Can a sustainable structure cost less and be more durable than common constructions just by treading lighter? The recreational pathway connection work to the lower side of the falls of Otter Creek in Vergennes, Vt. stopped when bids for an elevated steel and wood portage and overlook structure proved excessive. When a project contractor proposed a fabric formed concrete structure, prime engineer TCE brought in Engineering Ventures to develop a schematic design by Broadleaf Landscape Architecture. A lighter footprint construction was thought to be more economical on the steeply sloped, tight site between an operating electric power-generating dam and a Gilded Age industrial building. In addition to the construction cost issues, the public stakeholders were concerned about the snow and ice maintenance requirements, heightened by a waterfall’s mist, for the structure proposed originally. The new structure incorporated sustainable design aspects and lower impact construction methods to come in under budget and address durability concerns.
Figure 2: Fabric-formwork wall system.
From prior experience with fabric-formed structures, Engineering Ventures understood that geotextile formwork supported from standard sawn lumber could be built from pieces carried and constructed on site by workers alone (see Figure 2). With a soft riverbank at the bottom of the site (that was flooded twice during the project) and several no load zones and overhead power lines, a design that limited mechanized site access requirements proved vital. No crane was used for the structure or foundation.
Figure 3: Fabric-formed surfaces.
The fabric provides an economical means to cast shapes with large reliefs by eliminating the need for limited use panels at sides and soffits. On the riverwalk, that translated to a structure reflecting the adjacent stone and bridge works of Vermont’s oldest city. In addition to the fabric texture impressions (see Figure 3), a more durable surface is created. The denser surface comes from the fabric’s ability to allow for the release of excess water and air from the surface. (For a reference, see “Fabric-Formed Concrete Structures” by Prof. Mark West, C.A.S.T, University of Manitoba.) No sealer or coatings were applied to the fabric-formed surfaces. Also, the fabric was able to meld to the slope, unforeseen conditions, and existing site feature irregularities, reducing schedule time and form panel wastage. This was significant in the fabric-formed shallow frost-protected foundation.
Additional sustainable measures were specified. The wall form fabric was shaken out and laid down for slope and insulation stability, redeployed as a geotextile. The form system lumber from the project was entirely repurposed; smaller pieces as fuel for local sugarhouses and larger pieces moving ahead to the other constructions, including barns and formwork. Fly ash was specified to reduce Portland cement use and to increase durability, but it also enhanced the workability of the mix in the formwork.
Taking advantage of the continuous concrete wall system’s ability to distribute load and bridge small abandoned underground features that would have otherwise required removal, a shallow frost-protected foundation was designed with the geotechnical engineer. This took the place of the individual footing system proposed originally, which at a five foot minimum frost depth on the slope, needed excessive, energy intensive excavation, stockpiling and shoring. The reduced depths allowed for hand digging of the foundations. No earth was moved by mechanized equipment and only minimal shoring was needed. Fabric was used not only for the side forms of the footings but also for the bottoms (see Figure 4) at the contractor’s request to protect against the effects of precipitation and fall mists.
Additionally, the fabric forms were rather seamlessly adaptable to adjustments in the aesthetic design proposed by the pathway structure contractor, Arro Design, for construction phasing and actual mix performance (see Figure 5).
Figure 5: Fabric-formed beam.
Figure 6: Downstream view from pathway top.
While the use of fabric as a formwork is not new, the proliferation of geotextiles has opened new possibilities. The project saw the convergence of sustainability and cost benefits from a lighter material and low energy use. Durability and beauty contribute to its foreseen endurance, keeping it out of the landfill, and as a fitting pathway passage along Vermont’s longest river.
Russ Miller-Johnson, P.E. is with Engineering Ventures, PC in Burlington, Vt. He serves on the SEI Sustainability Committee and is a member of the Vermont Green Building Network. He can be reached at email@example.com.
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