Rehabilitation of Steep Soil-Nailed Slope System Integrates Vegetated Geocell Fascia

Nov. 1, 2001
In the winter of 1989, construction of the Kressview Springs condominium in Cambridge, ON, was underway. An 850-ft.-long driveway embankment was constructed, ranging in height from 7 to 36 ft. and passing over areas of a peat deposit up to 8 ft. deep. Conditions of the construction site, situated on the north bank of the Eramosa River valley, required the embankment to conform to a steep, contoured landscape; tolerate differential settlements as great as 2 ft.; and support heavy vehicles during construction.
These requirements were fulfilled by Presto Products’ vegetated Geoweb (geocell) earth retention system. More than 12 years later, the vegetated geocell system continues to perform as expected. Referred to as the “flower pot” wall by residents, the open front cells of each geocell section form horizontal terraces where vegetation can flourish. The open cells capture rainwater while controlling groundwater evaporation, creating a natural environment for vegetation.The layered geocell earth retention system meets all structural requirements, provides construction flexibility, and is aesthetically pleasing with a completely vegetated face. The system meets site challenges, even when the subgrade is compressible, unstable soil.Phase II Slope Reconstruction In the summer of 1998, a new geocell earth retention system was installed at Kressview Springs condominium. The condominium corporation decided to construct a vegetated retaining wall 280 ft. long with a maximum height of 30 ft. in front of the building’s main entrance. This vegetated geocell wall system replaces a geosynthetic facing on a steep soil-nailed slope in need of rehabilitation.“Considering the successful performance of the existing Geoweb wall system over the past years, the system was the obvious choice for the reconstruction work,” says Mike Walsh of AGS Canada, Presto’s distributor.The Original Geosynthetic System Click here for larger view.The existing steep soil-nailed slope was originally constructed in 1988 prior to the geocell wall supporting the driveway embankment. The original 1:2 slope on the river-valley bank was excavated to a near vertical cut approximately 33 ft. high. The cut was made so the slope had three terraces 6.5 ft. wide to provide better visual appeal to the condominium’s residents.The slope material comprised dense glacial till, ranging from silty clay to silty fine sand. Lenses of more clayey soils were found throughout. These lenses acted as horizontal dams causing several water tables to flow to the face, thus providing a source of water for icing in the highly frost-susceptible silty soil.“A soil-nailed system with a geosynthetic facing was chosen to permit drainage throughout the face of the structure while allowing for potential soil movement,” explains Walsh.The system was constructed by first drilling and grouting 600 1-in.-diameter Dywidag soil nails into the face of the slope. The soil nails ranged in length up to 39 ft. and were installed in a series of horizontal rows to facilitate connection between the nails. The geosynthetic fascia was then applied and anchored using the soil nails.Installation of the New Geocell Wall System The reconstruction utilized the existing soil nails but replaced the geosynthetic fascia with the geocell. The geocell wall system, at 30 ft. high, permitted drainage throughout the face of the structure while accommodating potential soil movement.“The rehabilitation of the steep slope began with removing the geosynthetic fascia and all vegetation that had grown over the past 10 years, leaving a grid work of exposed soil nails,” describes Walsh.The contractor, Cambridge Landscaping Inc., prepared a level pad constructed from granular B, Type II, 1 ft. thick and compacted to 95% minimum Standard Proctor Dry Density (SPDD).Next, the geocell sections with a black textured fascia and internal perforated cells were placed and expanded on the granular pad.“The cell-wall perforations create good frictional interlock with the aggregate and allow lateral drainage through the system. Drainage is especially important given the seepage of groundwater at various elevations behind the wall,” explains Walsh.Each 8-ft.-wide x 2-ft.-long x 8-in.-deep geocell section was expanded and held open with stretcher bars. Adjacent sections were pneumatically stapled together to complete a wall course.Each course was checked and adjusted to achieve proper elevation, setback, and alignment. With the exception of the outer cells, the course was then overfilled with granular B material and compacted with hand-operated equipment. Once compacted to 95% SPDD, excess granular was raked away and the next course was installed. The outer row of cells was filled with topsoil to allow vegetation growth. This procedure was continued until the full wall height was achieved. Throughout the construction, geocell sections were anchored at soil nail locations and elevations.The geocell sections were attached to the soil nails using a polyester geogrid, galvanized steel pipe, and a special soil-nail head attachment. This process was achieved by first installing the soil-nail heads to the soil nails, then using the heads to support and hold the galvanized steel pipe in place. The polyester geogrid was wrapped around the steel pipe and placed over the filled geocell sections at the required elevations. Geocell sections were then placed over the grid. With the grid taut, the geocell sections were infilled, and the infill was compacted at the required elevations. Construction Challenges The finished wall with only front access forced the contractor to resort to a creative means of placing infill material within the higher wall courses.“The lower sections of Geoweb were filled with a typical front-end loader,” recalls John Verhoeven of Cambridge Landscaping. “As the wall height increased, a Loadall with telescoping boom was used to fill the midsections. When the height was beyond the reach of the telescoping boom, a 25-ton crane was brought in.”The crane lifted a large bottom-dump concrete bucket filled with granular infill material. The laborers, receiving the bucket at the top of the wall, dumped the granular into the geocell sections.The owners chose not to use conventional hydroseeding but accepted the contractor’s recommendation to place individual plants in each of the front cells. The selected ground cover will spread and eventually cover the entire face of the geocell wall. The terrace areas were subsequently planted with a variety of small shrubs.The vegetation has developed as expected and is gradually transforming this geocell wall into a mass of vegetated life. The owners of the condominium are quite pleased with their new view and look forward to its seasonally changing appearance in the future.“While hydroseeding provides initial ground cover, the grass grows dormant and results in a bare appearance,” concludes Verhoeven. “That is why we selected semi-evergreen that ensured continued and healthy plant growth.”Completed Project ScopeIn total, 5,000 ft._ of wall face was reconstructed in three wall lifts varying in height from 6 to 30 ft. More than 1,000 geocell wall sections and 12 rolls of geogrid were utilized for the wall structure.

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