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.
One of the goals of the project was to demonstrate that it is possible to break away from straight rammed earth walls and build elegant curving elements. This required the construction of curving formwork, which was assembled from standard wooden 2″x4″ elements and bendable plywood. To account for the huge outward thrust created by the compaction, horizontal support triangles were built from using the same wood. Additional horizontal pieces were screwed in between the supports to prevent bulging and cracking of the plywood.
(From Left to Right): formwork assembly, manual spreading of the earth, bendable plywood sheets being installed, pneumatic compaction of the soil
The actual ramming of the earth was done by coating the formwork with a 3-4 cm thick layer of lime stabilized earth (mixed on site), which was then filled with local Princeton soil from the Princeton University construction yard. This soiled had been screened through a 1 inch mesh and moisturized to the optimal water content of about 10%. The soil was lifted into the formwork using an excavator, after which it was spread by hand, and then compacted using pneumatic backfill tampers.
(From Left to Right): Excavator filling the wooden framework with earth, Flattening the earth with tampers, Compressing the earth with pneumatic tampers
Subsequently, the formwork was disassembled and peeled away from the compacted earth. This is a fairly easy process thanks to the thrust exerted by the dirt. The majority of the lumber used for the formwork will be recycled and reused in a next construction project.
Finally, the top of the wall was covered with a water-resistant membrane and planking of untreated cedar wood.
Author: Jacob Essig & Tim Michiels
Project By: Tim Michiels & Sigrid Adriaenssens
Construction team: Tim Michiels (project coordinator), Eric Teitelbaum (coordinator formwork construction), Amber Lin, Jacob Essig, Victor Charpentier, Sigrid Adriaenssens, Olek Niewiarowski, Princeton University Civil Construction: Steve, Paul and Mike.