Helical pier vs. Driving Concrete Piles

[windsomniac]

I am going to be building a new house bordering a manmade canal in florida. The lots were constructed with poor fill. Typical houses in the area are built with 10"x10" conc piles driven to 15 ton capacity, about 12' on center, grade beams, and block walls. I am considering using helical piles instead of driving piles. I think helical piles are more cost effective and are less disturbing to neighbors. Any thoughts?

 

[PEinc]

I would not use the helicals for a house in Florida along a canal with poor fill soils. I would be concerned with corrosion of the steel helical piers. There is a reason why the other houses are on concrete piles.

http://www.PeirceEngineering.com

 

[ron9876]

Augercast are probably cheaper and won't bother the neighbors. Went to a seminar on helical piles a few months ago and they sure didn't spend much time discussing corrosion. If you arev near the coast I wouldn't consider helical piles.

 

[csd72]

Corrosion of helical piles in normal situations is not an issue but this is not a normal situation.

look at galvanizeit.org for some reference material on galvanised steel corrosion rates in salt water soils.

It can be done but it needs to be very carefully managed.

Personally I would agreed that concrete piles are probably better but I would be concerned about maintenance of cover around the reinforcement.

Precast driven piles is definately a safer more durable option.

 

[a2mfk]

Freshwater, ICW or gulf side canal? Will answer a lot of our galvanized/corrosion questions.

Pre-cast concrete pile still have steel in them that can and probably will corrode. Axial capacity-wise, you can get a lot out of helical piles and they will be much cheaper to install. I understand the others concerns regarding corrosion, you can definitely get galvanized augers and pipe...

Now if your house is going to be a stilt-type house, then a concrete pile that will cantilever above grade to form the building columns is a good option. I have used that before on a bay in SW Florida where storm surge scour was a huge issue.

HTH,
Andrew Kester, PE
Florida

 

[windsomniac]

The canal is on salt water. The house will not be on stilts. It will be grade beam & foundation wall.

I was talking with a pile driving company who drove pilings for the neighboring propertys. He said the soil is muck down to about 45-50 feet then bedrock. He said helical piles in this situaion would cost 3 times more than driving piles.

Sounds like driving conc piles is the best option.

 

[msquared48]

If these are helical screw pile, they also take special equipment to install, and the number of contractors to do this, with the equipment, is far less than augercast. Stick with what is normal in the region. There is a reason.

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

 

[PEinc]

Using helical piles that go through muck to bearing on hard rock would be questionable, IMHO. There could be a bearing problem of the lead section's point bearing on hard rock and there could be a lateral support (buckling) problem for the long, slim shaft in muck. If the point hits hard rock and the helices are still in muck, the helices will provide little or no bearing and may not develop sufficient torque.

http://www.PeirceEngineering.com

 

[PSlem]

Regarding the tip on rock argument the bearing area of a 2-7/8" OD pipe is .045 sq. ft. For a 20,000# pile, the rock would have to exceed 222 TSF while the helix 6" above is in muck.

 

[csd72]

Also think about the point on your precast piles, may be best to use a flat point.

 

[a2mfk]

I don't work for a contractor directly or indirectly, but I am very familiar with this side of the business in Florida...

What county are you in?

Have you had SPTs drilled on your site?

40-50 ft of muck over limestone would be very surprising, so say my coworkers (geologists). My point is to take a contractor's opinion with a grain of salt, like asking a roofer if you need a new roof...

Helicals can be installed down 50+ ft into limestone no problem, actually, with a lot of clay it can be an issue in penetrating through the clay layers. Depending on your area it has been done for less than $2000/helical, also depending on the depth to limestone. Depending on the model and company, I have seen 20k++ axial allowable capacity.

Mike is right, this is a specialty. But there are areas of Florida where there are a lot of these types of contractors because of residential foundation repairs due to soil issues.

PEinc also has a valid point on the lateral buckling stability of a 3" steel pipe driven down 50ft. We've discussed that in our office before, not a lot of good technical info out there on that. How much lateral support does soil (especially organics) at various depths provide? Unsure, may involve on-site load testing to know for sure.

That's about all I know, HTH!

Andrew Kester, PE
Florida

 

[windsomniac]

Its the aqualane shores neighborhood in Naples in collier county.

 

[PEinc]

Also, not all helical piers use pipe shafts. Some use 1.5" to 1.75" square shafts which are even more slender than the pipes. From my experience, there seem to be more helical pier/anchor contractors than there are auger-cast pile contractors.

http://www.PeirceEngineering.com

 

[BigH]

See if there is any soils information in your area - as alluded to earlier, the 'muck' may be "easy driving" of the pile and not truly a poor soils condition - notwithstanding that the upper portion may be poor as implied. OP didn't say how deep the piles were driven in the area. If you have to go to rock, why not use steel H-piles? They drive easy and will seat well on the rock and you can provide enough sacrificial steel so that you'll be long 6 ft under before having to worry about it. If concrete piles are driven to rock and driven too hard when rock is reached, there is always a possibility of damage to the pile (a friend ran into problems in Northern BC due to this).

 

[PSlem]

Caltrans states an H pile will corrode on both exterior faces while there will be no interior corrosion in pipe piles. A pipe pile has stagnant interior air only drawing in fresh air with a seasonal drop in the water table. IBC specifies 3/8" minimum thickness for H piles. A helical pipe pile gets you more than 3/16" wall and would be galvanized for a salt water application.

 

[PSlem]

Regarding buckling Bjerrum (1957) says it only needs to be considered if Ip/A^2< or = stress^2/(4khdEp) where stress is yield stress of pile and A is cross sectional area of pile. For a 2-1/2" sch 40 pipe pile with a 50,000 PSI yield and Ep = 30,000,000 PSI and a khd of 75 PSI for a soft clay
1.45/1.59^2 = 0.57 >50,000^2/(4*75*30,000,000) = 0.28
A consideration for driven piles would be is there access to bring in equipment and 50' piles? Will the neighbors call the police to stop the disturbance the first time the hammer hits? If a diesel hammer will it spot the neighbor's car or house?

 

[ron9876]

I believe if you check with local geotechs in that area they will tell you that augercast piles are the way to go. Doubt very seriously that there is 50' of muck in that area.

 

[a2mfk]

Get a couple of SPTS and/or consult with a local geotech. My resources say you may have some muck from the man-made canal (like 5-10ft) then probably sand to limestone. For the others on the list, FL limestone is fairly soft in most areas compared to other types of bedrock.

The Tampa area has a lot of helical and "Push-pin" pile contractors that may serve that area with a local contractor.

Good luck!

Andrew