Residential Reconstruction

As part of Hawaii’s National Pollutant Discharge Elimination System (NPDES) Erosion Prone Area Improvements Program, the City and County of Honolulu was able to establish potential funding and to authorize a streambank stabilization project for a 330-foot (100 meter) stretch along the eastern bank (see Figure 1 A) in 2014. The project guidelines for the project focused on a softer, more natural reinforced vegetative system, in contrast to the concrete lining on the opposite side of the stream channel (see Figure 1 B).

Project Development
Initial geotechnical design for reconstructing and vegetating the streambank was based on a compost-sock structure with included geogrid wraps to provide a geosynthetic-reinforced “wall” ranging from 4 to 8 feet high. ParEn Inc., dba Park Engineering of Honolulu, HI, was responsible for civil design and developed the construction drawings and specifications. STRATA Inc. of Boise, ID, conducted the geotechnical fieldwork, soil testing, and geotechnical design of the wall.

Geotechnical Evaluationand Design
Due to limited equipment access to the project site, geotechnical investigation and sampling relied on hand tools (such as a post-hole digger) that allowed for excavating three test pits to a maximum depth of 3.7 feet (1.13 meters) to identify and describe subsurface soils. Laboratory testing included the following:

• Natural Moisture Content & Dry Density—ASTM D2216, ASTM D2937
• Particle-Size Analysis—AASHTO T-27 and ASTM D1140
• Atterberg Limits (liquid limit and plasticity)—ASTM D4318
• Classification of Soils for Engineering Purposes—ASTM D2487
• Direct Shear Strength of Soil—ASTM D3080
• Laboratory Compaction Characteristics of Soil (Maximum Dry Density and
• Optimum Moisture Content)—ASTM D1557

The soil was classified as Silty Sand (according to the Unified Soil Classification System), having moderate shear strength with an estimated cohesion of 460 pounds per square foot (psf) and an angle of internal friction of 31 degrees. Compaction testing indicated an estimated maximum dry density of 88.5 pounds per cubic foot (pcf) at optimum moisture content of 31.5%.

For geosynthetic reinforced-soil wall design, the focus is on evaluating engineering stability against the following potential failure modes (Koener, 1998, Berg et al., 2009):

1. base sliding
2. overturning (i.e., rotational tipping of the wall)
3. exceeding foundation bearing capacity
4. internal failure of the geogrid (i.e., rupture or pull-out)
5. large-scale global slope failure

All of these potential modes were checked and evaluated using accepted geotechnical stability analyses, which showed that standard design criteria were met for a wall system 5 to 8 feet high, with 4-foot wrap embedments of PYRAMAT-75, and with an overall face angle of 1H:3V (71 degrees) which included a mid-height step to be used as a planting bench.

“Site accessibility, which was through a residential side yard, was a big concern for this project,” says Steven Harano, P.E., project manager with ParEn Inc. “PYRAWALL was ideal in that the material could be hand carried and rolled out at the project site. The tiered landscape concept allowed for incremental levels to receive vegetative ground cover, which provided a more ideal and natural look to the streambank. Another desirable aspect was the high life-expectancy of the material.”

Site Grading and Subgrade Preparation: Because this geosynthetic wall system must be founded directly on native soil or approved structural fill, subgrade preparation involved vegetation removal and grubbing, followed by using a mini-excavator with smooth-blade bucket to remove topsoil and organic detritus in order to reach native, undisturbed soil (Figure 4) and provide a smooth platform to place the initial wrap-face lift. Line and grade for the wall base were surveyed and marked with paint or string-line.

Wrap-Wall Construction: The HPTRM roll width of 8.5 feet (2.6 meters) allows for a bottom width of 4 feet, a lift height of 1 foot, and a top-flap layback width of 3.5 feet. After the internal braces were installed at a regular 24-inch spacing, the long section of HPTRM was placed on the level pad, the front edge properly aligned, then the fabric stretched taut in both directions and pinned down (Figure 6). Backfill soil then was placed and spread, then compacted in two 6-inch lifts (Figure 7) to fill the wrap-lift to the brace height.

Because the HPTRM wrap-face system is flexible, the curve required by the wall alignment was achieved easily for each successive lift by cutting the horizontal HPTRM sections perpendicular to the wall face, and then overlapping the cut fabric accordingly to match the curve shape.

During the construction period of July to October 2018, the site was affected by minor flooding due to Hurricane Lane in late August. The high flow in Kaneohe Stream reached the base of the wall pad but did not cause any damage to the partially constructed wrap-face wall. However, the heavy rainfall and wet site conditions caused a work delay of about two weeks while waiting for the soil to dry out and allow proper compaction again.

Vegetation Planting: Typically, during infilling of each wrap-lift, grass seeds or stolons can be added to the soil directly behind the HPTRM at the face. If desired, additional seeding can be achieved post-construction by hydroseeding the completed wall. However, for this project the design specified planting plug-stock through small cuts in the woven fabric (not prone to raveling), which included vetiver grass along the wall base, hopseed sprawling shrub on the planting bench, and white leadwort ground cover at the top (Figure 10).

If needed, temporary irrigation can be used to help new plantings get established. For shoreline applications, lower wrap-lifts planned to be below average water level may be filled with coarse granular backfill (to allow free movement of water), then overlying lifts transition to finer-grained infill soil above the waterline to support wetland vegetation. This wall system also is ideal for saltwater applications using brackish-tolerant plant species, because the geosynthetic components are chemically inert and do not corrode.

By using internal seeding and post-seeding via hydroseeding, excellent grass cover can be established quickly even in northern climates, as shown by a similar wall constructed near St. Paul, MN, in 2019 (Figure 11).