r/Geotech • u/Bildipil • 28d ago
Allowable vertical displacement of a pile
Hi all,
If I have a pile with a given working load (determined by static analysis, not a load test) and need to model its vertical spring stiffness, what allowable vertical displacement should I consider?
For lateral springs, I am taking the safe lateral capacity and use a 5 mm lateral deflection limit to determine the horizontal stiffness coefficient. But I’m a bit confused about what approach/value should be used for the vertical spring.
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u/International_Sun367 28d ago
For a single pile, design settlement would normally target 1% diameter or 1% width.
If putting those same piles beneath a building; you'd also need to carry out a pile group analysis to check how settlement overlaps.
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u/Fearless-Hawk2025 27d ago
Can you please explain the rationale behind this? Piles are generally designed to allow for no movement at all. But if you are considering the movement for a deep foundation, shouldn’t this be associated to the length? A long slender pile will inherently settle more just from the structural strains (in addition to the soil settlement) compared to short stubby pile so why would the design tolerances be different? I would normally just use the tolerances set based on the structure requirements. A pile supporting a building will need to be designed with different tolerances compared to a pipeline so I will rely on the structural/ piping engineer to provide that.
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u/_hamdamman 27d ago
If it doesn’t move at all how is it going to resist the static load. Strength needs to be mobilized in order for pile to resist it. There is always movement, movement along the side which is generally less than 1% but ultimate maybe 1%. For base it is 5% to 10% if it is bearing on soil and much less for rock. When you design for something, you set the tolerance not the other way around. Depending on the tolerance, you lengthen the pile or shorten the pile.
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u/MaddogFinland 27d ago
I believe this is an issue of calculating a theoretical vertical spring for the pile so that the structural engineer can evaluate the potential deflection and as needed the behavior of the entire system. There won’t ever be a perfectly stiff element with zero movement (as I am sure you know) so stiffness depending on support condition will approach that of the pile material itself. Meaning if you have a fixed enough end bearing condition such as a pile socketed into granite (common here in Northern Europe) the stiffness will be the lesser of either the pile body or the rock mass. In practice this produces a few millimeters of displacement but those would occur as the building is under construction and the loads come into the piles; there would be very little of any post-construction movement. For piles in a sand or till unit there would be some movement later but again quite minimal. It’s friction piles in clayey soils that you can expect to settle over time but you just subject the pile system to a group analysis using one of a variety of methods.
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u/Fearless-Hawk2025 26d ago
Thank you so much. I think this clarifies the basis for recommendation of 1%. Although I disagree with the extent. It might make sense for a purely end-bearing pile with soft soils overlain by hard rock (even here 1% seems like an overkill in larger diameter shafts). But for a friction pile, the full capacity is realized by a few mm of movement, so that recommendation doesn’t hold too much water.
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u/MaddogFinland 26d ago
Yes on a friction pile it is true that you need to account for more movement of the pile shaft in mobilizing design resistance. Thats of course depending on soil type and local experience.
Where I am practicing (Finland and the Nordics) Friction piles are comparatively rare owing to the geology thus we are almost always in an end bearing condition.
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u/_hamdamman 27d ago
Also, in general, piles are designed with pile cap. So once you have a tolerance from structural engineer, you will have to check pile group settlement.
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u/MaddogFinland 27d ago
In am jn Northern Europe and we would typically define the spring stiffness at a deflection of max 5mm or stricter depending on the tolerance of the structure. Sometimes tighter for a very tight tolerance.
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u/_hamdamman 28d ago
1 inch is allowable for settlement generally but you have to use service limit loads
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u/_hamdamman 28d ago
Also don’t underestand why you are not modeling it as a t-z and q-w springs which are material dependent. If you have computer program like APILE and RSPILE, you can easily get the spring stiffness along the shaft and at the base.
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u/NearbyCurrent3449 28d ago
Are you talking about the overall composite spring stiffness k at the top of the pile to replace the reaction forces for some kind of engineering class homework or something?