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SF Tower settlement Part III 18
- Thread starter dik
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Yes, in the production piles the center bar is hollow and is used to tremie the grout in. I think in the "indicator piles" it was a solid bar and the grout was tremied in in a separate grout pipe. The hollow bar is described in the Structures Magazine article.
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From 48Hills: "What’s up with the Leaning Tower of Soma? The supes want to know"
The article rehashes much of what has been in the news but includes a link to some of PhD licensed geotechnical engineer, Robert Pyke's communications with SFDBI & SG&H.
The article rehashes much of what has been in the news but includes a link to some of PhD licensed geotechnical engineer, Robert Pyke's communications with SFDBI & SG&H.
Not sure what you are referring to RFreund. In the recent test piles and casings, not much. Mostly just more careful monitoring of plug depth in the casings and water and air pressures for the piles. Back at the time of the indicator piles, the first 24-inch pile did not carry the casing down into the Franciscan when they drilled the socket, and the socket did not hold when they tried to do a pullout test. So, they did a second indicator pile carrying the casing down into the socket and then withdrawing it as they tremied the grout in. I assumed that they stopped withdrawing the casings around the top of the Franciscan but recently I read somewhere that they pulled them up to 20 feet above the top of the Franciscan - not sure about this. As discussed above, for the production piles the grout is pumped down a hollow central rod. In the two indicator piles the central bar was solid and had Osterberg-type load cells inserted in it and the grout was pumped down a separate tremie pipe.
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Thanks for the link epoxybot... looks like half a day's read for this weekend. I thought the info at the comment below was interesting and I thought the coclusion was a bit weak.
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
Dik, were you referring to Tim Redmond's conclusion being a bit weak? I assume so. I think that in his third last little paragraph he mixes up Dr. Pyke's position with Mr. Hamburger's. I don't believe that Pyke has opined on what tilt might be acceptable, although I don't know what he said to Redmond. And even Hamburger has said no problem in earthquakes up to a total tilt of 48 inches, not more tilt of 48 inches. And I think he acknowledges that other problems with elevators and plumbing might arise if the tilt exceeds 30 inches. I might add that I am not sure that anyone can calculate what tilt might be acceptable in a major earthquake with any accuracy.
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Walnut said:
Sorry, I should have been more specific. As I understand they started to install the "repair piles" down to rock. However, this accelerated the settlement so they stopped. The articles that I was reading said they then "modified the installation method". I'm curious to know what about the original method caused the accelerated settlement or what they changed to avoid this in the piles they are currently installing.
RFreund well that's a long and incomplete story! But the short answer is that I think the basic problem with installing the 24-inch piles is that they are using a rock bit. Fine for drilling into the Franciscan but not so good for drilling through soils. This really only came to light due to the monitoring report written by Ben Turner of Dan Brown & Associates, which I am guessing is why he was sent home! Some excerpts:
During drilling, air pressure varied from approximately 190 psi to 220 psi, water pressure varied from approximately 200 psi to 240 psi, and the water flow rate was approximately 20 gpm.
During each of these stops, the operator lifted the drill approximately 1 ft off the base of the excavation (i.e., by withdrawing the drill string up into the 24-in casing) and air continued to be circulated through the face of the tool and up the return line at approximately 200 psi. It is necessary to keep circulating air through the drill in order to continue flushing remaining spoils up and out of the return line; if the air pressure were to be abruptly stopped, the spoils would fall out of suspension and could cause clogging of the drill upon restarting. This is notable because of the potential for the drill to unintentionally “mine” soil during these intervals when air is circulating but downward drilling progress is not occurring. The greatest potential for unintentional mining would be in clean sand layers such as were encountered near the base of the Alameda Formation.
The pour log indicates that partway through pouring from the fourth grout truck, while the 24-in casing was being lifted such that the casing tip passed through the depth interval of approximately 260 ft to 256 ft below the top of guide wall, the grout level dropped from 19 ft to 37 ft below the top of guide wall. As grout pumping continued and the casing was withdrawn to the final tip depth of 243 ft, the grout level continued to drop to a low point of 67 ft below the top of guide wall before beginning to rise again. In total, the grout level dropped 48 ft over a period of about 30 minutes, corresponding to about 5 yd3 of grout loss plus the additional volume that was pumped during this time.
Note that the air and water pressures used to lift the cuttings are like 7 atmospheres! They did make a change to the device in the drill bit that cuts a slightly larger hole than the nominal diameter of the drill bit, but I doubt that made much difference. The biggest change was that Center Rock, the supplier of the drill bits and DBA were more carefully monitoring the pressures and so on. That didn't make much difference either.
During the actual installation of the piles there was a quite dramatic drop in the pore pressures in the Old Bay Clay. It is not very clear, at least to me, what this results from. Does the high-pressure reverse circulation have the effect of "sucking" water out of the clay, or is it just overmining creates a cavity into which water can flow? This drop in pore pressures and increase in effective stresses is then rather quickly reversed but some permanent settlement should result from that cycle of loading, in addition to settlement that results from overmining. For some unknown reason there has typically been a delay of several days before grouting, so that does not help. I believe that Jaxon van Derbeken is chasing down that story.
During drilling, air pressure varied from approximately 190 psi to 220 psi, water pressure varied from approximately 200 psi to 240 psi, and the water flow rate was approximately 20 gpm.
During each of these stops, the operator lifted the drill approximately 1 ft off the base of the excavation (i.e., by withdrawing the drill string up into the 24-in casing) and air continued to be circulated through the face of the tool and up the return line at approximately 200 psi. It is necessary to keep circulating air through the drill in order to continue flushing remaining spoils up and out of the return line; if the air pressure were to be abruptly stopped, the spoils would fall out of suspension and could cause clogging of the drill upon restarting. This is notable because of the potential for the drill to unintentionally “mine” soil during these intervals when air is circulating but downward drilling progress is not occurring. The greatest potential for unintentional mining would be in clean sand layers such as were encountered near the base of the Alameda Formation.
The pour log indicates that partway through pouring from the fourth grout truck, while the 24-in casing was being lifted such that the casing tip passed through the depth interval of approximately 260 ft to 256 ft below the top of guide wall, the grout level dropped from 19 ft to 37 ft below the top of guide wall. As grout pumping continued and the casing was withdrawn to the final tip depth of 243 ft, the grout level continued to drop to a low point of 67 ft below the top of guide wall before beginning to rise again. In total, the grout level dropped 48 ft over a period of about 30 minutes, corresponding to about 5 yd3 of grout loss plus the additional volume that was pumped during this time.
Note that the air and water pressures used to lift the cuttings are like 7 atmospheres! They did make a change to the device in the drill bit that cuts a slightly larger hole than the nominal diameter of the drill bit, but I doubt that made much difference. The biggest change was that Center Rock, the supplier of the drill bits and DBA were more carefully monitoring the pressures and so on. That didn't make much difference either.
During the actual installation of the piles there was a quite dramatic drop in the pore pressures in the Old Bay Clay. It is not very clear, at least to me, what this results from. Does the high-pressure reverse circulation have the effect of "sucking" water out of the clay, or is it just overmining creates a cavity into which water can flow? This drop in pore pressures and increase in effective stresses is then rather quickly reversed but some permanent settlement should result from that cycle of loading, in addition to settlement that results from overmining. For some unknown reason there has typically been a delay of several days before grouting, so that does not help. I believe that Jaxon van Derbeken is chasing down that story.
Milliontown
Geotechnical
Wow, I had no idea they were using a down the hole hammer for the entire depth of the pile. Even with casing that is unusual to me for such soft soils. I would have through a bit more consideration would have gone into the installation procedures because of the high profile and cost of the project. I would have proposed a continuous casing (temporary or permanent) with a standard soil auger to rock before using the DTH bit. It would be more costly, but not as expensive as repairing a repair.
@Walnut - thanks for the explanation. It seemed like most articles just say "there was more settlement during repairs, so they modified the installation procedure" and moved on, but never actually talked about why there was more settlement or what they did to correct this.
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In the event the tower takes on a real serious tilt, who cleans up the mess and who pays for it? The condominium owners? As an added expense, should San Francisco have them put together a fund to 'clean up'?
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
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