Drilled Shaft Embedment Length Practice 3

[bridgebuster]

I was wondering what others do regarding the embedment length of a single drilled shaft, e.g. one that is supporting a noise wall or traffic pole. This is something I've never thought about because the situation came up.

For a single shaft is it customary to increase the embedment length for example 3 to 5 feet as a precautionary measure, in the event that someone migth excavate in close proximity to the shaft to install, for example, a utility line?

I asked one of the geotechs here and he says there's no reason to add extra length.

 

[FixedEarth]

See attaced.

 

[PEinc]

So, whose criteria are these?

http://www.PeirceEngineering.com

 

[bridgebuster]

FixedEarth - PE makes a good point regarding the origin of the criteria. However an allowance of 1 to 2 diameters seems prudent. I came across a design example by ADoT and they disregard the top five feet.

 

[FixedEarth]

bridgebuster - I used to do about 3 drilled pier jobs a year a decade ago. So everytime I had to relearn the analysis. Now, I do at least one drilled pier project a week. Sometimes I only provide geotechnical criteria for design and other times I do the substructural portion as well.

The above criteria meets geotechnical stability and serviceability of drilled piers in soil. I presented them here for you and future readers to benefit from it. It is a rational way to compute required embedment.

My firm has inherited a project last year where a client with a 2.5 ft tall retaining wall was given a 27.5 ft deep drilled pier embedment. Once I went through the criteria above, we came up with about 15 ft embedment. That project had a soil creep force of 1,000 lb/ft and a descending slope was near by, which you may or may not have.(The client did switch to us and the structural engineer and contractor were satisfied) On another job we bid, I saw a geotechnical report with a 4 ft high solar panels and the soil report recommended a 25 ft deep pier.

So I know a lot of people are just throwing embedment depths that are solely based on lack of knowledge on geotechnical stability and serviceability. May be other contributors can add about rock socketed piers, piers on expansive soils, etc.

 

[bridgebuster]

FixedEarth - I appreciate your responses and didn't mean any disrespect by my previous comment. At work I always get sucked into the annoying problems. Here's why I'm asking:

Last week one of the civil guys (they're working for a contractor) asked me "is it OK to put a 3" diameter shaft for a traffic signal 14' deep, 1' away from a water pipe 5' deep?" I'm assuming the water pipe is there and so my concern was damaging the water pipe. My assumption was wrong. They want to excavate next to the shaft to put in the water pipe.

My gut feeling: not a good idea. I said the safe thing to do would be to put a guy wire onto the traffic pole and excavate using a steel sheeting box to prevent movement (maybe it's belt and suspenders). The drilled shaft size and depth was taken from a DOT standard sheet. There's no indication that the embedment has any fluff. That's what got me thinking.

One question: Did you mean 25' high retaining wall? I agree with you about people coming up with embedment depths that could be erroneous. I have to design some shafts for a sound wall; one of the geotechs (the one who said no need for fluff) said he did a project with the same wall height (25') and post spacing (15"). I asked him what was the embedment of the shafts. He told me 50' minimum. I mentioned this up with one of the structural guys familiar with that project, he rolled his eyes. He told me that site has excellent soil.

Again, thanks for your help and I apologize for the misunderstanding.

 

[FixedEarth]

Yes it is hard to believe, but that retaining wall was only 30 inches tall. No surcharges were present other than a 150 psf driveway. I have respect for your experience & contribution and am not offended at all.

Now this is my opinion - the soils engineer should dictate the minimum pier diameter, minimum pier embedment, and minimum pier to pier spacing. Final design, as long as they meet the soils report criteria should be fine, even if the wall designer uses 2 times the minimum embedment value.

Just few hours ago, I reviewed a drilled pier plans that used 22 ft embedment, 18" pier diameter & spacing of 4.5 ft. I called in my soils report 12 ft minimum embedment, 18" diameter and min. spacing of 3B (48 inches). So I am happy to sign off, because he met our minimum geotechnical guidelings. Who am I to say, why did you use additional 10 ft pier embedment!

It is really too bad about the 50 ft pier embedment job with the good soils.

So let me add an example of when one thinks deep pier embedment is "more conservative" so it is better. When you have a given shear, axial downward, uplift and moment, you get favorable output (less deflection, less induced passive pressure, etc) with larger diameter than with deeper embedment of smaller diameter, everything else being equal. Try the beam on elastic foundations from Hetenyi to verify this statement.

If you want to get more in depth information on drilled piers, get yourself a copy of "Advanced Foundation Engineering" book by V.N.S. Murthy, 2007 +/-.

 

[bridgebuster]

Thank you FixedEarth. I downloaded the FHWA's LRFD Drilled Shaft Manual (2010). It's 972 pages but I have a very long commute to work. It'll keep me occupied.

 

[jayrod12]

I can't believe no one has mentioned it yet but depending on whether the piers are utilizing skin friction or end bearing also has an impact on embedment depth. So does frost action.

Where we are we need to discount the top 10 feet of any pile/pier in an unheated area to counteract frost action and so 20-30 foot embedment is the norm. We also are not doing 3' diameter but it's something to consider none the less.