Drilled Shafts vs. Auger Cast Piles

[Mickney]

I welcome feedback on the following issue:
I am geotechnical engineer investigating foundation alternatives for a site in Charlotte, NC. It will be a 30 story high rise structure. Gravity loads range from 600 kips to 6000 kips; lateral loads range from 10 kips to 190 kips; uplift loads range from 300 to 2500 kips. There are also a few moment frames incorporated into the structure. Soil profile is a residual soil profile with depths to competent bedrock (200 ksf bearing) between 20 and 50 feet below ground surface. Partially weathered rock ranges in thickness between 0 and 20 feet. Due to the variable subsurface profile and variable foundation loads, I have recommended drilled shafts to rock, with rock socket and/or anchors to resist uplift and lateral, where necessary. I have been asked to investigate the feasibility of auger cast piles as a foundation alternative. I question their feasibility due to the shallow depth to bedrock and the high and variable loads. I welcome feedback on the feasibility of auger cast piles from engineers who have used them on similar projects. Thanks for your time.

 

[GeoPaveTraffic]

ACPs should be usable under the conditions that you describe. The biggest problems are likely to be the uplift loads in areas of shallow bedrock. Another thing that could be a problem would be the amount of wethered rock.

I would suggest an extensive load test/drilling test program where the production piles are installed EXACTLY like the test piles. You can use the reaction piles to test the uplift capacity and lateral load tests can be difficult to set up, but you can run them.

 

[eric1037]

A few things to consider:

1. For the loads you have described, the auger-cast piles would have to be relatively large to be economical. I think the biggest ACP diameter is about 36 inches. Even at that size, the pile caps would be fairly large.

2. You will need full-length reinforcing cages. These get hard to install at lengths of 50 feet. It's not impossible, but difficult.

3. ACP are very dependant on contractor experience. Therefore, you will want several load tests for each pile capacity. These get relatively expensive to perform.

4. If you have to extend any significant depth into the rock, there could be major difficulties. Especially if there is a hard, weathered layer above the rock. ACP don't install well into rock.

If this were my project, I would stick with the drilled shafts for the following reasons:

1. You can visually verify bearing material.

2. You can verify you are the appropriate depth into bearing material.

3. It is easier to install the beefy reinforcement required.

4. Large-diameter drilled shafts would not require the huge pile caps.

There are other considerations, but these are some that come to mind immediately.

 

[TDAA]

I think eric hit the main points and I concur. It isn’t that the ACP could not be done, but there is a lot more visual QC for the large diameter. You can send someone down and test the bearing surface, drill pilot holes to verify the rock, etc.

Keep in mind that it is more likely that you need to address the group effects with the ACP’s.

I would also request there be PET testing done on either alternative, to look for defects in the foundations. Those are some pretty hefty loads.

 

[SchroederJM]

Eric1037

I am curious on part of your response to drilled piers vs. auger cast piles.

"2. You will need full-length reinforcing cages. These get hard to install at lengths of 50 feet. It's not impossible, but difficult."

Why will you need full-length reinforcing cages? Is it even possible to get a cage in 16" or 18" diameter piles?

 

[fattdad]

I don't think I'd go for the ACPs. My gut agrees with the use of a drilled shaft. I'm interested in the strength of the soils that are above the bedrock. Are they residual soils that are primarily fine-grained (i.e., sandy silt, elastic silt, clay, fat clay) or more along of a micaceous silty sand? What are the range of blow counts? Any pocket penetrometer/Su data? I guess I'm just wondering whether the soils are uncapeable for big conventional foundations (may be, eh?). Assuming that there's no way other than going to rock, I'd go with the drilled shafts.

f-d

p.s., this is a fun forum for geo-geeks!

 

[eric1037]

SchroederJM:

For ACPs to resist lateral loading and uplift, full-length reinforcing would be required.

I have seen many full-length cages for 18" ACP. I don't think it's feasible for 16" diameter. However, for the loads described, I would bet you would need the largest diameter ACPs available to be economical.

 

[SchroederJM]

Like Mickney, I too have been asked too look into design of an auger cast pile system. The project I am working on has been previously designed using drilled piers, however, now the owner wants to look into auger cast piles as a possible savings. The soils conditions on site are bad. Basically, they have dredged the bottom of a lake to fill in a corner of the lake to build upon. The soil is basically all sand and bedrock is approximately 55-60ft below grade. The soils report shows allowable end bearing of 20,000psf at weathered shale for ACP's and 50,000psf for drilled piers. Allowable side friction is not available for ACP's but 5ksf for drilled piers. Therefore, we will embed ACP’s 5’-0”. The drilled pier design was embedded 1 pier diameter into bedrock. The loads coming down from my structure range from 400k to 1200k.
From geotechnical point of view, what type of system would you recommend? Have you ever heard of any complaints on construction of drilled piers? I assume there are plenty of contractors able to do drilled piers.

 

[eric1037]

SchroederJM:

I would be careful trying to embed ACPs 5 ft. into weathered shale. The contractor may not be able to achieve that.

Based on the end bearing, it appears that the geotech report is having the ACPs bear on top of the weathered shale and the drilled piers bearing on more competent rock. I would verify design bearing elevations/depths with the geotech. You should also be able to get some skin friction for the ACPs. It will probably be similar to the drilled piers though.

If the issue is just loose sand, you may want to consider vibrocompaction or vibroreplacement as a possible alternative. I have seen post-vibro allowable bearing pressures as high as 8,000 to 10,000 psf for shallow foundations.

Obviously, I don't know all of the particulars of your project, but you may want to discuss vibro with your geotech.

 

[Mickney]

SchroederJM: My understanding of ACPs is that they are mainly a friction type pile with just a small amount of end bearing capacity. And I have read and been told repeatedly that they cannot penetrate dense materials. I don't know what your soil profile is like, but it may be conducive to Geopiers.

 

[GeoPaveTraffic]

SchroederJM,

ACP's could work in the conditions you describe. They would develop load capacity from both friction and end bearing in the shale. eric1037 is correct that penetration might be an issue, however, it might not be depending on the properties of the shale. The amount of torque that moderen installation equipement can create is very high.

eric1037, I have to disagree with something you posted a while back. Full length reinforcing cages are not required for ACP's to resist lateral loads. For lateral loads the cages could be as short as 15 feet. It all depends on the amount of lateral load, the soil condistions and the amount of fixity of the pile top and the pile cap. As for uplift loads, a single bar down the center of the pile to the bottom is generally more than adaquate to provide uplift capacities.

Placing cages in any ACP can be difficult. It is very important to install the cage as soon as the auger has been withdrawn and the top of the pile cleaned. If there is a delay, then sometimes the cage cannot be installed. If that happens, the pile needs to be imeadiately redrilled, repumped, and the cage installed. This should be included in the specifications. As for diameter, cages can be installed in any size ACP, however, the amount of steel that it is possible to install drops quickly as the diameter drops.

 

[fattdad]

SchroederJM: what are the structural loads for your building?

f-d

 

[SchroederJM]

fattdad,

Vertical loads are 400k to 1200k and ther could potentially be an uplift of 150k.

 

[eric1037]

Geopavetraffic:

I agree that full-length cages may not be required for the lateral loads described by Mickney. However, with uplift loads of up to 2500 kips, you may need more than just one bar down the center. I guess I should have said "may be required". Also, if you are socketing into rock, I would anticipate the need for full-length reinforcing. Maybe not a cage though.

to clarify, full-length cages are not required in all cases of uplift and/or lateral loading.

Thanks for pointing it out.

 

[kleo]

I would be glad to discuss ACP's with you for your project(s). We have successfully installed them in residual soils (Atlanta). There are other pile types that may work as well.

Regards,
R. Stone
Morris-Shea Bridge Co.
morrisshea.com

 

[DRC1]

I have seen augercast installed and I am not overly impressed. We have used mandrel driven piles and I would think they would be competive with augercast and resolve some QC issues. We have predrilled with large DTH's to clear unaugerable obstructions. DTH's could be used for 5 ft rock sockets. I can't imaginge augering through 20,000 psf shale. It will be slow. Once you have bearing, do you need the rock socket? It will add significant expense to the job.