Before the large swirling structure in Forbes garden could be constructed, a set of tests walls were built to master the construction workflow. The tests walls will also be used to test a different set of erosion protection measures, as one of the goals of our research experiment is to assess the erosion resistance of rammed earth in New Jersey. The first test wall was built out of unstabilized earth with no erosion protection implemented for reference. The second wall was also unstabilized, but plants will be grown on top of this wall in the hope that their roots will slow down the erosion process, while their leafs protect the dirt from driving rain. The third test wall was stabilized on the outside with a 10% lime-earth mixture, which was applied only at the outer 3 cm. This technique is a traditional rammed earth construction technique originating in Spain and referred to as “calicascado” which can be freely translated as “lime shell”. The 4th and final test wall was built unstabilized earth once again again, but half of it was coated using a silicone spray, while the other half was coated with a lime wash. All of the test walls were built with a reusable plywood formwork on top of a blue stone slab..
After the successful completion of the test walls, we moved on to the much larger spiraling wall inside Forbes garden. As explained in the previous blog post, the spiral consists of a lower bench area and a taller wall, separated by an opening. At its lowest point the bench is 40 centimeters high, and at its highest point it is 3 meters tall. Both rest on a blue stone foundation. Again, different erosion-protection measures were implemented. The bottom 15 cm of the entire wall was made out of a 25% lime- earth mixture, and placed on a water impermeable membrane to avoid capillary rise. The outside of the bench and most of the rest of the spiraling wall was stabilized using the calicascado technique after its promising results on the test walls. A great advantage of this technique is that it allows for a minimum volume of soil that needs to be stabilized with lime and thus requires less material transport. To compare the durability of the technique once again a section was left unprotected. Additionally, one section of the wall was entirely lime-stabilized using 6% lime as an extra test.
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 .
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 Design: A Site-Specific Golden Spiral
In celebration of earth day, we want to show you the explorations of earth construction at the Form Finding Lab, in particular our focus on rammed earth. Rammed earth buildings are constructed by pouring soil into a form work, similar to the one used to make concrete elements. This soil is then compacted in successive layers, either by hand or using tampers to create highly compacted dirt walls like those in the Tuscon Mountain house.
For his senior thesis, Aaron Katz (’16) studied rammed earth buildings and the earthquake loading capacity of rammed earth walls. His research project evaluated the application of limit state analysis developed for masonry to assess the overturning of rammed earth walls during earthquakes.