Stress distribution of a rock socket
Stress distribution of a rock socket
(OP)
We have a bridge project where the bridge pier spans a subway. In order to avoid loading the subway it was proposed that we put caissons on either side of the subway to carry the load from the bridge. The top of the subway is approximately at the top of the rock so the caissons will be socketed into rock alongside the subway walls. From the outside edge of the rock socket to the subway wall is 5 ft. There is no room to move the caissons further away because then we have conflicts with buildings.
Can anyone give me a reference to determine the stress that will be imparted to the subway wall from the rock socket. The rock sockets are 16 ft. long extending to about the bottom of the subway and are carrying some pretty substantial loads.
Is there any way to minimize the stress on the wall? Also we have concerns about whether the vibrations during construction will be too much for the subway wall or the surrounding buildings?
Thanks
Can anyone give me a reference to determine the stress that will be imparted to the subway wall from the rock socket. The rock sockets are 16 ft. long extending to about the bottom of the subway and are carrying some pretty substantial loads.
Is there any way to minimize the stress on the wall? Also we have concerns about whether the vibrations during construction will be too much for the subway wall or the surrounding buildings?
Thanks





RE: Stress distribution of a rock socket
Dik
RE: Stress distribution of a rock socket
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RE: Stress distribution of a rock socket
Is the rock good competent rock? Can you provide description, UCS, RQD info etc. If the rock is good then there may not be much stress transferred to the subway?
RE: Stress distribution of a rock socket
Drill an oversized shaft to the bottom of the subway.
Drill the correct diameter shaft for the rock socket inside the oversized hole.
Use corrugated steel pipe (same diameter as the rock socket hole) as stay-in-place concrete form inside the oversized hole.
After caisson construction, backfill the annular space outside the corrugated steel pipe with sand.
We constructed several hundred 4' diameter caissons using the method.
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RE: Stress distribution of a rock socket
What kind of loads are being delivered by the caissons? Just axial or moments as well? If it includes the latter.....you will have a stress distribution on the side walls of the socket. I have always approximated this like a rigid post embedded in soil. There are a couple of wood reference handbooks (that you probably have laying around the office) that can give you this stress distribution.
Usually coring work doesn't produce high levels [i.e. amplitudes] of vibration. If you are worried......you may want to get with the contractor and find out what he is using (and then contact the manufacturer directly).
RE: Stress distribution of a rock socket
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RE: Stress distribution of a rock socket
Also, If the rock is solid intact rock, will the subway even know that the cassion/micropile etc has been constructed?
PEin, for your micropiles, what size and spacing would you typically use? Wondering would they have sufficient capacity?
RE: Stress distribution of a rock socket
Me personally, I like SlideRuleEra's solution.....it pretty much eliminates any load on the subway tunnel.
But if you had to put a load on it....the first question is what type of loads are we talking? (Which is an unanswered question I had earlier.) If it's a load on the socket side walls from overturning moments.....then that will translate into a shear stress on the rock between the tunnel and the socket. Allowable shear stress on rock (probably the most critical among all the stresses developed) is discussed in this thread:
http://www.eng-tips.com/viewthread.cfm?qid=260979
As far as the force/stress distribution on the tunnel wall from the forces generated by the socket......it would seem to me to be prudent to do some sort of 3D/solid FEA solution (or just assume a conservative "spread out" of the pressures). Then compare that to some of the allowable compressive/shear stresses for the rock. (Hopefully there was a good sample taken of it.)
My approach would be: if the "rock" in the FEA model didn't give (i.e. displace significantly) and didn't exceed any allowable stresses.....the tunnel probably wouldn't see a thing.
You'd have to be careful about your model though: I would probably discount some width of rock near each edge due to cracking from the installation of each (i.e. the socket and the tunnel). That would bring up another question: how was the subway tunnel put in? Blasting? That's another joker in the deck.
I'd take all the guess work out of it by running with SRE's idea.
My 2 cents.
If you've got good bedrock.....you don't have to worry about capacity when it comes to micropiles. I've gotten hundreds of kips out of them before. A bonus with them: their installation produces virtually no vibration. Good suggestion by PEinc.
RE: Stress distribution of a rock socket
Regarding the OP?
You would calculate each foot of length and its vertical shear stress imparted by the interface friction. Convert that to a point load acting at that depth and use the radial elastic solutions (and the closest position of the subway) to figure the delta sigma H and the delta sigma V for the location of the subway.
You'd accumulate stresses and create a design stress envelope.
Maybe I'm missing something?
f-d
ípapß gordo ainÆt no madre flaca!
RE: Stress distribution of a rock socket
Axial - 2034 K
Shear - 445 K
Moment - 3233 K-ft.
Rock Type is Schist/Gneiss. First 5 feet is weathered then very hard competent rock - Co > 6000 psi.
PEinc - We looked at micropiles but we don't think we can get enough batter out of them to resist the shear. This is because of the limited space we have at the surface and the depth of the subway.
Sliderule - I like this option. We discussed it as well. Our deflection criteria is, 0.5" at service and 1.0" at strength. We weren't sure if the sand would allow us to meet that. Do you have any experience with this and how you modeled it?
Thanks again everyone.
RE: Stress distribution of a rock socket
Micropile spacings can vary but usually they are spaced at least 3 diameters apart.
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RE: Stress distribution of a rock socket
The 4' diameter caissons (rated 400 tons, if memory is correct) are typically about 30' long. Fifteen to twenty feet (length of the corrugated steel pipe) through poor quality overburden and weak sedimentary limestone. Rock socket 15 to 10 feet long in hard sedimentary limestone. Caisson design relies solely on friction in the rock socket. No point bearing considered because of the prevalence of solution voids randomly located in the limestone (always the possibility of an undetected void just below caisson tip). The station survived a direct hit from a Category III hurricane that slightly exceeded design wind speed with no structural damage.
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RE: Stress distribution of a rock socket
LPile could probably model the situation for you. (Or pretty much any other FEA software as long as you get the right spring constants.)
RE: Stress distribution of a rock socket
Is it possible, somehow, to cast the top of the pile against the rock on one side and backfill with sand only on the side nearest the subway? That way, the lateral load can be taken all to the rock on one end of the bridge girder.
RE: Stress distribution of a rock socket
RE: Stress distribution of a rock socket
for a larger diameter hole (oversized to fit the 4' diameter cmp as suggested by slide rule) drilled through schist/gneiss, than wouldn't they use a reverse circulation) rock bit, not a core drill? seems like a fair amount of vibration from that next to the subway. something like this?
RE: Stress distribution of a rock socket
F-d, i think one of the tricky parts would be determining the portion of the load taken by side friction and the portion taken by end bearing. I suppose it would be conservative to assume its all taken in side friction of the rock socket. That may even be how the OP designed.
RE: Stress distribution of a rock socket
Good point. Although I have put in piles of that size into bedrock and heard no complaints about vibration. But it never hurts to ask contractors about what they plan to use.
On the other hand it's a point for micropiles: I've asked those contractors a bunch of times if it will produce any appreciable vibration. They all say no. (Some of them will even put it in the contract upon request.)
I can pretty much promise you those distances are typically based on group action factors. (I.e. available skin friction capacities, lateral pressures in a group in soil, etc.) When they give you that number, they aren't thinking about pressures developed on rock from a socketed in pile delivering moment. I've asked them about this type of thing in the past and they've never been too sure. (Everyone of them gave a different answer.) I specifically had one where the moments were so high (not that far off from what the OP has).....I thought the pile might get pried right out of the socket. So I did something similar to my long-winded post above to be sure. (Didn't have to worry about an adjacent subway though.)
RE: Stress distribution of a rock socket
Dik
RE: Stress distribution of a rock socket
RE: Stress distribution of a rock socket
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RE: Stress distribution of a rock socket
PEinc, we don't think there's going to be enough room to fit a pile cap for micropiles.
We decided to present the option of drilling a larger diameter hole along the subway and backfilling the annular space with either sand or bentonite. We saw the use of bentonite on another project. Does anybody have any other experience using this?
RE: Stress distribution of a rock socket
We used it, in powder form, for the (relatively) impervious liner for a coal bottom ash pond. Layers of compacted soil, both above and below were used to contain the bentonite. For the next pond, switched to a manufactured bentonite pond liner (the carpet-like structure of the liner, plus a soil layer covering, provides bentonite containment).
Also have used a perimeter bentonite slurry trench to cut off flow of groundwater into a temporary excavation.
Surely there is something better than bentonite... at least the type bentonite I'm familiar with.
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