SR99 Bored Tunnel-Assessment of Settlement Impacts ... - SCATnow

Proposed SR 99 Bored Tunnel -Assessment of Settlement Impacts toBuildings1.0 Executive SummaryAn initial assessment of impacts to the buildings along the SR 99 roadway alignmentdue to the construction of the proposed bored tunnel alternative is presented in thisreport. Tunneling is expected to result in settlements of varying magnitude, whichdepend on many factors. The evaluation process started with an assessment of theexisting condition of buildings along the alignment, followed by an analysis topredict settlement associated with tunneling. The amount of settlement assumed inthe analysis is a baseline from which refinements or mitigations can be measured.Settlement corresponding to 0.5% of the tunnel cross sectional area (termed “lostground”) was assumed. This amount of settlement is not fixed or final, but areasonable assumption on which to initiate the assessments. On the basis of thepredicted settlement, an analysis was performed to predict building damage. For thebuildings with the greatest potential damage, further structural engineering analysisand evaluations were performed. Finally, a qualitative evaluation of the potentialimpacts to the buildings was made by structural engineers knowledgeable in the typeof structures encountered along the tunnel alignment. The unmitigated buildingimpacts are expressed as the damage that may take place. Damage is quantified bythe nature of the anticipated repairs that may be required. This assessment of thepotential unmitigated settlement impact for each building will be followed by furtherstudies of alternatives and mitigations, when appropriate, as the documents forconstruction are prepared.The south portal construction for the proposed SR 99 Bored Tunnel Alternativeproject will extend north from Royal Brougham Way along Alaskan Way with theheadwall for the bored tunnel occurring just south of South King Street. Passingunder Alaskan Way and the existing viaduct, the tunnel would then traverse a largeradius curve beginning just south of South Washington Street, pass under WesternAvenue and continue diagonally under Seattle’s central business district to FirstAvenue. Near Stewart Street, the alignment would traverse a large radius curve tothe north, extending beyond and diagonal to the street grid of Seattle’s Belltownneighborhood. The tunnel would then travel under Denny Way into the SixthAvenue North right-of-way and transition to a cut-and-cover section north ofThomas Street.Buildings along the alignment range from single-story unreinforced masonrybuildings with wood framed roof structures to multi-story residential and officeThe Alaskan Way Viaduct & Seawall Replacement Program March 2010Proposed SR 99 Bored Tunnel - Assessment of Settlement Impacts to Buildings 1Public Disclosure Request 11-0123 for Elizabeth Campbell - 2nd installment