Tower Foundation Design 2

[GeoGrouting]

I am considering a monopole tower foundation design. The height is about 25 m. I would appreciate a couple of design references/manuals/texts that I can use for design. I am familiar with NAVFAC and wonder if NAVFAC has any chapter on tower foundation design. Thanks

 

[Mccoy]

Considerations should be given to cyclic loading conditions.

I fully agree, that shouldn't be forgotten when dealing with wind, seawaves, earthquakes and man-made vibrations.

 

[GeoGrouting]

Can Chapter 13 Foundation Design of the Highway Signs, Luminaries and Traffic Signals (AASHTO LTS-4)and ANSI/TIA-222-G be found online, please? Thank you.

 

[oldrunner]

I don't think so.

 

[jheidt2543]

Geogrouting:

The attached paper "Design of Laterlly Loaded Piles and Caissons in a Layered Soil", by Naik and Peyrot may be of some help. They have a pretty good example problem using their method for a drilled caisson foundation for a large utility tower.

 

[geoman110]

You can use both footings or auger caissons for the tower. The design issues for footing are straight forward. For auger caissons, I personally do not like and trust this passive design methodology. It is not very accurate and does not simulate the stress condition. In order to check or design the lateral resistance of a auger caissons or piles, I prefer and use Broms Theory.
I also have a question for bluefoot and Mccoy:
How do you consider cyclic loading in the design? Do you reduce the soil strength parameters (how) or something else?

 

[apsix]

I usually use Broms, but it doesn't work well for shallow, larger diameter piers in clay, due to the recommendation to ignore the top '1.5 x dia.' of embedded length.
This can cause a larger diameter pile to have a lower (theoretical) lateral capacity than a smaller diameter one.

 

[GeoGrouting]

apsix

a) The discarding of 1.5 m from the top, is it exactly a Brom's requirement? Thought it was allowance for depth of drilling which may be 0.6 to 1 m usually.

b) Also have seen amny references from Brom. One likes to read his own article but do not know where to find it.

c) How do you evaluate Das version of the Brom's theory.

 

[Mccoy]

I also have a question for bluefoot and Mccoy:How do you consider cyclic loading in the design? Do you reduce the soil strength parameters (how) or something else?

You may act on the parameters by reducing them, in 2 ways taht I know

1) rigorous 6 costly= dynamic lab tests smulating the design frequency, so that you'd have a phi_dynamic, c_dynamic, Su_dynamic....

2)Literature correlations = not always satisfactory

Or you might use a winkler-type model.

This way, you wuold consider a static horizontal (also vertical) reaction and a dynamic reaction.
A classical source for dynamic reaction is Gazetas, 1991

 

[geoman110]

apsix/GeoGrouting

There are two different things: ignoring the upper 1 m or 1.5 m or ignoring 1.5 x diameters (as apsix mentioned). Ignoring the upper 1m or 1.5 m is typically coming from issues such as soil disturbance due to frost-thaw action or any other phenomenon that you may have to reduce the soil resistance in the upper portion. On the other hand, I have not encountered anywhere the requirement for ignoring the 1.5 x diam

Mccoy, thanks for the info. I personally think for regular tower (e.g. tall light pole, telecommunication, etc), reducing the parameters from literature is good enough. For a more complicate tower load (e.g., wind turbine), the dynamic lab test is the way to go. However, I am not familiar with the winkler-type model or I may have studied it long time ago and completely forgotten about it. I would check it out.

 

[jheidt2543]

Geogrouting:

The paper I uploaded discusses Brom's original paper, as well as others, then extends Brom's work to include multi-layered soils. It also lists many references you could follow up on if you wished.

 

[geoman110]

Hi jheidt2543
Somehow I can not locate the file you have uploaded. How can I access to it?

 

[apsix]

Geogrouting

The advice re 1.5xdiameter is for Cohesive soils only and is stated in the California DoT FALSEWORK MANUAL (amongst others):

"LATERAL LOADING IN COHESIVE SOILS
The ultimate soil resistance for piles in cohesive soils
increases to some maximum value at approximately 3 pile
diameters below the ground surface then remains fairly constant at greater depth. Literature suggests using a soil
distribution of zero between ground surface and a depth of 1.5 times the pile diameter (1.5d) and then using a value of 9 times the undrained shear strength for the remainder of
the pile depth."

I understand that it's from Broms' original paper on the subject.

I don't know Das's version.

 

[jheidt2543]

Geoman110:

If you click on the link in my first post, it should take you to another page, click on that link and it will download the paper in .pdf format. I just tried it and it does work.

 

[geoman110]

apsix,
the text from California DoT (as you quoted) does not appear referring to Brom's method. I might be missing something in here.

 

[geoman110]

I have to correct what I said above. It is referring to Brom,s method. The concept is that due to lack of confining stress in the upper portion of soil, the resistance on the 1.5x dia. should be ignored. This might be the case for a driven pile with small diameter, however, for a large diameter caissons, the confining stress would be developed due to the size of the loading (diameter of caissons). As I noted before on this thread, I believe for a large diameter caisson ignoring 1.5 x dia is not necessary unless there are other parameter such as frost/thaw issue that can reduce the soil resistance on within the frost depth (not 1.5 x dia).

 

[apsix]

geoman110

From another source (AS2159-1978):
"the value of Nc in equation ... is generally taken as 9, but where the ratio of the depth to the diameter of the base is less than about 4, progressive reduction in the value of Nc to 5.6 at the surface will be necessary."

Do you have any basis for your assertion?

 

[geoman110]

apsix,

You and I may be talking about the same issue. Allow me to elaborate and see if we are at the same page or not. This dialogue started when someone earlier in this thread mentioned in using Brom’s method, we have to ignore the upper 1.5 x dia of pile. My point is when someone is using Brom’s method, there is no need to “ignore” the upper portion of the pile unless there are other reasons for soil to lose the strength (e.g., frost/thaw, erosion, etc). The loss of lateral resistance in the upper portion of the pile is already included in the original Brom’s method.

Here is the way I am looking at it: Brom’s Method which was originally presented in graphical form provides Ultimate Lateral Resistance of the piles based on several variables such as e/L, Su, etc. One of the most important variables in his graphs is L/d (embedment length/diameter). In his graphs, if the L/d is low, the associated factor for Ultimate Lateral Resistance would be reduced extremely. For example, for a pile with e=0, at L/d=8, the factor is about 20 and for a L/d of 4, it is 5 and at L/d of 2, it is practically zero. As you can see, any lost associated with the low lateral resistance in the upper portion is already included in the graphs and if we use Brom’s method/graphs and also decide to ignore the lateral resistance of the upper portion of the pile, we are deducting it twice (Brom already done that in his graphs). All we need to do is to get the associate L/d ratio (e.g. L/d=4) and other variables (e.g., e/L, etc) and use the graphs and obtain the Ultimate Resistance factors (everything is already calculated even low resistance in the upper portion).

Put all of that in perspective, when I design a pile/caisson subject to lateral load, first I use Brom’s method/graphs to obtain ultimate lateral resistance and then if the pile/caisson is subject to freeze/thaw cycle, I ignore the resistance for the upper portion of the pile which is subject to frost action. This may not make any considerable difference at all for a caisson or pile whit diameter of 750 mm (18”) or larger. However, if we are talking about a pile with diameter of 300 mm (12”), the way I approach it would make a noticeable difference for short piles as there are reduction in the lateral resistance in the upper 2Xdia. of the pile (already incorporated in the Brom’s method) and then in addition to that, I ignore the lateral resistance for the upper portion of the pile (say 1.2m depth-frost penetration depth).

I hope this could clarify my opinion. I am not correct, I greatly appreciate everyone input.

 

[apsix]

geoman110

We do agree, we're just approaching it from a different perspective.
If using Broms' graphs the ignored 1.5 x dia is already included. Your approach is correct.
I've developed a spreadsheet using Broms' theory; one of the inputs is depth of pile to be ignored, which I usually set to 1.5d.

Sorry about any confusion.

 

[geoman110]

apsix,
Thanks for the reply. I am happy that the issue is cleared. As I noted before, generally, I directly use the graphs provided by Brom; however, your spreadsheet idea is appealing also. Particularly, if you need to ignore more than 1.5 X dia. for the reasons I mentioned before (e.g. frost depth). The spreadsheet sound easier to work with for this situation

 

[UtilityStructure]

You should see if your work will invest in LPile for pier/pile analysis (if you need to design often).

The software is easy to learn/use, but generally will require a geotechnical report done unless you want to make some assumptions.

Or if you have STAAD you can use STAAD foundation. I'm not so familiar with that program.

Cheers!