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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 the field by the geotechnical engineer during grading. Fill above the keyways shall be benched into firm, competent soil or bedrock and drained as appropriate. Unless otherwise recommended by the Geotechnical Engineer, benches shall be constructed at vertical intervals of not less than five feet. Mitigation Measure 4.4-3f: Subsurface drainage systems should be installed in all key ways, swales or natural drainage areas, and landslide removal areas in accordance with the recommendations of the geotechnical exploration. Depending on the actual conditions encountered during grading, similar subsurface drainage facilities may be recommended within existing stock ponds, springs, low-lying areas, or at lots where wet conditions are encountered during excavation. In addition, new sources of seepage may be created by a combination of modified topography, future irrigation practices, and potential utility leakage. If discovered, these conditions should be brought to the attention of the Geotechnical Engineer so that remedial actions may be initiated, if necessary. Mitigation Measure 4.4-3g: Structural retaining walls shall be constructed in accordance with the recommendations of the geotechnical exploration and provided with drainage facilities to prevent the buildup of hydrostatic pressures behind the walls. Mitigation Measure 4.4-3h: Where steeper slopes greater than 3:1 are desired, supplemental slope stabilization measures such as geogrid enforcement and/or the use of select fill shall be employed. Drainage benches shall also be provided at a 30-foot minimum intervals, and construction shall follow the structural setbacks of the CBC. Accordingly, structures shall be: (I) set back from the top of the slope by a minimum of one-third of the slope height or a maximum of 40 feet; and (2) set back from the bottom of the slope by a minimum of one-half of the slope height or a maximum of 15 feet. Alternative setbacks may be addressed on a per lot and foundation design basis. Significant Impact Impact 4.4-4: Expansive soils and bedrock materials are present within project boundaries and construction on expansive soil could cause foundation and other structural failures, particularly at lots traversed by a cut/fill transition. 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 expansive soils and bedrock materials to a less-than-significant level: Mitigation Measure 4.4-4: Cut lots shall be overexcavated 2 feet, scarified 12 inches, and recompacted so as to provide a zone of at least 3 feet of reworked and compacted soil. Additionally, cut/fill transition lots should be overexcavated so as to provide a minimum of 3 feet of a uniform thickness of engineered fill within the entire foundation area. In addition, exposed soils shall be kept moist by watering for several days before placement of concrete. LUND RANCH II(PUD-25)RESIDENTIAL PROJECT 12 FINDINGS