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

Borehole extensometers, in which the lengths of segments along vertical drilled holesare monitored, deliver the earliest warnings on impending large settlements. Thelowest portion directly above the tunnel would settle first, which can be measuredand set to generate alarms. Borehole extensometers are expected to be included inthe scope of the instrumentation for the actual tunnel construction.A very advanced method is InSAR monitoring, which uses earth-orbiting satellitesand compares multiple radar images to determine settlement values to an accuracy of¼ inch. Since satellites only pass the Seattle area every 24 days, there would be adelay of a month or longer to get results. For this reason, it is not considered apractical monitoring method for construction of the tunnel.In most practical applications, a combination of sensors and methods are employed,but considerable additional efforts are needed to relate the different methods at eachlocation.Since most settlements generally occur within a one-week period when theexcavation is directly below a building, some expensive sensors can be reused manytimes. For instance, if the project duration is one year, then the instruments could bereused approximately ten times (two weeks of baseline measurements, one weekmeasuring for active settlements, and two weeks of tailing-off measurements, plussome time to allow for the relocation of the sensors). The length could be dividedinto ten zones, and sensor placement could be planned within such zones. However,the fixed monitoring elements of the project should remain in place for several yearsafter completion of the project should additional, follow-up monitoring be required.The Alaskan Way Viaduct & Seawall Replacement Program March 2010Proposed SR 99 Bored Tunnel - Assessment of Settlement Impacts to Buildings E-2Public Disclosure Request 11-0123 for Elizabeth Campbell - 2nd installment