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Helical Pier Capacity 2

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lsmfse

lsmfse

Structural
Nov 19, 2001
147
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.
 
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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?
 
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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
 
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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.
 
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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.
 
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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.
 
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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!
 
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Helical piles of this type of capacity will be special order only. In-stock helical piles usually only go up to about 150k.

csd
 
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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.
 
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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.
 
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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!
 
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I appreciate all the input. Does anyone have an idea of the cost of mini piles vs drilled caissons?
 
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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???
 
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Not familiar with the terminology. Please advise what you mean by micropile.
 
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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.
 
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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?
 
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Google "Micropile" and you will get a cornucopia of info. It is nothing like an augercast pile.
 
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