SBCT Final EIS - Govsupport.us

Chapter 5 – Environmental Consequencescombination of steep slopes, easily erodible soils, and the damage or modification to land cover orsurface drainage that would occur due to use of the roads by SBCT vehicles during maneuver trainingcould increase the potential for slope failure. Concentrated use of the roads by the Stryker vehiclescould load weakly supported slopes and accelerate potential for slope failure in these areas.Stryker vehicles using the Helemano Trail to transport Soldiers during training activities at the SBMRBAX range would increase the potential for slope failure adjacent to the road. As described inChapter 3, this road has segments that traverse highly erosive soils or steep slopes.Use of the Dillingham Trail during maneuver training activities under in DMR would also increasethe potential for slope failure adjacent to the road. As described in Chapter 3, this road has segmentsthat traverse highly erosive soils or steep slopes. Most of Dillingham Trail would follow existingroads and would be on relatively gentle stable slopes. Parts of the proposed route would approach therim of the gulches of Poamoho Stream and Kaukonahua Stream. The route could cross areas ofunstable slopes, or construction of new roadways or modification of the existing roads could reduceslope stability through creation of new cuts and fills or drainage problems.Over the long term, use of Dillingham Trail by heavy vehicles may destabilize slopes in areasunderlain by soft saturated soils. In addition, vibrations caused by heavy vehicle use may inducefailure of unstable slopes, or loading on unstable steep slopes may induce failure of the roadway.Repair of failed slopes could require additional cutting, filling, or shoring, with the potential to alternatural land contours and drainage patterns further. Landslides themselves may become the locus offuture slides because the failed soil may be poorly drained. Some of the clay soils on the coastal plainnear DMR are not considered highly suitable for road fills and are subject to shrinking and swellingor soil creep (slow downslope movement in soils with low strength).Use of Drum Road during maneuver training activities in KTA could result in slope failures due tovibration or loading, but the proposed improvements to the road are expected to reduce these impactscompared to current conditions. Although there are many steep slopes within PTA and the KeamukuParcel, most slopes are underlain by shallow bedrock or exposed rock outcrops, so there is littlepotential for slope failure.Regulatory and Administrative Mitigation 6: Overall, the increased potential for slope failureresulting from use of roads and trails for SBCT maneuver training is considered potentiallysignificant because slope failure could alter the landscape, obstruct stream channels, interrupt use ofthe road, and create safety problems for personnel. Regular monitoring and early maintenance of theroadways and adjacent slopes, as mandated in the USAG-HI ITAM annual work plan, as well asproposed improvements to existing roads, however, would reduce the impact to less than significant.Less Than Significant ImpactsExposure to Soil Contaminants. Exposure to chemical contaminants in soils at maneuver trainingareas could occur through several pathways, including direct contact with contaminated soils,ingestion of soils, or through inhalation of windblown dust. Exposure estimates are based onassumptions about the amount of soil that might be ingested by a person who works in an area withcontaminated soils. It is a generally accepted principle of risk assessment that not all exposures resultin unacceptable health risks and that there are certain thresholds of exposure below which the healthrisks are so low that they cannot be distinguished from background risks. Results from soilsinvestigations conducted by the USAG-HI at SBMR and PTA in 2002 were discussed in Chapter 3and are summarized below as they relate to the potential for exposure during maneuver trainingactivities at the proposed BAX ranges and other maneuver areas.February 2008 5-14 2/25th SBCT Final EIS