Phi and cohesion for hard limestone?
Phi and cohesion for hard limestone?
(OP)
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.
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.





RE: Phi and cohesion for hard limestone?
RE: Phi and cohesion for hard limestone?
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:
h
RE: Phi and cohesion for hard limestone?
RE: Phi and cohesion for hard limestone?
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!
RE: Phi and cohesion for hard limestone?
RE: Phi and cohesion for hard limestone?
RE: Phi and cohesion for hard limestone?
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.
RE: Phi and cohesion for hard limestone?
There are other ways to evaluate phi and c than using the triaxial test cell.
f-d
¡papá gordo ain’t no madre flaca!
RE: Phi and cohesion for hard limestone?
RE: Phi and cohesion for hard limestone?
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!
RE: Phi and cohesion for hard limestone?
RE: Phi and cohesion for hard limestone?
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.