Geocell, grid, and reinforced-soil in restoring eroded steep slopes

By Stan Boyle and Kathy Robertson

Abstract

In October 2003, stormwater overflows from an intense, short-duration storm eroded approximately 20,000 yds.³ of soil from the bottom and side slopes of a ravine between the Tacoma Narrows Bridge and a subdivision near Tacoma, Wash. The erosion ruptured two storm drains and left undermined slopes that continued to ravel and retreat. Measures were implemented to help stabilize the eroded slopes and prevent further retreat that could affect the Tacoma Narrows Bridge expansion project and a residence above the opposite ravine slope. The measures also addressed regulatory agency concerns about sediment-laden water discharging from the ravine into the Narrows that could damage sensitive sand lance habitat on the beach at the mouth of the ravine.

Ravine restoration measures consisted of constructing unreinforced soil, flexible geocell-faced reinforced soil, and geocell-geogrid reinforced soil slopes that mimic the former steep ravine. After slope construction, a mixture of drought tolerant and native vegetation was planted to provide erosion protection and restore the natural ravine environment. The geocells had the flexibility to fit against, and transition between variable slopes and create steep slopes with planting benches for trees, pockets for smaller plants, and terraces that slow stormwater runoff. The perforated geocell walls allowed roots to spread. The geocell ravine repair design was an effective, economical solution that saved the client the cost of potential litigation, satisfied regulatory requirements, and expedited construction, which was completed in less than 5 months during the 2004 construction season.

This article presents project history, features, and challenges, and discusses design and construction of the geocell-faced, reinforced-soil slopes and vegetation restoration.

Introduction

On Monday, Oct. 20, 2003, during an intense rainstorm, stormwater overflowed into a ravine along the north side of the west anchorage being excavated for the Tacoma Narrows Bridge expansion project, approximately 6 miles west of Tacoma (Figure 3). The rain exceeded the intensity of the 100-year storm by about 20% and overwhelmed the 24-hour/10-year stormwater controls for the bridge expansion project and drainage systems in the vicinity.

As flooding continued, concern rose about the safety of the existing bridge abutment and anchorage, which could be undermined if stormwater overflowed and eroded the bluff above the Tacoma Narrows, a fast flowing tidal channel between Tacoma and Gig Harbor, Wash. Stormwater was diverted into the ravine to protect these structures. Figure 2 presents a project site plan.

The diverted stormwater overflows added to runoff already in the ravine from adjacent roads. The resulting high discharge eroded the ravine bottom to the level of the Narrows beach, ruptured two storm drains buried in the ravine bottom, gullied and undermined the south slope next to the bridge, and undermined the north slope below the subdivision and an access road used for the bridge construction. The erosion also ruptured an old storm drain on the south slope and removed an old gravel road in the ravine used as a trail to the beach by the subdivision residents. Flows from the ruptured storm drains contributed to the erosion.

The storm eroded approximately 20,000 yards³ of soil from the ravine, some of which was deposited as an alluvial fan at the Narrows shoreline. The ravine bottom was lowered by approximately 12 vertical meters. Ravine sideslopes were left undermined and over-steepened, continuing to ravel and slope retreat in subsequent storm events. This continued erosion threatened the east abutment of the Tacoma Narrows Bridge expansion and a residence above the north side of the ravine. The erosion also created sediment-laden discharges from the ravine into the Narrows, potentially degrading sensitive sand lance habitat. Figure 1 shows post-event conditions.

If unrepaired, the ravine would continue to degrade, affecting public safety, creating economic loss, and damaging the environment. Recognizing these hazards, the regulatory agencies, permitting authorities, and bridge expansion design-build contractor (Tacoma Narrows Constructors) set a deadline for ravine stabilization by Nov. 1, 2004. Earthwork had to be completed by mid-September 2004.

While permanent stabilization measures were designed and permits for their implementation obtained, temporary erosion and sediment control measures were installed to reduce sediment delivery to the Narrows. These measures included constructing a sediment trap about 18m from the ravine mouth, above the beach (Figures 2 and 3); temporarily piping stormwater past the site; and removing sediment deposited on the beach.

Project goals

The primary goals of the ravine repair were:

• prevent further erosion that could affect the bridge construction project and the residence above the opposite ravine slope.
• avoid damaging sensitive sand lance habitat on the Narrows beach at the mouth of the ravine.

The project was not intended to prevent naturally occurring erosion, such as the bluff retreat occurring above the Narrows shoreline. Completing the project before November 2004 was critical, but because of the wet ravine conditions, work could not begin until mid-June 2004.

With the support of stakeholders (i.e., subdivision residents, Tacoma Narrows Constructors, Washington DOT, the general public, and various permitting and regulatory agencies), the project team was able to:

• obtain environmental and building permits in less than 6 months.
• design a flexible, low-maintenance, cost-effective solution that rebuilt the eroded, steep ravine in 3 months.
• complete the project without complaints from landowners, the general public, or agencies in 5 months.

Site conditions

The damaged area extended approximately 80m inland from the beach and measured approximately 55m from the top of the south slope to the top of the north slope. The approximately 25m-high south ravine slope was eroded to approximately 45° with locally steeper areas at the toe. The eroded portion of the north ravine slope varied from approximately 35° in the lower 6 vertical meters and upper 12m, with the intermediate 12m steepened to about 70-80°, with locally vertical areas. The top of the north slope had eroded to within approximately 3m of a sport court for the residence above the north slope.

Pieces of stormwater pipes and concrete plus quarry spalls (i.e., 8-in. minus angular rock) placed early during the storm event, were on the surface and buried in debris in the ravine after the storm. Fallen trees covered the ravine bottom (Figure 1).

Dense sand with varying amounts of silt and gravel was exposed in the lower two-thirds of the north slope. The sand is overlain by a 3- to 4.5m-thick layer of hard, interbedded silt and clay, which is itself overlain by 0.3 to 0.6m of colluvium.

Soil exposed on the south slope consists of loose silty sand, which records indicate was fill placed during construction of the existing Tacoma Narrows Bridge. Approximately 1.5 to 3m of very loose sand, silt, and gravel was deposited in the ravine bottom and along the toes of the slopes near the end of and following the storm event.

Despite being plugged and diverted uphill of the site, the ruptured storm drains leaked and continued contributing flow into the ravine after the storm events. Small seeps observed on the lower section of the south slope added to the flow that kept the ravine bottom conditions wet.

Steep slopes and existing roads cut off upland ravine access except at one location along a construction access road that was being used for the bridge expansion project. Primary access for ravine repair was via this access, while access from the shore could be used only for emergencies.

Project elements

The ravine repair project focused on improving slope stability and reducing long-term erosion. The repair backfilled the ravine bottom and buttressed the failed sideslopes. Finish slopes vary from about 1H:1V (horizontal to vertical) to flatter than 2H:1V.

Repair measures included:

• constructing an access road into the ravine.
• removing debris, vegetation, topsoil, loose soil, and concrete from areas where fill would be placed.
• installing subsurface drains to collect and drain natural seeps.
• backfilling the bottom of the ravine with clean sand and gravel (Figure 4).
• building 1.5H:1V to 2H:1V slopes using geocell-reinforced soil (Figure 5).
• building 1H:1V to 1.5H:1V slopes using geogrid-reinforced soil with geocell slope face stabilization (Figure 6).
• replacing damaged stormwater conveyance pipes and systems.
• replacing the old trail with a forest-type path.
• covering unreinforced soil with a biodegradable turf reinforcement mat (TRM), protecting the slope against surface erosion until vegetation could be established.
• planting drought-tolerant and native ground cover, shrubs, and small trees that provide short- and long-term erosion protection, reducing the need for watering, and re-establishing vegetation in the ravine.

The flexible design accommodated the complex, variable site conditions that changed with each storm event, allowed rapid field modifications, and reduced potential construction delays. Workers could readily shape the geocells and geogrids to match the existing terrain.

Materials

The Washington State Department of Fish & Wildlife required that the ravine backfill and slope fill be clean sand and gravel to reduce the amount of fines washing out onto the beach and into the Narrows. Table 1 presents the gradation the agency approved for fill and backfill, which also conforms to WSDOT (2002) requirements for backfill for reinforced soil and gravel borrow. Fill was compacted to a minimum 92% of its maximum Modified Proctor (ASTM D1557) dry density. To enhance plant growth, organic soil was mixed with the fill material placed on the surface of unreinforced slopes and in the exposed cells of geocell-reinforced slopes.

Confinement at the slope face was required for slopes steeper than 2H:1V (Figures 5 and 6). Geocells, 200mm high by 200mm nominal dimension, with perforated cell walls, were selected because they:

• were easily transported into the confined work areas.
• had the flexibility to fit against, and transition between, variable slopes.
• could be used to create steep slopes with planting benches for trees, pockets for smaller plants, and terraces to slow stormwater runoff.
• allowed roots to spread via the cell-wall perforations.
• allowed water to drain between cells.

To limit erosion and potential soil loss, geocell walls exposed at the slope face were not perforated. The contractor used geocells, and geocell-faced slopes 1.5:1V and steeper (Figure 6) were reinforced with geogrid.

Reinforced slope design was based on procedures presented in Elias and Christopher (2000). Long-term design tensile strengths of 37kN/m and 72kN/m were specified for geogrids placed within 3.6m of the top of slope and geogrids at greater depth, respectively. For ease of construction, the contractor used a single geogrid that satisfied the higher strength requirement for the project.

High-density polyethylene (HDPE) pipe was used in the subgrade drains, to replace the ruptured storm drains, and for new storm drain pipes. This pipe type was selected for its ease of handling, durability, and strength. Pipe joints were fusion-welded for strength.

Construction

The contractor mobilized on-site June 14, 2004. Work was performed 10-12 hours per day, 6 days a week, except for approximately 30 days in August while an outfall pipe for the bridge expansion project was installed through the area.

Earthwork and stormdrain repair were completed by mid-September. Vegetation and planting operations were completed in October. The access road that the contractor constructed down the center of the ravine to reach the work area was restored to a narrow forest trail upon construction completion.

Before placing fill, the contractor removed debris, loose material, and the temporary sediment trap that had been installed near the ravine mouth. Subsurface drains were installed at the base of the fill and along the toe of the south ravine slope to collect and convey seepage to the Narrows.

Fill was delivered to the site by backing dump trucks down the 15-18% grade of the access road or dumping material from the trucks over the top of the south ravine slope. A wheel-loader and excavators transported the fill to placement locations. Unreinforced material in the bottom of the ravine was spread with a bulldozer and compacted using a smooth drum vibratory roller.

New, continuously welded HDPE storm drain pipe was buried in the fill, connecting existing upgrade catch basins to the existing outfall pipe. New storm drain catchbasins, pipe and outfall were installed to collect runoff from the bridge site to avert a similar future occurrence. The outfall for the subgrade drains discharged onto a rocked-reinforced section of the slope rebuilt across the mouth of the ravine above the beach.

Workers placed and stretched the geocells and geogrid material to fit the variable terrain, and used walk-behind compactors to compact soil placed within geocells and the reinforced zone. The lightweight, flexible geocells and geogrid reinforcement allowed workers to place and field-adjust them on variable steep slopes within confined areas (see Figures 7 and 8), achieving continuous coverage and smooth transitions.

An excavator placed fill material in geocells and over the geogrid (Figure 8) and workers raked the fill into place. The fill kept the geocells shaped and positioned. Where the design minimum specified lengths would have required excavating into steep ravine slopes, geogrids were extended to the natural slope face, reducing earth pressures assumed to act on the back of the reinforced zones, and the tails wrapped back, providing pullout resistance.

Planting benches formed by the geocell-faced slopes were relocated, modified, or eliminated, as necessary, to match actual conditions and expedite work progress. After completing the earthwork and stormdrain repairs, workers placed the TRM on the unreinforced slope faces, and then planted mostly drought tolerant, but also native, ground cover, small shrubs, and trees that have good root-holding capability. Geocell cell walls were cut or removed to create large planting pockets for shrubs and trees, and allowed room for roots to expand. Riprap was placed at the mouth of the erosion channel, above the ordinary high tide line and vegetation planted either side of the riprap to protect the slope toe.

Photographs of the project soon after planting in October 2004 are presented in Figures 7-9.

Summary

A combination of unreinforced soil and geocell- and geogrid-reinforced soil were used to repair an erosion-damaged ravine and reduce slope retreat that could potentially have an impact on adjacent structures. These techniques were selected because they were adapted to variable, changing slope geometry, constructed in confined work areas on steep slopes, were rapidly installed, and provided for erosion control and easy planting.

The repaired slopes include an approximately 27m elevation difference between toe and top of the easternmost geocell- and geogrid-reinforced slope. The slopes have performed well during the two years since construction. The successful partnering of the bridge expansion project and ravine repair contractors and designers, the bridge owner [Washington State Department of Transportation (WSDOT)], regulatory and permitting agencies, and neighboring homeowners were integral to the timely completion of the project.

The professionalism, high level of communication, coordination, and trust between all parties averted potential litigation, expedited project approvals, and reduced project costs. This project exemplifies the adaptability and flexibility of geocell- and geosynthetic-reinforced soil construction, and the value of stakeholder partnering.

Stanley R. Boyle, Ph.D., P.E., is vice president at Shannon & Wilson Inc. in Seattle.
Kathy Robertson, P.E., L.E.G., formerly with SvR Design Co., is a principal at Pickets Engineering LLC in Kirkland, Wash.
This article is a modified, magazine version of a paper presented at the Geosynthetics-2007 Conference and Trade Show in Washington, D.C., January 2007.

References

Elias, V. and Christopher, B.R. (2000), Mechanically Stabilizeed Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines, U.S. Department of Transportation Federal Highway Administration, FHWA Demonstration Project 82, FHWA-NHI-00-043, 393 p.

GeoProducts LLC. (2004), EGA 30P geocell product information, www.geoproducts.org

Huesker Inc. (2004), Fortrac 80/30-20 geogrid product information, www.hueskerinc.com

Washington State Department of Transportation (2002), Standard Specifications, M41-10.

Acknowledgements

Success of this project and its timely implementation was the result of partnering, cooperation, and coordination of a number of interested and affected parties, including: SvR Design Company (SvR) the civil engineer, landscape architect, and project manager for the project; Shannon & Wilson Inc., which provided geotechnical engineering, geology, natural resource, and environmental services; St. Paul Travelers, the insurer; J. Walling Associates, insurance adjustor; Quigg Brothers, Inc., the general contractor for the ravine repair; Terra Dynamics, the landscape subcontractor; Tacoma Narrows Constructors (TNC), the design/build team for the bridge expansion project; Shipwatch Homeowners Association; Washington State Department of Transportation, which owns right-of-way in the ravine; Duane Hartman & Associates, surveyors; and Pierce County and Washington State Departments of Ecology, Fish & Wildlife, and Natural Resources, the regulatory agencies.