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HomeMy WebLinkAboutM_Scope Change Narrative_ApronR Mitigation Hangar SEPA Addendum_200305_v1.pdfApron R Central Scope – Detailed Write Up INTRODUCTION Boeing is proposing to complete infrastructure and maintenance work to Apron R Central (proposed project) located at the Boeing plant in the City of Renton (City). This work is required in order to address deteriorating or inadequate infrastructure conditions at the project site. The plant is located at the southernmost end of Lake Washington between Rainier Avenue North and Interstate 405 (Parcel ID 0723059001). Boeing operates its facilities under a Development Agreement for Renton Plant Redevelopment approved by the City in 2003. The current zoning of the site and adjacent areas is Industrial Use. The groundwater table at the project site varies from 3 to 5 feet below the ground. The subsurface soils at the site include soft and loose, un-consolidated alluvial soils from the ground to depths of about 60 feet. Existing drainage at the site is collected via catch basins and conveyed to outfalls on Lake Washington. Apron R Central is approximately 2.300 acres in area and is located in between Boeing plant buildings 4- 20 and 4-81. The project site is primarily used to stage airplanes in designated stalls to undergo systems checks, make minor modifications, and/or to temporarily store the airplanes before they are transported to the airport. Most of Apron R Central was constructed in the early 1990s and has since failed to meet required water quality bench marks. The proposed Apron R Central project has been developed to repair known water quality deficiencies by removing and replacing aging water quality infrastructure with new facilities. The project will remove surface obstacles for aircraft operations and improve utility systems. In general, Apron R infrastructure improvement projects include the following elements;  Removal of existing water quality structures;  Construction of stormwater collection, conveyance, and water quality facilities;  Relocation of existing utilities;  Replacement of the existing pavement with new concrete pavement; PROJECT CHANGES The “Apron R Central” project described above has been reduced in scope from its parent project called the “Mitigation Hangar". The new project has been reduced in scope to facilitate replacement of the 4- 01 Wet Vault/Oil Water Separator without constructing the Mitigation Hangar structure. Removing the Hangar structure reduces the project area, eliminates the need to pre-load a building footprint, and prevents the re-route of critical infrastructure. Instead, the existing use and general configuration of the Apron will be maintained. Improvements will include revised storm drainage, water quality facilities, removing above ground utilities, grading, and new paving to support continued operations in the outdoor staging stalls (CI Area) and parking areas. Overall, the total project site area will be reduced in size from 2.619 acres to 2.300 acres, a reduction of 0.319 acres. The previously planned Mitigation Hangar building required the project limits to extend to the western face of the 4-81 building. The new, reduced project shifts the limits to the west, keeping the canopy structures adjacent to the 4-81 building intact for continued operations. A pre-load for the building structure is no longer required eliminating the need to install 511 linear feet of temporary ecology block walls and import roughly 18,800 cubic yards of soil. Instead, this area will be graded and re-paved for drainage improvements. Critical utilities previously located underneath the building footprint will no longer need to be removed and re-routed. The 10” existing fire main just outside of the 4-81 Building will stay intact along with all of its connections to the internal 4-82 Building fire protection system. Exterior lighting and utility feeds from the 4-81 Building will also stay in place. Above ground domestic water, compressed air, power, and data will be moved underground but will be positioned similarly to the current utility configuration. DRAINAGE Storm Drainage improvements as a part of the reduced project will still include abandonment of the 4- 01 Wet Vault/Oil Water Separator and associated infrastructure, providing replacement water quality facilities upstream of the outdoor staging stalls, providing secondary containment for outdoor staging stalls, and re-routing select conveyance systems as required to avoid utilities while accommodating a new treatment system. Replacement of the 4-01 Wet Vault/Oil Water Separator will include providing water quality treatment and oil control facilities for stormwater run-off from pollution generating surfaces which are currently tributary to the vault. The northern 2.003 acres of the project site will be graded to collect runoff and convey it to facilities constructed within the Apron R project limits. The remaining 0.297 acres of the project site and additional 5.220 acres of off-site tributary area will collect and convey runoff to newly built water quality facilities constructed within the Apron R Central project limits. The new water quality facilities consist of three vaults: a new combined oil/water separator and bypass vault with a dual modular wetland vault system for enhanced treatment. The elimination of the Mitigation Hangar building allows the outdoor aircraft staging stalls to remain in operation. There are three designated staging locations within the Apron R Central project area. New pavement in these areas will be graded to drain to vaults fitted with 100-gallon capacity “Safe Drain” containment systems. The “Safe Drain” system consists of a stainless-steel containment basin with a manual valve that can be closed in the event of a spill. The intent of the “Safe Drain” is to prevent a spill from entering the underground conveyance system and is intended to supplement operational BMPs already in place. The vault will act as a traditional catch basin when the valve remains open, but provides a second level of redundancy to Boeing’s Spill Control and Containment System. Existing drainage conveyance pipes will be re-routed to newly constructed water quality facilities within the Apron R and Apron R Central project sites. Drainage is positioned to avoid the abandoned 4-01 Wet Vault and newly constructed underground utility lines. A total of 1,504 linear feet of new 12 to 30 inch HDPE pipe will be installed connecting 19 new drainage structures. Lot 6 bioretention cells proposed with the Mitigation Hangar project will still be constructed with the reduced project because the treatment area swap outlined is still required. UTILITIES Currently, above ground utilities serve the outdoor aircraft staging stalls. Above ground utilities and other obstructions are not desired on the Apron near manufacturing operations. To facilitate continued use of the project site, all above ground utilities will be moved underground. Hardstands R5, R6, and R7 will be removed and replaced once all paving and grading operations are complete. Each hardstand will be fed with underground power, compressed air, domestic water, and data. Utilities originate from the west face of the 4-81 building, route directly to Hardstand R5, and route to Hardstands R6 and R7 from there. This configuration will not be changed and new utilities will tie in to the existing feeds from the 4-81 Building. Bolted down bollards will protect each hardstand from vehicle impacts. Underground Fire Protection System improvements will include the abandonment of the existing 8” fire system main located within the project site, removal of 2 existing yard hydrants, and the installation 3 new yard hydrants fed by existing fire mains spaced at a maximum of 300-feet. PAVEMENT, MARKINGS, AND SITE FEATURES The existing project site is impervious surface with a combination of asphalt and concrete pavement surfacing. The entire project area will be graded and resurfaced with 14.5” thick unreinforced slabs with doweled joints. Irregularly shaped panels around drainage structures and other features will be reinforced. Pavement markings and associated signage within the project area will be installed for aircraft staging stalls, pedestrian walkways, crosswalks, and fire hydrant gore striping. Existing site features demolished for construction will be replaced in similar locations. Features will include a bus shelter and associated bollard protection, ecology block walls, black vinyl fencing, and load/unload ramps and platforms.