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

Calculations utilizing the B&C method of determining building distortions andstrains for each building were performed using an Excel spreadsheet and VisualBasic software. An explanation of this method is provided in Appendix D. Manualcalculations were performed to verify the accuracy of the spreadsheet calculations.Calculated strains were compared to the limiting strain values to determine the levelof expected damage likely to be experienced. Results of the analyses are presented inSection 6.As the Tunnel Boring Machine (TBM) advances, a small volume of soil is empirically“lost” in the excavation process, even under the most carefully controlled tunnelingconditions. This lost ground may be defined as the small percentage of soil that isexcavated over-and-above the absolute minimum volume of soil excavated undertheoretically perfect tunneling conditions. The loss of this small volume of soil ismanifested as settlement of the ground surface above the tunnel alignment, the socalledsurface “settlement trough.”The shape, depth, and width of the settlement trough depend on many conditions,including soil type, depth of tunnel, and tunneling method. For the SR 99 Tunnel,the shape, depth, and width of the settlement trough have been estimated byShannon and Wilson, using classical settlement calculation methods, similar to thosedescribed in Attewell, Yeates, and Selby [1986].The front of the settlement trough develops along the surface in response to theadvancement of the TBM. For granular soil conditions (K=0.35), the advancementof the settlement trough can be described as follows: at any given time, the leadingedge of the settlement trough is located at the surface ahead of the TBM by adistance of approximately 0.8 to 1.0 times the tunnel depth. For K=0.5, thesettlement trough is located at the surface ahead of the TBM by a distance ofapproximately 1.0 to 1.3 times the tunnel depth. At that same time, at the surfaceposition directly above the TBM, the settlement trough has reached a depth ofapproximately one-half the maximum trough depth that will eventually develop afterthe TBM has moved well past its current location. At this same time, the settlementtrough has already reached its maximum depth behind the TBM by a distance equalto about 0.9 times the tunnel depth. Thus, for a vertical longitudinal section throughthe ground surface (a vertical section parallel to the tunnel axis), the front of thesettlement trough describes an “S” shaped curve: zero settlement at a location 0.9times the tunnel depth ahead of the TBM; one-half the maximum settlement at theposition of the TBM; and the maximum settlement at distances equal to or greaterthan 0.9 times the tunnel depth behind the TBM. In fact, the shape of this curve isdefined in analysis as the S-shaped Cumulative Distribution Function for a normal,or Gaussian, probability distribution.Once the TBM has passed any given position (by a distance of at least 0.9 times thetunnel depth), the settlement trough is “fully developed”, meaning that thesettlement at the center of the trough has reached its maximum value. For a verticaltransverse section through the ground surface (a vertical section transverse to theThe Alaskan Way Viaduct & Seawall Replacement Program March 2010Proposed SR 99 Bored Tunnel - Assessment of Settlement Impacts to Buildings 22Public Disclosure Request 11-0123 for Elizabeth Campbell - 2nd installment