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The video doesn't mention that the tower is a lot of reinforced concrete, not steel as in most high rises. That makes it much heavier than other friction pile b

The video doesn't mention that the tower is a lot of reinforced concrete, not steel as in most high rises. That makes it much heavier than other friction pile buildings which is at minimum contributing to the problem. IMHO I think it's insane anyone proposed not drilling to bedrock and that the city ever allowed it. In an earthquake zone bedrock piles should be an absolute requirement for tall buildings. Last I heard the modified drilling procedure didn't help and the accelerated sinking hadn't fully stopped so right now there is no plan to resume. When the pile drilling started lean was 17". In less than a year of that work it's now 22". By building settlement standards that's blindingly fast and makes me think the soil on that corner is in much worse shape than anyone assumed... possibly bad enough it won't hold in a large earthquake. It may be a more complicated interaction than just compression due to the load. I'm not sure what they can do at this point. Have a bunch of crews get the piles installed as quickly as possible and hope you can do it before the sinking becomes dangerous? Given it is a concrete building rather than steel they probably can't do much to reduce the weight but moving everyone and everything possible out might help a little. To counteract the leak maybe they can drill on the other corner (that's always been part of the long-term fix) to get it to settle at a faster and thus more even rate? Definitely a difficult problem.

hyperbovine 5y ago
I have no idea why this comment would be downvoted other than disgruntled property owners.
- xenadu02 5y ago
  I'm not sure either but I did find confirmation that San Francisco no longer allows friction piles for any high-rise buildings in areas with "infirm" soils. Piles must be driven to bedrock.
- kelnos 5y ago
  The downvotes are likely because the parent clearly didn't read the entire article, and presented information in conflict with it. The engineering team is testing modified drilling techniques so they can resume repair work, this time (hopefully) without causing more accelerated settling. This is all outlined in the article, and it's frustrating to see people commenting when they haven't read it.
ArchitectAnon 5y ago
I’m not an engineer but I am an architect and I have worked closely with engineers on skyscrapers. Drilling all the way down to the bedrock is risky. When you drill a deep pile you need to use bentonite slurry to stop the pile collapsing in on itself, the pile auger is withdrawn and then a long pipe is used to introduce concrete from the bottom of the the pile which should displace the slurry out of the top because it is less dense than the concrete. However, this process doesn’t always work and you get bentonite inclusions in the concrete which are found when the pile is tested ultrasonically afterwards. I don’t remember the stats but lets say it’s something like 1 in 10 piles fail the test when you drill to 50m deep bedrock through clay. You then have to drill two more either side of the original pile and bridge over the defective pile but these each have the same risk of inclusions, so there’s an increased risk the workaround will fail. As you can imagine this can get pretty complicated and expensive quickly. As an architect I was involved because we needed to have a backup plan for how the basement would be reorganised if there were a lot of piling failures requiring extra bridging structures.
- montag 5y ago
  Very interesting. Thanks for sharing. You make a good point that it’s not so simple as just drilling to bedrock.
  - pas 5y ago
    It's not that complicated though. There's nothing stopping the builders to get the bad column out and try it again. And again. Yes it's expensive as fuck, but this is still the foundation we're talking about. This (and the load bearing core) is where cost must come after safety. Yes, naturally safety needs to be quantified and that just gets us back to cost again in a sense, but that's why there's a minimal safety codified (in the building code). It's all about cost. There are many ways to get those nice columns in the ground, it's possible to drill them and put annular rings on top of each other (just like with regular wells) then fill it with concrete. More expensive than the bentonite slurry method? Sure, but it's still simple.

hyperbovine 5y ago

I have no idea why this comment would be downvoted other than disgruntled property owners.

xenadu02 5y ago
I'm not sure either but I did find confirmation that San Francisco no longer allows friction piles for any high-rise buildings in areas with "infirm" soils. Piles must be driven to bedrock.
kelnos 5y ago
The downvotes are likely because the parent clearly didn't read the entire article, and presented information in conflict with it. The engineering team is testing modified drilling techniques so they can resume repair work, this time (hopefully) without causing more accelerated settling. This is all outlined in the article, and it's frustrating to see people commenting when they haven't read it.

ArchitectAnon 5y ago

I’m not an engineer but I am an architect and I have worked closely with engineers on skyscrapers. Drilling all the way down to the bedrock is risky. When you drill a deep pile you need to use bentonite slurry to stop the pile collapsing in on itself, the pile auger is withdrawn and then a long pipe is used to introduce concrete from the bottom of the the pile which should displace the slurry out of the top because it is less dense than the concrete. However, this process doesn’t always work and you get bentonite inclusions in the concrete which are found when the pile is tested ultrasonically afterwards. I don’t remember the stats but lets say it’s something like 1 in 10 piles fail the test when you drill to 50m deep bedrock through clay. You then have to drill two more either side of the original pile and bridge over the defective pile but these each have the same risk of inclusions, so there’s an increased risk the workaround will fail. As you can imagine this can get pretty complicated and expensive quickly. As an architect I was involved because we needed to have a backup plan for how the basement would be reorganised if there were a lot of piling failures requiring extra bridging structures.

montag 5y ago
Very interesting. Thanks for sharing. You make a good point that it’s not so simple as just drilling to bedrock.
- pas 5y ago
  It's not that complicated though. There's nothing stopping the builders to get the bad column out and try it again. And again. Yes it's expensive as fuck, but this is still the foundation we're talking about. This (and the load bearing core) is where cost must come after safety. Yes, naturally safety needs to be quantified and that just gets us back to cost again in a sense, but that's why there's a minimal safety codified (in the building code). It's all about cost. There are many ways to get those nice columns in the ground, it's possible to drill them and put annular rings on top of each other (just like with regular wells) then fill it with concrete. More expensive than the bentonite slurry method? Sure, but it's still simple.