The Goshen to Pleasant construction project is an example of a well-planned and well-implemented erosion and sediment control plan.
When a highway construction project works well to control erosion and the emphasis is not only on containing sediment that comes from disturbed ground, the project deserves a little notoriety. One such job is the construction project involving the Goshen to Pleasant Section on the Willamette Highway, otherwise known as Oregon Highway 58 in Lane County. At the time the project was scoped for environmental concerns, erosion was expected to be an issue that would require careful planning and design. The project involved a great deal of excavation and grading that could affect a major river and other drainages. The Willamette Highway travels southwest from Eugene, splitting from Interstate 5 at the town of Goshen and heading through the Cascades to the town of Oakridge. The highway is heavily traveled with cars and trucks mainly making the trip from Eugene and points along Interstate 5 through the Cascade Mountains and on to Klamath Falls and farther south. An elementary school has access to the highway, so school buses and lots of traffic are routed to and from the school on the highway. Logging trucks, goods and services vehicles, and recreational travel make up a great deal of the traffic. Traffic was a concern on the project because of the high average daily traffic and the demand for the road during the recreational season when people travel from the valley to the mountains.
The project area spanned 3 mi. and a variety of geomorphic surfaces such as floodplains, low and high terraces made up of mixed alluvial soils, and uplands with soils formed in glacial outwash material. Topographically, the project is divided into two distinct sections: the western half that passes through the floodplain to the Coast Fork of the Willamette River and associated lowlands. The eastern half rises onto the upland bench where the community of Pleasant Hill is located. The drainage pattern associated with the lowlands at the west end of the project is characterized by sloughs and dry drainageways, which link into the Coast Fork. The highway crosses the river, four small streams, and an overflow channel. Roadside ditches through this section are broad and grass-lined.
 |
| A bulldozer with a slopeboard attachment finishes the cut slope for seed placement. |
According to Soil Survey for Lane County, OR, vegetation reestablishment was expected to be successful thanks to the deep soils that are rated high for pasture production. The Willamette River crosses the project area with Chehalis silty clay loam soils, which are occasionally flooded. The terrace soils consisted of the Salem gravelly silt loam and the Coburg and Malabon silty clay loams. The project crossed an area of sloping ground with the Salkum silty clay loam on 2-8% slopes and 8-16% slopes. The finer-textured soils would normally be considered an erosion hazard if it weren’t for the fact that they are permeable and have low runoff potential. The soils of most concern were the Salkum soils on the steeper slopes of 8-15% where runoff and the erosion hazards are moderate.
The construction project involved grading, structures, paving, signing, and illumination in order to reconstruct and widen the highway. New construction resulted in realignment of road connection, bridgework, construction of subsurface drains, bridge scour protection, installation of drainage facilities, and the relocation of a historic monument. The widening created new fill slopes that caused the moving of roadside ditches and the formation of new ditches.
The main contractor on the job was Fowler Construction who, according to Oregon Department of Transportation (ODOT) inspector Chuck Remus, was found cooperative to work with. Utility work occurred first with the power utility coming in during the early spring and summer months. A meeting with officials from the Eugene Water and Electric Board during early project development helped to ensure that adequate erosion control measures were taken during the moving of power poles and the trenching operations. Graveled construction entrances and access roads were placed prior to the utility work to reduce tracking onto the pavement and to control runoff.
A grubbing operation loosened the top 6 in. of sod and was followed by scalping. Larger trees and limbs were ground and sold to the nearby Kingbury Charcoal plant by the contractor. After the slopes were denuded, a protective layer of straw with a heavy tackifier was applied to hold the soil until the slopes could be worked to permanent grade and the final seeding was placed.
Permanent seeding was done on three occasions throughout the project, although the majority occurred in late September and early October 1999. The seeding and mulching was subcontracted to Anderson Construction. The cut slopes were shaped with a slopeboard, a wing out from the bulldozer with a hydraulic adjustment. During the first pass a cut was made into the slope face with the dozer arm raised. A second pass followed in which the arm was lowered to serrate the soil surface. The result was a surface roughening of about 30 in., with grooves running parallel with the slope contours.
Upon site review it appeared that the erosion from the cut slopes was well controlled by the combination of good surface treatment and excellent vegetation establishment. At the time of the site visit, 70% of the work had been completed, including permanent grading and vegetation establishment, curb and storm drain in the urban section, and half of the retaining wall construction completed. Remaining work includes shoulder widening and paving. Completion is expected in fall 2000.
No matter how well a highway construction project is run, problems will occur. According to the contractor, traffic was tough to deal with and the contractor was not allowed to constrict traffic between the hours of 7 a.m. and 9 a.m. and 2 p.m. through 6 p.m. To keep the project on schedule, the contractor worked around the clock for six weeks. Another problem was that they had to waste a majority of the soils resulting from the cuts and could not use it for fill, as it was too wet. This resulted in disposal of about 225,000 yd.3 of material.
The total cost of the construction project is projected to be $8.9 million. The contractor costs for erosion control is $113,292. The Erosion and Sediment Control Plan (ESCP) bid estimate was $132,591, or less than 1% of the total project cost. ODOT’s erosion control team in the Geo-Hydro Section developed the plan. The ESCP was developed for a worst-case scenario as the project schedule projected excavation work to occur during the rainy winter season. The contract for construction was let in January 1999. The plan included supported and unsupported sediment fence, straw-bale sediment barriers, inlet protection, aggregate check dams, aggregate construction entrances, temporary slope drains, temporary sediment traps, diversion dike swales, and permanent seeding and mulching.
The revised plan differed from the created plan. The plan developed by ODOT called for almost 6,500 ft. of diversion dike at the toe of fills. The idea was to save on the use of sediment fence by creating a small dike at the toe and seed the berm so that it would become stable. Sediment-laden water could collect in the swale, where it could slow the velocity and settle out soil particles. The revised erosion control plan submitted by the contractor did not use the diversion dike swale approach, as drainage on the project was revisited and drainage ditches were created at the toe of fills instead.
In conclusion, the project was very successful in controlling erosion thanks to a commitment from all of the necessary players. The ESCP was well designed with field visits and planning in addition to participation with others in the project development process. The construction project manager and inspector were familiar and trained in what to look for on the job for erosion control efforts and expected the contractor to be diligent. The contractor paid special attention to protecting the slopes on a temporary basis and then when doing permanent seeding using good seedbed preparation and within the seeding window to obtain adequate moisture for germination.