Helical Piles 1

[abusementpark]

I am just learning about this type of foundation anchor/pile.

Can anyone give an overview of the advantages of these types of piles and when they should be considered?

Any comments/insight are greatly appreciated. Thanks.

 

[PEinc]

Advantages: fast, minimal equipment, grout not always needed.

Disadvantages: relatively low capacity, hard to install in soils that are dense or rocky, materials can sometimes be expensive.

Use them only when you are certain they can be installed to the reuired depth with bearing on each helix.

http://www.PeirceEngineering.com

 

[Ron]

Agree with PEinc...good, quick, easy installation, low capacity. Have used them in a swampy area for a large, covered walkway, subject to lateral wind load, uplift, as well as the typical DL+LL.

 

[JAE]

We've used them quite a bit - many good applications for them but some applications (per the above comments) might be not-so-good.

Historically helical piles started out using a single square bar of steel (1 1/2" sq) with cut and canted helical blades that formed the screw.

Recent years have seen more use of the pipe pile where a 2 7/8" dia. pipe with the helical blades is used.

I've used them mostly with underpinning work. Sometimes with new structures where traditional spread footings can't be designed to work - such as adjacent to an existing foundation.

I had a small, 9000 s.f. building that had sunk 9 inches at one end (due to poor grading and the presence of collapsible soils). We used the pipe piles - about 8 ft. on center around the perimeter grade beams and actually hydraulically jacked the whole building back into place - they were bolted onto the sides of the foundation walls. These weren't helical piling but actually "push piles" which were hydraulically jacked downward using the weight of the building.

Two ways to design/specify these things -

1. Prescriptive specification - the designer takes the soil data from a geotechnical report and uses helical pile design criteria to determine the number and diameter of helical plates as well as the estimated required depth. In the field ,the installer correlates the design to readings on the installation equipment that provides information on required torque.

2. Performance specification - the designer communicates with local helical pile suppliers and gets rough estimates of reasonable maximum pile capacities. Then the designer simply indicates on their plans the number, location, spacing, and required load capacity of piling. The helical pile supplier then provides piling that is certified to meet the required loads. During installation the pile capacity is determined from the torque required to install the piling.

A.B. Chance (

http://www.abchance.com/)

is a major supplier in our area of helical piling but check with your local firms. As you might guess, the performance specification is usually the best way to go - but again - depends on your local piling firms as to their preferences as well.

 

[JLind]

JAE is right on the money. A.B. Chance has a helical pile design software called Helicap. You can use this to aid in helical pile designs. If this is your first, second, or even third design, I would contact the local A.B. Chance distributor for design questions. I have these analysis completed by our design engineers daily.

A few things to consider with A.B. Chance Helical Piles:

Positive: Quick Installation, No spoils, Generally requires small equipment for installation, Load is determined at the time of installation, no vibration

Negative: Cannot be installed in hard rock layers. Square shaft may buckle in weaks soils where the blow counts are 5 or less. You would need to go with a helical pulldown micropile or pipe shaft helical anchor. Lateral load capacities of 1Kip or less. Capacity can reach 200 kips with minimal deflection as long as the soil can give it to you in end bearing capacity.

 

[geoman110]

I agree with all of the above cons and pros.
I used them in a situation where there was not enough overhead to use any other conventional equipment. One particular case was a 4-storey building planning to be 9-storey. Helical was used on the ground floor to increase the bearing capacity of the existing footings in order to take the higher load. I am not sure if they end up being more economical that other conventional deep foundation for a brand new construction without any special restriction (e.g., vibration). Every time I put them as one option beside other type of deep foundations for a brand new construction, the client end up to use a conventional method as being cheaper (at least in my area the cost comparison is like this).
I am still not sure how the helical behave and perform in seismic situation (earthquake)!

 

[Geostructsparks]

Helical piers installation also goes up to 6" pipe sections or larger (with a helix on the bottom of course to increase the bearing). Talk the Chance about that. I think they can get up to 200 to 300 kips. In Nevada, they can go into fairly stony ground.

Advantages are there is no pile driving vibration or noise (think small to large excavator with hydraulic drive, not diesel hammer); compared to drilled shafts there is no excavated spoils or groundwater control. We recommended these to support a replacement retaining wall adjacent to Lake Tahoe (environmental permitting, tight space for rig), where drilled shafts would have extended into the groundwater table and required mud or pumping, pile driving would have been noisy and the correct size crane would have to have blocked half of the highway at the top of the wall. Easy to install off-vertical (including as a tieback in the right soil). You don't need too many or any load tests as the rig torque is an indicator of the axial capacity.

Disadvantages, in my high-seismic area I don't really trust the small diameter helical piers to provide lateral capacity, and in an earthquake, you would have batter sets of piles in all 4 directions - there goes the cost. It is possible to have a larger-diameter upper section and a more-conventional shaft in the helix section.

 

[msquared48]

Another disadvantage - I did a PEMB foundation using them several years ago, and there wqs only one contractor available to do the job. Requires special equipment too. IE, higher costs.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Mickney]

In my experience, I have used helical piers for foundation underpinning and to help arrest isolated areas of building settlement.

I will point out that installation is usually performed by monitoring the torque from the surface. Therefore, it is critical that good subsurface information from soil test borings is used to design these foundation systems, particularly as it relates to an adequate foundation depth.

Case History: I was asked to review a project where the southeastern corner of a three-story building was constructed on 15 feet of new structural fill placed over an old trash pit. The building was supported on spread footings bearing approximately 3 feet below grade. As you would expect, settlement occurred. It was recommended by the geotechnical engineer to install a system of helical piers to help arrest the settlement and "jack" the footings back up to original grade. The geotechnical engineer performed several soil test borings that revealed the presence of 15 feet of well compacted fill, underlain by 10 feet of trash fill, underlain by very firm to dense residual SAND soils (SPT>25). Two (2) years after the helical anchors were installed, the same building distress appeared at the previous locations. Upon reviewing the installation data, numerous piers were installed to a depth of 17 feet (i.e. they were terminated in the trash fill). The specialty contractor installing the piers must understand the subsurface conditions, coordinate the installation with the project geotechnical engineer and ensure they are installed to the required depth.

 

[PEinc]

When installing helical piers or anchors, it is not always possible to "ensure they are installed to the required depth." When the lead helix hits bedrock, an obstruction, or very dense soil; that may be as far as it can be screwed. Then, the designer or contractor may be screwed.

http://www.PeirceEngineering.com

 

[MDE327]

They are fantastic for repairing foundations that have settled for various reasons.

Mike Drinkwater, P.E.

http://www.mdrinkwaterengineering.com

 

[csd72]

http://www.abchance.com/resources/technical-literature.asp

http://www.structuremag.org/OldArchives/2004/june/Deep Foundations.indd DLS.pdf