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Stepped pile design

Stepped pile design

Stepped pile design

Contractor is having a difficult time sourcing an 8ft diameter casing for the desired length (>40ft). They are capable of sourcing a 7ft diameter casing quite easily, though. Anchor bolt template requires an 8ft diameter (min.) to accept the template with adequate cover. So the "new idea" is to use a stepped pile: 8ft diameter to accept the anchor bolts and then constrict it to 7ft diameter down to the bearing surface.

I'm analyzing the lateral pressure, settlement, and capacity using Allpile. I am looking for some insight in the behaviour of the pile at the step location. See the attached sketch.

Questions that arise for me:
1. What is the appropriate way to detail the step joint? I need to develop the rebar in the 7ft section by extending them into the 8ft section. Ideally I would just run them straight up to the top but I don't have the space with the anchor bolt template diameter (unless I provide greater than standard cover to run the bars on the inside.
2. If there is a large zone from 12ft to 35ft depth that is soft soil (saturated loose to compact sand, susceptible to liquefaction), wouldn't the step joint be even more susceptible to movement and instability?

The foundation is for a telecom tower: low axial, high bending. Everyone is "aware" of the soil implications on site. Groundwater is at the surface. They have decided that a large pile cap with multiple piles is really not feasible.

I'd rather run a single pile to whatever depth than a stepped pile and I'd rather have a new site rather than this one, but I'm at least trying to work with what's there. Maybe just run the pile all the way to middle earth???

Also, is there a way to model the liquefaction scenario in Allpile? I've tried 0kPa skin friction in the zone and also the distributed load feature but, frankly, they don't seem to make a super noticeable difference.
EDIT: I'm looking into getting the ground motion parameters as per the building code foundation requirements for sensitive site soils.

RE: Stepped pile design

1 - I think it is best to run straight up as you have indicated and allow extra cover. The contractor wont appreciate a difficult / complex bar bending arrangement. Keep it simple and make life easier for everyone. Its one pile so a little extra concrete isnt a big deal. If you could post the anchor bolt template arrangement I am sure the structurals could give you some reinforcement recommendations.

2 - If the pile is poured in one hit then it wouldn't really matter. When the soil liquefies the pile shouldnt settle as it will be (should be) founded in non liquefiable material. Yes there is reduced lateral support in an earthquake situation but you have 12ft of non liquefiable (maybe you dont?) at the surface and then your bearing layer which should give you some support. As long as the pile is not near a lateral spreading area (river, steep change in topography etc) i dont think it would be an issue. Wind loading is your critical load I suspect but I think you dont consider your wind loading and earthquake loading simultaneously as it is too conservative.

You cant model liquefaction in Allpile, you need liquefy pro or something similar. Also liquefaction assessment are typically based on free field assessment (i.e. no load). If you are looking at modelling a load and liquefaction together you need a FEM program like Plaxis or Flac.

RE: Stepped pile design

It won't look near as worrying if drawn to scale. You've drawn 8' on 4'.

How long are the anchor bolts and what size is the reinforcement? Seems like the 7' reinf could terminate below the bolts and simply gap-lap with the 8' reinf if nothing is massive.

RE: Stepped pile design

Based on the information provided, there is no red flag to me that a pile cap w/ several piles below would not work.

What is the reason it is not feasible? Seems like it would be the best option for constructability.

Any chance of modifying the anchor rod pattern?

RE: Stepped pile design

@Motorcity: I believe cost is the main deterrent for a pile cap w/ 3 or 4 piles. Structurally, it provides the best option to reduce movement. I'm considering designing two options and identifying one as a better option to limit movement...but I'm a little hesitant of that practice.

There's probably little chance to modify the anchor rod pattern. Outer diameter of the ring is about 76", rods are on a 72" dia. and 52" in length.

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