Helical anchor allowable bearing pressure 3

[HaveGunWillTravel]

I am designing helical anchors using the individual bearing capacity method. I typically limit the bearing pressure of the helices to less than the allowable soil bearing pressure given in the project geotech report. For instance, I recently was asked to design helicals to replace concrete-shaft drilled piers that had an allowable end bearing of 15000 psf, so I limited the helical bearing to less than 15000 psf. Is that necessary? I have not found such a design check in any of the literature I have or by an internet search, but it seems reasonable to me. So, is that check necessary?

 

[csd72]

Seems reasonable if it is in soil but not if it was bearing on rock.

You also need to check the structural strength of the screwpile.

I would specify that they check the torque value also and base the acceptance of the load capacity on that.

 

[PEinc]

I would not use the allowable bearing pressure given in a geotech report for spread footings (shallow foundations?) when designing helical piers (deep foundations?). The soils could be very different at the two different levels and the allowable bearing capacity was probably given for a much wider footing. The assumptions used to determine the allowable bearing pressure for footings may not apply to the small diameter, deeper helices. Do a helical pier design based on your interpretation of the borings at the expected bearing level and then confirm the pier capacity with load tests.

http://www.PeirceEngineering.com

 

[bksstructural]

I agree with PEinc. The geotechnical engineer gave an "allowable" bearing capacity for a drilled shaft. You don't know what his true factor of safety was in publishing that value. I have known geotechs to up the factor of safety for lightly loaded structures just to make sure the engineer uses a decent sized shaft. Additionally, I have seen publications that suggest a different Nq value for helicals than that used for drilled shafts.

 

[Geostructsparks]

Agree with responses above. In particular, bearing capacity for a larger diameter shaft increases the B*gamma/2 term proportionally (but not the depth or cohesion terms), so the bearing capacity of a large diameter shaft in sands would be higher. On the other hand, if they consider settlement response of the tip, allowable bearing capacity for a large diameter shaft may be much lower since mobilization of bearing pressure may be limited by the overall settlement (BC mobilized at 10 - 15% of diameter). If for shafts roughly the same diameter as the helical pier, then may be OK.

As noted above, the geotechnical recommendation could be uniformly conservative, since the geotech may have a high factor of safety on their tip resistance. Low allowable bearing might be presented in part because if stronger and weaker portions of the deposit are available (variable non-homogeneous strength) one can't rely on the tip bearing on the strongest layer (either because one can't know exactly which depths the highest strength is, or one can't rely on the contractor not to overdrill, whereas you can advance the helical until torque is reached), or in particular they may have assumed cleanout of the shaft tip may not be perfect, particularly for small-diameter shafts.

 

[PEinc]

I've never seen a project geotech report give bearing pressures for helical piers. I suspect that the bearing pressures given in the geotech report were for spread footings bearing at a higher elevation than the helices would be located.

http://www.PeirceEngineering.com

 

[dik]

From my drawing notes for a recent project where the geotechnical engineer has provided design data:

FOUNDATIONS (SCREW PILES)

THE FOUNDATION DESIGN IS BASED ON USING CONTINUOUS SCREW PILES

PILES HAVE BEEN DESIGNED ASSUMING A SKIN FRICTION VALUE OF 375 PSF AND AN END BEARING CAPACITY OF 4177 PSF

FOR EXTERIOR PILES, THE TOP 6.6 FT BELOW FINISHED GRADE HAS BEEN NEGLECTED FOR SKIN FRICTION

FOR INTERIOR PILES AND BASEMENT WALL PILES, THE TOP 3 FT HAS BEEN NEGLECTED FOR SKIN FRICTION

SKIN FRICTION HAS BEEN ASSUMED FROM THE ABOVE DATUM TO ONE HELIX DIA ABOVE THE FIRST HELIX

BEARING AREA HAS BEEN ASSUMED TO BE THE HORIZ PROJECTED AREA OF THE HELIX LESS THE DIA OF THE PILE FOR THE UPPER HELICES AND THE FULL PROJECTED AREA FOR THE LOWEST HELIX

PILE WALL THICKNESS AND SIZE SHOWN IS MINIMUM AND SHALL BE INCREASED AS NECESSARY TO ACCOMMODATE INSTALLATION EQUIPMENT

SCREW PILES SHALL BE PLACED NOT CLOSER THAN 2-1/2 HELIX DIA FROM ADJACENT PILE U/N

PITCH OF HELICES SHALL BE 6" UNLESS APPROVED BY THE GEOTECHNICAL CONSULTANT

SPACING OF HELICES ALONG THE PILE SHAFT SHALL NOT BE LESS THAN 3 TIMES THE HELIX DIA

SPACING OF HELICES SHALL BE A WHOLE NUMBER MULTIPLE OF THE HELIX PITCH

PROVIDE A 1" THICK MIN STEEL PILE HEAD WITH SUITABLE MEANS OF CONNECTING TO THE INSTALLATION EQUIPMENT

PROVIDE A ZINC RICH EPOXY PRIMER FOR THE TOP 10 FT OF ALL PILES AND THE PILE HEAD


Dik

 

[PEinc]

I guess I've seen it now. Interesting. Continuous screw piles but helices are not continuous. It sounds like the spec is referring to Chance's soil screws.

http://www.PeirceEngineering.com

 

[SoilRocks]

After reading all the responses, I would urge you to be careful. Most helical piles are designed more as friction elements than end-bearing elements. The geology in your area may dictate end-bearing for support. But you can refuse right after you get to dense material because your torque shoots up. Getting embedment can be important and vary your end-bearing. You evidently have a helical contractor in the area that is going to install these. I would talk to them to see if anyone has done any load tests in the particular formation you will be bearing in to get some typical values for end-bearing. Also, I would require some load-deflection testing of the anchors. If you are being conservative because of the uncertainties, these tests may pay for themselves.

 

[dik]

The helices for this project are 2' diameter and located 6' apart; the shaft is 13" dia od.

The piles have a capacity of from roughly 50K to 75K with the first helix located approx 16' from grade. Clay is approx 60' deep.

Dik

 

[dik]

Load testing is already part of the project specs...

Dik

 

[bksstructural]

I disagree with SoilRocks. Most helical pile are designed as end bearing pile, not friction pile. The design is normally based upon the individual bearing of each helix. Most of these pile have small shafts and skin friction is normally neglected in the design. I recently designed a 24" single blade helix on a 7" shaft bearing on sandstone with overlying strata of soft and stiff clays. The pile was load tested to 342K. A similar pile, 7" shaft with 16" and 20" helices was load tested to 420K in a shale. In both instances, the load came from bearing, not skin friction.

 

[dik]

Is anyone aware of any literature that provides guidance on the design of the 'screw pile' itself, not the load carrying capacity?

Dik

 

[PEinc]

Usually the manufacturers literature gives ultimate strenghts of their anchor or pier components. They also often recommend a safety factor of 2. The parts are usually proprietary so it can be hard to design for anything other than their recommended strengths.

http://www.PeirceEngineering.com

 

[bksstructural]

Dik, I provide specialty design to a helical pile contractor. Everything we do is custom. What are you looking for? You can contact me direct if needed.

Bks

 

[dik]

I was wondering if there was a methodology of designing the screw auger piles that did not involve doing finite element modelling... given the torque, soil type parameters, diameter, wall thickness, spacing of helices, length, etc.

 

[bksstructural]

dik,

There's no need to do any type of finite element modeling, unless you were trying to analyze the plate bending capacity. I normally do a yield line analysis to determine plate thickness. The design of the shaft itself for axial and lateral loading is no different than any other steel pile. The biggest difference is you will need to analyze the shaft and connections for torsional loading. The torsional load itself will be dependent on the soil/pile capacity. There are well established torque to bearing capacity correlations for "normal" shaft sizes; i.e. for a 1.5" square shaft, a Kt of 10 is accepted. This means you can multiply the torque (in ft_lb) by 10 to obtain the ultimate capacity of the pile (in lbs). The Kt drops as the hub (pile) diameter and helix plate thickness increase. For 2.875" pile the Kt is 8 to 9, for 3.5" pile its 7-8. At 7" it will be down to 4.5. To structurally design the pile, you first have to determine the ultimate capacity of the system. If you are designing the pile for a known load, this would be the required allowable capacity times a factor of safety (2 normally). If you are checking a particular blade configuration in a soil profile, you would calculate the ultimate bearing capacity using either the individual bearing capacity method or the cylindrical shear method. Once you know the ultimate capacity, you have to select a pile size (diameter and thickness) capable of handling the required torque, axial, and lateral loads. It can be somewhat trial and error, especially if you are working with a large array of pile sizes and working on non-typical stuff.

Is that what you were looking for?

bks

 

[dik]

Thanks... dik