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Helical vs. Concrete Piers (cost effectiveness)

BOBWALTON (Geotechnical) (OP)
14 Jan 06 11:10
Howdy:  After doing a search on helical foundations, I discovered the following post: thread256-115353 by renRosen which has been placed in the archive.  I would like to revive the question:

renRosen: "I'm starting the process of having built for me a home (2700 sf) on a hillside outside of San Francisco.  The slope is pretty severe (1(rise)/2(run)).  Bedrock is around 10 feet down.  Foundations here are very expensive ($100K-$200K) and almost always use concrete piers and gradebeams.  I've heard that helical piers are ofen less expensive and quite effective, but they don't seem to be used very often here.  I'm trying to figure out whether helical piers can be used on a job like this and whether they tend to be more cost effective.   Any insights (including thoughts on other foundation systems which could be explored) would be greatly appreciated."

Then came one reply.

GeoPaveTraffic:   "I would stick with concrete piers and grade beams.  They have the advantage of being a stiff system, whereas most of the helical piers are not very stiff.  Another consideration is that concrete piers can be ground partway into the surface of the bedrock, the helical piers cannot be.

Given the potential for earthquake loads and slope/mud slides in the area, I would go with as stiff a system as possible well tied together.  That way if one pier fails, the others can take the load and keep the house from moving."

As a former pole house builder and helical specialist, I would like to learn if anyone has successfully designed and constructed a hillside foundation of high capacity helical piles- 6" to 24" diameter tubular steel columns which are screwed into the soil using high torque- 10 to 20 foot-kip (or greater)?  The arguments of GeoPaveTraffic may be valid as to slender square-shaft helical piers which have very little lateral capacity and may not penetrate formational soils.  However, using a boom truck or hydraulic excavator with a high-torque hydraulic drive, I suspect that high-capacity galvanized steel helical screw piles would make a dandy hillside foundation- much more cost effective than caisson grade beam and just as stiff!         

Walton Foundations
www.waltonfoundations.com

PSlem (Geotechnical)
15 Jan 06 7:50
I doubt concrete piers would withstand a mud slide, and even if they did, the cost to restore slab support, repair pipes and get a new CO may be too high.  Helical piles work fine as new support under houses.  Several companies have larger tubular piles, but only a couple have the very large ones mentioned.  Cantsink does a 6000'# 2-7/8" shaft pile in GA which is well suited for residential loads.  In the above example of 10' to rock it would cost less than an additional $35 per lineal foot of foundation and grade beam with the only alteration to standard footings being the addition of negative moment rebar.  

BOBWALTON (Geotechnical) (OP)
17 Jan 06 23:07
PSlem:

Thank you for your response.  Almita Manufacturing of Alberta, Canada is making
screw piles up 12 inches diameter and I believe they are starting to use their piles for cantilever shoring walls.  I was not familiar with Cantsink and appreciate the reference.

I am glad to see engineers interested in helical screw foundations as a substitute for concrete pier and gradebeam.  

My question was actually directed at a different concept.  On a steep slope, such as the one confronting renRosen, one has the option of constructing a floor diaphram to be anchored where the floor intersects the slope.  The rest of the floor would be supported by vertical columns.  Each column would be a round-shaft helical pile driven into formation.  Depending upon soil conditions, grade beams would not be used.  This is not a new concept, just a new application for helical piles.  I'm wondering if it has been done yet.  

Walton Foundations
www.waltonfoundations.com

PSlem (Geotechnical)
22 Jan 06 5:36
I have seen some new houses where the wall was anchored and the floor supported, but if you are just anchoring against earth pressure, I have done several cantilevered retaining walls on two rows of helical piles.  I batter the outside row to compensate for the horizontal thrust.  A house like you describe would be similar.  This minimizes the number of piles and their cost.  Plus the piles exhibit less movement in compression.  

BOBWALTON (Geotechnical) (OP)
3 Feb 06 10:53
PSlem:

Thank you for your idea on battered piles.  I am presently working with another engineer studying feasibility of cantilever helicals and I will pass this along to him.

Also to return to the issue of cost, I recently bid a large room addition foundation on a fill and partly on slope (1.5 to 1.0).  The geotechnical engineer called for a concrete caisson and grade beam foundation of mostly 24" diameter caissons.  There was about 300 LF of drilling, spoil removal, and concrete.  

Then I figured an alternate using 3" round-shaft galvanized helicals and reduced grade beams to the level ground (subject to engineering review).  The alternate came in 36% cheaper.  The engineer told me that he is not seeing a problem with lateral soil movement so he may not require tiebacks.  If he does, I can throw in a few square-shaft helicals and still be way under the caisson price.          

Walton Foundations
www.waltonfoundations.com

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