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A geosynthetic wrap-face vegetated wall system

October 1st, 2020 / By: / Case Study, Feature

For highway embankment construction 

By Stanley M. Miller and Drew Loizeaux

A recent creek enhancement project to facilitate the passage of bull trout in Pend Oreille County of northeastern Washington state required removal of a 6-foot (1.8-m) diameter concrete culvert (Figure 1), realignment of a well-traveled county road and a new 58-foot (17.7-m) span bridge. The new alignment would cross a low-lying area adjacent to the Pend Oreille River, which is subject to seasonal flooding in late spring due to water releases from an upstream dam. Limited right-of-way access provided a relatively narrow footprint for the planned earth-fill embankment approaches needed for the new bridge. Thus, they would require steep side slopes or near-vertical retaining wall structures, such as a concrete-panel mechanically stabilized earth (MSE).

FIGURE 1 Concrete culvert to be removed

Due to geotechnical concerns over differential settlements expected with soft foundation soils, and because project stakeholders and the Pend Oreille County Public Works Department preferred a vegetated-face alternative for the embankment slopes, a geosynthetic wrap-face vegetated (GWFV) wall system was selected (Figure 2), known as PYRAWALL. This reinforced-soil system uses 1-foot (0.3-m) high uniform wrap lifts and relies on three components (Miller 2017): 1) a polypropylene high-performance turf reinforcement mat (HPTRM) to form the wrap-face lifts and secondary soil reinforcement; 2) internal fiber-composite braces that hold up the face and allow filling and compaction of the soil lifts; and 3) geogrid layers that extend back into the embankment for primary soil reinforcement.

FIGURE 2 Partially completed GWFV wall along west embankment approach

Site conditions

To accommodate the new bridge, realignment of the road would require a west approach embankment 340-feet (104-m) long with a maximum height of 18 feet (5.5 m), and an east approach embankment 210-feet (64-m) long with a maximum height of 13 feet (4 m). Public and private borrow sources within 3 miles (4.8 km) of the project site contained approved granular, structural fill, which was classified as Well-Graded Gravel with Silt and Sand (GW-GM, according to the Unified Soil Classification System), with less than 8 percent passing the No. 200 sieve. A private borrow source for topsoil also was located nearby.

Soil borings completed along the proposed realignment showed mixed alluvium soil with generally high groundwater levels; i.e., within 8 feet (2.4 m) of the ground surface. Subsurface soils included silt, silty sand, silty gravel and poorly graded sand with silt or gravel, with some zones (often between 10 and 20 feet [3 and 6 m] below ground surface) having very low standard penetration test (SPT) blow counts in the N=2 to 5 range. Higher blow count materials (N=20 to 50) were encountered at depths from 30 to 50 feet (9.1 to 15.2 m).

Geotechnical considerations

Where the two approach embankments terminate at the bridge abutments, steel sheet-pile walls were designed to provide a bulkhead for the embankment fill and to provide long-term erosion and scour protection along the creek bank on either side. In addition, micropile foundations were designed for both bridge abutments to provide adequate bearing capacity for the single-span bridge. The micropiles (hollow-bar, grout-injected Grade 70 steel) and concrete pile caps were installed behind the sheet-pile walls on either side of the creek. Thus, the GWFV walls on the lateral sides of the embankments would need to be curved into and behind the sheet-pile walls to provide a competent termination of the geosynthetic walls at the bridge abutments. This would be relatively easy to accomplish because the HPTRM can be cut and then pulled and overlapped to form curves with tight radii (Miller 2016).

GWFV wall design 

Any geosynthetic-reinforced soil mass must resist the lateral earth pressure of the retained soil tending to deform and slide downward and outward due to the pull of gravity (Rimoldi 2016). It also must resist the lateral load contribution from surcharge loading in the backslope area, such as permanent loads (due to surface structures) and temporary loads (due to construction equipment or vehicular traffic).

External wall stability analysis was conducted for the tallest proposed GWFV wall section using both allowable stress design (ASD) and load and resistance factor design (LRFD) methods to evaluated base sliding, overturning (eccentricity) and bearing capacity. Calculated results are summarized in Table 1 for the tallest wall section with an 11-foot (3.4-m) long geogrid reinforcement zone.

TABLE 1 Stability analysis results for tallest GWFV wall section (18 feet [5.5 m]).

Internal wall stability analysis focused on rupture potential for the geogrid primary reinforcement, with 1-foot (0.3-m) vertical spacing for the lower lifts, then 2 feet (0.6 m) elsewhere. Wall calculations showed the minimum required long-term design strength (LTDS) to be approximately 1,300 pounds/foot (19 kN/m). However, subsequent global stability analysis for large-scale rotational failure paths (Abramson et al. 2002) indicated that stronger geogrid layers would be needed in the lower portion of the embankment to provide adequate resistance to potential “compound” failure paths that could pass through the reinforced zone. The final design specified that the lower geogrids have a minimum required LTDS=3,600 pounds/foot (52.5 kN/m) and the remaining geogrids have a minimum required LTDS=1,800 pounds/foot (26.2 kN/m).

The above analysis results were based on static-loading conditions. Stability calculations for dynamic (seismic) conditions also were conducted by pseudo-static methods, using a locally estimated horizontal seismic coefficient of kh=0.09 (refer to Abramson et al. 2002, p. 354 and 394), and those analysis results also met accepted design criteria.

A generalized schematic showing various components of the GWFV wall designed for the bridge-approach embankments is presented in Figure 3.

FIGURE 3 Schematic of GWFV wall for highest section of the west bridge-approach embankment (maximum length of geogrids is 11 feet [3.4 m]).

Overview of GWFV wall system

The rolled HPTRM used for the face wraps is 8.5-feet (2.6-m) wide by 120-feet (36.5-m) long. As illustrated in Figures 1 and 3, this width provides a typical wrap layer with 4 feet (1.2 m) along the bottom, a 1-foot (0.3-m) near-vertical face, and 3.5 feet (1.1 m) folded back over the soil infill lift. The geosynthetic provides excellent erosion protection as vegetation seeded in the infill soil (“internal seeding”) emerges and becomes established. The open weave of the fabric also is conducive to vegetation being established by hydroseeding (“external seeding”) after the wrap-face structure has been completed.

The unique lofted, three-dimensional feature of this HPTRM allows for the insertion (weaving) of bracing-bar components, which stand up the 1-foot (0.3-m) high section of the fabric, forming a face against which infill soil is placed and compacted. These bracing bars are fabricated using a high-strength fiber-composite material consisting of nylon and fiberglass.

Primary geogrid reinforcement is added to the system by inserting (“sandwiching”) the geogrids between successive wrap lifts, applying a thin soil layer to inhibit any fabric-to-fabric contact. Wood stakes or metal pins can be used to stretch both the HPTRM and the geogrid taut and hold them in place while soil backfilling occurs. A construction photograph presented in the next section (Figure 5) shows both types of fabric and a typical granular soil used to coat the inserted geogrids.

Embankment construction

Initial soil stripping and grubbing began at the site in April 2018. The footprint areas for the two embankments were cleared down to native, undisturbed soil using dozers or smooth-bucket trackhoes with laser-leveling controls. Some localized areas did not respond well to proof rolling and compaction; i.e., the soil moisture content was high enough that the soil deformed beneath the compaction equipment by pumping rather than compacting to a tight, solid condition.

Subgrade preparation

Those areas with pumping subgrade were overexcavated by about 2 feet (0.6 m), and the wet soil was removed and wasted. A heavy-duty, nonwoven separation fabric was then placed on the ground and a 2-foot (0.6-m) layer of well-graded rock (quarry spalls) was placed and compacted with five to six passes of a vibratory roller to a tight, unyielding condition. Next the fabric was wrapped back over the top of the rock fill to encapsulate it and provide a top layer of separation fabric in preparation for laying a leveling course of approved structural fill. This subgrade enhancement treatment is shown in Figure 4.

FIGURE 4 Subgrade enhancement using nonwoven separation fabric to encapsulate a 2-foot (0.6-m) thick layer of compacted quarry spalls; granular leveling layer on top

GWFV wall installation 

Once the subgrade was compacted and leveled to the proper grade, the toe-line of the wall was surveyed and marked with paint lines along the ground. Then a base geogrid layer was laid out, with a length prescribed according to the wall design specifications. If necessary to keep the geogrid flat, stakes were driven through it after stretching it taut in its position. A thin layer of structural fill soil was spread across the geogrid, and the first HPTRM wrap was moved into position with the face braces already installed (Figure 5). The face of the lift was positioned along the painted toe-line, then staked in position along the front and back of the base layer, using a stake about every 6 to 8 feet (1.8 to 2.4 m) along the fabric length. The local area around braces was backfilled first to hold them down and to keep them vertical, followed by filling across the entire width of the wrap. Compaction of the first 6-inch (15.2-cm) soil lift was accomplished using a walk-behind plate compactor near the face and a large vibratory roller at distances farther than 2.5 feet (0.8 m) from the face.

FIGURE 5 Initial placement of geogrid and first lift of GWFV wall

A second lift of soil was then added to fill up the remaining wrap height and was likewise compacted; topsoil was used directly behind the face fabric (about 0.8-foot [0.25-m] thick), with structural fill comprising all the remaining backfill. A prescribed grass-seed mix was scattered along the edge of the topsoil at the face; then the upper flap of the HPTRM wrap lift was pulled back over the soil infill and stretched taut and staked. This completed the initial wrap lift along the base of the wall, as shown by the example in Figure 6.

FIGURE 6 Completed first lift (wrapped layer) of GWFV wall

The toe-line of the next lift was surveyed (located using a high-resolution GPS) and again marked with paint, this time along the top of the completed wrap lift. In regard to maintaining elevation grade, the slight bulging in the face wrap resulted in the actual lift height being slightly less than 1 foot (0.3 m), meaning that the thin layer of soil spread across the wrap top (and, thus, on either side of the inserted geogrid) made up that difference to help stay on grade as additional lifts were added. The second lift was completed in similar fashion to the first, and the entire process repeated to build the wall higher (Figure 1).

It is important to note that wall construction was facilitated by beginning at the lowest elevation portions of the wall base, then progressing upward as grade changes dictated. This means that foundation steps were required along the base of the wall for each of the bridge-approach embankments. Care was taken to make sure these step heights corresponded directly to the height of wrap lifts; that is, 1 foot (0.3 m).

High-water inundation

Project scheduling called for partial completion of the embankments prior to the planned June release of spring runoff water upstream on the Pend Oreille River. This temporary high-water period extended for several weeks and inundated the lower 4 to 5 feet (1.2 to 1.5 m) of the GWFV walls on the south faces of the embankments, potentially saturating portions of the embankment toes (Figure 7). Due to concerns that face saturation, followed by receding waters, could pull soil infill out of the wrap lifts, the project team decided to apply an environmentally approved tackifier (binder) to those lower wrap lifts expected to be submerged.

FIGURE 7 Toe of GWFV wall submerged temporarily due to rising water levels of the Pend Oreille River

During this high-water period, work proceeded on the bridge abutments by starting installation of the micropiles and pile caps directly behind the previously installed sheet-pile walls. This work extended until completion in early August. The receding floodwaters caused no noticeable damage to the GWFV walls. The second phase of embankment construction commenced in mid-August.

GWFV wall tie-in at sheet-pile walls

To tie in the GWFV walls to the structural walls at the bridge abutments (i.e., steel sheet-pile walls and concrete pile caps), the HPTRM lifts were curved into and behind the sheet piles up to the elevation at the top of the piles (Figure 8). Wrap lifts higher than that were curved into the concrete pile cap. The curves were accomplished by cutting the bottom and top wraps perpendicular to the face, then pulling the HPTRM across itself (overlap) to form the desired curves, with internal braces spaced as needed to retain the shape.

FIGURE 8 GWFV wall tie-in at west bridge abutment

Vegetating the GWFV walls

In addition to the grass-seed mixture applied internally to the wrap lifts, project specifications also called for planting selected shrubs or vines during the fall of 2018. These rootstock plants were inserted through slits cut into the ministeps along the wrap-lift edges. Examples of the plant species included wood’s rose, honeysuckle, snowberry and wild clematis. The grass mix contained primarily rhizomatous species, which reproduce and spread through runners just beneath the ground surface (Figure 9).

FIGURE 9 Completed west bridge embankment in September 2018

The project area, located north of Newport, Wash., receives an average annual precipitation of 28 inches (711 mm). The selected plant species are well-adapted to the area and should have a good survival rate once they are established. The county considered the possibility of using irrigation water if precipitation was scant during the fall and early winter, but this was not necessary.

Conclusion

A GWFV wall system was selected to construct bridge-approach embankments for a culvert-elimination fisheries project in Pend Oreille County in northeastern Washington state. Suitable structural fill soil and topsoil were located at nearby borrow sources, and large construction equipment (such as dozers, trackhoes, large side-dump trucks and vibratory roller compactors) could be used effectively due to ready access from the existing roadway. Compaction control was monitored regularly throughout embankment construction using a nuclear moisture-density gauge per project specifications.

After the construction crew had been trained and gained some experience with the wall system, installation rates typically ranged from 25 to 40 square feet (2.3 to 3.7 m2) of wall face per hour. For project sites with more difficult access or limited to smaller construction equipment, the installation rate likely could be reduced by 50 to 75 percent. Figure 10 shows the completed west embankment with excellent vegetation cover 22 months after wall completion.

FIGURE 10 Fully vegetated west bridge embankment in July 2020

Stanley M. Miller, Ph.D., P.E., is professor emeritus in the Department of Civil Engineering at the University of Idaho and a consulting engineer based in Moscow, Idaho.

Drew Loizeaux, M.S., P.E., is an engineering specialist in Propex GeoSolutions for Propex Operating Company LLC, in Chattanooga, Tenn.

All figures courtesy of the authors.

References

Abramson, L. E., Lee, T. S., Sharma, S., and Boyce, G. M. (2002). Slope stability and stabilization methods, 2nd ed, John Wiley & Sons, New York. 

Miller, S. M. (2016). “Combining geosynthetics to construct steep vegetated slopes and walls,” Land and Water, 60(6), 31–38.

Miller, S. M. (2017). “Internally braced, geosynthetic wrap-face vegetated walls and reinforced soil slopes,” Geosynthetics, 35(5), 24–33.

Rimoldi, P. (2016). “Geotextiles used in reinforcing walls, berms, and slopes,” chap. 15 in Geotextiles: From design to applications, ed. by R. M. Koerner, Elsevier, Amsterdam, The Netherlands.  


SIDEBAR: Project Highlights

A geosynthetic wrap-face vegetated wall system

OWNER: Pend Oreille County Public Works (Newport, Wash.)

LOCATION: Indian Creek Bridge on LeClerc Road South, Pend Oreille County

GENERAL CONTRACTOR: N.A. Degerstrom Inc. (Spokane, Wash.) 

DESIGN ENGINEER: Nicholls Kovich Engineering, PLLC; and STRATA Inc. (Spokane, Wash.) 

GEOSYNTHETICS PRODUCT: PYRAWALL

GEOSYNTHETICS MANUFACTURER: Propex Operating Company LLC