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Casebook: Going green

September 1st, 2007 / By: / Case Studies, Landscapes

An occasional reference section on landscape design using technical textiles and geosynthetics for aesthetic ends.

The long-term success of a geosynthetics installation depends not simply upon the presence of geosynthetics but on how well they work in conjunction with natural elements, particularly vegetation.

Plants and trees possess their own unique performance and aesthetic characteristics. We may not be able to duplicate all of these traits in a manufactured material—a geotextile will never grow self-stabilizing roots for instance—but we can certainly leverage their power at the job site.

The condition and presence of green (and blue) spaces has a direct impact on human health and our general well-being. Whether we wish to preserve and improve the spaces we have or build new ones where they are needed, the combined use of geosynthetics and plant life has many benefits.

Case studies

Reclaiming shorelines

When 60% of the shoreline of Lake Phalen in St. Paul, Minn., was dredged and faced with riprap, authorities believed their erosion problems were over. But seasonal water level changes, increased dramatically by local development, scoured the banks and caused turf-covered verges to drop off steeply into the water.

To remedy the erosion, landscape fabric was installed to contour gentler grades and to stabilize the soil in order for native plants to take root. Two fabrics were utilized. One, placed on the lower shore, consisted of coconut fibers sandwiched between three UV-stable carbon black nets. The fabric provides a perfect blend of nutrients for vegetation and long-term durability against pounding waves.

The second fabric, installed on the upper shore, is 100% biodegradable, which gives 18 months of protection to establish a prairie habitat. The fabric takes the shore’s contour more readily than rigid fabrics and its design prevents ducks and other creatures from getting snagged.

To date, plants have enjoyed a 90% survival rate. Minimal weed invasion has occurred and no substantial erosion is evident. Park patrons and pedestrians have enjoyed the results of the project since its completion in 2005.

Aquatic habitats

Polypropylene geotextiles perform well when exposed to water and other inorganic compounds. They are appropriate for use as pond liners, in bank weed control, for garden fountain systems, filtration, dividers in ponds, pile wraps, lagoon liners, turbidity curtains, and even as artificial habitat.

Rain Garden Ornamentals near Kaneohe, Hawaii, produces ornamental plants and fish for the aquarium trade. The business utilizes a nonwoven polypropylene geotextile to create artificial habitats. The material is cut into panels roughly equal to the depth of the ponds. A hem of about 24mm diameter is sewn along one side of the panels and one end is sewn shut.

Sand is poured into this hem and the open end is sewn closed. Material above the hem is sliced into strips about 50mm to 76mm wide. The modified “aqua” geotextile is simply laid across the bottom of the pond—the hem sinks, the strips float to the surface.

The new habitat is not toxic to fish and provides cover, which reduces some aggressive chasing. Vegetation grows on the strips, and microinvertebrate grazers also emerge. This growth creates a self-contained food chain, with fish as top-level predators. In a month to six weeks, enough nutritious bits of plant and animal matter provide meaningful amounts of forage for the fish.

Phytotechnologies

Phytoremediation is the use of plants to remediate or contain contaminants in the soil, groundwater, surface water and sediments. The term phytotechnologies has been introduced to cover a range of technologies and applications related to phytoremediation, including stabilization, degradation, accumulation, etc.

Over the last two decades, phytoremediation has emerged as a feasible alternative to more active and costly technologies, especially for large areas with relatively low levels of contamination in shallow soils or groundwater. The technology is rapidly gaining acceptance within regulatory agencies.

In Florida, ferns are being used to draw arsenic from ground contaminated by a wood treatment facility. A combination of species and microorganisms have been applied at a West Virginia Superfund site. Many other projects are underway, but it can take years before mature vegetation provides a full “pumping” effect.

Geosynthetics, as part of an interdisciplinary approach to remediation, can benefit (and benefit from) phytotechnologies in numerous applications. In a Virginia Superfund site, a lightweight geosynthetic cover is being used in conjunction with grasses and trees to increase evapotranspiration, which may transform a sulfate-contaminated area into a recreational and economic resource.

Basic research has already demonstrated “proof-of-concept” at the laboratory and greenhouse level. But more full-scale field applications need to be implemented throughout different climate zones to evaluate the long-term performance of these systems in real-world climates and growth conditions.

Urban landscapes

The design of the new Federal Building and United States courthouse in downtown Minneapolis, Minn., includes 19 tear-shaped mounds, or drumlins, created to guard the front plaza entrance. The drumlins are steep, grass-covered, and oriented in a pattern so that shadows are cast on the building during certain hours of the day and months of the year.

The plaza sits on the top deck of an underground parking ramp, requiring the drumlins to be lightweight. Four different geo-products were used in conjunction with a soil fill.

The top deck of the ramp was waterproofed and a protection board installed. A prefabricated drainage composite was placed to collect rainwater, which drained to the stormwater system. The composite consisted of a polyethylene core wrapped with a geotextile.

Each drumlin footprint was filled up with blocks of expanded polystyrene (EPS), a geofoam. Another geotextile was placed over the blocks, and a soil fill of 70-80% sand and 0-20% clay was added. A geogrid was introduced within the mounds, tensioned by hand, filled with sandy topsoil and compacted. Sod followed. Though imposing, the finished mounds are extremely lightweight.

Green Roofs

In 1994, William McDonough + Partners won a competition to lead a team of architects and designers in the construction of a new corporate campus for clothing retailer The Gap. Located in the hills above San Francisco International Airport, the complex’s buildings feature grassland roof coverings that mimic the local ecosystem.

The green roofs offer a number of benefits. Natural aesthetic enhancements attract local birds and butterflies. Acoustic insulation mitigates the sounds of adjacent freeway traffic. Thermal insulation lowers the costs of heating and cooling.

Extended durability of the roof membrane is ensured, since the waterproof membrane is protected from weathering by the natural covering. The local environment is enhanced through the production of oxygen. And the plantlife provides stormwater retention.

Green roofs are fast becoming a mainstream technology. A green roof generally mimics typical soil strata, with some differences. Modular green roofing systems consist of drainage layers, filter cloth, growing media, and plants that are prepared in movable, interlocking grids.

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