Is the Bertha repair pit safe?
Today I found an article on Bertha from Fox News. The article was claiming a report from an engineering firm that the circular shaft constructed for removing Bertha faced catastrophic failure (here is the link). Fox News referred to a draft report, while the final engineering report had different language. I wanted to check this out, so I prepared a quick model of the shaft and analyzed with our deep excavation software.
While we had to make some very rough assumptions, it seems that as long as the circular ring action is ensured that everything should work as planned. Here is what we did:
https://www.deepexcavation.com/en/seattle-viaduct-access-repair-shaft-analysis
While I did this out of professional curiosity, they can always hire us if they want a full independent review. Got to make a living as well!
Owner, Terrain Geotechnical Consultants Ltd.
10 年I don't see this as a design issue, it is a very easy problem to solve I arrived at 5.2 N/mm2 (725 psi) on half a sheet of paper, the problem as I see it is the adoption of the wrong solution for this shaft. 120 feet or 37 metres is a long way for a secant wall to maintain its interlock, a diaphragm wall would have been a less risky approach. Also a partial secant wall through the first 80' feet of water bearing soils, yes, followed by conventional precast concrete shaft rings through the stable soils. I always reinforce piles in this situation even though in theory they are just in ring compression, then if you lose the interlock you can recover the situation by casting an internal reinforced concrete ring beam(s) and let the piles span between these. The "as we go" approach referred to in the article is probably the only way to proceed, the situation can be recovered by advance permeation grouting "as you go" followed by careful excavation, this is standard practice for recovery of such a situation and I fail to see why this is such an issue.
Principal Geotechnical Engineer @ Inland Empire Geotech | Geotechnical Engineering & GeoDesign
10 年Dimitrios Konstantakos, not sure what went wrong with their waterproofing and how difficult to fix it will be. Despite the issues with this project, we can appreciate the attention it is bringing to our field and hopefully allow us to further educate the public not only about the hazards and risks associated with projects, but about our value as engineers to take these difficult problems and solve them.
Samuel Probert, there was a chart showing about 1.5 inches of settlement near the shaft, and as much as 0.5 inches almost 1500 feet away. Leaking could explain these settlements.
Principal Geotechnical Engineer @ Inland Empire Geotech | Geotechnical Engineering & GeoDesign
10 年A few weeks ago, I talked with some of the drillers who installed inclinometers in the piles and they told me that the wall was leaking a substantial amount of groundwater. This leaking and the lowering of the watertable may be what is causing the observed settlements around the excavation. I don't know if they have since solved their leaking issue.
Yes it does seem to be on the conservative side. But again this is a very high profile and sensitive project and I think it is justified.