Helical Piles - Round Versus Square

[SPR2]

Hello,

I am looking for some technical information to help with a problem I find myself in dealing with helical piles.

We have specified a particular helical pile system for a project with a relatively large number of piles. 640 lead sections in all.

The problem is that we have been asked to consider an alternative system with consideration given to capacity, corrosion resistance, drving torque etc.

The two systems are:

1 - A square shafted helical pile surrounded by a column of grout placed at the time of driving. Pile material is galvanized and surrounded by the grout. Sections are approximatley 7 feet long and spliced along the way. Column of grout contained within pvc liner due to peat soils condition in the upper portion of the pile.

2 - A tube shafted helical pile. Pile material is not galvanzied but supplier plans to oversize tube thickness to address corrosion to some extent. This particular installation method uses a side mounted torque applicator which allows longer pile lenghts between sections.

The problem is that everyone I speak to is connected to, is a supplier/ installer of one type or the other.

I would like to hear from a designer type person who has some familiarity with both tube type and solid square shafted type piles who could offer some thoughts as to when one type may be preferable to another.

If you think you can help please respond.

Thanks,

Stephen Riley

 

[cassion]

Piers can be fabricated with varied structural pipe diameters and wall thickness to addres torque and lateral load. Then have them galvanized we pay about 14c per pound. You can build them in sizes up to 20' lead section. These piers when designed by an engineer are not proprietary and are routinely fabricated and installed around the country.

 

[SPR2]

Thank you for the comments. We have been in contact with several firms that fabricate and install the tube style shafts.

The problem I have specifically is comparing, from a technical perspective: "a square shaft pile with a grout column" versus a "tube shafted helical pile".

I have been told that the square shaft is cheaper and easier to drive (less torque required) than a tube type, but cost is not the only consideration. I have a tube installer give us a better price so it becomes a non issue.

The question remains, is there a technical reason to chose one type over the other?

Steve

 

[cassion]

The information you are looking for is
Helix Pier Engineering Handbook 2000
By Howard A. Perko
you may be able to find this on the web.
We also build and install helix tube piers.fyi.

 

[BOBWALTON]

SPR2: Speaking as an independent geotechnical contractor with experience installing both square and pipe shaft helicals, there are several considerations in making your selection. With 640 lead sections, this is an interesting job. I would only bid lump sum if I was confident of uniform soil conditions so I could control the depth on each installation. Otherwise, I would bid so much for each setup and so much per foot.

Whether you bid lump sum or by the foot, the idea is to minimize depth to provide the best price for the customer. (Watch out for guys who bid by the foot and then run up the price by driving deeper than necessary)

If an engineer has specified a minimum depth, I would examine his reasoning. Depending on the soil, you might max out on torque before hitting the minimum depth and this causes problems. The important thing is to establish the horizon beyond which you can generate capacity for your piles.

More to your question, pipe shafts generate more torque resistance but also greater pile capacity due to the direct relationship between installation torque and axial capacity. Of course, you can increase capacity on square or round shafts by putting on additional helicies. The point is, you need to look closely at these issues in deciding which method will be cheaper.

Lateral capacity is an important consideration. If there are significant lateral loads, the square shaft pier becomes suspect- and the pvc lined grout column must be examined closely.

I would also be concerned about potential seismic loading. The larger diameter steel pipe would seem to provide better stress resistance if seismic loading is a concern. In fact, this is a very good area for technical argument on your question but I would leave it to others to weigh in here.

Galvanizing is always the first defense against corrosion and as a Contractor, I would want to provide galvanizing on either square or round shaft. In order to go without galvanizing on the pipe, you should hire a corrosion engineer to provide an estimate of duty life. I would try to avoid anodes or impressed current if possible due to cost and maintenance requirements. The concrete column will provide added protection in the oxygen zone beyond which corrosion is not a big deal.







Walton Foundations

http://www.waltonfoundations.com