Helical Pier Lateral Capacity 1

[Rabbit12]

I recently started a new job and got tasked to work on desing of an industrial facility that has had several jobs completed prior that are similar. Thus they had developed systems for supporting certain elements in this facility. Not wanting to re-invent the wheel per se, I've tried to get to a comfort level with what they have been doing.

The foundation "systems" utilize helical piers. These sites tend to be remote and we haven't to this date been getting any geotechnical data. My experience with helical pier design is limited, but from my experience it is possible to specify a minimum installation torque and correlate that to an axial pier capacity. I don't have much on an issue with this aspect. We assume a worst case scenario, plus the loads are low.

The issue I have is we are relying on the piers to resist lateral loads. For example, we are using cantilevered columns (fixed at the base and supported by helicals) to support piping by using lateral soil resistance values. Doing some research I was able get a copy of section #5 of AB Chance's Technical Manual. Section 5.7 in the introduction of the manual says that square shaft helicals do not provide any significant resistance to lateral loading, but round shaft helicals do. I can't figure out why there would be a significant difference in the two. I emailed the AB Chance technical marketing director and he didn't answer my question.

Anyone know why there would be a difference in lateral capacity of the differenty types of shafts? Anyone have any experience in lateral design of helicals that can provide a little direction? Maybe I'm over-thinking this issue.

 

[WillisV]

It has to do more with the size than the shape. Square diameter shafts are typically smaller than 2" in width, while round diameter shafts are available from 2 7/8" up to 4 1/2". If you area really going to rely on the shafts for lateral resistance, I would highly recommend doing a lateral test on-site and using the minimum depth and torque used in the lateral test as the basis for the remainder of installations.

 

[JLNJ]

If you are using three or more you can batter them so that the load is all axial.

 

[TLHS]

Your issue is going to be flexibility, not ultimate strength against lateral and moment loads. If you're talking about single T-Posts for pipes and you're effectively treating them as pure vertical supports with incidental lateral loads then this arrangement is likely fine as long as you're okay with some movement in the post. If you're talking about cantilevered columns as part of a frame, they may not quite work the way you expect unless you take the foundation flexibility into effect. Your foundation will be *very* flexible in comparison to your frame, so you have to model that somehow or your moments and deflections could be significantly off.

Another reason you may not want to use square shafts for lateral loads, is that while installing, the corners of the section will severely disturb the material and likely leave a very loose area of soil next to the sides of the shaft (in the area between the circle formed by the diagonal of the section and the actual walls of the section). So when you apply a lateral load, it'll have limited resistance until it deflects enough to hit the solid soil outside of that zone. I have absolutely nothing to back this up though, it's just me postulating.

 

[EngineeringEric]

The helical cap element name escapes me... there is a product "grout collar" (i made up the name now. not the real name) that is placed on the top helical lead or as needed. the collar is a plate say 12" diameter that the helical pile pulls into the ground. this leaves a open 12" dia x X' deep hole. the hole is then filled with grout/concrete and it is this sleeve can be used to help resist lateral loads. It is not much still and everything stated above still applies but it can help.

These are used when unpinning is used on a slope to provide additional stiffness to the helical pipe encase some soil is eroded the column won't buckle. they can be used to help you as well if designed right.

The best idea is to batter piles and use them as tension only elements.

 

[Rabbit12]

Thanks for the responses! All great points.

Unfortunately battering the piles is not an option as they are using one single vertical pier that creates a fixed column base with no concrete cap. Helical manufactures also somehow can grout around the shaft which can give increased resistance to lateral forces through soil bearing (essentially what Eric is referring to). But before I suggest a change like that, I have to make sure I'm 100% correct. Being new I have to tread lightly.

TLHS:

Yes. My main concern is actually the flexibility of the supports. One of the current details has a 2" diameter threaded rod just below a plate at the top of the helical (the plate is a baseplate for a W6 column). This gives the option of 2" of field height adjustment of the plate. But, the flexibility created by this link in my mind is huge. The other issue is fatigue of that link. With wind loading, it could see a significant number of cycles in it's life.

I also agree with your on the hollowing out of the hole with a square helical. I also have nothing to back it up, but in my head it makes sense.

 

[Brad805]

I suggest a soil spring analysis to resolve your concerns. We use screw piles quite frequently for signs and other lightly loaded structures and that is what I do. We have never used a square pile ever. I have a hard time wrapping my head around the idea behind the idea of square, but most of our screw piles are in the 4.5" to 12" diameter sizes (some heavily loaded situations) so they have never made much practical sense.

When you do your analysis I suggest you keep in mind that it is quite frequent for these to wander during the installation for a variety of reasons. The degree of misalignment depends on the installer as well as the equipment. I have had many a discussion with suppliers about the problems caused by adding a plate and stiffeners to install the column over in the correct location.

I would caution about having a lot of trust in the torque/axial capacity correlation. Without geotechnical information for the site it might be better to use an allowable end bearing.

 

[TLHS]

Brad,

Why don't you have a lot of trust in the torque/capacity correlation? I've taken a look at test results in a number of references and they seem to correlate pretty strongly. If you've got a proper safety factor, I'm pretty comfortable with it. Final torque isn't quite enough on it's own, though. You should have continuous monitoring to flag whether there's significant weak zones above that could effect things.

There's a bit of risk that you could have a soft zone underneath that somehow isn't enough to reduce your required install torque.

 

[Brad805]

I have dealt with screw pile failures for large bin structures as well as one tank settlement failure. Both those failures were the result of contractors not achieving an appropriate torque spec, but in the process of those studies I spoke with a few different geotechnical engineers researching the screw pile capacity theories and they indicated the science does not fully support the correlation that suppliers use. I think the ultimate torque value is a great indication of the strength of the soil your bearing element is in, but unless I have appropriate goetechnical data or load tests, my backup check is the allowable bearing capacity based on my knowledge of the local soils.

Over the years I have developed a distaste for screw piles. This is largely the result of the suppliers we have to deal with and the industry as a whole. They are a great solution for remote sites, uplift forces, lightly loaded structures, or small loads, but in many other cases the cost savings is marginal at best. None of our suppliers ever include the cost for the steel cap plates, welding or offset details when piles are grossly out of alignment. Once you figure out the cost for those components we do not find there really is a great savings.