Can sonotubes be left in drilled piers? 1

[Kom3]

Hello all,

I designed a drilled pier that is 5' deep with 6" above ground. Upon doing a structural observation, it is found that the contractor used a sonotube for the full length of the drilled pier (from bottom of embedment to top). It is my view that the sonotube can rot since it is made of cardboard and a half inch perimeter gap can form once it rots. Is leaving the sonotube in the ground an issue? Is there any issues from a constructability point of view if they remove the sonotube before they backfill and compact the surrounding soil?

Regards,

 

[BAretired]

Sonotube over the full height prevents skin friction from developing; with a 5' depth, skin friction is usually ignored in the upper 5' or so in any case. If soil bearing over the area of the shaft is adequate to carry the applied load, it doesn't matter whether the sonotube is left in place or removed.

BA

 

[Kom3]

Hi BAretired,

The loading is relatively light but I am concerned about overturning. If the sonotube rots and a half inch gap perimeter forms, would this affect the overturning capacity of the pier since the soil around it is not "compact"? Will a half inch gap actually form though?

Thanks!

 

[STrctPono]

I can think of no compelling reasons for why to leave it in and several reasons for why to remove it. Just have them remove it.

 

[Kom3]

Hi STrctPono,

I want to remove it as well but I wanted to know if it is common or even industry standard to use a sonotube for ground pier foundations. I am a structural engineer and I don't want to dictate the means and methods especially when there is no issues and I am just overthinking their approach. As with all the contractors that I have dealt with, they are likely going to say that this is how they have been doing it for decades without problems. Therefore, I want to explore all the possibilities before I challenge them on it. They might push back hard since it could be a lot of work removing the sonotube from the ground once the concrete is poured.

Regards

 

[BAretired]

Hi Kom3,

If it's a drilled pier, the contractor would have had to drill the hole larger than the pier in order to accommodate the sonotube. After the cardboard form disintegrates, one half inch gap around the pier is a reasonable expectation. Actually, the gap will be variable because the drilled hole will not be perfectly circular cylindrical. If you are relying on soil to provide passive resistance to lateral movement, the soil should be compact and should be in contact with the pier. Ideally, the concrete should have been placed in the drilled hole without a cylindrical form.

If in doubt, it is best to remove the form and compact the soil around the pier.

BA

 

[Kom3]

Hi BAretired,

Moving forward, I think I will specify that the sonotube not be used for underground piers. I question the half inch gap because even if the sonotube does rot, the decayed matter is still there and takes up space since mass can't just disappear (although it might shrink in volume a little bit, I don't know if it will). I think the existing sonotubes that the contractor placed in field should be fine because in my analysis, I used the minimum code presumptive values and did not double it per IBC 1806.3.4 which gives increase lateral capacity for pole like structures where half inch deflection is acceptable. Since I was conservative to ignore this provision, I believe it should negate any possible issues caused by the sonotube decaying. Please let me know if you think that is a reasonable assessment. As for soil compaction around the sonotube, I agree with you that it should be compacted.

Regards,

 

[STrctPono]

Kom3,

Wait, the concrete is already poured?

I was thinking that your inspection was pre-pour.

My thought was that if they had not poured the concrete yet then they should be able to easily extract the sonotubes as they are pouring the concrete. It's basically a temporary casing. They can leave a small portion of it at the top to form the 6" extension.

As for means and methods, I guess that depends on how your spec is written or whether you show it on your drawings. I know our specs always require the contractor to remove any temporary casing unless a permanent casing is specified. You could argue that your drawings show the concrete poured neat to the soil surface without a void material between.

 

[hokie66]

Sonotubes are for forming circular columns, not bored piers. It is unacceptable to leave the sonotube there, unless for some obscure reason you want a gap around the pier.

 

[phamENG]

What's your soil type? I've had to have them left in place in sandy soils near the beach before. Water table was just a couple feet below grade, so once the auger hit water the sand just washed into the hole. THey had to set up pumps and slide the tubes in about 6 inches below the finished depth and backfill that with stone. Poured the foundation and had to leave it - otherwise the excavation would have collapsed. In that case, though, there wasn't much concern. When the tube rotted away, the sand moved in pretty easily to take its place. We just readjusted calcs based on our geotech's recommendations for reduced lateral soil capacity.

 

[canwesteng]

Sonotubes are left in when you want to break friction with the soil to prevent frost jacking (have had this recommended by geotechs to me).

 

[KootK]

When I use sonotubes for foundations, I mostly consider the finished product to be:

a) A pad footing of odd proportions allowing it to be constructed integrally with the pier that brings the concrete to grade for the attachment of stuff above.

b) Good for only nominal lateral resistance. That imposed by things like deck posts etc.

I'd be willing to do more aggressive things only under the direction of a geotechnical engineer.

Are sonotube foundations actually installed via drilling? I always figured that the contractor just dug a hole, stuffed the sonotube in, and back filled.

Can a sonotube be removed from a buried pier? That sounds... difficult.

I designed a drilled pier that is 5' deep with 6" above ground.

The scale of this sounds very much like what I've described above. Can you elaborate on the particular application here? I can't see anybody bringing a drill rig out for some deck post piers.

 

[Kom3]

Hello all,

The application is for a small rack holding equipment that is conservatively being designed as a 5' tall x 7' wide solid sign (Wind governs by wide margins). There is a drilled pier on each side (2 total) supporting the legs. Some of the foundation has already been poured with the sonotube (I didn't realized they were keeping the sonotube in the ground until later - I initially assumed it was just form work to be removed but moving forward it will be a red flag if I see it in the ground). Some has not been poured. I don't have a geotech report for this location so I'm assuming sand, silty sand and using lateral bearing pressure of 150 psf/ft per the IBC presumptive load bearing values. However, a nearby geotechnical report that has a boring about 200 ft away says we can use 400 psf/ft (The geotech refuses to offer any support if his boring is more than 50 ft away from the site). Therefore, I feel this design has a lot of conservative elements in the analysis since I'm not even doubling the presumptive lateral pressures I'm using as mentioned above. If I fine tune this analysis, I can get embedment to be less than 3'. My only concern is how detrimental is the sonotube to the overturning capacity.

Regards

 

[phamENG]

Careful assuming that the report from "nearby" has much to do with your site - the geotechnical engineer is spot on with his 50' limit. I did a design a couple years ago where there was a historical stream bed running through the middle of the site (dried up about 250 years ago). You'd never know it by looking at the site. Borings on either side were great - low rise building could be supported on shallow footings. Those borings were less than 50 feet apart. Thankfully, the geotech was familiar with the underlying geology and got a boring in the stream bed - the building would have cracked in half from all the soft, unconsolidated organics in the old stream. We had to put the whole thing on piles.

 

[wannabeSE]

Kom3 has some reasonable justification to let the sonotubes stay in place for this particular case.

I question the general rule that sonotubes can be left in place above ground. Don't get me wrong, I have seen plenty of sonotubes left in place. I just question whether this is a good practice. Aren't they organic material like wooden formwork that should be removed. I don't have any experience with frost jacking, but I don't see how the sonotube prevents this. Won't the tube absorb moisture and freeze the soil to the tube? Or, is form release used to prevent the tube from sticking to the concrete. Or, is the tube assumed to rot and provide a gap? Is wooden formwork ever left in place to prevent frost jacking?

 

[Compositepro]

I have the same question as KootK. How can you remove a Sonotube from a poured pier that is underground? And why use one unless it is to act as a release from the soil?

 

[Kom3]

Hi Compositepro,

I talked to the contractor. The reason why they used sonotubes in the first place was because the surrounding soil was sandy and had the tendency to fall in without the sonotube (similar scenario to phamENG). After they poured, they just left it there. I talked to the contractor about my concerns and we have agreed that moving forward, they will remove the sonotube after pour or not use it at all. I won't demand that they strip the ones already in place for reason specified above. They have assured me that they do compact the soil around the sonotube. Thank you everyone for your valuable inputs.

Regards

 

[phamENG]

One thing to keep in mind - if you're using the "flagpole" foundation equations in the IBC, they rely on undisturbed soil to work. If they excavated, put the sonotube in, and backfilled it may not be good enough. That's a scenario where I'd want a geotech to specify the compaction scheme to restore the lateral resisting abilities of the soil.

 

[hokie66]

phamENG,
The use of sonotubes in that way just sounds like a poor man's augercast pile.

https://www.structuremag.org/wp-content/uploads/2014/09/C-BB-Augercast-Piles-by-Jason-Black.pdf

 

[phamENG]

Sort of. Just not as deep. Where I am, it isn't practical to take a "drilled pier," as we like to call them more than about 6 feet. High water table and loose soil make it really impractical. (If I remember correctly your handle is from VT - not sure if you ever made it down toward the coast and our...lovely?...swamps.) They also aren't used for vertical loading of any significance - just lateral loads from sign poles, light poles, lightly loaded posts for small car ports or other minor structures.