RES 16819 - Laserfiche WebLink

Significant Impact Impact 4.4-3: Some residences would be constructed near slopes with colluvial materials. These slopes, and slopes constructed as part of the project, could become unstable and potentially result in on- or off-site damage. Finding Changes or alterations have been required in, or incorporated into, the project which mitigate or avoid the significant effects on the environment. Facts in Support of Finding The City of Pleasanton has adopted the following measures which will reduce potential impacts related to slopes with colluvial deposits to a less-than-significant level: Mitigation Measure 4.4-3a: Debris catchment walls shall be constructed along the rear sides of the building pads on Lots 14, 15, and 22 through 25 where they border adjacent slopes. The walls shall be a minimum of 5 feet high and designed in accordance with the recommendations of the geotechnical exploration in regards to retaining walls. Walls may not be required if pad grading provides sufficient space via rear yard slopes or retaining walls. The final design of the walls or catchment area shall be reviewed by ENGEO when the detailed project plans are prepared. Proper access to the catchment areas shall be provided and all debris catchment walls or designated catchment areas shall be periodically maintained by removing and disposing of accumulated slope detritus. Mitigation Measure 4.4-3b: Debris catchment measures shall be constructed along Lots 5 and 25 which are located at the mouths of existing drainage courses. The catchment measures shall include a minimum 5-foot berm placed across the mouth of the drainage with clog-resistant surface drainage inlets provided. The final dimensions, placement, and design shall be determined in the detailed project plans. Mitigation Measure 4.4-3c: The final corrective grading plans shall show detailed locations of keyways, subdrains, debris benches, and subexcavation areas. The sequence of grading issues, such as placement of various cut materials in specific locations, should also be evaluated during review of the final 40-scale grading plans. Mitigation Measure 4.4-3d: The Geotechnical Engineer or qualified representative should be present during all phases of grading operations to observe demolition, site preparation, grading operations, and subdrain placement. After grading operations commence, geologic observations of cut areas should be made at frequent intervals so that revised geologic recommendations can be incorporated into updated grading plans as grading proceeds. During slope grading, all cut slopes should be viewed by the Engineering Geologist for adverse bedding, seepage, or bedrock conditions that may affect slope stability. In the event that adverse conditions are identified, overexcavation and reconstruction of the slopes may be required. Track rolling to compact slope faces is not sufficient. All fill slopes should be overbuilt at least two feet and cut back to design grades. Mitigation Measure 4.4-3e: Keyways equipped with a subsurface drainage system shall be constructed. Each keyway shall be keyed into firm natural materials unaffected by shrinkage cracks. The keyway shall be filled with materials derived from the Livermore Gravels compacted to 95 percent relative compaction for non-expansive materials and 90 percent relative compaction for expansive materials. The required depth of the keyways shall be determined in LUND RANCH II (PUD-25)RESIDENTIAL PROJECT 12 FINDINGS