For light filters through the masses, forming an ever-changing weave which depends on atmospheric conditions and the shapes of the stones: the effect is something entirely new, rather like a fascinating brise-soleil.
~ Dominus Winery
Gabions (from Italian gabbione meaning “big cages”) are cages or boxes filled with rocks, rip-rap, concrete or sand and soil. The filled cages are typically used for erosion control and other civil engineering applications. They have also been used in a military context, with earth or sand-filled gabions used to protect artillery crews from enemy fire. In the tradition of Herzog & de Meuron’s Dominus Winery in Napa Valley, California, mills studio proposed to build the DTS Project House with exterior screen / privacy walls composed of Gabions filled with rock. Beyond their ability to fulfill certain functional requirements, the two walls composed of rock filled gabions are intended to provide a very specific character to the architecture and endeavor to physically express the family’s institutional purpose.
The Gabions filled with rock address multiple general and specific aspects of the architectural agenda and design intent, including:
1. Provide privacy from directly adjacent neighbors.
2. Provide shading of east and west glass walls.
3. Act as a thermal mass keeping house cool during the day and releasing heat at night.
4. Filter natural daylight on the interior.
5. Focused light streams physically express daily and seasonal passage of time.
6. Filter artificial nightlight on the exterior.
7. Expresses how the institution of the family provides protection for the individual and acts as a filter between the individual and the community.
8. Is an organic element providing context for the manmade – nature and industry.
9. Recycles rock existing on site.
10. Turns “Landscape” of site vertically so that landscape is an integral multi-level experience of living on the hillside.
11. Roots the building to the ground and establishes a context of stability for the floating cantilevers.
12. Provides a line of protection from which one can escape to liberation.
13. Makes an ancient technology modern.
14. Connects the building to the neigborhood’s historic rock walls.
Natural light filtering privacy / screen walls were an integral design element of the DTS Project House from the very first concepts. mills studio explored various options for the design and materiality of these walls, and determined that gabions filled with rock contributed more to achieving the architectural agenda than any of the various options.
Rock filled Gabion cages employed as an exterior wall requires a series of decisions and their implementation that affect the final visual and functional results. The first and most important decision is whether or not to use rock filled Gabions as an architectural element. Although mills studio could provide a list of conceptual reasons to use rock filled Gabions and could provide images that convey a certain amount of information, it is necessary to experience actual stone filled Gabions in person to see how they truly affect each of our senses and if they function visually and physically as mills studio intends for the DTS Project House.
Arrangements were made to visit the Dominos Winery. In preparation of the visit, the Owners and mills studio reviewed all available information, including several research papers on Dominus Winery in order to develop a list of relevant issues that must be addressed in the design of the gabion walls. Since we assumed this may be our one and only access to the Dominus Winery and that the first experience of significant architecture can be somewhat overwhelming, we wanted to know specifically what to look for and look at during the visit. So in addition to seeing how the Gabion walls relate to the other architectural intentions and elements, there were specific architectural and construction details that we wanted to study and document. A detailed list of questions was sent to the staff at Dominus Winery asking about issues from initial design to long term maintenance.
“After researching this idea, a few concerns have come to mind that can be best addressed by someone with firsthand experience with gabions. This is why I contacted you. As requested, here are a few of my general concerns that I’d like to discuss in a short conversation with the appropriate person on your staff:
1) Do the gabions attract nesting birds, insects or other wildlife?
2) If so, does this present a maintenance problem? What are the nature of such problems, if any, and how serious are they and how are they managed?
3) Does it become necessary to clean or provide any other type of routine maintenance to the gabions? If so, what needs to be done and how often?
4) Does the wire in the cages rust, discolor or otherwise deteriorate over time? Have there been any other problems with the metal?
5) In retrospect, are there any ways that the construction of the gabion systems could had been better executed or realized from either an aesthetic or functional perspective?
6) Do you have any further counsel, advice or ideas that you can offer as far as this building element (gabions) are concerned? Am I asking all the right questions?”
The visit to Herzog and dee Meuron’s Dominus Winery was inspiring. Based upon their visit to Dominus Winery the Owners were convinced that rock filled Gabions were the appropriate choice for the DTS Project House’s privacy / screen walls. The visit to Dominus Winery and a detailed inspection and documentation of the installation and how it has aged, provided a checklist of installation details that needed study and resolution. Although the Dominus Winery provided the best single example to research and study since the scale of the gabion walls are similar to the DTS Project House walls, mills studio studied, visited, and documented numerous other examples of rock filled gabions with differing contexts, purposes, and constructions details which all lead to different aesthetic impacts.
The construction implementation requires several categories of decisions with numerous details in each category. The categories include the following:
1. Structural Support
2. Gabion Cages
3. Gabion Filling
Although mills studio divided the construction aspects of the privacy / screen walls into three categories, the content of all of the categories are completely intertwined and most all choices determined in one category greatly affect the choices made in other categories. One has to start at both ends of the process simultaneously. One starts with a desired impact that the details of the structural support and gabion cages will provide, but at the same time the density and weight of the gabion filling determine the design of the structural support and the gabion cages. For instance: the sizing and thickness of the gabion cages is determined by a combination of what the building structure calls for and allows, how the structure relates to the fenestration of the house separating inside from outside, how light will or will not penetrate, and aesthetics.
The Gabions filled with rock must be supported and or restrained so as not to move or become out of alignment as a result of gravity or earthquakes. The DTS Project House walls composed of rock filled Gabions are 33’ high above the sloped hillside grade with the top of the wall sloped to follow the grade. The Dominus Winery sets its supporting structure on the inside of the Gabions so that stone is attached to the side of the structure. Therefore, the full height of the wall is uninterrupted by structure and the rock within the gabions is allowed to settle over time and warp the gabions cage volumes. mills studio looked at ways to address the cage deformation within the positioning of the support structure relative to the gabions and the detailing of the support structure. One significant difference between the DTS Project House and Dominus Winery walls is that the top of the DTS wall is sloped which means that the stone in the trapezoidal like gabion cages will want to move laterally down the slope of the hill. The support structure must resist this lateral movement.
At Dominus Winery there are definitively two different sides to the gabion wall, an outside and an inside, with the steel support structure only located and seen on one side of the wall – the inside. This structure of vertical, horizontal, and diagonal steel tubing creates its own pattern and in places offers a competing aesthetic to the winery’s filtered light. mills studio looked for ways to position and detail the support structure in ways to reduce the visual competition and treats both sides of the privacy / screen wall equally.
Gabion cages are made up of a metal wire mesh that holds the rock in place and provides a visual presence. The size of the wire and wire mesh spacing are directly related to the size of the rock to be restrained. The size of the wire and wire spacing and the color of the material and how it ages all determine the visual impact of the cages relative to the rock. If and how the size of wire and its spacing would change if different size stones were used in different cages was also a consideration. In most cases the wire cages are six sided and in most cases gabion cages are both stacked on one another and positioned end to end so. The DTS Project House looks at ways to restrain the rock more vigorously to better maintain the physical and visual alignment of the cages. mills studio also looked at options for “opening” the cages to allow adjustments to the stone filling necessitated by settling over time. Because the top of the two gabion walls is sloped, the top gabion cages must have an irregular shape with a sloped top. Thus, more restraint is required on the downhill side face of the cages to resist the stones wanting to move laterally downhill.
The Gabion filling has the most effect upon the visual quality and functional performance of the privacy / screen walls. The color, the size, and shape of the individual rocks that fills the gabions determine the aesthetic impact, how the light is filtered, and the amount of ventilation the walls allow. And then the layering or non-layering of the rock in the gabion, and mixing or non-mixing of different colors and sizes of rocks further affect the visual impact, light filtering, and ventilation. The composition and layering of stone sizes manipulates the amount of light and air penetration, or transparency and ventilation. The less densely packed stones allow more transparency and ventilation. The larger rocks, which leads to less density, provide a more porous wall and the smaller stones lead to a more densely packed wall. The more densely packed the gabions, the more shade, thermal protection, and enclosure the wall will provide. Decisions had to be made regarding mixing stone sizes in the same gabion cages and / or mixing individual gabion cages with different size stones. The aesthetic and visual impact would be very different based upon the choice of mixing options.
Various rock options were looked at and evaluated for the weight and density of the material which greatly affects the design of the support structure. The weight also affects the installation process as the material has to be lifted to the level required for installation and that height is 33’ above grade for the highest gabions. mills studio looked at how the stone’s shape would or would not reduce the propensity for settling over time and looked at how the stone installation process would or would not affect future settling. The stones will not only want to settle from top to bottom, but also from high side to low side in the top cages. The color of the stone options was an important consideration as the stone is a vertical landscape / hardscape element from both exterior and interior that establishes the buildings connection or disconnection from the earth. The color of the rock is also an important interior design element as the rock is the largest surface area in many of the interior spaces. The cost of the material, where the material would come from, including how far away from the site, how it would get to the site, how it would be unloaded and transported to the required location for installation were all important considerations.
While evaluating rock options, recycling rock generated from the on-site excavations showed itself to be an option. It had been assumed that the density of the bedrock on site would not lend itself to providing usable rock, but the major excavations changed this assumption. The most vexing question was determining if we would generate sufficient quantities of rock to completely fill the gabions, and if not, what would be mixed in and how would it be mixed in. Where we could just bring in more material until the gabions were filled if we picked a stone from a yard, we would not really know if we had sufficient material from the site until it was all used up and if the gabions were not filled, what options did we have. We also had to determine if we could shape the stone to desired sizes and if it would hold up over time when exposed to the elements.
Recycling what was already on site was certainly an attractive alternative as it both reduced the need to remove excavated material from the site and bring in material from off site. The possibility of the actual site material literally rising from the site and morphing into a vertical landscape and the color of the gabion walls being generated directly by the site was too exciting not to exhaustively try and make work. Thus, we completed all major excavations with the intent that all usable rock material would be saved and used in the gabion wall construction. All rock of 4” in any one dimension was saved and stored exposed to the weather to confirm it was sufficiently dense for use.
The saved stone was then laid out on the ground in an approximate thickness of the gabion walls to determine the approximate surface area of stone accumulated. This gave us some idea of how the amount of saved stone relates to the amount required. This helped us determine if we should assume that we would have to mix in material not generated on site and the amount we assumed we would be short influenced our considerations for mixing on site and off site material from the beginning of the installation so there was not a demarcation, or if we waited until the all on site material was used and then filled the remaining cages with off-site material providing for the need to determine where the demarcation lines should occur. The potential need for demarcation lines might affect the design of the structural support structure depending on the intended visual impact of these demarcations.
Assuming we would need more material than accumulated on site, we saved all broken concrete chunks of 4” or bigger from the few existing foundations and slabs. We also looked for other construction sites in the neighborhood that might generate usable stone from the same type bedrock. Several were found and material brought the short distance to the site. mills studio also was commissioned to design a subterranean addition in the same neighborhood with its excavation potentially generating usable stone material. Although it may be coming from the same ranges of bedrock, it was assumed that all off-site material would at the very least have subtle variances to our on-site rock material.
Although the first question regarding recycling the site’s excavated rock was to determine if sufficient material would be generated, we also had to determine if the range of rock sizes and shapes could be turned into the intended aesthetic and functional gabion filled wall. Thus, we constructed a full size 10’ wide x 10’ tall mock-up on site, in the same north-south direction of the permanent walls, to test and evaluate options for the support structure, options for the wire cage construction, options for how the stone filling would be installed, and options for all other details affecting the aesthetic and functioning of the permanent walls. The mock-up was also intended to show us how the actual installation and settling of rock material changed the actual amount of material required relative to the simple surface area calculations previously generated. We would take the results generated by the mock-up and extrapolate for the surface area of the two permanent walls, and this calculation would have to serve as the best information available for making all decisions about integrating off site material.
Only together do the darkness of gravity and the radiance of light bring forth the beautiful appearance of life, perfecting and completing a thing to become what we call truly real.
~ F.W.J. Schelling