Our Summer Rammed Earth Experiments 1/3: The Golden Spiral for Forbes Garden


Dirt—as in clay, gravel, sand, silt, soil, loam, mud—is everywhere. The ground we walk on and grow crops in also happens to be one of the most widely used construction material worldwide. Earth does not generate CO2 emissions in its generation, transport, assembly or recycling and this in contrast to more conventional building materials such as concrete and steel. In rammed earth construction a mixture of  clay, silt, sand and gravel is compressed into a formwork to create a solid low-cost load-bearing wall. Despite the renewed architectural interest in contemporary rammed earth construction in (semi-)arid climates of the USA, little is known about its potential in the erosive humid continental climate of New Jersey. Because of the great potential of rammed earth as a local building material, we decided to design and construct a spiral rammed earth structure in Forbes Garden that will be an enduring representation of Princeton’s effort to create a campus containing sustainable and elegant zero carbon architecture.

The Material:  Dirt

The Form Finding Lab’s team established the suitability of Princeton soil for earth construction though an extensive set of laboratory tests. The team, led by PhD candidate Tim Michiels and supported by undergraduate student Amber Lin ’19 and summer intern Jacob Essig, subjected a series of compacted samples with different water contents to compression tests (the rammed earth samples had an average compressive strength of 1.35 MPa). The team also experimented with lime additives  (3%, 5%, 10%, and 25%)  to test the compacted dirt’s resistance to weathering on a series of prototype walls (See image above title).  All these results informed the design of the structure that was designed for Forbes Garden as part of the Campus as Lab Initiative .

Testing of compacted samples with different dirt compositions to establish unconfined compressive strength. The local soil was composed of 19% gravel, 42% sand, 24% silt and 15% clay.

The Site: Princeton Garden Project

The Princeton Garden Project at Forbes College is a student led initiative that supports and advances sustainability and food awareness  on Campus. Following with its mission of sustainability, the rammed earth spiral is a sustainable experiment made with local and abundantly available materials intended to enhance the existing organic garden and transform it into a space for research and learning.

The Garden Project, the ideal collaborator for a rammed earth project (image credit Garden Project at Forbes College)

The Design:  A Site-Specific Golden Spiral

Spirals occur all throughout nature. For example, we see them in the trajectories of sunflower seeds and pine cone kernels in Forbes garden or in the pictures of Karl Blossfelt (1865-1932).  We adopted this familiar shape and designed the Forbes Garden spiral as a golden spiral, a type of logarithmic spiral whose growth factor is the golden ratio. We positioned the spiral with respect to the sun in high summer so that the structure would cast shade. To further enhance the visual aesthetic experience of visitors, we worked to ensure that there was a straight line of sight from the lower part of the swirling wall, which serves as a bench, towards the Gothic Cleveland tower and carillon which dominates the Graduate College landscape. To invite visitors to spend time in the garden and experience the raw and minimal character of the  structure, we designed the lower part of the spiral as seating. The size of the swirling curve was fixed by anticipating the range of comfortable distances from a fire pit, which will be placed in middle of the semicircular slower section. As the bench slopes gently upward from a minimum of 40 cm, it allows comfortable seating tailored to garden-enthusiasts of different heights. The bench is furthermore separated from a wall that reaches a height of 3 m to allow for different seating heights on the lower end, as well as to ensure sufficient clearance for a sense of openness of space at the higher end, where a shading roof is planned. Additionally, the wall’s varying height needed to be adapted to match the slope of the uneven terrain of the garden. These constraints, together with structural stability considerations, informed the development of the 3D shape of the Spiral. Stay tuned for our next post  to find out how we figured out how to build such a complex geometry in dirt!

Spirals in the plants photographed by Karl Blossfelt
Site plan showing sight lines, seating and the sun’s position at high summer
3D rendering of the golden spiral for Forbes Garden Project

Author: Jacob Essig

Project by: Sigrid Adriaenssens & Tim Michiels

read part 2 | read part 3

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