Rock Slope Stabilization Measures at the Pali Tunnel Route 30, Maui, Hawai'i

摘要

On October 15, 2006, two earthquakes with magnitudes of 6.7 and 6.0 struck in close succession just off the northwest coast of the Island of Hawai'i. Damage caused by the earthquakes has exceeded $100 million; however no deaths and only minor injuries were reported. Numerous rockfalls and landslides occurred in road cuts, embankments and natural slopes. Because of the lack of redundancy in the highway systems of the islands, road closures due to rockfalls or landslides can have debilitating effects on the transportation systems. Most rockfall and landslide events occurred on the Big Island, and on the southeast coast of Maui.The Hawai'i Department of Transportation (HIDOT) is implementing a program of rock slope stabilization throughout the islands, including placement of rockfall drapes on the steep rock slopes along Route 30 on the south side of the western end of Maui (which did not suffer damage from the earthquakes). HIDOT implemented these programs long before the October 15, 2006 earthquakes. As part of on-going slope stabilization, the department elected to stabilize the rock slopes above the portals to the Pali Tunnel, located at Milepost 10.4 on Route 30. This two-lane paved roadway can have ADT counts up to 60,000 vehicles per day, as the roadway provides the most direct route for tourists from the Kahului Airport to the resorts in Lahaina, located on the western side of Maui. The roadway also serves as the primary route for emergency and commercial vehicles serving the resort areas. Therefore, HIDOT limited traffic stoppages for rock slope mitigation construction to 20 minutes, and these could only occur during the night.rnDifferential weathering typical of young volcanic rocks, coupled with precipitation events and ground motions induced by earthquakes, are the principal causes of rockfalls in Hawai'i. The bedrock at the tunnel consists of thin to medium bedded Wailuku basalt flows, separated by thin to medium bedded, discontinuous beds of clinker and scoria, and thin beds of lateritic soils. Initial construction of the Pali Tunnel in 1951 did not include stabilization of the slopes above the portals, as these were fresh cuts at the time of construction. Weathering has diminished the stability of the slopes above both portals, which required light scaling prior to placement of rockfall drapes. However, the slopes above the west portal rise thousands of feet to the north, and contain large, loose boulders on the verge of toppling and rolling onto the highway. To reduce maintenance associated with preventing rockfalls from reaching the roadway, HIDOT selected an innovative, commercially-developed hybrid rockfall barrier/drape to contain rolling rocks from high on the slope. The hybrid contains elements of a 2,000 kJ rockfall barrier, with ring nets extended as a drape below the barrier. To protect traffic while construction of the hybrid was underway, a lower 1,000 kJ rockfall barrier was constructed on a lower portion of the slope. Hybrid systems have recently been implemented in Washington, California, New Jersey and Colorado, and are undergoing field testing in Italy.