Construction
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5.0 Construction 5.1 Construction Approach This section provides an overview of potential construction activities and timing. Major activities would include: The major civil construction activities that could cause environmental impacts and community disruption • Civil construction: utility relocation, foundation and column include: placement, guideway construction, and track work, followed • Demolition (buildings, by construction of other facilities such as stations, park-and- pavement) and debris removal ride lots and structures, and ancillary facilities. • Clearing and vegetation removal • Fill and excavation • Systems installation: installation of the electrical system that • Utility extensions, relocations, or powers the trains. disruptions • Drainage changes • Testing and startup activities: communications, safety, and • Staging area use emergency systems tested and certified before beginning • Construction activity in or near a revenue operations. wetland or water body • Elevated structure construction Activity would be most intense during civil construction, which • Retaining wall construction would take between 1 and 4 years in any given portion of the • Pile-driving or auguring piles corridor. • Blasting (not likely) • Temporary partial or total road The following sections describe the methods for each major or lane closures and detour routes construction component. Several sections quantitatively • Temporary, partial, or limited compare the impacts of the Preferred Alternative against a low- access cost project and a high-cost project, using a conceptual design • Building temporary vehicular and pedestrian detour routes level construction cost estimate. Low- and high-cost projects are • Delivery of materials and as follows: equipment • Low-cost project: Preferred Alternative with the Kent/Des Moines At-Grade Station Option, S 272nd Star Lake Elevated Station Option, and the Federal Way I-5 Station Option • High-cost project: SR 99 Alternative with the potential additional S 216th West Station Option, the Kent/Des Moines HC Campus Station Option, and the potential additional S 260th East Station Option Many impacts are discussed qualitatively because the specific approach to construction will be refined as Sound Transit obtains additional information on site conditions and makes decisions related to construction contracting methods. It is expected that the project would be constructed in at least two phases, with the first phase Federal Way Link Extension 5-1 Final EIS November 2016 5.0 Construction being Angle Lake to Kent/Des Moines, and the additional phase constructing from Kent/Des Moines to S 272nd Street and the Federal Way Transit Center. As described in Chapter 2, if the ST3 package is approved by voters, the schedule will be revisited. Site-specific conditions, permit requirements, and market conditions at the time of construction, among other factors, affect how a project is built. Sound Transit will coordinate with each jurisdiction on land use approvals, right-of-way use and land disturbance permits, and other permits required for construction. Although the SR 509 Extension Project could be under construction concurrently with the FWLE, this chapter only analyzes FWLE project impacts. Potential combined impacts of both projects being constructed at the same time are discussed in Chapter 6, Cumulative Impacts. 5.1.1 Types of Construction 5.1.1.1 Elevated Light Rail Construction Construction of elevated guideway would involve the following: • Demolition of existing structures • Clearing and grading • Relocating utilities • Preparing construction access • Building a temporary construction road where access is not available from existing roads • Temporarily closing some traffic lanes and detouring traffic when working within existing street right-of- way • Constructing the guideway structure Construction would begin with preparation work to build foundations such as shallow spread footings, deep-driven or augered piles, or drilled shafts. Concrete columns would then be constructed. The elevated superstructure could be steel, cast-in-place concrete, pre-cast concrete, or segmental concrete. Sound Transit would primarily use segmental construction for elevated guideways, where the segmental box girders are typically poured offsite and Columns Under Construction for the Central trucked to the project location to be placed by crane. Link Project in Seattle Federal Way Link Extension 5-2 Final EIS November 2016 5.0 Construction False-work could be required in places where cast-in place False-work concrete is used to support the superstructure while the False-work is temporary support concrete is poured and the cast concrete gains enough strength structures used during construction of a structure not yet able to support itself. to support itself, or while the steel beams are joined through welding or bolting. It would require temporary traffic detours until a sufficient portion of the elevated structure is complete. Some other construction activities would also require short-term partial or full street closures, as described in Section 5.2.1. 5.1.1.2 At-Grade Light Rail Construction Construction methods and impacts for at-grade guideways would be similar to typical road construction, as follows: Construction of Elevated Guideway Showing False-Work • Demolishing existing structures in the project footprint and relocating conflicting utilities • Performing shallow excavations to construct the subgrade, Construction Footprint track, and station platform slabs The construction footprint used in analyzing impacts includes areas beyond the permanent footprint • Building a temporary construction road where access is not needed for construction access or available from existing roads staging. It does not include all temporary construction easements. • Installing drainage structures and below-grade light rail infrastructure 5.1.1.3 Trench and Retained Fill Light Rail Construction Trench and retained-fill guideway construction would be similar to at- grade guideways, but may be more intensive and of longer duration because of required retaining walls. Trench construction would involve excavating trenches and building retaining walls. Dewatering may be necessary in places. Retained fills would require building retaining walls and placing fill. Sound Transit would use soil nail, soldier pile, and secant pile retaining walls. Soil nail walls would be most often used because they are the most cost-effective and have the shortest construction time. Soldier piles walls are also cost and time efficient for shallower excavations up to 10 feet. Secant pile walls would be used in deeper excavations and where groundwater is present. The type of wall to be used would depend on depth of the trench and site conditions. Federal Way Link Extension 5-3 Final EIS November 2016 5.0 Construction Construction under roads would use cut-and-cover construction methods, typically with metal plates over the trench to maintain Retaining Wall Types Soil Nail: A type of excavation using traffic flow. grout tension-resisting elements (nails) designed for permanent Construction staging areas are needed before, during, and for a support. This method enables construction to occur from the short time after construction work, for the following: existing ground surface down to the desired excavation depth. Typically 3 • Construction to 6 feet of soil is excavated from the • ground surface and nearly horizontal Equipment storage holes are drilled into the exposed • Construction materials delivery and storage face at 3 to 6 feet on center. The tension-resisting steel is inserted and • Demolition or spoils handling grouted. A drainage system is installed on the exposed face and • Contractor trailers reinforced shotcrete is applied. This • Access roads procedure continues to the desired depth of excavation. • Construction crew parking Secant pile: A type of wall formed by constructing intersecting All profiles would have construction staging areas along the reinforced concrete piles. The holes alignments. Contractors would use the property in which the for the piles are either drilled or augered and then reinforced with facility is being constructed, property that Sound Transit either rebar or steel beams. Primary piles are installed first and then acquired for right-of-way, or other properties as negotiated by secondary piles are constructed in the contractor. between when the primary piles have gained sufficient strength. The Following construction, staging sites may be used for the project piles overlap by about 3 inches. Soldier pile and lagging: Soldier or redeveloped by others consistent with the current zoning. piles and lagging uses vertical steel piles with horizontal lagging. In undeveloped areas, 50- to 100-foot-wide construction areas Typically, H-piles are drilled or driven at regular intervals along the planned could be necessary to maneuver equipment and materials along excavation perimeter. The lagging the corridor. Sometimes the project could use areas acquired for material can be timber, steel, or precast concrete is inserted between project right-of-way, but other times it would require temporary the H-piles. The lagging effectively construction easements from adjacent properties. Following resists the load of the retained soil and transfers it to the piles. construction, these areas would be restored to preconstruction conditions. Construction might require temporary road closures. Where this would occur, traffic would be rerouted to