uniaxial compressive strength vs bearing pressure

[stallion0002]

I have a foundation system that I need to provide anchors across the concrete/rock interface. I am concerned with the grout-to-rock bond strength since the rock is weathered fractured limestone with an allowable bearing capacity of 8000psf. (Yes, that is correct...only 8000 pounds per square foot.) I saw an article that recommends that the grout-to-rock ulitmate bond strength could be estimated as one tenth the uniaxial compressive strength.

The geotechnical report that is available makes recomendations on the bearing capacities but does not address the uniaxial compressive strength. Are these realted in any way to each other? Other than performing additional tests, is there any method to determine the uniaxial compressive strength?

 

[blgeo]

I believe the 10% rule you are referring to usually is only applied to 'competent, massive rock'. Judging by your description this is not the case. Trying to backcalculate any parameters from that allowable bearing pressure is a waste of time.

A contractor who installs anchors in your project area would be a good resource (presuming you can't contact the geotech who wrote the original report).

 

[BigH]

"bearing pressure" is not a measure of rock-grout strength - as you have noted. How "deep" is your weathered fractured limestone before you get to competent material? When I have had several metres of it, we used to just drill the anchors deeper - small cost to take an anchor a couple of metres deeper once the drill is set up and drilling. You might wish to check out NAVFACs from VulcanHammer's website for their section on rock anchors. You not only have the rock to grout case to consider but also the grout to anchor and the anchor cone pull-out. As for the allowable rock to grout - like was mentioned by blgeo - check out local specialty contractors for a "handle".

 

[Riggly]

I agree with the above two posts. By the way, what kind of structure is it? How many anchors? Did the geotech report address rock anchors?
Fracture limestone - is that even a proper location for the bond zone? You might have to pump a lot of grout. On the other hand pumping a large amount of grout could fill some of the voids and seal the fractures, and ultimately improve the pull-out capacity of the anchor.

Concerning the tests, I always believe a few more tests are cheaper than the cost of failure.
You may want to check the compressive strength of massive rock, not weathered fractured rock. For preliminary you should fine range of strength of competent rock in NAVFAC, but the local contractor is preferred.

 

[stallion0002]

Thanks for all your thoughts.

The foudnations is for a conveying system for a material handeling project. To complicate matters, the location is on the toe side of a client owned access roadway embankment and is right next to and possibly partway into a state owned creek. When it rains, it pours...hopefully not, that could just casue even more problems. LOL. Because of site conditions, we are limited with the access and availability of equipment to get in to excavate/drill for the foundations.

With all the concerns, we want back to the drawing board and reconfigured the conveying system so that the foundations in question is away from the creek and embankment toe. With the new location of the foundaions, the geotechnical engineer has said we can use 25psi as the rock-to-grout interface shear strength.

 

[Star2009]

hi all i need some clarification about skin friction bored piles. what is the normal construction practice? how we can do the Pile testing? i don't have experience in this skin friction pile.

 

[fattdad]

unconfined compressive strength represents the value sigma 1 minus sigma 3 (i.e., the diameter of a Mohr's circle when sigma 3 equals 0). The corresponding shear strength would be half this value (i.e., the radius of the Mohr's circle). If you are dealing JUST with a single value of shear strength, the correlation to ultimate bearing capacity is 5.7 this shear strength value.

That's your correlation.

When dealing with rock mass, this is meaningless as you can't depict defects in the rock mass when doing an unconfined compressive strength test in the laboratory.

There are all sorts of ways to look at the strength of a rock mass. I refer you to my thesis for more information (and citations to better references then my actual thesis - ha).

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

f-d

¡papá gordo ain’t no madre flaca!