Phi and cohesion for hard limestone?

[craigmcg]

I'm designing a sheet-pile seawall that will be founded in what borings indicate to be primarily hard limestone (>100 SPT blow counts). The geotechnical report give values for design of Phi=0 degrees and cohesion = 100000psf. The extremely high cohesion combined with low friction angle makes me raise an eyebrow to this. Anybody have any thoughts on this?

Furthermore, the location of the sheet piles will be, in some areas, in very close vacinity to what would resemble a 10' dropoff in the limestone, or a cliff. What would be the best way of determining a safe distance to which the wall could be set back from this ledge? It is a little difficult with my limited knowledge given the friction angle of 0 degrees.

Thanks a lot.

 

[dirtsqueezer]

I say this all the time, but you might just want to refer back to the geotech. Angle of phi is a vasue assigned to a soil, not a rock. That's probably why it's 0, kind of analogous to a null set. As far as cohesion, that also applies to soils! And I'm not sure the units are correct... They are probably tasking about allowable bearing pressure. I might just find another geotech firm, show them the report, and have them do your setback for the shoring. I did thosefor my old firm, but it's not just a number you pull from a table. You need some site recconossaince, and an experienced geologist to recommend, maybe even rock borings. I say find another firm because it sounds like something's out of place. Why not give someone else a shot?

 

[msucog]

i agree...i wouldn't go throwing 100ksf out there for rock without core data (at least in my part of the world). however, the geotech might be much more comfortable with the local conditions than i can infer by reading information second/third hand over the internet. but 100ksf seems to the higher side relative to rock. "drop off" in limestone might be something else...notice the remark "without cavities" in the linked source below. what part of the country are you in craigmcg?

by the way, here's a good source (navfac/ufc):

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_220_03fa.pdf

see page 6-2

here's the link to the entire collection:

http://www.wbdg.org/ccb/browse_cat.php?o=29&c=4

 

[PEinc]

To me, the bigger question is how or why you need to design a sheet pile wall in hard limestome. You can't drive the sheet piling into the limestone; therefore, you will not be retaining the limestome. Please give more information on what you are trying to do.

 

[fattdad]

Phi and C are perfectly fine expressions for shear strength of a rock mass.

Here's my thesis:

http://home.comcast.net/~fatt-dad/thesis.pdf

There's some background on rock mass properties.

f-d

¡papá gordo ain’t no madre flaca!

 

[dirtsqueezer]

How do you do a triax test on limestone?

 

[MSEMan]

Before going any further you may want to ask how to drive sheet piles into 100+ blow count material. If you have good limestone to found on you may find that building an MSE wall inside a coffer dam will prove more cost effective.

 

[msucog]

to compare cost effectiveness with mse walls for anything, i suggest the client be made fully aware of the added exploration, evaluation and material testing costs associated with the things (which is typically not disclosed to the client at the front end of the project from my experience). the wall designer sets the testing protocol so the designer should discuss this with the client. i won't get on a soap box about this today, however, i would like to see the mse world change some of their "tactics" so that everyone can have a more positive attitude toward the things. until then, the only positive comment i will have is that they look nice while they're still standing.

as far as phi/c of limestone, i'll take fattdad's comments informatively and will read the thesis once i get the chance since it seems interesting from what i've read so far. however, i don't normally think of rock in this manner and don't recall seeing rock properties reported in this manner. but i'm sure lots of folks don't normally think in terms of shear wave velocity either but i happen to be somewhat familiar with that particular characteristic. so i'd be interested to learn more about this if someone has real world experience where phi/c of intact rock is useful. also, how would you determine the values? i'm definitely not an expert in rock mechanics but should probably know more about the subject (i'd say i should know more about everything we do since i still learn something new every single day). fattdad, if 100ksf is actually the bearing capacity, does that seem reasonable to you for upper limestone (given: we don't know what core data is available)? if 100ksf is actually the c, does that seems reasonable to you?

i'm wondering if craigmcg might be near karst formations. if it is suspect, one relatively quick way would be to use refraction microtremor and seismic refraction surveys to further evaluate the top of rock profile and to evaluate whether there might be cavities below the top of rock. with one set up of the traverse, you could gather data for p-wave refraction, s-wave refraction, and ReMi. p-wave will help identify groundwater and some properties of the soil/rock and s-wave will identify the top of rock as well as confirmatory information about the soil/rock. however, seismic refraction will not likely identify a lower velocity layer below a higher velocity layer. however, ReMi has the ability to see lower velocity layers below higher velocity layers (i.e. sink holes, rock lenses, etc) as well as assess the shear wave velocity of the soil and rock to depths of up to 100-300 feet deep.

 

[fattdad]

100 ksf sounds high for unconfined compressive strength (unless it's for the intact rock mass, i.e., not considering ANY discontinuities). It may or may not be appropriate for your bearing stress - hard to tell in cyberland.

There are other ways to evaluate phi and c than using the triaxial test cell.

f-d

¡papá gordo ain’t no madre flaca!

 

[dirtsqueezer]

What other ways? I know about the Mohr-Coulomb diagram, minor and major principle stress, and something about with rock it's reversed than how it is for soil. Is this something along the basic lines of the principle?

 

[fattdad]

Firstly, recognize that a rock mass has discontinuities and at the global scale these discontinuities together with the intact rock behave different than you would measure in the laboratory. To consider the intact rock you could do a triaxial test (it might be difficult to do, but maybe. . . .), but you would then have to adjust this peak strength to account for the discontinuities.

If you look at my thesis, there is a RMR (rock mass rating) that provides a flow chart that considers properties of the intact rock and the discontinuities to give a 5 part rating. Each of these 5 classes has a phi and c attributed to it. That would be one way to assign a rock mass failure envelope.

Another method that can be used to evalate a failure envelope for rock mass is to do field mapping in areas with failed slopes and do back-calculation. This works well for soft-rock engineering or when the failure criteria is directly influenced by bedding planes or other linear features.

There are also down-hole methods to test for rock strength. The Goodman jack comes to mind, but I'd have to look further into this to refresh my memory. I may have something in my thesis about that. . . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

You can download a free program from the rocscience website that was developed by Evert Hoek. It provides estimates of Phi and C for rock masses based on a GSI (Geological Strength Index) and some other descriptive values including slope height. The GSI takes discontinuities into consideration. It is a good first aprroximation.

 

[Touma08]

The book by Hoek & Bray is a good reference on this subject. C & Phi are very much a function of the rock mass status (degree of fracture, spacing of fractures, nature of in-seems...).

I also have to go back and re-ask PEinc's question: Why the sheet piles in rock ?? I assume that you'll be doing predrilling then refilling the hole with sand or grout after pile insertion... naturally the holes will be overlapping in order to benefit from the interlock between the piles.