An independent deep excavation analysis model for the repair shaft of the Seattle Viaduct (14/Jan/2015)
The repair works for the Seattle Viaduct (Bertha) have been again on the news. Fox news pointed out to a "draft" report by an engineering firm that used words as catastrophic failure (here is the article). The final report used different language, but the press always wants to jump on anything that sounds scary. We wanted to check the circular shaft with our deep excavation software DeepEX and see if we could find something that raises an alarm.
Seattle Viaduct Access Shaft Jan/14/15 (Official Project Website)
First, a little background for those unaware with the story:
Bertha, the tunnel boring machine for the viaduct has been stuck/damaged for the last few months. As part of the repair works, a circular shaft approximately 83ft in diameter and 120 ft deep when finally excavated was proposed and is being constructed. From information that we could extract from the web, the shaft was constructed with large diameter piles (10ft) that are overlapping. It is also our understanding that additional grouting is taking place to create a sufficient compression ring where the large diameter piles have to be constructed next to what appears to be protection piles along the tunnel allignment. Where the tunnel is to bore through, the shaft thickness has been extended with more overlapping pile rows so that increased arching is available.
While we are missing some geotechnical and structural information, we made a series of rough but conservative assumptions for soil properties, water levels, and structural properties to analyze the circular shaft. Some rough geotechnical section could be assumed since the geotechnical baseline report is available online (close to Main Street on map):
Fill for 15ft
Soft clay/silt to 80ft depth
Hard glacial overconsolidated deposits below 80ft
Hydrostatic water pressures are assumed starting from a 10ft depth
Geotechnical section for Seattle Viaduct based on Geotechnical Baseline report by Shannon & Wilson (available online)
Based on these assumptions, and using the circular shaft analysis capabilities in DeepEX, we came back with maximum hoop force of 529 k/ft (vertical foot). With an assumption of 1% verticallity tolerance, equivalent compression stresses in the order of 704 psi were computed. We believe that these stresses well within the capacity of the used structural concrete and grout. A more detailed analysis would be required to consider the effects of the openings with rings, but:
a) We need to have all the data
b) We cannot give out such a freebie. If they need it, they can call us.
While our analysis is "preliminary" with rough assumptions, it indicates that the current shaft design appears to be stable and functional.
Image for pile layout for Seattle Viaduct Repair Access Shaft (available from project website)
Model of circular shaft for Seattle Viaduct Repair Access shaft with DeepEX deep excavation software
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