Helical pier slenderness or... column bracing by soft soils
Helical pier slenderness or... column bracing by soft soils
(OP)
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?
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?
RE: Helical pier slenderness or... column bracing by soft soils
RE: Helical pier slenderness or... column bracing by soft soils
We have soft clay (with sand above and below) so.... there's hope!
RE: Helical pier slenderness or... column bracing by soft soils
Never used this on a house. Did use it on a federal (military) multi-family project once.
RE: Helical pier slenderness or... column bracing by soft soils
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"), Fy = 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:
www.SlideRuleEra.net
RE: Helical pier slenderness or... column bracing by soft soils
RE: Helical pier slenderness or... column bracing by soft soils
www.SlideRuleEra.net
RE: Helical pier slenderness or... column bracing by soft soils