Stress distribution of a rock socket 1

[peelwvu]

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

 

[dik]

Can you move the caissons away from the tunnel, and put a large transfer beam over the tunnel?

Dik

 

[PEinc]

Why not just use cased micropiles bonded into the rock below the bottom of the subway?

http://www.PeirceEngineering.com

 

[EireChch]

Firstly i will start by saying that I do not have much rock mechanics experience but even so I think you will need to do some FEM analysis to determine the stress on the subway. Something like Plaxis or FLAC.

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?

 

[SlideRuleEra]

If the shaft can be drilled deeper, a method we used for electric generating station foundations may avoid the problem:

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[WARose]

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.

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.

Also we have concerns about whether the vibrations during construction will be too much for the subway wall or the surrounding buildings?

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).

 

[PEinc]

We just had a job with the same problem. Used micropiles.

http://www.PeirceEngineering.com

 

[EireChch]

A number of methods have been put forward about how to approach the problem, however no one has given a proposal for how the OP is the determine the stress on the subway wall. Correct me if I am wrong but I don’t think there is “standard” method for assessing this? Not like boussenisq theory for soil?

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?

 

[WARose]

A number of methods have been put forward about how to approach the problem, however no one has given a proposal for how the OP is the determine the stress on the subway wall. Correct me if I am wrong but I don’t think there is “standard” method for assessing this? Not like boussenisq theory for soil?

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.)

Also, If the rock is solid intact rock, will the subway even know that the caisson/micropile etc has been constructed?

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.

....for your micropiles, what size and spacing would you typically use? Wondering would they have sufficient capacity?

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.

 

[fattdad]

My take, go with Slide-Rule's approach.

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!

 

[peelwvu]

Thanks everybody. Here's a follow up. We're looking at loads of the following:
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.

 

[PEinc]

EireChch, micropiles can carry very high loads. I have done micropile jobs with design loads of at least 250 tons (500 kips). Casing pipes vary in size, but 5.5" to 13.375" outside diameter are available. Larger diameter casing are used foe higher loads and for micropiles with lateral loads. Wall thickness can vary, but about 0.4" to 0.595" wall thicknesses are available. Check

http://www.skylinesteel.com

Micropile spacings can vary but usually they are spaced at least 3 diameters apart.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

peelwvu - We used this method in the early 1980's, so no computer modeling was performed. However the generating station is in high wind (hurricane) and high seismic area (coastal SC), so significant consideration was given to lateral loading. For several reasons, all foundation loading was taken care of with a very conservative design.

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[WARose]

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?

LPile could probably model the situation for you. (Or pretty much any other FEA software as long as you get the right spring constants.)

 

[hokie66]

I like SRE's approach too, but with some modification. The shear and moment would scare me with the sand buffer.

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.

 

[Okiryu]

What is the minimum distance between piles recommended by the geotechnical engineer? If that distance is less than your 5 feet between the socket and wall, perhaps there may not be significant stresses acting in the wall. Otherwise SRE’s suggestion is good.

 

[cvg]

Usually coring work doesn't produce high levels [i.e. amplitudes] of vibration.

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?

 

[EireChch]

PEinc/Warose - cheers for the information. Micropiles are less common this side of the pond i think. Hence when i think of them I always think of a small dia (5.5'') with one bar in it, but good to know they go up to 13.375''. As WARose said, there is probably huge capacities available in that rock. Structural capacity of the pile itself would be controlling, not sure if a micropile would have capacity for the large moment. Maybe, maybe not. Depends on what the soil is above the rock too.. OP can you tell us that?

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.

 

[WARose]

[blue](cvg)[/blue]

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?

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.)

[blue](Okiryu)[/blue]

What is the minimum distance between piles recommended by the geotechnical engineer? If that distance is less than your 5 feet between the socket and wall, perhaps there may not be significant stresses acting in the wall.

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.)

 

[dik]

PEinc: Skyline steel just had a webinar (like 15 minutes ago) on reinforcing and there was a bit of a note on micropiling construction, installation, size, and capacity. I've requested a *.pdf file of the presentation and will post it on line.

Dik