Scott Gibson, Michael Maines and Helical Piles
rockies63
| Posted in General Questions on
Scott, I’m not a Prime member so can’t reply to your article on helical piles but wanted your opinion on this video from Cornell Engineering in Australia on the problems associated with helical piles.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Scott,
You were looking at helical piers, and also a sort of space-frame foundation that didn't need much excavation. What did you end up using?
For those who don't have 29 minutes* to watch the video, he talks about a number of issues and offers solutions.
One issue that he does not offer a solution for is if the soil is unstable and expansive. His comment: "There's no justification for designing a foundation on low-classification soil simply because you're supporting it on screw piers. The foundation system still needs to stand on its merits."
One issue is that helical piers have low resistance to sideways forces. The piece at the surface is only around 2" thick and doesn't have much horizontal resistance to begin with, and in the drilling process you disturb a column of soil around the pile. He recommends a pile cap, which is a concrete cylinder poured around the pile at ground level. His recommendation is to start the hole with a post-hole-digger, then put the helical into the bottom of the hole, and then fill it with concrete. The size and depth of the hole needs to be engineered.
Often helical piers are used because the soil on the surface is prone to settling or recession. When that happens the house appears to rise out of the ground. His response is basically it is what it is. If the soil has contracted, don't attempt to fix it by bringing in more soil, the soil may expand again and push the house off of its foundation.
Sometimes a pier can't be put exactly where it needs to go. When that happens the pier gets a sideways moment if the load is off center, which it isn't engineered to handle. That can be addressed by using ground beams, horizontal poured concrete beams at ground level connecting the pier to the two adjacent ones.
I'm working on a house right now where the engineer has specified helical piers for tree preservation. He has specified that the piers should end 9" above grade, and an 18"x18" reinforced concrete grade beam should encapsulate the pier heads and connect them all together.
*(I watched it at 1.5x speed. He's a slow talker.)
Thanks for the summary! And for the description of your project. As a fan of tree preservation and avoiding concrete, I find it a little unfortunate that you still need concrete, but it's presumably much less than in a conventional foundation.
DC, thanks for weighing in--I'm not watching a 29-minute video, even on 1.5x speed! Your summary sounds accurate. On a recent project I designed a mini-basement to provide lateral strength and for mechanical transitions, but for simplicity we went with Byggmeister's WarmForm instead. (With a low-carbon concrete specified, of course.)
Nothing worrying in the video. Many of the comments were specific to their warm Australian climate. His preference for encasing the tops of the posts with concrete piers would be problematic when things froze.
As DC said, his basic assumption is the they are used when you can't get bearing close to grade, so many of his concerns are for very deep (5m) posts - which aren't very common.
One interesting observation was that used under a waffle slab or grade-beam where there are expansive soils is that the soil movement at grade may alter the forces each post ends up carrying, as the beam is subject to differential uplift.
Thanks Malcom. Concrete pier caps would be a problem, which is why I mentioned using angle bracing, which transfers lateral loads to the floor diaphragm.
I don't know about other brands, but Techno Metal Post (TMP) comes with a plastic sleeve that is designed to allow vertical soil movement without affecting the metal post.
Our local installer said that the deepest he's gone is about 30 feet, through fill over a peat bog. They just keep going until resistance to drilling indicates adequate bearing capacity. I might worry about that in a seismically active zone, or would at least want to check with an engineer.
Here in DC there's a neighborhood called Spring Valley that was used for chemical weapon testing during WWI. After the war they bulldozed the site and it was redeveloped as housing. I was talking to someone who put an addition on a house there and he said they had to go down 40 feet with helicals to get adequate bearing.
I have Techno Metal Post (TMP) under about 1300 sq ft, the heads are out about a foot with metal brackets welded on. The reason was tree preservation and I'm in a valley that was once an extremely large lake (almost 8000 sq miles) thus the "soil" is largely clay, sand and gravel. The height "about 1 foot" was in part due to regulations (power line clearances), typical issues with "in-fill" lots. Only one pile out of about 30 had to be deeper. Beams are set and screwed into the brackets. The floor joist are set between the beams (not on top of the beams). As all the beams (screwed into the brackets) form a network (together with floor joist), I would imagine head movement would be somewhat minimized. Two PEs (structural engineers) reviewed the plans.
Have experienced several earthquakes, a 6.5 in Idaho and others closer by (the house is still standing).
And, I have watched/listened to that entire Australian video.
Michael, so would you need to have a structural engineer specify the size of the braces and their angle/attachment points to the floor structure and posts? In your photo you show tall wooden posts on top of brackets so the bracing would be wood to wood but what if you had used beams on top of the brackets? Would you still have needed braces?
What type of brackets did you use and how are the wooden posts attached?
And for those interested, here's a video from 30x40 Design studio in which the architect and his structural engineer discuss the loads and forces acting on a pier and beam foundation and the challenges of attaching the right sized braces and what to use to attach them (it starts at minute 10).
https://www.youtube.com/watch?v=29-xtjX8rAk
I might get you guys to summarise all of my videos.
If you're more visual, I also have a transcript on my website (so you can read as fast as you like). Loved the feedback. Cheers from Australia.
Matt