GEOWEB Retaining Wall

Project Name: Hawaiian Landslide Repair

Project Location: Wheeler Army Airfield, Oahu, Hawaii

Landslides Damage Critical Infrastructure at Army Airfield in Hawaii

During a 40-day rainstorm, six landslides occurred on military service roads at the Wheeler Army Airfield along the side of a ravine. Surface water runoff on the roads triggered embankment failures in clayey silt fill soils and saprolite.

Debris avalanches developed along one of the roadways, eroding the slope and roadway base, leading to cracking and material loss of the asphalt road. The debris flows created vertical scraps and deep gullies draining into the main ravine below the road. Exploratory testing of the site revealed that the soils were primarily pahoehoe lava flows, which although weathered, are known for their high shear strengths.

Landfill Repair: Product Evaluations & Selection

When the US Army Corps of Engineers (USACE) advertised the design-build project, their conceptual designs considered only cast-in-place concrete walls on micropiles, a shotcrete wall with concrete drive slab, and drilled shafts plus steel H-beams and pre-cast concrete panels. However, these options were deemed unfavorable due to high costs, challenging site access for construction equipment, and excessive temporary excavation needs. Additionally, the highly humid tropical environment made adverse conditions for some of the potential options.

The winning design-build team, Hals Hays Construction and AMEC Environmental & Infrastructure determined that pile structures were too expensive and difficult to construct safely near the edge of the landslide scarps. They ultimately selected the GEOWEB® Retaining Wall System due to its design advantages, ease of construction, lower lifetime costs, and immediate availability, even in remote locations.

The GEOWEB Green Wall: A Flexible & Sustainable Alternative

The GEOWEB system was selected as the MSE wall facing due to the system’s flexibility during installation, tolerance to settlement, and availability on the island of Oahu. Additionally, the green geocell facing blended with the lush tropical vegetation of the ravine.

During installation, the GEOWEB panels were stacked vertically, with the geogrid reinforcement placed every four rows. Both the GEOWEB cells and the backfill areas over the reinforcement were infilled with crushed and screened gravelly sand—local on-site material. The on-site fill was used for cost reduction purposes, as there was no need to transport engineered stone to these remote locations. The gravel sand was the ideal fill material to use because it is free-draining, which help minimize the risk of pore pressure buildup behind the wall face.

The outer edge of the GEOWEB wall sections were infilled with topsoil and seeded to allow for vegetation growth and seamlessly blends in with the tropical environments in Hawaii.

A Value Engineered Solution to Landslide Repairs

This project demonstrates the advantages of using geosynthetic materials and showcases the effectiveness of the design-build contracting process. AMEC designed the GEOWEB system to repair the landslides using safer, lighter, greener, and less expensive materials than the original concept of pile supported structural walls.

The engineer’s cost estimate for the design-build RFP was approximately $5 million, based on the conceptual steel pile and concrete retaining walls. The selected design-build team completed the project for approximately $2.9 million by constructing GEOWEB geocell-faced walls with geogrid reinforcing and drainage improvements instead of the conceptual walls presented in the RFP. Excavation costs were about $970 per square meter of wall face, and the wall construction with imported gravel backfill was $3,200 per square meter of wall face.

This value engineered (VE) project saved $1.5 million (around 50% of what the initial solutions were estimating) by constructing MSE walls with the GEOWEB cellular confinement facing and geogrid reinforcement, instead of structural walls with vertical piles.

GEOWEB Walls: A Winning Solution

The GEOWEB geocell system offered several advantages that made it an ideal solution for stabilizing the landslide sites:

• Lightweight and Transportable: The materials were easy to transport, carry, and position, requiring only small, lightweight equipment for construction.
• Flexibility and Settlement Tolerance: Geosynthetic materials allowed for flexible wall layouts and tolerance to differential settlement in challenging terrain.
• Non-Corrosive Properties: Geosynthetics were non-corrosive, an important feature for the tropical environment.
• Aesthetic Integration: The green GEOWEB geocell fascia blended seamlessly with the surrounding tropical vegetation.
• Cost-Effectiveness: The materials and construction methods provided a cost-effective solution for stabilizing the failed slopes.