Lateral Capacity Piles | Options

[bookowski]

Maybe this should be in geotech forum... but

I have a relatively long (~175ft) building that is relatively light (steel job). Due to nearby below grade structures we are required to use piles even though the soil is decent. The problem I'm having is that the number of piles required for vertical load is far less than that required for lateral load. The piles that we are using based on the geotech report/design have 100T compression capacity and 4T lateral. Piles are expensive so adding piles for lateral base shear only is a tough pill to swallow.

I'm thinking that this must be a common problem. Do you ever use a shear key for an overall building? If you add up all the pile caps they pretty much do it on their own.

 

[oldestguy]

Provide a cross section sketch. Who requires the piles? Is the nearby structure sensitive to pile driving vibrations?

 

[SlideRuleEra]

The traditional cost effective way to resist high lateral loads with driven piling is to batter some of them. This can be a problem with seismic loading, but there are ways to deal with that.

http://www.SlideRuleEra.net

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[bookowski]

Structure is adjacent to subway, they require no loads imposed on soil at/above - it's typical here and you are required to use piles to deliver load below the sub.

I'm told (by multiple sources) that no one batters piles in my area because in general these are very narrow sites, 25ft range typical so with 60ft to 100ft piles it's difficult.

 

[bookowski]

But my question was - has anyone used passive pressure on keys of some sort in a situation like this? I haven't seen it or been able to find anything but I don't see why it isn't valid assuming the soil is sufficient.

 

[WARose]

[blue](bookowski)[/blue]
But my question was - has anyone used passive pressure on keys of some sort in a situation like this? I haven't seen it or been able to find anything but I don't see why it isn't valid assuming the soil is sufficient.

The only time I've used a "key" is with a retaining wall. But to answer your question: Sometimes I have successfully used the passive pressure developed against the slab of a building (if I am satisfied that the ground adjacent to it will remain undisturbed, paving will generally guarantee that) to resist lateral loads.

 

[KootK]

One story? Lateral system is braced frames on perimeter grade beams? I've not seen keys exactly but, for low, low rise, I've seen:

1) Turn a blind eye altogether.

2) Assume passive / at rest / active pressure against caps and grade beam corners depending on your belief system. Evaluation of the movement required to develop the resistance is always a thing.

3) Make some assumption about the friction generated on the sides of your grade beams as a result of translation through the soil.

4) Tie your grade beams and pile caps to the SOG and assume that you're engaging every damn pile you!ve got.






I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

[blue](KootK)[/blue]

3) Make some assumption about the friction generated on the sides of your grade beams as a result of translation through the soil.

A note for bookowski: be careful with this. (Really any kind of friction if I understand your problem correctly.) Reason being: a lot of your vertical load should run directly into the piles......ergo it won't be available for frictional (sliding) resistance.

 

[IFRs]

Surround the pile with a partially buried or buried cap, ring or collar of concrete to increase the vertical contact surface area, this will increase the lateral force needed to actuate more soil and give you greater lateral resistance

 

[Retrograde]

I have mixed long piles for vertical load with additional short piles to take the excess lateral load the long piles could not take.

 

[bookowski]

Kootk - 10 story, dual system with braced frame + moment frames, two basement levels, yes to grade beams tying caps together. Because of adjacent bldg on one side and subway on the other I shouldnt be counting on any passive resistance at the basement walls (even though it's clearly there at 2 stories buried). I'm already assuming that over two basement level diaphragms I'm able to engage every pile. It's not really clear yet that this is entirely valid but if I don't do this I'm sunk. I might use a thicker reinforced slab on grade with ties to the caps to justify it more.

IFRs - This was basically an idea that the geotech had. We have 9.5" piles and he suggested that maybe for the top X feet they could enlarge it to increase the lateral. He wasn't sure about the analysis off the top of his head but thought it might be possible, if tricky to build.

Retrograde - This idea also came up and might be an option if there's no other way.

The allowable pile load is based on 1/2" lateral displacement. We're in good soil - it seems to me that there has to be a lot of cumulative other stuff helping, friction and passive etc. If I was on junk soil I wouldn't be so gung ho for that approach.

 

[IFRs]

I only mentioned because I had done it in the past. Adding additional levels to pipe racks they were not designed for, the thermal expansion of the pipe induces lateral loads into the foundation, we used an air or water lance to excavate around the piles and used the ground to form rectangular caps around the piles joining pairs of piles. The vertical height of these caps was calculated to activate enough soil to provide restraint. I think we did this every 20 feet for 500 feet of rack. The geotechs were on board the whole time.

 

[KootK]

Ah, that's a much larger scale thing than I had imagined with respect to lateral loads.

- If the soil pressure against the caps gets it done, I could certainly get behind that. I've seen engineers backfill with lean mix in attempt to reduce the displacement at which resistance kick in.

- Maybe the geotechnical engineer would consider improved values if you detailed a fixed head condition.

- How short is the short plan dension?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[structSU10]

Whenever I have gotten a lateral capacity for a pile, it has primarily has been related to the permissible deflection of the pile rather than strength. The 'sway' for a pile could be up to 1/4"-1/2" to develop 4T based on some that I have dealt with. That kind of movement is enough to develop passive pressure on the caps generally.

 

[bookowski]

Fixed head is another option. Short dim is ok. It's all wind controlled, 725k base shear in long dirn, a fraction of that in the other since it's long and skinny.

SU10 - Yes, the allowable for this is "1/2 the load which produces 1" of lateral deflection" - so 1/2" basically, assuming it's linear. I agree that this should be plenty to engage passive pressure, especially if we used Kootk's lean concrete backfill idea. Just need to get the geo on board. I'll go back and press this idea.

 

[Shu Jiang PEng SE]

Continuous underground wall foundation is an option in this case.

 

[KootK]

I asked about the short dimension because I was wondering if it might be feasible to run a stocky grade beam between foundation walls to act as a shear key. Preferably, one would hit a column or two on the way. Maybe not great for services.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RobertHale]

What is the amount of base shear you are looking at dissipating? Could you use something like a battered helical pier as a tension element? Just a thought.

Robert Hale, PE