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Chapter 4 Environmental Setting, Impacts, and Mitigation Measures Threshold Would the project result in the exposure of persons to or generation of excessive groundborne vibration or groundborne noise levels? Impact 4.8-9 Construction activities associated with the proposed project could generate or expose persons or structures to excessive groundborne vibration. While implementation of mitigation measures MM4.8-1 through MM4.8-4 would minimize this impact, it would not reduce it to a less -than -significant level. This is considered a significant and unavoidable impact. Although construction -related vibration associated with individual development projects constructed pursuant to the Transit Zoning Code would be temporary there are two potential impacts that could occur. First, vibration at high enough levels can result in human annoyance. Second, groundborne vibration can potentially damage the foundations and exteriors of historic structures. Groundbome vibration that can cause this kind of damage is typically limited to impact equipment, especially pile drivers. Construction activities that would occur pursuant to the Transit Zoning Code have the potential to generate low levels of groundborne vibration. Table 4.8-13 (Vibration Source Levels for Construction Equipment) identifies various vibration velocity levels for the types of construction equipment that would operate within the City during construction. Table- Equipment Approxmmote VdB 25 Feet 50 Feet 75 Feet 100 Feet Large Bulldozer 87 81 77 75 Loaded Trucks 86 80 76 74 Jackhammer 79 73 69 67 Small Bulldozer 58 52 48 46 SOURCE: Federal Railroad Administration 1998 In addition to the construction equipment shown in Table 4.8-13 (Vibration Source Levels for Construction Equipment), vibration that would be experienced from the use of impact pile drivers could reach as high as 112 VdB at a distance of 25 feet (HMMH 2O06). Like noise, groundbome vibration will attenuate at a rate of approximately 6 VdB per doubling of distance. The groundborne vibration generated during construction activities would primarily impact existing sensitive uses (e.g., residences, schools, and hospitals) that are located adjacent to, or within, the vicinity of specific projects. These sensitive uses could sometimes be located as close as 25 feet to the construction site or as far as several hundred feet away. Based on the information presented in Table 4.8-13, vibration levels could reach up to 87 VdB at sensitive uses located within 25 feet of construction. For sensitive uses that are located at or within 25 feet of potential project construction sites, sensitive receptors (e.g., residents, school children, and hospital patients) at these locations may experience vibration levels during construction activities that exceed the FTA's vibration impact threshold of 85 VdB for human annoyance. So long as construction occurs more than 50 feet from sensitive receptors, the impact associated with groundborne vibration generated by the equipment would be below 85 VdB and thus would be less than significant. However, as specific site plans or constructions schedules for projects requiring impact pile drives are unknown at 4.838 City of Santa Ana Transit Zoning Code (SD 84A and SD 848) EIR