Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Helical Pier Capacity 2

Status
Not open for further replies.

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.
 
Replies continue below

Recommended for you

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 Sometimes the casing is withdrawn as additional grout is placed through the casing.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 As the auger is withdrawn, grout is placed under pressure from the auger tip. Any steel is placed after the auger has been completely withdrawn.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


 
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.
 
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.
 
I agree with DRC's hard granite comment above. I dont come across much granite here in Australia, but your point is noted.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor