The substation site is located adjacent to State Route 237 (SR 237) and North First Street. There, the major noise source is traffic on SR 237. Along the Kifer - Nortech route, traffic along Lafayette Street and Highway 101 is the major noise source. Along the Trimble - Nortech route, traffic along North First Street, Trimble Road, Zanker Road and Highway 101 is the major noise source. Flight operations of the San Jose International Airport (SJIA) also contribute to ambient noise levels at the substation and along the power line routes. Construction noise levels at and near locations on the project site would fluctuate depending on the particular type, number, and duration of use of various pieces of construction equipment. The effect of construction noise would depend upon how much noise would be generated by construction, the distance between construction activities and the nearest noise-sensitive uses, and the existing noise levels at those uses. Construction noise would be intermittent, extended over a period of five months at the substation site. Construction of pole foundations, erection of poles, and stringing of lines would also generate short-term noise along the proposed power line alignments.
The noisiest phases of substation construction would generate approximately 90 Leq at 50 feet (U.S. EPA, 1971). The receptors nearest proposed construction activity would be Oakcrest Estates Mobile Home Park, south of, and across SR 237 from the substation site. Although noise from substation construction would generate noise levels up to approximately 90 dBA, these noise levels would attenuate to 72 dBA or less at the nearest Oakcrest Estates residences, approximately 750 feet away.
Given ambient noise levels at these residences, construction noise would be noticeable; however, many residences are not occupied during the daytime. Construction noise would be noticeable, and possibly annoying, to residents at home during the daytime, but it would be a short-term effect. During nighttime, temporary construction-related noise could be more noticeable (since background noise is lower) and could annoy the closest residents given the more sensitive nature of the nighttime period. Therefore, without appropriate limitations on allowable hours of construction, this temporary impact could be significant.
Substation transformers and cooling fans on the substation site would generate operational noise. The three transformers proposed for the site would each generate a noise level of 69 dBA at partial load and without fans operating and a noise level of 72 dBA during peak load periods. Long-term noise levels resulting from the transformers is predicted to range from 56 dBA Ldn at a distance of 160 feet from the substation fence line to 49 dBA Ldn at a distance of 340 feet. Applying a 5-dBA "penalty" to account for human sensitivity to the pure tone component of transformer noise, the resulting noise would meet the City of San Jose's criteria for residences more than 340 feet from the transformers. PG&E predicts a resultant noise level of 36 dBA at the nearest residences, in Oakcrest Estates. This projected noise level at the nearest residences due to the transformers is well below the ambient noise level during daytime and evening hours.
Construction of the power lines would require the use of cranes, drilling and digging equipment, compressors, tampers, generators, trucks and other equipment. Each of these operations would be of limited duration and any given location would be affected only by local construction activities. Given the spans between the poles, placement of one or two poles may be the full extent of the noise effects of heavy construction experienced at any one location. Projected short-term noise levels at 100 feet range from 68 to 93 dBA for all construction operations. These noise levels could be audible and noticeable to residents and sensitive receptors in the vicinity of the various construction activities.
Ground vibration resulting from construction operations may be felt by persons in nearby buildings, but no damage is expected to result. If vibration-sensitive operations, such as semiconductor manufacturing or lithography, exist near prospective power pole locations along the power line routes, such operations may be adversely affected during a portion of the five-month construction period.
Under certain conditions (such as wet weather or dirty insulators), power lines can create corona-generated noise, usually associated with a buzzing or crackling. Sound levels beneath the power lines would be less than 30 dBA, as would sound levels at all buildings along the power line corridors. Given the number of rainy days per year within the Santa Clara Valley, and PG&E’s practice of high-pressure washing of insulators, corona-generated noise, while occasionally audible, would not be considered a substantial increase to the typical urban noise environment.
The following mitigation measures would reduce the potential impacts of construction noise and construction vibration to less than significant levels:
Mitigation Measure X.a-1: To reduce the construction noise effects, PG&E shall ensure that noisy construction activities at the substation site and near residences along the power line route shall be limited to, as much as practicable, the least noise-sensitive times of day and week (e.g., 7:00 a.m. to 6:00 p.m., Monday through Friday, not including federal holidays, in residential areas), unless there are overriding traffic and/or power interruption concerns.
If such activities occur, PG&E shall notify the CPUC project manager of noisy construction activities in residential areas outside the aforementioned hours within seven days. Written variances for Mitigation Measure Xa-1 may be required if such activities outside the agreed upon hours are disturbing residences near PG&E construction activities.
Mitigation Measure X.a-2: If concerns are raised about potential vibration from project construction, PG&E shall respond by informing those individuals about the nature, locations and schedule of construction, and by minimizing vibration-causing construction activities to the extent practicable.
PG&E shall monitor activities at the site and document compliance with measures X.a-1 and X.a-2.
PG&E shall provide the CPUC mitigation monitor with documentation of compliance actions in regular progress reports.
PG&E shall inform CPUC of such concerns raised by the public and of PG&E actions in response.
b) As discussed in the response to Item X.a., the noise levels resulting from project operation would be less than ambient noise levels and would be considered a less than significant impact.
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