Helical pier slenderness or... column bracing by soft soils

[HouseBoy]

I'm working with a geotechnical engineer's recommendation for helical piers to be extended thru a very soft layer of soil to reach bearing (rated by the torque) at a much greater depth. Boring logs indicate the sampler dropped under its own weight (140 lbs) 12 ft and 15 ft in the two boring holes. The helical pier installer did a test pier that reach 47 to 50 feet before reaching the target bearing capacity. 30k is the design load (unfactored D+L) and the "Practical Load Limit" for the test pier is listed by the installer as 80k.
Borings were terminated at 20 and 30 ft.

I'm wondering about the pier shaft acting like a slender column thru the 12 and 15 ft zones of soft soils.
Shaft is 2 7/8" diam 0.262" wall thickness. Seems marginal for slenderness.

I have a few messages in to the soils engineer but have not heard back in a couple of weeks. Maybe he was on vacation or he's very busy..... I'm just trying to get some background info on this deep and slender component.

Project is a single family residence located in Northern Michigan so snow is a big number (i.e. not a dead load). I estimate DL/LL are about even contributors to the design load (wood frame house with crawl space foundation throughout. Most of the pier loads will be 25 to 30k.

I'm just thinking that when the boring probe can drop under its own weight, then the bracing effect of the surrounding soil might not be that much. Any research on this or good references for this particular aspect?

 

[phamENG]

Only anecdotal, and probably not much help. I believe it comes down to the type of soil. I recall two projects with this issue - one with soft clays and the other with soft, silty organics. If I remember correctly the geotech green lighted the clay to support the pile but for the organics we had to switch to driven timber piles because the standard helical piles were too slender and the large diameter (4" round HSS) lead sections were too expensive.

 

[HouseBoy]

Thanks pham.
We have soft clay (with sand above and below) so.... there's hope!

 

[phamENG]

Indeed. Another possible (though likely unwelcome) solution is to do a grouted micropile. Helps to boost the buckling strength of the section in questionable soils. It's just a helical pile with a flat plate a bout a foot above the helical leads. It displaces a bunch of soil around the pile as it goes down. The contractor builds a large box around the pile and fills it (and keeps it filled) with near zero slump grout/slurry. As the helical goes down and displaces the soil, the grout flows down and 1) helps keep the soft stuff from collapsing and 2) hardens to give you better buckling resistance. Doesn't really gain bearing strength for the pile as that's still based on the size of the largest helical disc but strengthens the flimsy shaft.

Never used this on a house. Did use it on a federal (military) multi-family project once.

 

[SlideRuleEra]

Boring logs indicate the sampler dropped under its own weight (140 lbs) 12 ft and 15 ft in the two boring holes.
30k is the design load...
Borings were terminated at 20 and 30 ft.
I'm wondering about the pier shaft acting like a slender column thru the 12 and 15 ft zones of soft soils.
Shaft is 2 7/8" diam 0.262" wall thickness. Seems marginal for slenderness.
Any research on this or good references for this particular aspect?

The geotech has not called since they have no data below 30 ft. So, do it yourself with available data, you have plenty to make an informed decision.

The 12' and 15' "soft soils" meet the definition of "fluid soil" (N=0) for pile lateral bracing purposes (Fluid Soil = Water).
Lateral bracing starts 5' into firm soil (N>5) or 10' into soft soil (0<N<5). For the best case (firm soil) unbraced length is 25' (5' + 15' + 5'). Treat the pile shaft as a column. Looking at an extra strong 3" steel pipe (wall thickness of 0.300"), F_y = 36 ksi as proxy for the helical pile shaft, rated load, from AISC 9th Edition, is 11 kips for 19' unbraced length. Longer unbraced length is not allowable since Kl/r > 200.

Also, the first 5' embedment of a helical pile is considered unbraced (except under best circumstances).

Bottom line, forget about helical piles for the conditions described... and, IMHO, don't waste (your client's money) on schemes to try to make helical piles work in fluid soil. Select a different type pile... and get a new geotech.

All of the above info can be derived from generic info in the "Ram Jack Engineering Manual"

Note: I have no connection with Ram Jack. We deal with much "fluid soil" (pluff mud) on the southeast coast:

http://www.SlideRuleEra.net

 

[phamENG]

I stand corrected. The depth of your library never ceases to astonish, SRE. Thank you for posting that.

 

[SlideRuleEra]

phamENG - Thank you. Fluid soil is a real trip. I've been in "bottomless" pluff mud. You displace your weight in the fluid. This means when you are upright, you are in the fluid up to your armpits... since the fluid has a density higher than water. You can sort of walk/swim slowly through it, without ever touching firmer soil. The good news is, that contrary to the unfounded rumor about quicksand, you are not "pulled under". You just bob-along till you get out.

http://www.SlideRuleEra.net

 

[phamENG]

Sounds...fun? I did have an interesting case once where WOH material went 85 feet before we got competent soils. It was a residential flood mitigation project, and existing building was on 30' timber piles in the muck. The geotech worked up a system of timber friction piles to go between the existing to improve capacity to account for the extra level of CMU walls. So the piles were driven through the first 6 feet of 'crust' and then pushed with the hammer to the specified depth. Poured grade beams, built the foundation walls, and then put the house back on top (elevating in a flood zone). So that house is supported on those 'fluid' soils. No issues that I know of.