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Isla Rhizolith

APTUM Architecture

Shared By

Symone Garvett

Project Name

Isla Rhizolith

Project Status


Year Completed




  • Julie Larsen
  • Roger Hubeli


  • Fabrication and Construction: CEMEX Research Group


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Project Description


The installation, Isla Rhizolith | Rhizolith Island, constructed in Cartagena, Colombia, is a prototype of a floating concrete breakwater. The installation and exhibition was part of the RC 2016 (Reunion del Concreto), an international Expo and Academic Conference on Concrete held in October 2016 in Cartagena. The island is a concept and prototyping project that investigates the potential for high performance, floating concrete structures to revitalize Colombian shorelines along vulnerable urban sites with ongoing flooding and mangrove depletion.

The Incubator
Continual urban growth, agriculture, and storm surges all contribute to devastating amounts of vegetation and habitat loss, as well as annual flooding along the shoreline of Colombia. Due to this devastating depletion of mangrove forests that naturally control sediment and shorelines from erosion, the installation and speculative urban proposal aims to achieve a new shoreline infrastructure through emphasis on newly developed concrete materials, formal expression, and performance. The Naval Academy and the City of Cartagena in Colombia are interested in ways to combat flooding and loss of mangroves with a soft infrastructural approach that is responsive to different site conditions and still aesthetically enriches the public quality of the shoreline.

What is a Rhizolith?
To derive a formal and performative logic, we began with the concept of ‘Rhizolith’, which derives from “fossilized root systems that were once encased in mineral matter” and formed through “a process of chemical weathering and cementation.”[i] Rhizolith Island is conceptually aimed to begin as an artificial Rhizolith that slowly returns to a natural form of Rhizolith over time. The design of the breakwater prototype, as an ‘artificial root system,’ is comprised of ‘root-like’ concrete modules that work like an artificial Rhizolith. The project uses new composites of concrete mixes, from high strength to ultra lightweight, to strengthen the island’s ecological performance as a new coastal infrastructure. The islands work as a ‘seed’ for the revitalization and protection of newly planted mangroves on the islands. The ultra high-performance concrete (UHPC) head of the module works as a breakwater and encases newly planted mangroves, while the concrete fins below water create new habitats for marine life. Over time, the concrete elements survive long enough to protect the mangroves as they grow but are designed to intentionally ‘fail’ and break apart once the mangroves reach maturity. To ensure longevity of the mangroves, the inevitable ‘failure’ of the concrete is designed to break apart naturally once mangrove roots become robust and naturally moor into the newly formed sediment below the modules. The island begins as a completely artificial, concrete structure but eventually is taken over by the mangroves; thus bringing the ‘Rhizolith’ back to a natural state.

Floating Concrete
The islands need to float as a breakwater to ensure they can accommodate challenging site conditions, such as locations where sedimentation washed away due to storm surges, lack of quality sedimentation for mangrove roots to attach, or to ensure mangrove seedlings stay above water till roots grow naturally down to the seabed. The modules aggregate into a field of floating islands; working together as a buoyant field that acts as a soft breakwater. The island attenuates waves through the aggregation of many small 2-meter wide concrete elements. The individual elements of the breakwater are constructed of two pieces; a head and a fin, made with two different proprietary concrete mixes; one for strength, the other for lightness. The fin of the element is made of UHPC (ultra high performance concrete) that is needed for it to be strong and stabilize the structure to moor into the seabed over time. It’s weight and strength stabilizes the elements in the water and provides a basis for the development of a marine habitat. The head of the element is made from a concrete mixture that is lighter than water and guarantees the island floats even if there are holes in the shell.

Shoreline Acupuncture
The Rhizolith Island prototype is a fragment of a larger breakwater that, once aggregated into many islands, can be deployed along the shoreline in an ‘acupuncture-style’ shoreline strategy. The strategy aims to achieve three goals: protect the shoreline from further sediment depletion, grow mangroves where needed, and become a public form of infrastructure for the local community and tourists. As the first phase, the mangroves are planted and protected in substrate located in the head of the concrete element; which serves to stabilize the mangrove seedlings. The concrete elements are designed with voids to allow the roots of the maturing mangrove to grow and spread beyond the elements. Simultaneously, the element fins serve as stabilizers and create pockets of space with voids puncturing the surfaces to provide space for new ecosystems, similar to leaf litter and decomposing debris, so that flora and fauna begin to inhabit the structure. In the second phase, the mangroves continue to grow on the floating elements, as well as on shore, as newly deposited sediment slowly accumulates; allowing for further growth of mangrove trees along the shoreline. In the final phase, the floating elements eventually moor into the seabed and further work as a type of Rhizolith breakwater system to reduce additional erosion of the sediment. The site strategy reflects the long-term goals of restoring the shoreline over time, using the concrete island as an incubator for the mangroves to grow and thrive. Eventually the roots of mangroves break through the concrete and take over to be a permanent, natural buffer that soaks up water and reduces flooding during storms.
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