SBCT Final EIS - Govsupport.us

Chapter 3 ⎯ Affected EnvironmentRock falls would be the most common mechanism of failure because the slopes contain relatively littlesoil cover.The northwest part of Oahu is within an area that has about a 10 percent probability of experiencingground accelerations of more than 10 percent of gravity due to earthquakes during the next 50 years(Klein et al. 2001). Liquefaction is the sudden loss of strength of saturated soil or sediment that resultsfrom increased pore pressure caused by vibration or seismic shaking. Loose sandy sedimentswith a high water table are particularly susceptible to liquefaction. The combination of loose beachand dune sands and a shallow water table present at DMR make liquefaction a potential hazard at theinstallation.3.1.1.3 Kahuku Training Area and Kawailoa Training AreaPhysiographyThe KTA ROI is on the northern side of the Koolau Mountains (including portions of KLOA), inlandof the Kamehameha Highway and does not extend to the shoreline of Oahu. Elevations range fromnear sea level to about 1,860 feet amsl. The topography varies from relatively flat on the coastalplains to nearly vertical bluffs on the cliffs to the east (USAG-HI 2004).GeologyKTA lies on the northernmost exposure of the Northwest Rift Zone of the Koolau Volcano (USAG-HI 2004). Most of the area is underlain by Koolau basalts that were deposited at the end of its eruptivecycle. The Northwest Rift Zone contains dense volcanic dike intrusions, most of which arealigned in the same direction as the rift zone, on a northwest trend. The dike orientation tends to parallelthe flow direction of streams and gulches in the northern part of KTA, but is perpendicular to thesurface drainage and erosion patterns on the east and west (USAG-HI 2004).SoilsKahuku Training Area. Almost the entire southern (upland) half of KTA is classified as Kapaa siltyclay at 40 to 100 percent slopes (USAG-HI 2004), which formed in weathered volcanic rock materialsand occurs on steep drainages, gulches, and ridgelines in mountainous areas with high rainfall.Kapaa silty clay soils are deep and well drained, with fine to moderately fine subsoil on gentle slopes;however, runoff is very rapid and erosion potential is very severe on steep slopes, where most of thesurface layer has been removed by erosion (USAG-HI 2004).Paumalu-Badland Complex soils are found in a broad band north of the Kapaa soils (USAG-HI2004). Paumalu soils, which make up about 40 to 80 percent of the acreage in this complex, exhibitmedium to rapid runoff and moderate to severe erosion potential. Badland, which consists of nearlybarren land that remains after the Paumalu soils are eroded away by wind or water, includes rockyand stony land with rapid runoff and very severe erosion potential.Kemoo-Badland Complex soils dominate the band to the north of Paumalu-Badland soils. Kemoosilty clay accounts for about 40 to 80 percent of the area covered by Kemoo-Badland Complex soils.Kemoo silty clay soils are well-drained red to dark reddish-brown blocky soils found on elevationsbetween 300 and 1,200 feet amsl, where the rainfall ranges from 35 to 60 inches. On steep slopes,runoff is medium to rapid, and the erosion hazard is moderate to severe. On gentle slopes (2 to6 percent), runoff is slow to medium and the erosion hazard is slight.February 2008 3–11 2/25th SBCT Final EIS