drilled shaft pier vs helical
drilled shaft pier vs helical
(OP)
We are planning a 400 sq foot deck attached to a house that has a hillside out the back. Our geo-eng. spec'd 30" piers 12' into the bedrock. The total pier length would be 27'. The deck would require 4 of these. I have inquired about helical piers. He said they are not for use with lateral loads. Although, I have found a company that manufactures a LRD for use with a helical. The house was built in 1959 and has some type of piering system although I doubt 12' into the bedrock. We want something that is adequate but don't want to install something that is more than we need. Appreciate any advice.





RE: drilled shaft pier vs helical
Contrarily, the 50's design of foundations was more customary and in accord to what the average behaviour is. Obviously, the average behaviour is far far from the development of a worst case scenario. As far, for example, as the normal operation of the hotel was from plunging to the river in the typhoon of two days ago. Imagining the hill sliding past your socketed piers like butter, er, makes one willing to search for another site, and this is one thing an owner wanting to have some house there is not surely wanting to consider. So the practical approach is go with the recommendations of the geotech, you'll be in the warrant of their recommendation and in the end it may turn reasonable in current practice terms.
RE: drilled shaft pier vs helical
If worst case happens, is house founded on similar foundations? It's been fine for 60 years.
RE: drilled shaft pier vs helical
My point to the geo-tech is that when the big one hit I won't be standing on my deck because it will have been crushed by the collapse of the house. We had a structural engineer take a look at the house before we purchased it and he found no issues with it.
Thanks for your input.
RE: drilled shaft pier vs helical
The number of required piers is related the way the deck if framed. By using larger framing members, you could reduce the number to a minimum of 2.
RE: drilled shaft pier vs helical
RE: drilled shaft pier vs helical
Any ballpark figures on what these four drilled shaft piers would cost to install? I have a drilling company coming out next week to give me a proposal but I don't want to waste more time if the cost is going to be prohibitive.
RE: drilled shaft pier vs helical
I'd ask your geotech for alternates to what he proposed initially. Perhaps, in place of 4 large piers a larger number of smaller diameter micropiles could satisfy. Micropiles (5" to 7" dia.) placed both vertically and in a battered array will carry the lateral load of a deck and can also be keyed into rock with relatively small sized equipment when compared to drilled pier machines.
Helicals can carry a lateral load when battered and tied to vertical units, but can not be keyed into rock.
RE: drilled shaft pier vs helical
The large diameter and embedment of the pile is probably based on a calculation assuming the pier behaves as a cantilever. If you tie-back the top, you may get a more reasonable size. Also, with this concept the required embedment should be much less.
One option that could be explored is whether you can use a helical anchor for vertical load and a second helical anchor on an angle to resist the lateral load. You could also try micropiles (1 vertical, and one at a 45 degeree angle). See attached. These system can serve dual purpose.
Another option is a drilled pier with a cap beam and a micropile or helcial tieback installed on an angle into the cap beam. Depending on your spacing of piles and cap beam I'm sure you could get 16"-18" piers.
RE: drilled shaft pier vs helical
Some helical pier manufactures have caps that fit on the helical pier shaft and provide for lateral stability, LRD or Lateral resistance device as you mentioned. That should be plenty of support.
For a deck, I'm curious what lateral loads are being braced by a 30" diameter pier? The wood posts don't transfer heavy lateral loads.
I do understand why 12ft into bedrock was specified, due to very small deadload to resist the uplift pressure on the pier, therefore deeper bedrock penetration. But again, this is a deck, some movement can be expected.
Why not just use 18" x 18" x frost depth pads? The house was built so long ago that the soil has been exposed to the additional moisture from lawns and such that any swelling or settlement has taken place. Any future movement should only be from year to year variations in moisture.
RE: drilled shaft pier vs helical