At the inaugural Chicago Architecture Biennial in 2015, visitors confronted a curious installation consisting primarily of the world’s oldest building material: rock. The 12-foot-tall monolith, with a heavy mass supported on three columns, appeared at first glance to be made entirely of gray pebbles. Closer inspection revealed the presence of braided string interspersed between the stones. The “Rock Print” structure, developed by ETH Zurich’s Gramazio Kohler Research and MIT’s Self-Assembly Lab, revealed the possibilities of digitally fabricated stone construction without the use of cement.
To build the installation, the researchers employed a robotic arm to lay coils of filament between layers of aggregate. The binding interaction of the string and randomly placed rocks resulted in “poly-dispersed jammed structures,” a physical phenomenon in which granular substances behave less like fluids and more akin to unified solids. In the absence of a chemical binder, the installation appeared to defy gravity. After the event's conclusion, the team dismantled the structure by removing the string—returning the materials to their original, reusable state.
The rock-jamming approach continues to be a productive research trajectory. In 2018, the ETH Zurich's Gramazio Kohler Research expanded the scope of its investigations by building an outdoor pavilion in Winterthur, Switzerland. In this case, 30 metric tons of gravel and 75 miles of string were employed to construct 11 different 12-foot-tall columns that supported a flat steel roof. Unlike the Chicago installation, which was cordoned off from the public, the Winterthur construction was deemed sufficiently safe to allow visitors to walk inside it.
Swiss Federal Laboratories for Materials Science and Technology (EMPA) scientists Martin Arraigada and Saeed Abbasion have recently explored the jamming idea further in roadbed applications. Traditional hot mix asphalt (HMA), used to surface most roads, contributes about 145 pounds of CO2 emissions for every six-tenths of a mile of a vehicle lane. The bitumen in asphalt is also harvested from crude oil, and the extraction contributes other forms of air pollution. EMPA researchers employed robotic arms to lay string in various patterns, with five successive layers of roadbed gravel in between. They then conducted mechanical load tests with a rotating pressure plate, comparing the performance of the string-reinforced sample to an unreinforced one. Arraigada and Abbasion determined that the jammed roadbed could resist a half metric ton of pressure without much gravel movement, whereas the pressure plate sank into the less cohesive, unreinforced sample.
Jammed structures are part of a burgeoning interest in “aleatory architectures.” According to researchers Sean Keller of the Illinois Institute of Technology and Heinrich Jaeger of the University of Chicago, these are constructions composed of randomly configured elements that can collectively adapt to changing circumstances. They consist of simple individual elements (gravel, for instance) that exhibit complex characteristics in aggregate. Although knowledge of the behavior of these structures has been limited thus far, new research reveals novel opportunities for material capacities. “Findings from a surge of recent studies using more complex, non-spherical particle shapes have made it possible to start designing granular aggregates with target properties previously out of reach—such as aggregates that are not only stronger or tougher, but that combine high porosity with high strength, or that are self-strengthening under load,” wrote Keller and Jaeger in the journal Granular Matter.
Like rock printing, aleatory architectures include various piles, scaffolds, and stratifications that consist of self-similar elements that create robust structures when combined. Additional examples include Kentaro Tsubaki’s “Tumbling Units” project, a construction composed of interlocking ceramic modules formed as hybrid tetrahedrons; Yusuke Obuchi’s pavilions composed of jammed aggregations of disposable chopsticks; and the “Aggregate Architecture” installations by Karola Dierichs and Achim Menges that feature self-supporting assemblies of x-shaped units. Keller and Jaeger argue that this work “demonstrates that aleatory methods, with suitably chosen, self-confining particles, can produce essentially all of the basic architectural protostructures, including walls, arches, and domes.”
Jammed granular constructions represent an uncanny combination of primitive and advanced material knowledge. On the one hand, they are based on the earliest forms of human structures, made of layers of found materials; on the other hand, they represent a frontier of scientific inquiry, highlighting the fascinating interplay between statics and fluid dynamics. Simply put, aleatory architectures offer significant potential advantages for future construction. They are infinitely reusable and reconfigurable by design, as long as the individual components remain viable. These structures eliminate the need for cement, mortar, glue, and other adhesive binders—materials that contribute an outsize percentage of the built environment’s carbon footprint. Jammed constructions also exhibit an inherent porosity that may be ecologically beneficial: for example, for use in roadbeds that are pervious to stormwater. Although the widespread adoption of such an experimental approach may be far-fetched, the environmental benefits alone mean we should ramp up our architectural jamming research.
