Design of Drilled Shaft Foundation 2

[SharkswithLasers]

I would like the following information for use in studying for the PE exam:

Given: I have a boring log that indicates field "N" values are fairly consistent, ranging from 10 to 15 bpf from the surface to 35 feet bgs, soils are silty sands, with a few thin (~2-inches) clay lenses. Water table at 18 feet bgs.

#1) Select soil design parameters for use in the drilled shaft foundation design for the construction of a 180-foot monopole used for a telecommunication tower. State your assumptions (loads, etc).

#2) How would the soils info be used in designing the drilled shaft foundation. Concrete specifications, reinforcement, etc. Is the use of 1.7D + 2.6L typical for communication towers?

I'm not taking the geotech depth exam, but need to see how to use the field data to recommend soil design parameters and drilled shaft foundation design.

 

[Focht3]

You need a new site!

Sands near the ground surface offer very poor lateral resistance. Where did you get this problem?

 

[MikeDB]

Regarding item 2, reference TIA/EIA-222-F standard for foundation requirements. It looks like the safety factor varies from 1.5 to 2 depending on the type of foundation.

 

[SharkswithLasers]

Focht3: The "problem" was adapted from a site boring log (Texas) provided in Drilled Shafts and Caissons (ASCE publication?).

So what you are saying is that not enough skin friction (i.e., pull out force) could be developed to counter act the wind forces in these medium dense sands, correct?

If something like this came up on a PE exam, the "N" values would have to be higher, and then the problem could be done. Or is it possible to construct a very wide diameter drilled shaft foundation?

Either way, it seems you are saying it is not economical (find a new site).

 

[MikeDB]

The blow count could be used to estimate soil weight (110pcf) and angle of internal friction (30). Embedment length and shaft dia. could then be determined from a Broms equation or a program like LPILE or CAISSON. I think I would design for a higher water table, depending on when the boring was taken and how close to the coast it is. Sites for cell towers are usually dictated by coverage area, so this may be the best site. Drilled pier can work for this location, however at that height, a guyed tower will probably be more economical, but that is not what you asked.

 

[Focht3]

SharkswithLasers:

A 180 foot monopole telecommunications tower in silty fine sands with 10 to 15 bpf to 35 feet within 30 miles of the Gulf of Mexico:

You will be dealing with 130 mph hurricane force winds (design); the expected mode of failure for a monopole would be overturning, not uplift. Although a tornado spawned from a hurricane could lift the monopole and foundation, it should have already failed in overturning or failure of the pole-foundation connection (due to the high winds associated with a tornado big enough to pick up a monopole of this size.)

(I inferred the silty fine sand and location within 30 miles of the Gulf based on the location (Texas), the sand thickness (the site is probably near the coast) and the depth to free water. My principal area of practice is in Texas, and I've done a lot of monopole design - both direct embed and bolted tower types.)

In my experience (in Texas), monopoles stop at about 100 feet. So-called self-support towers top out at about 180 feet - and guyed towers are used for heights above 180 feet. These are rules of thumb - the height boundaries are guidelines, not limits.

It is possible to design a drilled pier foundation to support this design, but it would probably have an extremely large diameter - I would guess at least 8 or 10 feet in diameter (and it could exceed 12 feet.) And the embedment would likely be over 30 feet, so you would have to fight the groundwater. Not a very practical choice.

Did you develop this problem, or was it given to you?

 

[SharkswithLasers]

Thank you for the comments.

The drilled shaft designing procedure was something I wanted to be familiar with. Even though it seems that a guyed structure would be the most likely given this scenario.

I developed this "problem" to go through the motions of designing a drilled pier foundation. I planned on doing the same (for this 'site') for a lattice tower (self-supporting) structure and a guyed tower. Then comparing them all. Perhaps it would have been better to forget about the site location and provide anticipated loads, adn then still go through the motions, even if "intuition" and rules of thumb would dictate otherwise.

 

[Focht3]

Okay, I'll go along with the exercise. Here are some suggestions:

Monopole: Lateral load controls. If bearing controls, you've busted the lateral load analysis (or it isn't a true monopole.)

Self-support: Uplift controls. Clays: reduce your allowable uplift to 90% of the compressive force skin friction. Sands: reduce your allowable uplift to 70% of the compressive force skin friction.

(Wide Base) Lattice towers: Bearing should control, but check uplift. "Efficiently" designed foundations should also be checked for lateral load stability.

Factors of safety will depend on the consequences of failure. This topic is too complicated for this message thread.

Good luck!

 

[jdmm]

Sharkswithlasers

I would drill it with a backhoe. A 180 foot monopole is not very big and uplift/overturning is the main concern. For silty sands with the water table at 18 feet a buried spread footing would likely be more appropriate. With a rule of thumb of 25% in the ground the drilled foundation depth would be 45 feet. It seems to me that a driven pile would be more appropriate. However, if you are in a seismically active zone you have a problem below 18 feet with liquefaction.

Sorry to say that this is a dumb question where the solution is presented with the problem.

 

[MikeDB]

I must respectfully disagree with jdmm.
We typically dig with a backhoe and drill with an auger.
A 180' Monopole is HUGE.
The reactions at groundline are shear, axial compression and large moments. There is no uplift at all.
In 25 years I have never even heard of a 25% rule, nor would I consider using such a rule since the moment capacity is a function of the square of the embeded length.
I would only consider spread footers if it was a 3 or 4 legged tower.

 

[Focht3]

A spread footing (in silty fine sands) is not a good solution - the excavation would be over 60 feet wide / round. It isn't economical, and frequently the sites are only 75 to 100 feet square. That's not a lot of room for error.

Truck-mounted drill rigs are feasible, although the sand would need to be cased or drilling fluid used to stabilize the hole (which adds a whole lot of other problems.) A driven pile could be used, but the connection would be problematic - it isn't done (around here, anyway) very often.

In the monopole business, 180 feet is huge. They are manufactured in 45 foot sections, then galvanized. (The galvanizer's vat is "only" 46 feet long.) The connection is a press fit to a dodecahedral shape; no weld or bolting. The overlap is typically two to three feet; you would need five sections - plus the foundation - to fabricate the structure. And you would need a crane capable of placing that last piece. Another big piece of equipment.

Regarding the 25% rule of thumb on embedment: these guidelines are more common that I care to admit. I've had to fight them a lot. The worst one is the 10% rule used by some electric utilities: a 65 foot long tangent structure would only get 6.5 feet of embedment. Not 7 feet - 6.5. Damn scary. As a result, 25% doesn't seem so bad to me - but the structure needs a proper design, not a rule of thumb.

MikeDB: I'm glad you've had 25+ years in the electric utility business and never heard of the __% rule. It seems to be a EE problem: I guess ignorance breeds bliss.

Cheers!

 

[1714217]

The N value from the SPT will by no means give enough information for you to arrive at value for the angle of internal friction of the silty sand. The percentage of silt will make a great different on the angle of internal friction. If you know thie percentage, you could use Zeevaert`s criteria and have a good idea of it's value, from here on, knowing tjhe depth of the water table, the N value can give an estimation of the weight of the material, which togheter with the internal friction will allow the calculation of the lateral resistance of the drilled shaft by any of the available methods.