6 - GPA19-01; AA19-01_651 W SUNFLOWER AVENUE

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Legacy Sunflower Apartments Page 84 Mitigated Negative Declaration – March 14, 2019 exceed the City of Santa Ana’s exterior noise standards to any sensitive land use. Therefore, noise from the proposed parking structure would have less than significant noise impacts. The noise levels generated by the project would not impact surrounding land uses. Furthermore, the project would not be significantly impacted by traffic noise on either Flower Street or Sunflower Avenue with the incorporation of Mitigation Measure No. 10. Therefore, project noise impacts would be less than significant. b) Generation of excessive ground borne vibration or ground borne noise levels? Potentially Significant Unless Mitigation Incorporated. The way that vibration is transmitted through the earth is called propagation. There are three main types of vibration propagation: surface; compression; and shear waves. Surface waves, or Raleigh waves, travel along the ground’s surface. These waves carry most of their energy along an expanding circular wave front, similar to ripples produced by throwing a rock into a pool of water. Compression waves, or P-waves, are body waves that carry their energy along an expanding spherical wave front. The particle motion in these waves is longitudinal (i.e., in a “push-pull” fashion). P-waves are analogous to airborne sound waves. Shear waves, or S-waves, are also body waves that carry energy along an expanding spherical wave front. However, unlike P-waves, the particle motion is transverse or “side-to-side and perpendicular to the direction of propagation”. As vibration waves propagate from a source, the energy is spread over an ever-increasing area such that the energy level striking a given point is reduced with the distance from the energy source. This geometric spreading loss is inversely proportional to the square of the distance. Wave energy is also reduced with distance as a result of material damping in the form of internal friction, soil layering, and void spaces. The amount of attenuation provided by material damping varies with soil type and condition as well as the frequency of the wave. Vibration amplitudes are usually expressed as either peak particle velocity (PPV) or the root mean square (RMS) velocity. The PPV is defined as the maximum instantaneous peak of the vibration signal in inches per second. The RMS of a signal is the average of the squared amplitude of the signal in vibration decibels (VdB), ref one micro-inch per second. The Federal Railroad Administration uses the abbreviation “VdB” for vibration decibels to reduce the potential for confusion with sound decibel. PPV is appropriate for evaluating the potential of building damage and VdB is commonly used to evaluate human response. Decibel notation acts to compress the range of numbers required in measuring vibration. Similar to the noise descriptors, Leq and Lmax can be used to describe the average vibration and the maximum vibration level observed during a single vibration measurement interval. Figure 18 illustrates common vibration sources and the human and structural responses to ground- borne vibration. As shown, the threshold of perception for human response is approximately 65 VdB; however, human response to vibration is not usually substantial unless the vibration exceeds 70 VdB. Vibration Impacts There are several types of construction equipment that can cause vibration levels high enough to annoy persons in the vicinity and/or result in architectural or structural damage to nearby structures and improvements. As shown in Table 23, a vibratory roller could generate up to 0.21 PPV at a distance of 25 feet; and operation of a large bulldozer (0.089 PPV) at a distance of 25 feet (two of the most vibratory pieces of construction equipment). Groundborne vibration at sensitive receptors associated with this equipment would drop off as the equipment moves away. For example, as the vibratory roller moves further than 100 feet from the sensitive receptors, the vibration associated with it would drop below 0.046 PPV, depending upon the soil type and specific usage of the piece of equipment. 6-106