replaceable teeth which dig and fracture earth or rock as the drums spin. Other special function heads include jack-hammer like spikes, compression fracture micro-wheel heads like those of Tunnel Boring Machines but in miniature, a slicer head like a gigantic chain saw for dicing up rock, and the simple jaw-like buckets of traditional excavators. A debris collector in front of the crawler sweeps up the excavated material onto a conveyor belt which deposits it in a pile or bin behind the machine. Some models include special laser guidance systems affording auto-leveling, precise grading, and digging with millimeter precision. A typical roadheader under normal conditions can excavate tunnels its own width as fast as 20 feet an hour and needs only one or two people to operate it. These machines have a multitude of uses. They can be used in mining, of course, but also feature in general and municipal construction, in the construction of wine storage caves, in demolition work, landscape sculpting and grading, and trench carving for pipe and telecom work. The roadheader has generally replaced explosives for all small scale excavation work and I suspect that, in the future, machines of this type will prove to be the most important pieces of equipment ever sent to the moon and other planets.

opal tunneling in Coober Pedy

unfinished wall texture

Constructing a home using a roadheader is a relatively simple affair. One simply prepares the face of the rock by carving it for the desired exterior form then the roadheader is driven into the rock face to excavate individual clear-span room chambers separated by thick walls. There is enough precision with this equipment that rooms with variable floor levels, steps, and carved-in counters, benches, shelve alcoves, and windows, doorways, and corridors can be cut as the rooms are excavated. But the articulation of the machines is limited by their bulk and for some fine features one still must resort to hand operated jackhammers, drills, and plain-old pick and shovel. The surface finish left by the roadheader is rough and grooved and this can be smoothed with other tools, covered in plaster or shotcrete, or left as is. Windows and doors are set by cutting shallow grooves for their frames and mounting the glass and doors roughly flush to the stone. Floors can be left as bare stone, covered in tile or flagstone, covered in concrete, or framed for wooden flooring. The choice is usually determined by the way utilities are installed, which can be retrofit along walls and through-wall conduits or placed in shallow trenches in the floor. Installation of septic systems can be complicated by the surrounding rock but in Coober Pedy they use the unique approach of septic boreholes which are drilled below the level of the water table so that blackwater can be simply dumped down them and dispersed.

homes of Coober Pedy

Using a roadheader, a 4000sf home in Coober Pedy can be excavated in a single day by two workers and finished in a week, its final cost in the area of $50k -roughly $13 per square foot. Even accounting for differences in monetary exchange, this makes such architecture one of the cheapest forms of construction possible -at least in Australia.

the 'lost' city of Petra

Excavated homes are nothing new. They have been around as long as civilization and every culture and country has its examples, though the most well known are in Spain, Turkey, Greece, China, and Australia. The use of this form of architecture tends to be predicated on the presence of relatively soft rock or hard earth that provides sufficient structural strength while being soft enough to be excavated by simple tools. But today more powerful mechanical mining technology and explosives have greatly expanded the potential of this approach to construction, the homes of Coober Pedy representing what may be the state of the art in the technology for this. Underground homes can, of course, be built anywhere but most rely on the use of heavy concrete shells with elaborate waterproofing which are expensive to build, something the much simpler excavated construction eliminates but with the dependence on specific type of rock. Until recently I was unsure if the geology necessary for this kind of construction existed anywhere in the US. But then I learned about Forestiere's Underground Gardens in Fresno California.

an excavated resort complex in Turkey

Early in the 20th century an Italian immigrant to California named Baldasare Forestiere was welcomed to this country in the typical American fashion; by being cheated out of his life savings by a con man who sold him a completely worthless parcel of desert 'farmland' with soil too hard, dry, and sun-beaten for anything to grow. Trained in horticulture and hoping to earn a living as an orchard farmer, Forestiere instead found himself working as a laborer on other people's farms in the region until one particularly hot California summer led him to a remarkable discovery. Living in a tiny shack on his property, Forestiere followed the example of his neighbors and sought refuge from the oppressive summer heat by digging a deep basement for his home. But at a certain depth he found that the useless hard earth of the property gave way to a rich soft earth which would, had it not been buried so deep, be perfect for an orchard. Hitting on a novel solution to this dilemma, Forestiere experimented with growing a tree at the bottom of an open atrium shaft. The tree grew just fine and Forestiere began creating the orchard he had dreamed of by digging a grid of atrium shafts in a network of tunnels. The trees would grow right out of the top of the shafts where he could harvest the resulting fruit with ease at waist level. Finding the environment of his underground orchard very comfortable, he began to expand the complex into an underground home and after some 20 years of solitary digging and construction had created a veritable fairy-tale palace underground, complete with ponds and gardens for plants of all kinds. Several generations of his family have since lived in this clever and unusual home. Today the Forestiere Underground Gardens are listed in the National Register of Historic Places and are open to the public, one of the more remarkable yet lesser known tourist attractions in California. Look Here for images and a video clip of this remarkable place.

These underground gardens proved that, at least in one place in the US, Coober Pedy style architecture was indeed possible. But what about in the places EIs usually need to go, like the desert Southwest? Digging deeper -figuratively speaking- I learned that there were a number of other excavated homes throughout the Southwest. For instance, retired prospectors of the legendary Superstition Mountains of Arizona would sometimes 'build' their retirement homes in the side of cliffs using some skillfully placed dynamite. Long before them, Native Americans dug homes from the soft pumice outcroppings found in some parts of Arizona and New Mexico. And most recently, geologist Bruce Black excavated a unique home with the help of a precision blasting expert some 100' up the face of a mesa cliff in Farmington New Mexico. This home cost about $25k to build, though required a fair amount of additional sweat-equity because of its challenging location. Now known as Kokopelli's Cave, it is open to the public as a bed & breakfast and has been featured in many articles and TV programs. Black and grey water disposal tends to be a bit complicated with this approach to construction because the solid rock does not lend itself to percolation. Blackwater disposal is actually the easier issue to solve, since readily available composting and incinerating toilets eliminate the need for septic systems. Greywater is more complicated because its volume is much higher, too high for composting or incinerating. However, spray dispersal will work in most areas and and the emerging technology of living machine processing offers another good solution.

Consulting with Bruce Black, I learned that the geology suitable for this construction -calcite bonded sandstone, pumice/tuff, and some types of soft basalt- were quite common throughout all the regions of Arizona and New Mexico where EIs have tended to congregate for sake of clean air. Hawaii also offers potential, though most basalt in the islands tends to be of a porous type. While one needs to look out for the common hazard of radon as well as make sure the structures excavated are well above the water table, it is clearly possible in a great many locations, especially locations which have little value for any other use because the bare rock makes farming, ranching, or conventional construction impractical. It seems to me that this may be one of, if not the single, lowest cost and most widely tolerable type of non-toxic construction option possible. The basic material is completely natural and non-toxic and free of metals. It needs little finishing and will readily accept naturally non-toxic finishing materials like plaster if desired for appearance. It virtually eliminates any need for heating and cooling systems, and their possible pollutants, except in the more extreme climates. Initial homes can be readily expanded incrementally and with an investment in sweat equity, albeit requiring a very high labor overhead without the use of machinery. It's fire-proof, and earthquake resistant. It's durability is effectively indefinite.

In studying this form of construction I've devised three basic home design strategies that seem the most practical. The first I call the Cliff-Face Linear Series. This is the approach used by Kokopelli's Cave and consists of a line of room chambers dug into the face of a cliff with all window openings along that face and connected by tunnel-like doorways or corridors. Rooms may take any shape and may include open terraces but all are arrayed in a line which roughly conforms to the contour of the cliff-face. Access can be by stairway or rampway shaft, by step terraces, catwalks, or even ladders like the cliff-dwellings of Native Americans. This type of structure is the least cost-effective of the three because the use of roadheader equipment is not possible when the room chambers are fashioned far above grade. However, roadheader use is possible where there is road access to the cliff top. This allows a rampway to be excavated to the depth of the rooms, which are then excavated from the back-side out.

The second type of structure is called the Step-Face Linear Series and represents the type of structure commonly built for new homes in Coober Pedy. This approach is the same as the previous approach but room chambers are excavated at or slightly above grade from an already nearly vertical rock outcropping or from a step-face cut into a sloping rock outcropping. Again, rooms may be of different sizes and shapes but all have windows to the front and are arrayed in a line roughly following the contour of the rock outcropping they are excavated from. This approach is the easiest for roadheader use and is easy for occupants to access, since they need not traverse stairs to reach their home. Room chambers can readily function as garages and need not be connected by tunnel to other rooms.

a step face cut entry and parking lot for a Coober Pedy store

The third type is the Sunken Atrium Network. This is the type of structure represented by Forestiere's Underground Gardens. Based on flat terrain where a suitable rock substrate is at or very close to the surface, this approach relies on atriums which are sunk into the surface to a necessary depth then surrounded by room chambers. Access is by a staircase in an atrium or by separate stairways or rampways, the latter most likely with the use of the roadheader and readily serving as a driveway. The atriums and their surrounding room chambers can be of any size or shape and, depending on the climate, rooms may or may not be interconnected, relying on the atrium for their access. This latter approach is most practical where the atrium is small enough to be enclosed by some kind of transparent/translucent enclosure. The atriums serve for all outdoor views and are therefore likely for use as gardens and pools. Their initial size tends to impose the key limit on expansion thus structures will tend to evolve in the manner of a network of atriums linked by shared rooms open on two sides. Consequently, likely designs that anticipate this pattern of growth will be based on symmetrical geometries of standardized room and atrium units. The chief limitation of this kind of structure is that it affords occupants no conventional exterior views. But then, the terrain this type of structure requires would probably not offer particularly appealing views anyway and one is free to cultivate any kind of outdoor environment within the confines of the atriums one can imagine. So the potential here is for a truly palatial and private habitat of unlimited size that far more than makes up for the lack of appeal of the surface landscape -as was well demonstrated by Forestiere.

Various hybrid versions of these three types of structure are possible. For instance, a Step-Face Linear Series can be combined with a partial atrium to create a more sheltered space. Likewise, atriums may be sunk behind a line of rooms in a Step-Face structure to make a private open space. In the case of a mesa-like outcropping the sunken atrium can be used to create a private open courtyard along the perimeter of the mesa edge containing room chambers with windows on two sides. A natural depression in an otherwise flat terrain can be exploited as a ready-made atrium for Step-Face excavation. Broad rock outcroppings with a naturally sloped edge can be used to create a Sunken Atrium Network entered by an at-grade access portal or partially open atrium.

Using these basic structural concepts, I've devised a couple of likely designs for modest homes, as illustrated below.

In this first design I employ a Sunken Atrium Radial Network with a hexagonal geometry. Two atriums are used, one as the main part of the home, the other as an entryway with two spaces used for garage and utility. This design assumed the climate requires isolation for the interior from the atrium in poor weather and so all the room chambers of the main home are linked. This could be eliminated by an atrium enclosure but it seemed to me that a thirty foot wide dome skylight would tend to be more expensive to construct than the continuous glazing surrounding the atrium. In a mild and dust-free environment the atrium could be left open and the rooms linked by it alone, the glazed opening replaced by shogi style screens offering a savings over the glass. Also, its possible that a tent skylight could be employed based on one of the lower-toxic architectural fabrics such as elastomeric coated canvas or fiberglass and teflon Sheerfill. But this would eliminate the view of the sky and limit the height of plant growth.

In this second design the Step-Face Linear Series is used. This is clearly a very simple structure to excavate, though much material may have to be removed to make a suitable step face and finding an ideal location much more difficult. These illustrations show the structure arrayed in a straight line but in practice these room chambers could be arrayed at some variation in angle or distance in separation in order to accommodate the shape of the rock face they are excavated from. Convex and concave curved faces would definitely create a different aspect for this kind of home. Note also the long plant-filled pool in front of the chmaber windows. In practice this may serve as a graywater recycling system.

While this is a very promising housing concept, it does suffer from two very difficult complications. While the geology necessary for this construction is apparently very common throughout the Southwest and possibly parts of Hawaii, finding specific parcels of property is shaping up to be an insurmountable problem. Other geologists have told me that most land with this geology is Bureau of Land Management and Native Reservation property. While this land does frequently become available, it is remote and requires careful examination to determine if it is suitable. Just the radon test alone will take two weeks to do. So it really needs to be done by people with local knowledge, not someone a thousand miles away.

The other problem seems just as intractable. This form of construction is only affordable if done by machine. Hand labor would be impossibly great and the use of blasting too dangerous for routine use and requies extremely great skill to perform with precision. Roadheader excavator machines are common throughout the world, with the one exception of the US. We're still decades behind most of the industrialized world in excavation and construction technology in the US and I've found very few people here who knew what these machines were or had ever seen or worked with them. There is one German manufacturer of roadheaders -Voest-Alpine- which quite recently began production of these machines on the US East Coast -possibly in response to the Boston Big Dig project, World Trade Center clean-up work, and the large amount of underground construction that has been going on in the Washington DC area since the inception of the Bush administration. But availability of contract excavation service with these machines remains limited to only a couple of California companies working primarily on wine cave construction in the northern part of that state. My attempts to contact these firms have elicited no response. Roadheaders are an extremely adaptable technology and they come in a vast assortment of sizes and variations. In Coober Pedy they commonly use a type of roadheader that is much smaller than the typical units and it is possible that this machine could be suitable for DIY use and affordable to a small group, much as mini-excavators and mini-loaders are. This offers the possibility of pooling the resources of a group to construct a whole community of these homes in a suitable spot. But I have been unable to identify the make or model of this smaller roadheader and while roadheader-type cutting head attachments are available for existing mini-excavators -only in Europe, of course...- these machines do not have the kind of articulation tunneling work requires. I have been looking into other rock excavation technologies, such as hydrojet cutting, but so far the roadheader remains the most practical tool. So for the moment the availability of the necessary excavation services this kind of housing requires remains an open question.

Clearly, this is a very promising approach to EI housing. It could be the cheapest and most widely tolerated solution of them all! But it remains to be seen if this is really possible here. This looks like another example of the US being too unsophisticated to do something that other countries can do cheaply and easily.