I need a sanity check - Drilled Pier depth determination in Lpile

[bigjeremy]

I've been questioned by Clients lately on the depths of my drilled piers and was curious how others handle determining their depths. Their past engineers would take them 10-15' deep and no further, so I'm wondering if I'm being too overconservative. These designs are typically small vertical loads and medium overturning moment situations for substation equipment columns/poles. Example loading would be 3k vert and 12k-ft moment on a 3' dia shaft, with a depth of 21' that I chose in the below 1st screenshot. I try to choose depths when the deflection approaches 0 in the Depth vs Deflection chart, but as one can see in the Top Deflection vs Pile Length anything after about 15' is diminishing returns. I can shorten it to 15' with minimal increase in the top deflection but the bottom kicks out a miniscule amount. I've read many people take their depths past the reverse curve of Bending Moment and Shear Force vs Depth charts where both approach 0, which would put my length at 40' as can be seen in the 2nd screenshot where I bumped the length to 50'. Drilled piers are a bit of black magic in our industry and seems many do it differently. Let me know your thoughts!

 

[ANE91]

I don’t know what your underlying assumptions are. LPile is great but has some limitations; I agree that it can be a bit black-box-y, given that it’s doing nonlinear soil-structure interaction with soil springs. Maybe those other engineers aren’t neglecting the top 5ft or so.

Have you tried running a hand-calc using Brom’s method?

 

[haynewp]

I assumed some low quality soils and checked it with the more simple IBC flagpole equations for embedment and 15 ft looks in the range, or slightly more, than what I would expect to see for similar loads for pipe supports. I don't know the deflection at the ground you are wanting to keep, but you can see the deflections in the L-Pile runs are very small. You can rerun L-Pile with the shorter depth. I don't know of anything that requires going down to where the pier deflection is 0 if it works at shallower depths.

 

[CARunderscore]

I'm not familiar with this particular size and application of deep foundations, but I had a run-in with LPILE's sister, GROUP, on a project. I found a bug that made analysis of multiple load combinations according to the FWHA drilled shafts standard a humongous pain. The fact that such a productivity-killing bug could exist in such a popular program makes me suspect that maybe a lot of people designing deep foundations are just kind of doing whatever. Well, that and the fact that I've met a few of them.

I've never had to deal with people looking at tip displacement. For bridge pier drilled shafts, if I recall, you demonstrate stability under 125% (or 150%?) of the lateral ultimate load to establish a minimum depth for overturning. Then, you can make it deeper if you need more axial capacity (which I imagine is unlikely for a drilled pier supporting a monopole).

I don't know that I like how shallow in the moment diagram the 15 feet falls.

 

[Flotsam7018]

Agreed that your deflection tolerance seems very conservative. In my experience the top deflection was the primary serviceability criteria and was typically a ratio of the depth or a maximum magnitude such as 1", whichever is less.

 

[dauwerda]

The utility I used to work at allowed the lateral deflection at the top of the drilled piers to be 1/2" for substation structures. Most stuff was in the 8ft to 15ft depth range.
For the T-line poles, lateral deflection was allowed to go up to 3".