07/15/2004 Env Bd Packet - Laserfiche WebLink

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• • 7:00a.m. to 10:OOp.m. 10:00 p.m. to 7:00 a.m. L10 L50 L10 L50 NAC -1 (Residential) 65 60 55 50 NAC — 2 (Commercial) 70 65 70 65 NAC — 3 (Industrial) 80 75 80 75 L10 represents the sound level that is exceeded for 10 percent of the time during a one -hour period. L50 represents the sound level that is exceeded 50 percent of the time during a one - hour period. Sound levels are expressed in dBA. A dBA is a unit of sound level expressed in decibels and weighted for the purpose of determining human response to sound. Individual home construction is not anticipated to cause any unusual or substantial impacts. Construction noise is an important issue for this project because of the need for a pile driver for construction of the bridge. The bridge will be supported on driven piles. The piles will driven at each end abutment and at the center pier. The piles will be driven approximately 40 feet to 65 feet below present grade. The pilings for the two end abutments and the center pier are expected to take three days for construction. Because of the depths of compressible organic soils at the bridge site, the proposer's geotechnical engineer, STS Consultants, has recommended the use of driven piles to support the bridge. For this project, they recommend steel pipe piles. The recommendation is that these piles should be driven with a relatively small hammer, to reduce noise and vibration effects. The pile driving hammer should have a rated energy no more than 30,000 foot pounds per blow. Typically, such a hammer operates a frequency of about 50 to 60 blows per minute, which relatively speaking is considered to be a low frequency. The hammer creates impulsive noise (series of repeated sounds) which would slightly exceed the noise standards for a Noise 1 classification area, namely 60 to 65 decibels. Thus the recommendation that pile driving should be restricted to no earlier than 8 a.m., and no later than 4:00 p.m., with no pile driving on Saturdays and Sundays. Typical construction vehicles generate noise. It is unlikely the vehicles will exceed state noise standards, and the restriction on working hours described above will reduce the impact. Nonetheless, it is likely that nearby residents will find the noise and activity unpleasant. Dewatering pumps typically run all day and night, and dewatering will be necessary for bridge construction. Though these pumps would not exceed state noise standards, they may be irritating when running all night. On another project in Lino Lakes, the contractor constructed a noise barrier: a three sided plywood wall with one -inch styrofoam on the inside. The top and side away from the residents was open for access and to allow the pump to still function. The use of such a temporary wall can reduce noise from dewatering pumps. Vibration: The other aspect of the pile driving activity is the potential for impacts from vibration. STS reviewed actual seismograph data taken adjacent to pile driving operations. The seismograph tests indicate that the frequency of the vibrations is less than Environmental Assessment Worksheet Page 17 Pheasant Hills 12th Addition