Rock Socket Wall Capacities 1

[WARose]

I came across this paper and thought I'd share (since this topic has come up before)......

http://www.eng.auburn.edu/files/centers/hrc/930-425-2.pdf

Great summary of the various theories of rock socket capacities (wall pressures) from applied moments.

I've seen several geotechs use the Fragio & Santiago method for socketing into hard bedrock.

 

[TehMightyEngineer]

Interesting; I've not heard about there being any research of rock sockets being utilized to support lateral loads like this. That might have been useful a year or two ago for a project that never took off.

I only had enough time to flip through the paper but it seems like a wealth of information. Definitely something I'll keep in my back pocket; but probably something I'd consult with an experienced geotech before I ever tried to implement. Thanks for the link.

Got any pictures of projects that have utilized this? The black and white, copied photos in the paper were interesting but not that clear I'm afraid.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[WARose]

Nah, no pics. It’s interesting because I’ve talked to geotechs who didn’t have a clue themselves what the walls were good for. (They just defaulted to the end bearing value……which is comparable but often not the same.)

It’s also worth noting that in many cases (according to LPile) there is an additional moment from the “depth to fixity” in the socket. (Just like in soil.) You wouldn’t think the hole sides would “give” enough to have an appreciable value……but it does. (The last they did for me had a “depth to fixity” of about 2.5’.)

Still another consideration is the withdraw capacity (i.e. uplift) simultaneous with the applied socket moment (creating (theoretically) areas of zero contact). In general, I’ve seen a lot of geotechs not require any reduction in side friction based on this.

 

[Settingsun]

Six papers on this topic in the history of geotechnical engineering. Got the variation in rock worldwide covered then...

They just defaulted to the end bearing value……which is comparable but often not the same.

A senior structural engineer told me he typically uses half of end bearing capacity, on the basis that geotechnical engineers have given him advice of half to two-thirds over the years. As this document you've posted shows, the geotechnical engineers are also flying blind unless there are funds for a load test.

I thought the Fragio & Santiago curves looked useful at first glance as they give an indication of the depth at which strong rock might not undergo brittle failure (as suggested by the Reese curve), and also what happens above that. The Reese curve for strong rock gives brittle failure at a small displacement while F&S predict plastic failure at depth and some residual capacity near the surface which seems more realistic. If the strong rock is at the surface, the Reese brittle model tends to result in a small pile capacity before it 'unzips' - you could model the pile as embedded to the other side of the world but (I think) Reese still predicts no increase in capacity once the load is sufficient to cause the strong rock at surface level to fail. I've had to use Reese for guidance in the past but modify the brittleness to get something realistic. Now I'll have a published basis for that modification. My own modification was a bit less conservative than F&S and based entirely on gut feel.

Thanks for posting this. Pity the Reese strong rock model didn't get a workout.