Helical piers instead of drilled piers?

[COEngineeer]

So we have this 300 sf residential addition. According to the geotech we need to use 26' min drilled pier penetrate minimum 10ft into bedrocck or refusal. Min 10" diameter with 25000 psf max bearing. Side shear 2500. Minimum dead load on the pier has to be 15000 psf.

Now we want to use helical piers instead. what are the things I need to pay attention to? Can I just spec the hellical piers to design load? Do i need to worry about the numbers the geotech eng gave us?

 

[JAE]

Drilled piers tend to have good diameter to length ratios as well as some "meat" in their diameter such that column loads and grade beams, etc. can usually be placed on a single drilled pier.

For helical piles, their shaft is usually a 1 1/2" square steel bar or possibly a 2" + dia. pipe. These do not possess the same bending stiffness and so are sensitive to eccentric loads.

Thus - you usually do not want to place a column on a single pile - rather three would be the minimum - centered under a column.

For a wall, such as for your house, you can use a line of piles under a grade beam or cap, but there is usually a recommendation to stagger them such that any eccentricity in the wall load can be transmitted to the piles in a more pure axial form.

Simply put - avoid any and all eccentricity on the piling as much as you can.

 

[COEngineeer]

So you dont recommend using it to support basement wall?

 

[COEngineeer]

Can you refer me to how to stagger them? I was going to space it at 10 ft under 9 ft grade beam.

 

[emmgjld]

Now you have asked the question which puts you in the 'neither fish nor fowl' realm. What do helicals in stiff soils really behave as? Do shorter to medium length (up to 30'-35') helicals, placed in a reasonably stiff clay, have a big problem with minor eccentricity? My experience in using helicals in repair and some new construction is No.
From a computation standpoint, Maybe.
The issue partially revolves around the amount of true side support of the shaft which remains after installation. Which actually gets into the real design issue.

In theory, the drilling/installation process of the helical has negated much of the potential shaft uplift, at which point, the required dead load can be reduced.

The first question is HOW much of the potential shaft uplift has actually been accounted for by the soil reworking.

The actual end bearing capacity of the soil/soft rock is not reduced but, as your numbers give only a minimum deadload, how much is end bearing and how much is 'side friction'?

I tend to remember we used 20,000 to 25,000 psf for end bearing in most of the Pierre Shale and 18,000 to 22,000 psf end bearing for the Shales/Claystones of the Denver and Dakota Formations.

Now the minimum to account for the expansive characteristics. It should be in 2 components, Min. End Bearing and Side Uplift. The typical method of reporting on the Northern Front Range is to bring these both together as a gross minimum deadload. So you have to get the geotech to give a minimum for end bearing, alone.

 

[HouseBoy]

If you are required to penetrate 10 feet into bedrock with a concrete pier, I can not imagine any helical pier that would provide comparable structural capacity.
What will a helical pier be "drilled" into? If a concrete drilled shaft must penetrate 10 feet, could a helical pier even begin to penetrate such material?
Ordinarily, with that much penetration, I would suspect that either the vertical loads are extremeny high or (more likely ) there is some lateral component to the pier behavior that is being sought by the soils consultant. Of course, the 10inch diameter won't resist much lateral force.

Perhaps I'm misunderstanding something here.
Why is the concrete shaft required to penetrate the bedrock so much?
Could a larger diameter pier be used to a much shallower depth?
Is there a lateral force that must be resisted?
What soils are being penetrated above the bedrock and whay are they not suitable for bearing?

 

[TDAA]

What are the things I need to pay attention to?

Pay attention to your geotechnical engineer. Talk to the geotech, as they can give you recommendations for using helicals.

Can I just spec the hellical piers to design load?

No

Do i need to worry about the numbers the geotech eng gave us?

Yes. Do you want the liability of designing a foundation without geotechnical recommendations? If anything, the numbers will change to what is appropriate for that foundation type.

In theory, the drilling/installation process of the helical has negated much of the potential shaft uplift, at which point, the required dead load can be reduced. The first question is HOW much of the potential shaft uplift has actually been accounted for by the soil reworking. The actual end bearing capacity of the soil/soft rock is not reduced but, as your numbers give only a minimum deadload, how much is end bearing and how much is 'side friction'?

It took a couple of reads to figure out that I was agreeing with emmgjld . In this area the DL pressure is speced to counteract some of the swell pressure of the subsurface materials, and is of course, based on the DL over the area of the pier. The rest is counteracted by the skin friction of the pier below the depth of wetting. The two together help to determine drilled pier lengths. The less DL, the more skin friction you need. When you switch to a helical, the shaft area is quite a bit different than the drilled pier and you have a different soil / structure interaction. The comparison is not a good one. One thing to keep in mind is that the helicals should bear below the depth of wetting, or they will heave from the bottom up, and not just along the shaft. There are a variety of other things to look at when changing the foundation type.

 

[TDAA]

houseguy, the pier is to counteract swelling soils in this area of the country. Also, it is likely the HP would penetrate into the bedrock, as mouch of the bedrock is "soft", poorly cemented sandstone or claystone (compared to granite, etc). See my post above to get an idea of why the lenght is required. The total lenght is based on the depth of wetting and the length required for the skin friction.

 

[COEngineeer]

I will call the geo tech guy tomorrow. I just thought helical pier is affected by uplift only by a little bit because the soil will push up and down and cancel each other. Therefore, you dont need to meet the minimum DL and minimum penetration.

 

[JackTrades]

The A.B. Chance helical piers that I've used in the past have a 1 1/2" square shaft which, I was told, would help in reducing upward lift due to expansive soils by not allowing the soil to "close" around the shaft. I used them quite a bit in the early to mid 90's in the Colorado Springs area and found them to work well, most of the time, for certain remediation and new construction projects. We once had the local installation contractor perform a load test with an 8" helice screwed down to about 25 feet which, I believe, put the helice a few feet into the transition layer at the top of the Pierre Shale. We managed 30 kips with a deflection of 1/4". The one problem I encountered on a few occations was when installing in an area with a gravel layer at the top of the shale, the screw would sometimes spin. I had the contractor sharpen the leading edge of the helice and cut a greater angle on the point bevel, but never got purchase into the shale.

 

[jhoulette]

I have used helical piers for most of the house additions I have done when drilled piers are supporting the existing structure with no problems. Talk to the Geo about design criteria for helicals to include a minimum depth, minimum torque and I believe most Geos will give you the conversion value to use for gravity loads to the min. torque.
For additions you usually just don't have the space for a drill rig to put in drilled piers.
Also, I used a lateral resistance device with helical piers. A web site I know of and supplier is

http://www.pacifichousingsystems.com/LateralResistanceDevice.htm

But you may not need a LRD if you can be braced in another manner.