ADVERTISEMENT
Columbia River Gorge11-01-00 | 16
img
 
Presto Products Company

Located in a scenic, natural area along the Oregon side of the Columbia River, the Tanner/Moffet Creeks project was constructed in 1998 to provide public access to the riverfront. The area where the Columbia River carves through the Cascade Mountains, known as the Columbia Gorge, was given protection with National Scenic Area status in 1986. The original paved highway, now known as Historic Columbia River Gorge Highway 30, has long since been divided into isolated sections by the construction of the Bonneville Dam and the I-84 highway.

The Design
The photo above shows the earth retention structure during the early stages of revegetation. Retaining walls were going to be necessary to keep the newly steepened side slopes in place, but they would have to be attractive and look natural.

When a plan was proposed to connect two sections of Highway 30 by constructing one mile of highway from Tanner Creek to Moffet Creek, the Oregon Department of Transportation (ODOT) took on both the engineering design and public relations challenges in coordinating the involvement of multiple public agencies. The plan included a bike and pedestrian pathway that will eventually become part of a 100-mile trail extending from Portland to The Dalles. Since the scenic corridor encompasses forests, creeks, waterfalls and shoreline visible from both the Washington and Oregon sides of the Columbia River, the ODOT engineering design staff had to coordinate input from numerous agencies including the Washington Department of Transportation, The Federal Highway Administration, The US Forest Service and local county and city governments. In addition, the team addressed the environmental and aesthetic requirements of the Historic Columbia River Highway Advisory Committee (HCRHAC) and the Columbia Gorge Commission.

Design Challenges

 The recently completed geocomposite earth retention project highlights an increasing trend toward application of geocell or cellular confinement product technology for fully vegetated repair and protection structures once limited to manmade reinforcements.

One of the greatest challenges during this phase was the design of a series of switchbacks to gradually bring the bike path from highway level down to the level of the creek at the point where Tanner Creek passes under the bridge. The site provides a spectacular view of the river and the Cascades. Retaining wall structures were necessary to keep the newly steepened side slopes in place, and needed to be attractive and natural to fit the environment.

"This part of the Gorge is an extremely lush area, so a decision was made by the project team, and approved by the HCRHAC, to design a wall that could be vegetated with native vegetation," said Larry Bush, Design Engineer for ODOT.

A geocell cellular confinement system with a vegetated fascia was selected. Final design was supplied by ODOT, and the geocell retaining walls were constructed as an FHWA Experimental Feature Project in recognition that this was the state's first experience with a geocomposite wall design of this nature.

Green Wall Acceptance

The project was subject to the regulatory review and approval of numerous public agencies. Their acceptance of this recently completed geocomposite earth retention project along the Columbia Gorge highlights an increasing trend toward application of fully vegetated repair and protection systems with cellular confinement systems. "Green walls" are gaining acceptance in situations that were once limited to the use of obtrusive manmade reinforcements constructed of concrete materials, shotcrete, rock rip rap, rock-filled gabion baskets, metal bin walls and sheet pilings, wood crib walls, used automobile tires and other even less desirable materials. With the support of Presto's representative, Soil Stabilization Products Company, the project became the state's first successful application of a geocomposite wall design.

The Green Solution
The cells within the interior wall used the standard Perforated GEOWEB System cell wall design, providing lateral drainage through the cell walls and increased root lock-up as vegetative cover is established on the face of the wall. The Oregon Department of Transportation plans to hydroseed the wall.

Six wall structures were ultimately required, one over 5 meters (16 ft) in height. All were constructed with high strength woven geotextile fabric for soil reinforcement and the Geoweb® earth retention system as the facia system. The system was specified with textured, colored outer face panels in a natural green to harmonize with the surrounding landscape. The exposed outer cells form horizontal terraces where vegetation can flourish and the open-celled system captures rainwater and controls groundwater evaporation, creating a more natural environment for vegetation.

"This was a great environment in which to demonstrate some of the unique features of the system because project requirements included strict criteria for conforming to the planned topography of the winding trail and 'fitting in' with local colors and supporting growth of native plant species," said Samuel Randolph, of Soil Stabilization Products Company.

Construction Glitch Solved

Six walls were ultimately required, one more than five meters in height. All were constructed with high strength woven geotextile fabrics for soil reinforcement. The Cellular Confinement System was used as the facia and manufactured with a texturized outer face colored in a natural green to harmonize with the landscape.

ODOT discovered the existence of a substantial natural spring behind one of the walls after construction was completed. Soils were being washed from the face of the system as the water source behind the wall penetrated the system and washed the topsoil fill out.

This discovery, left unchecked, could have caused serious saturation of the backfill material, and potential wall failure. Fortunately, the system's perforated interior cell walls provide a natural, built-in drainage feature. To drain the increase of water behind the wall, ODOT removed the non-perforated outer facia panels along the toe of the wall, exposing the interior perforated cells. An underground drainage pipe system was then installed to drain the water. The drainage system reduced the potential for hydraulic and hydrostatic pressure buildup, and protected the slope below from erosion problems caused by concentrated water flow.

As a result, repair crews simply excavated down at the toe of the wall in the impacted area and then installed a drain line to capture and reroute the excess water across the bike trail and then downhill. To establish vegetative cover quickly for these greenwall structures, ODOT hydroseeded the completed walls.

"This repair was done at minimal cost," said Randolph. "The system has the unique ability to accommodate field mandated changes that often arise after a design has been completed. This ability to resolve drainage problems or utility lines and pipe penetrations distinguish a cellular system like this one from rigid MSE wall systems like concrete block. Without this kind of flexibility, those awkward last-minute design revisions and change orders can be very expensive and time consuming."

Project Update
One of the most challenging problems was the design of a series of switchbacks to gradually bring the bike path from highway level down to the level of the creek at the point where Tanner Creek passes under the bridge. The site provides a spectacular view of the river and the Cascades.

"The retaining walls seem to be working well after being tested by some unexpected heavy water seepage," HCRHAC spokesperson Jack Wiles said. "The vegetation is doing well and masking the wall appearance."

Reporting on the status of the walls earlier this year, Bush stated, "The vegetation has established itself, and the walls are blending into the natural environment very well. The HCRHAC and general public are pleased with the result."

Project Recognition

This project was awarded the 1999 first place Award of Excellence from the Industrial Fabrics Association International (IFAI), Geosynthetics Category as part of the International Achievement Awards honoring the latest advancements in design and manufacture in the Industrial Fabrics industry. LASN

img


Related Content