Shoring Design Loading 1

[jayrod12]

A couple of us in the office are having a debate regarding shoring and I'm looking for more opinions.

Back story, there's a ton of shoring going in at a WWTP to facilitate expansion. The existing buildings (let's call grade elev. 100') are on driven piles with average t/o pile cap at elev. 88'-0". The new building foundations require an excavation down to elev. 76'-0" (24 ft change from grade, 12ft below existing t/o pilecap.

For some reason or another outside of our control the client/contractor is unwilling to excavate right up to the existing buildings which would result in the shoring only having to be 12ft tall, but rather are mandating that the shoring be full 24 ft height, but still as close to the existing buildings as possible (some areas we can get to 3 ft away, others 10').

Now to the question. When you are 3 feet away from the building would you still design it using the full 24ft height of soil in the soil pressure determination, or for the top 12ft next to the building do you treat it as a surcharge load and begin your triangular (or trapezoidal) pressure distribution at the bottom of the foundation wall.

What about if you are 10 ft away? And where between those two numbers would be your cutoff between full pressure on the wall and almost none until the underside of existing foundation?

Hopefully my sketch is somewhat helpful in deciphering my question.

 

[jayrod12]

OR option 3 (Which is maybe closer to reality if you ask me)

If you put the raker brace at the 12' mark, do a triangular pressure distribution for the upper 12' just like a standard cantilever retaining wall(0psf at top of wall, Ka*gamma*12 at 12' mark). Then for the lower 12' use a modification of peck's trapezoidal pressure for braced excavations in clays (Ka*gamma*12 at 12' mark, increases to Ka*gamma*24 at the 15' mark and stays at that value until the 21 ft mark and then decreases to zero).

Does this seem at all reasonable?

 

[PEinc]

If the building were not on piles, you would need to excavate down to the pile cap and then underpin the building. This would usually require removing the soil from against the foundation wall. This is done all the time. The contractor's concern seems overblown.

That being said, I don't believe that there is a good way to simply analyze the sheeting wall as shown in your sketch. This has been done different ways for different projects.

Perhaps you could remove only 6' of the 12' overburden and then design the sheeting wall for full earth pressure from an 18', single tier cut, without any building surcharge. You would be conservative and the contractor may feel comfortable than if he had removed the full 12' of soil.

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[jayrod12]

I think it's less of concern for my client (The contractor) and more of the fact he's getting paid unit prices per sq ft of shoring. They bid and won the job assuming full height shoring everywhere and so that's what they want to do.

Because of this I'm inclined to say design for worst possible case then. However this is a regular client that brings lots of work to the office and so I've been told by the higher ups to investigate this possible reduction of loading.

 

[dcarr82775]

You can't ignore the upper 12-ft. I assume the grade at the far right side of the building is about 100, and that soil is still pushing your building into the excavation. Depending on the original design you might be able to take the load from the upper 12ft and apply it as the basement slab, or you might have to split it up and apply 2/3 and the basement and 1/3 at grade.

There are 1001 ways you could do your loading (a trial wedge would account for the sliver of soil to answer one of your initial questions). I don't see anything wrong with your option 3, although I would do a couple things differently:
- I would not truncate the top portion of the apparent pressure
- I would add the contribution of the load from the far end of the building as point loads
- I would have some nominal uniform surcharge to account for the couple feet of soil between the building and the shoring.

Don't forget about checking base heave, not sure how crappy the clay is.

 

[jayrod12]

Pretty good clay. Base heave doesn't appear to be an issue.

I'm looking at modifying option 3 to a rectangular pressure distribution for the top 12 feet. I also like the idea of accounting for a point load at the base slab.

We will be accounting for a 250 PSF equipment and material surcharge everywhere even though it's unlikely to happen in that 3 foot gap.

 

[PEinc]

"We will be accounting for a 250 PSF equipment and material surcharge everywhere even though it's unlikely to happen in that 3 foot gap."
Are you serious?

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[jayrod12]

Yes. Maybe I didn't make it clear however the 3 foot gap area is intended to be closed off and remain untouched during construction, i.e. they already have excavation stockpile locations far away from the existing buildings, no excavation machinery is to be used in this said 3 foot gap. In fact, the only "surcharge load" in this area would be from the workers installing the lagging. I highly doubt a few guys with a bundle of lagging are going to put a surcharge load of 250 PSF on that 3 foot gap between the existing building and the shoring wall.

Perhaps you'd need to see a plan view to understand why my comment would apply however an NDA won't allow me to post it.

 

[SlideRuleEra]

Jayrod12 - Although it won't affect your calculations, I would want to know "How long are the existing piling, and were they designed as friction or point bearing piling?" For an existing building you may not be able to get exact answers, but I would make an attempt to find out (or make reasonable assumptions based on known information).

This would enter into the decision on how conservative (or not) to be with the loading assumptions. For example I would think twice if the piling are, say 20' long, but be less concerned if for 50' in length.

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[KootK]

In these situations, I often feel that it is the original building that gets short changed when it comes to engineering attention. Two possibilities that I shudder to even voice include:

1) Did the original building rely on passive pressure against the piles or pile caps to resist lateral? If so, those loads would now be transferred into the basement walls of the addition.

2) The existing piles now have an un-braced length of twelve feet. Bracing restraint will have to come from the shoring in the short term and the basement walls in the long term.

I think that the existing piling situation would affect the calculations. If the piles are solely end bearing, then there should be no surcharge due to the building at all. If the piles utilize skin friction over the length adjacent to the shoring, then that skin friction will translate into lateral surcharge. And that's a lateral surcharge that will be concentrated near the piles rather than distributed uniformly along the shoring.

Realistically, relaxation of the shoring will probably impair the ability of the piles to develop their friction capacity. Good for the shoring; perhaps bad for the piles. Usually, geotechs tell me to ignore the upper 6' or so for friction to account for soil disturbance and frost action. That might play in your favour here.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[PEinc]

Good point SRE.

I would not add a surcharge in the 3' space between the sheeting and the foundation wall. I hope they intend to monitor the building before and during construction. I would also try to lower the top of the sheeting wall so that the raker braces can be installed at the middle of or near the bottom of the pile cap/wall footing. It also may be hard to not lose backfill soil along the existing foundation wall when they are installing the lagging for the 3' wide area. This is old backfill that may not have been well placed and compacted.

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[JedClampett]

In my opinion, I would design for the full 24 feet in a triangular loading. I would use active pressure plus some surcharge. Whenever I asked a Geotechnical Engineer about this, they never gave me any relaxation on it.
But I have two questions:
1) For shoring, don't you delegate the design to the contractor? He hires a geotechnical engineer and/or a structural engineer to do these calculations and seal them. Then you do a cursory review, make sure there's no dead cats in there and leave it alone. Why expose your company to the liability of this being a problem (actually, there's no way to avoid liability, but you want to separate yourself from the design as much as possible)? And interfering in calculations is a sure way to assume liability.
2) If this is just an office discussion, I encourage it, but if you're trying to save your client money and the contractor already bid it, you're lucky to get a dime for every dollar saved.

 

[jayrod12]

We're the ones the contractor hired this time

We it is more office discussion as given the timeline and the answers that would be required from the geotech (Which would likely take 2 weeks given the required chain of command) we are going the conservative route.

The existing piles roughly 50' piles. They are designed I believe as mostly point bearing as the till layer occurs right around that depth. The new piles are expected to refuse at the same depth as existing. I agree that this fact weighs in to how conservative to be with the loading. And the existing piling isn't just perimeter piling but rather structural slab forest of piles under the entire extent of the existing building. To me that also plays in to how conservative to be.

Either way, it doesn't really change the design philosophy it's more an intellectual debate on what is reasonable.

 

[KootK]

Yeah, forest of piles feels good to me.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

Agreed, I'd be worried more if it was a dinky perimeter piles at some large spacing but it's something like 14" hex piles at 10'-12' spacing, everywhere.

The new expansion is in the range of 2300 hex piles driven to till. It's going to be a whole lot of pounding. almost 3 months of driving if everything goes perfectly (roughly 35 piles a day). Who knows how long if they start losing piles or hitting boulders.

 

[RFreund]

A few comments/questions:

If I understand correctly they are excavating one side of the building effectively turning this into a "walkout basement" condition.

1. There are many ways to look at this.
2. If this were permanente I could see making an argument for using 24' full soil but for temporary shoring I'd say that's a bit conservative. There are many redundancies here including shear friction on the sides of the building and the bottom of the structure and the lateral resistance of the piles to resist the pressure applied to the other side of the building.

3. Check out narrow retaining walls as for the 3' and 10' cases and how to reduce the lateral soil load accordingly. I would do a trial wedge most likely or look at FHWA-CFL/TD for some background. I may have a trial wedge spreadsheet on my website but I'd have to check. Anyway, this reduced pressure would be applied at top from 0 to d (it would not be zero). Then this soil is also applied as a surcharge on the soil below and I'd use an apparent pressure distribution (although you probably could justify triangular) You could use elastic methods (Bousinessq) or other methods to evalute the surcharge effect of 3' or 10' strip of soil. Again this is not the full height x the soil weight x Ka as there is not 12' of soil there.

So in the end option 1 seems conservative. Option 2, I would add a reduced load above and reduce the surcharge load you show.
Also FHWA has method for single and multilevels of bracing FHWA-IF-99-015. It can be adapted for stratified soils, I like.




EIT

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[jayrod12]

Thanks RFreund. Your last comment about option two is more or less what option 3 was. Might even be more conservative than you're suggesting.

Thanks for all the answers guys (and gals if there are any that were part of this discussion). Like every other time it's nice to see the varied opinions and reasoning.