Above: 24-in.-long ATRA anchors were placed in the four corners of the Geoweb sections to establish the square. Anchors were installed every other cell on the makai (ocean) side to provide additional anchorage from potential high surf.

The 1,000-ft detour roadway was constructed from both ends. Surf rising as high as 20ft. infringed on the road's edge during construction.

The Kamehameha Highway at Waimea Bay along Oahu's North Shore connecting Pupukea to Haleiwa was closed earlier this year on March 6 by a rockslide that caused several tons of rock to slide and fall on the highway, hitting and damaging two vehicles. The slide spread debris over a two-lane highway and caused an 18-in. deep x 3-ft.-diameter hole in the roadway. According to state geologist's assessments, the condition of the slope above the roadway was deemed unstable and unsafe for motorists to pass below and therefore was closed indefinitely. In the meantime, residents were forced to make the long drive around the Koolau Mountains.

Further investigations of the slide site revealed other potential problems. The 100-ft.-high cliff is composed of unstable, badly weathered rock that hangs over the roadway. In addition, the cliff is covered with exposed Koolau, the oldest rock above sea level that continues to break off. Physically removing the 2.5-million-year-old rock was one option to prevent a possible landslide. Not only is the brittleness a factor to consider, but when the soil becomes saturated with heavy rains, it pushes the rock forward.

The Governor of Hawaii, Ben Cayetano, declared a state of emergency in order to facilitate the process, bypassing the usual time-consuming construction procedures. State officials said it could be three to four months before the slope is fully repaired and the road made safe for traffic. During the closure of the only highway on the North Shore, traffic was rerouted from Waimea Bay northward via Highway 3 to the Windward Coast, a venture that didn't bode well with area residents, businesses, and tourists. The effects of the highway's long-term closure and loss of tourism revenue could cause an "economic catastrophe" according to some local businesses.

After further evaluation, State Department of Transportation (DOT) Director Kazu Hayashida reported a considerable amount of loose basalt rock and met with the primary contractor, Kiewit Pacific, to discuss the safest and most expedient method to remove the unstable rock. A preliminary plan includes using drills and demolition charges or water blasting to cut back the section of rock, removing the outer unstable 20 ft. of rock overhang.

"We cannot just use drills because of the limited access to the rocks," states Hayashida. "We will detonate small charges at some point, but they will not be strong enough to disturb the nearby church, heiau [religious site or temple], and homes. We're not trying to bring down the whole mountain. What we want to do is gradually cut away at the rocks until we get fresh rock that we know won't come tumbling down."

Native Hawaiians are opposed to any plan to blast the rock face, as it would harm the Pu'u o Mahuka Heiau and a sacred burial site above the highway. Within this cave area, bones were buried and sealed for preservation. Hawaiians believe their ancestors continue to give power to the heiau.

After much study, the option selected for a long-term repair included moving the shoulder of Kamehameha Highway at the rockslide site some 20-30 ft. away from the cliffside. Ironically, the DOT recognized the site as unstable for over 10 years but opted not to repair because of budget constraints.

Solution for the Waimea Bay Emergency Bypass Road

DOT officials, Kiewit Pacific, the Army Corp of Engineers, and Structural Systems Inc. met immediately to determine construction of a bypass road. After surveying the proposed area and its proximity to Waimea Bay Beach Park, the design team recommended that the road meet four important requirements: (1) withstand the North Shore's high surf conditions and the rainy season, when 30 in. of rain per week is not uncommon, (2) have minimum impact on the environment, allowing the site to be easily returned to its original natural state once the highway reopens, (3) support heavy vehicles over the soft sand subgrade, and (4) be cost-effective.

Following the team's fact-finding mission, a design by the US Army Corps of Engineers and DOT was accepted that created a temporary two-lane roadway approximately 24 ft. wide x 1,000 ft. long. The construction materials included geotextile fabric and Presto Products' (Appleton, WI) Geoweb cellular confinement system filled with native beach sand and surfaced with a native crushed coral.

"Based on past challenges, we've experienced tremendous success with the Geoweb cellular confinement system, which has a proven track record for its reliability over a wide range of applications," remarks Joe Enright, president of Structural Systems Inc., a Honolulu-based general engineering contractor. "Turnaround time was key to this project, therefore accessibility to a rapid load support system and onsite engineering personnel were reasons why the Geoweb system was selected."

Cellular confinement was developed as a means of constructing rubber-tired military-vehicle access roads over beaches. The three-dimensional polyethylene, honeycomb-like structure confines and strengthens cohesionless materials in its cells, preventing shear failure and lateral movement of the infill material. The system produces a stiff base with high flexural strength, acting like a semirigid slab by distributing loads laterally and reducing subgrade contact pressures. The flexible engineered system can be designed to handle loads as great as 40 tons.

Installation Process

Within 30 hours of the design team's March 12 job-site meeting, the Geoweb material was air-shipped from Wisconsin to Waimea Beach in order to expedite installation. Forty-three pallets were delivered in collapsed form, which were easily expanded to their full width and length (8 x 20 ft.) and secured with Presto's ATRA anchors prior to infilling.

"This is a classic example of job-site partnering whereby various government agencies work with local businesses to design and build this type of engineering solution," notes Enright.

In the initial stages of construction, DOT personnel readied the beach for road construction by clearing trees and building a ramp at one end of Waimea Bay. In efforts to remain sensitive to local cultural and political concerns, Kahu Samuel Safrey, a Hawaiian priest, performed a ground blessing at the site.

The bypass road was constructed by placing a layer of geotextile directly onto the sand. The first course of 8-in.-deep Geoweb sections was placed and infilled with the surrounding beach sand and compacted. A second course was placed, repeating the same procedure. The top 3 in. consisted of imported crushed coral placed on top of the two Geoweb courses, saturated and compacted. The 8- x 20-ft. sections of Geoweb were placed three across to create a 24-ft.-wide roadway some 1,000 ft. in length. Water-filled barriers were placed on both sides of the roadway to help delineate the two traffic lanes, protect the cells nearest the edges, and prevent cars from going off the secure road into the sand.

Late winter storms creating flooding and surfs as high as 20 ft. posed a threat to the integrity and foundation of the bypass road. The anchors - plastic clips attached to 24-in.-long rebar - provided additional anchorage to the Geoweb sections. The effectiveness of the anchors was tested a few days later. High surfs ripped out the center portion of an adjacent walkway, washing it out to sea. Although the waves also emptied sand from some of the roadway cells, the anchors prevented the road sections from completely pulling out.

The bypass road was completed March 18 at 6:30 a.m. and has been open to traffic since.

Currently traffic is limited to a 5-mph speed limit and a 4-ton weight limit. Met with initial skepticism and opposition, the Waimea Bay Emergency Bypass Road is being accepted by residents and businesses and functioning as planned.

Sources: The Honolulu Advertiser, Honolulu Star-Bulletin Hawaii News, and enews Hawaii