Settling of Building supported on helical piers

[NinerStruct]

Has anyone had any experience with a building underpinned with helical piers which then had additional settling? Back in 2003, an existing building built in 1919, experienced several inches of settlement on the north and east sides of the building. The firm I work for recommended helical pile underpinning and there did not seem to be any problems until last fall, when an additional 1/2" of settlement occurred in a smaller area at the corner. So cracks have appeared on interior finishes and in portions of the exterior brick face.

Subsequent surveys have shown little to no movement in the months afterwards. I’m having trouble determining the cause of the settlement. A soils report from 2003 indicates that there are fat clays under the building foundations, which are about 9 feet below grade. The helical piers were typically about 5 ft. apart and went about 22 ft. to 27 ft. below the bottom of existing footing. Is it possible that the drought from last summer had anything to do with this? Looking at precipitation graphs, it looks like we had pretty low moisture when the building originally had these problems, and there are two mature trees within 15-20 ft of the corner, but I would have assumed that the depth of the piers would have insulated them from the effects of the drought.

 

[Ron]

Yes...dessication of clays can cause this. Check the moisture content of the clay in which the helical piers are bearing and compare to prior geotech moisture contents.

 

[oldestguy]

Ron is right. I have seen significant shrinkage of fat clays below full depth basements. So, you may be experiencing a downward drag of the upper portions of the pile penetration, similar to having a layer of compressible peat part way down.

In cases like this we have solved the problem in the long run by cutting down the trees!!! Then, add water to the ground. Even watering the trees can do a good job, but it takes continued attention.

 

[NinerStruct]

Just curious, what is the deepest that you've seen droughts effect clays? Part of me would have thought that the depth of the helical plates would be beyond the effects of clay desiccation. Under a full depth basement, roughly 10 ft or so I understand, but is it possible to get as low as one of the helical plates if it's 18 feet deep?

The original engineer was still here during the first visit to the site, and felt that the drought likely had a large effect on this, if not outright caused it. But he has since retired, so I'm trying to pick up the pieces. I wrote a letter to the owner a while back and suggested that further investigation, such as a deep boring, would help us figure out if the moisture content had changed, and also mentioned that removing the trees would likely also be a good idea, but the previous engineer said he was just about run out of town when he suggested that years ago...

 

[fattdad]

So, there is a foundation load that resulted in too much settlement. You added helical piles as supplemental support, which transferred some of the foundation load to the helicies. After the helical piles were installed, there may have still been bearing stress from the foundation (i.e., it's not like the helical piles shouldered all the load). Even though you reduced the bearing stress, there still could have been some residual consolidation of the fat clay. As that residual consolidation progressed, there'd be more load acting on the helical piles. That would add to the settlement of the helical pile. Now you have to wonder whether the original foundation has the capacity to span from helical pile to helical pile.

If there was a prominent drought, there could have been shrinkage of the near surface soils that were formerly the bearing soils of the conventional foundation. That could also lead to increased load transfer to the helical piles.

Just a few of the thoughts that come to mind.

f-d

¡papá gordo ain’t no madre flaca!

 

[hokie66]

Could be a problem with the helical piles themselves. The ones that I have seen are skinny, often installed at an angle, and the connection to the footing is a bit tenuous. Being "22 ft to 27 ft" below the existing footing, a bit of bending in the pile could lead to settlement.

 

[oldestguy]

My experience as to depth below a basement for drying and shrinkage is limed by the inability to dig deeper, but since settlement of the basement occurred, there has to be several feet of shrinkage below there.

In your case, my thought was that the down drag on the piles at shallower depth than the tips would add to load on the tips and cause them to settle from that extra loading. You wouldn't have to have drying below the tips then.

As to watering trees, I am amazed as to how well this works to fix a problem, bringing foundations and floors back to where they started from in some cases. Cracks closed up also. It all started from "let's try this" approach.

 

[NinerStruct]

fattdad, I had been under the assumption that the piers had been designed to fully support the building, but I don't have any documentation as to what loads were given, so I couldn't check that. But you're saying that regardless of what they may have been designed for, they wouldn't have taken the whole load off the existing footings?

Also, the foundation was not continuous underneath this building (it was a very odd foundation). They tried to put helicals in between the footings as well, but that was one thought, as to whether or not there was a break in the foundation wall.

Hokie, I believe that the piles that were installed were 3 1/2" tubes, so they're not the 1 1/2" square bars, but still, I take your point. They did excavate down to look at one of the bracket attachments, to make sure it was still attached to the foundation or that the footing hadn't broken and things appeared to be intact, but that was only one anchor. It wasn't apparent whether the pipe had any bending either.

Oldestguy, when the problem first came up in 2000, that was the first thing they tried, was deep watering and it worked for a few years. They said that it even closed up many of the cracks within days. But in 2003, it settled a little bit again, so they wanted something "more permanent", and they put in the helicals.

I appreciate the suggestions, I would like to do these tests to find out the cause, but this also a building that is no longer in the Districts long term plans, so they're just wanting it to last a few more years, and don't want to dump a lot of money into finding out why it's settled, but at the same time, they feel like our firm is culpable for this because they "didn't get what they paid for" when we suggested they use helical piers. There's politics involved, as a council member's brother owns a push pier company and claims we wouldn't be having these problems if he'd gotten the job, but the bid was apparently twice the cost of the helicals and the previous engineer lobbied for the helicals, and so the guy is still a little bitter.

I basically wanted to know if it was possible, or unusual for a drought to have this type of effect on a building with underpinning. Or if there was anything that could have been done to avoid this. While I wasn't around when the decision was made, I don't know that I would've suggested anything different. And it's hard to explain these things to the owner without coming across as trying to pass the buck or just trying to wipe our hands of responsibility, but I really don't know what else we could have done.

Sorry for the verbose response.

 

[hokie66]

Your response is not verbose, it is just complete, and I thank you for that.

Was the entire building underpinned, or just the part where the settlement originally occurred? I know you said this occurred after a dry period, but could it be related to rebound swelling of the rest of the building, other than at the corner where the new cracking has occurred?

No advice on dealing with the political interference. The key issue is that all buildings require maintenance, especially when they are constructed on poor sites.

 

[FixedEarth]

Since your settlement is localized to a small corner, look at the subsoils in that area and the distance to trees in relation to their mature height. Are you sure it is a settlement and not heave?

 

[oldestguy]

OK, now comes the political part. You can always say something to the effect that other experts feel the helical piers was a good recommendation and the alternative would be to remove the trees. As to other modes of transferring the load down, that is a maybe and not a for sure thing either. Let the competitor prove his case with similar jobs.

In some cases like this things are so severe that the new supports have to adjustable top sections, such as screw jacks to periodically adjust for conditions beyond the control of only the new supports. You don't always know that this is required until experience tells you what you need.

 

[darthsoilsguy2]

brainstorming... don't necessarily need answers but just throwing them out for consideration.
how was the termination criteria derived? was there a load test or did that get Value-Engineered? if V/E, was the owner part of that decision? If tested, was it to 2x the allowable load? Was there a clear understanding by the installer about factored loads? Was construction observations performed? by whom? if not, was this V/E? what design/build elements are brought by the contractor for this work? Can we really know it settled 1/2" that fall, or maybe that was when the crack finally developed or was noticed? Has the use and loadings changed for the building or site? Is the property subject to vibrations? What is the overall percentage of helical piers appear to be functioning? Even though all settlement may actually be complete now, would the cost to install new additional piers and the original contract be less than the push-pier bid? How many piers are working the failing region? Is there a chance that one or more piers were damaged during installation? Knowing what you know now.... if you had to fix the neighbor's building would your recommendations be different?
Makes sense to me that there could be consolidation of clays since the helix would be taking load straight to these regions. But, there are enough variables at play that i would be difficult for someone to definitively pin it to you. If it gets down to it, here's the line "Filing a claim won't be a viable solution. My insurance will deny the claim because this work meets professional standard of care and liability insurance only covers when the standard of care is lacking. However, i do want to work with you to resolve this as quickly as possible."