AUGERED CAST PILES

[EBAT75]

Anyone interested in offering answers to these 2 questions?

 

[Br_Engr]

Shooting from the hip...

532.  Pi x d = 3.1416 x 16/12 = 4.2 sq. ft/ft of length

L of soft soil = 30' => therefore downdrag = 4.2 x 30 x 500 = 63,000 pounds (D)

533. End resistance = Pi x 16^2/4 = 201.06 sq in. => 1.4 sf therefore end bearing = 8 ksf * 1.4 = 11.2 kips

Total resistance required = 60 kips + 15 kips = 75 kips

Skin friction (SKF) resistance required = 75 -11.2 = 63.8 kips

L rqd = SKF/1.5/perimeter of pile

= 63.8/1.5/4.2 = 10.13' => say 11' (B)

 

[EBAT75]

The solutions match the answers given in the old samples book. But, unless there was errata I do not have,

532.  Wouldn’t the drag-down itself cause bearing failure, if skin friction is not mobilized.

63,000 lbs drag/1.4 sf= 45,000 psf  > 5 times allowable bearing pressure.

To limit the allowable pile end load to 11,200 lbs, the depth of embedment in the firm layer has to be (63,000 - 11,200 = 51,800 lbs)/(4.2 sf x 1,500 psf) = 8.22 ft and skin friction of 1,500 psf also must be at play simultaneously.

The older codes allowed a combination of skin friction and end bearing. Newer codes allow only one or other, not both simultaneously. (IBC 1810.3.3.1.4)

The answers match the solutions based on older codes.

533. Under current codes, if only end bearing is considered, bearing area required would be 75/8 = 9.4 sf bearing area - needing a belled end.

If only skin friction is considered, embedment needed would be 75/(4.2x1.5) = 11.9 ft, say 12 ft.

 

[Br_Engr]

I did not see any reference to a code, so I approached it strictly as an analysis problem.

For 532 I interpreted the question as what is the downdrag force. Nothing more. You are correct in that the downdrag would exceed the allowable end bearing, therefore there would need to be a length in the firm soil to counter balance. But that was not what they were asking.

Plus, stop at 30' or before and you might develop the full end bearing resistance anyway.

My background is bridge, so I cannot speak to IBC requirements.

 

[EBAT75]

I agree they were only asking for the down drag. Some questions are abstract, do not relate to a real world case. Why I extended it. Was not implying that you missed anything.

Stopping at 30 ft you still have 63 k down drag sitting on 1.4 sf firm stratum that can take only 11.2 k in bearing. No good for bearing still. 

Stopping before also will not help because the bearing capacity of the soft stratum will be even worse.

 

[Br_Engr]

Apologies, I meant to say.

Plus, stop at 30' or before and you might not develop the full end bearing resistance anyway.

Enjoyed this discussion!

 

[Br_Engr]

Anyone interested in offering answers to these 2 questions?

View attachment 19771

Quick question.  What is the source for this question?  A sample exam?

 

[EBAT75]

Looks like a sample, from the multi-part question format. A fellow I know who is taking the Civil PE asked me this. He had come up with different answers, why I thought of posting.

 

[dauwerda]

The older codes allowed a combination of skin friction and end bearing. Newer codes allow only one or other, not both simultaneously. (IBC 1810.3.3.1.4)

I would like to point out that the IBC does not prevent one from considering both simultaneously, it simply requires that a geotechnical investigation be performed and that said investigation allows for them to be used simultaneously. Essentially it just doesn't allow you to use both if you are using the presumptive values provided in the IBC.

 

[EBAT75]

Section 1803 - A catch all section.

1806 - Presumptive values “...used in design for supporting soils near the surface...”. i.e. Shallow, Not deep foundations.

1810 - Deep Foundations.

1. 1806 does not apply.
   
2. 1803.3.1.4. -  “.....shall not be assumed to act simultaneously unless...”.
   
3. This problem makes no reference to the basis for the values used, whether it was recommended by a geotechnical engineer’s report.
   
4. 1803.3.1.4 also says “....shall not exceed one-sixth of the bearing values.....at minimum depth as set forth in Table 1806.2 up to a maximum of 500 psf.” I felt the 1500 psf is high but chose not to detract from a theoretical sample problem. Also, where is the minimum depth in relation to Table 1806.2? And, that table has only presumptive lateral bearing pressure, not end bearing values.
   

Finally, in more recent years there is a tendency in codes to place caveats and fall back on terms like AHJ, Building Official, Registered Professional, demonstrated by analysis etc etc. The intent is to say be wary of designing for reliance on skin friction and end bearing to act simultaneously, yet leave the door half open. IBC is adopted, yet modified by AHJs to suit their own practices and preferences.

IMO, at least on deep foundations for large structures, without validating borehole and lab results with load tests, a geotechnical engineer would be hard put to recommend using both skin friction and end bearing. Unless the settlement reaches 0.4 inch, use skin friction only; beyond that use end bearing only. Not both. This has been recognized practice. It is about when one or the other mobilizes. Load tests don’t come cheap.

There is an old adage that also used to be in contract documents - Ground is a hazard.