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Helical Pier Capacity
2

Helical Pier Capacity

Helical Pier Capacity

(OP)
Hello all, I would like to get an opinion on the use of Helical piers .  We have soft soils over bedrock to a depth of +/- 30'.  Soils report recommends caissons.  Contractor has suggested Helical piers due to depth to ground water(10') since he says the cost to case the holes are significant.  Geotech approved the use of helicals and requires helicals to be "seated" in bedrock. Column loads can range in the 300-600 kip range, so multiple helix will be required.  My question is, how do you "seat" the helix in bedrock.  

RE: Helical Pier Capacity

He meant, keep drilling until you reach bedrock (30ft down) and turn it until you reach your design capacity.  I have never specified helical pier for that much load before, is it possible?

RE: Helical Pier Capacity

Sounds like a driven precast pile job to me.

RE: Helical Pier Capacity

Sounds like a multiple pilecap arrangement to get the anticipated load.  

I used these helical screw pile on a job about 8 years ago - a large premanuf metal building on fill with a structural slab.  Had problems with pile breaking off as they were screwed into place, and with the tips hitting large boulders necessitating relocation - typical in any pile job I guess.

Also had a problem with the contractor as he was the only one in the area with the special equipment and license to do the job.  $$$$$$$$  Just a thought if the equipment breaks down, as it did.  But the project did get completed and the building is holding up just fine.  

Mike McCann
McCann Engineering

RE: Helical Pier Capacity

(OP)
Driven piles are not practical at this site.  Basically, the Geotec was concerned about the helicals reaching bedrock & "spinning" creating an area of disturbed bedrock material. If the helicals arn't "seated", as the loads are applied during construction, settlment will occur. We are planning on using 6"-8" pipe shaft and in some cases filling with grout.

RE: Helical Pier Capacity

Helicals into bedrock just does not compute for me. Conversely, micropiles socketed into bedrock makes a lot of sense. With only 30-ft of overburden this seems like a very simple job. The loads you mention above are obtainable, generally speaking. The contractor just needs to have his hand forced in the appropriate direction, which is a different direction than helicals.

RE: Helical Pier Capacity

If the helical pier is installed through soft soil, it will advance until it hits bedrock.  In that case, only the shaft tip may be bearing on rock.  The auger flights may not be carrying any load, especially for a multiple helix pier.  I would not use helical piers drilled to the top of hard rock.  You will never be able to advance the helices into rock to accumulate their individula capacities.  Try driven piles or micro-piles.

RE: Helical Pier Capacity

We do drilled shafts below the water table all the time.  I'm not sure what the big deal is, just install casing, get to bearing, pour concrete and pull the casing.

I would not use helical piles for end bearing.  Matter of fact, I'm not sure I'd use helical piles for column loads of 150 to 300 tons.  Maybe I'm just old fashioned. . . . .?

f-d

¡papá gordo ain’t no madre flaca!

RE: Helical Pier Capacity

Helical piles of this type of capacity will be special order only. In-stock helical piles usually only go up to about 150k.

csd

RE: Helical Pier Capacity

If driven piles are not an option due to noise restraints or the like, I think you should revert to your original recommendation for bored piers.  As fattdad says, they are installed all the time where water is present.  If you can't pump the water out, tremie the concrete.  Helical piles are not the answer.

RE: Helical Pier Capacity

But of course hokie66, I'm sure the contractor knows this and could do it. My guess is he found a rock bottom price from some small startup helical contractor and sees a possible way to increase his margin $$. Any helical guy that would terminate their product on rock is probably hard up for work and can't put food on the table and therefore cannot be trusted. Now lets all go do the right thing and build it properly and not waste anymore time on something so foolish as this.

RE: Helical Pier Capacity

Amen

RE: Helical Pier Capacity

I'll bet there are some caisson contractors (can I still use that word) that are hungry this time of year too.

Second "amen".

f-d

¡papá gordo ain’t no madre flaca!

RE: Helical Pier Capacity

(OP)
I appreciate all the input. Does anyone have an idea of the cost of mini piles vs drilled caissons?

RE: Helical Pier Capacity

Costs are dependent of many conditions; site access, geology, geographic region and more? Not too far off the mark for most of North America, I'd budget micropiles at about $100/lf for capacities of up to 200-kips per. To get your total load carrying capacity you can pair or group this together, otherwise you can put in larger micropiles (macropiles) to carry upwards of 1000-kips each but at unit prices greater than $100/lf. Caisson prices???

RE: Helical Pier Capacity

Not familiar with the terminology.  Please advise what you mean by micropile.

RE: Helical Pier Capacity

A micro-pile or mini-pile is a relatively small diameter (about 12" or less), drilled and concreted foundation member similar to a small caisson or drilled shaft.  The micro-pile may or may not have a permanent outer steel casing shell.  The micro-pile also has an inner core of steel, usually some type of reinforcing bar or threadbar. Sometimes the core steel and/or the casing extend for the entire length of the micro-pile, sometimes not.  Micro-piles are very similar to heavy duty tiedown anchors, except they support load in compression instead of tension.

RE: Helical Pier Capacity

Sounds similar to what I call an auger cast pile.  So the casing advances down the hole with the auger.  Is the concreting done as the auger is removed or after?  When is the inner core inserted?

RE: Helical Pier Capacity

Google "Micropile" and you will get a cornucopia of info. It is nothing like an augercast pile.

RE: Helical Pier Capacity

Micropiles are drilled with casing, flushed out and grout poured or pumped into the cavity. Sometimes the casing is left in place as a structural element of the pile. Sometimes the casing is withdrawn as additional grout is placed through the casing. A single bar is sometimes placed in the center to provide additional capacity.

Augercast piles are bored using a hollow continuous-flight auger. As the auger is withdrawn, grout is placed under pressure from the auger tip. Any steel is placed after the auger has been completely withdrawn.

lsmfse,

It sounds to me like one of these two methods would be much more appropriate than a helical pile. Helical piles develop their capacity from the helices bearing on the soil, not in end-bearing. The Contractor in this instance is not giving you good advice.

Jeff


RE: Helical Pier Capacity

Large column loads can be dealt with by installing multiple screw piles under a pile cap - simple.

Seating the screw piles into rock is not new. The pile will not 'spin' just short of the bedrock because it is the helix that advances the pile, not the tip. Besides, there's a higher pressure under the tip which can cut into the rock until the helix reaches it - this is called seating.

Having said that, make sure the pile design does actually have a 'tip' per se and not just a tube cut at 45 degrees. A cut tube will not penetrate the rock.

RE: Helical Pier Capacity

I would think the ability of the helical pile to seat depends greatly on the rock. If you have a soft sandstone it may be possible, a hard granite, I doubt you would get any penetration. Large loads on soft soils with sleder shats raises questions about buckling. I would talk to Atlas or A.B. Chance Engineering deapartment directly. They will not want to see the product used inapproperately so they should give you some objective guidance.
50 tons is readily achiavable with a mini pile and higher capacities have been achived. A down the hole hammer allows seating into most rock. A newer twist is mini cassions that are somewhat larger in diameter and have a small cage as opposed to a central bar.

RE: Helical Pier Capacity

I agree with DRC's hard granite comment above. I dont come across much granite here in Australia, but your point is noted.

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