Grout to Rock Bond Stress

[SEshore]

There are several publications that list average bond stresses between grout and rock for differing types of rock, but none of the information I have at my disposal(FWHA & PTI) correlates grout compressive strength of grout to bond stress.

Intuitively, it seems like there must be a minimum grout compressive strength needed to be able to achieve the ultimate grout to rock bond stresses, but I cannot find any information as to what that may be.

PTI recommends a minimum grout strength of 3,000 psi for all grout, so it may be reasonable to assume that ultimate bond stresses can be achieved if the grout is at least 3,000 psi. But at what point does the compressive strength of grout begin to negatively impact grout to rock bond stresses?

 

[PEinc]

I'm not sure you will ever know. If a tieback anchor is tested successfully, the grout strength is OK, no matter what the strength. If the anchor fails, there could be several reasons, grout strength being one of them but also the least likely reason. Grout strength is rarely the cause of a failed anchor. I never call for grout cube testing.

http://www.PeirceEngineering.com

 

[SEshore]

I agree that it doesn't matter if a tieback is sucessfully tested. If you had a micropile job it might be a different story, since you don't typically test all of your production micropiles.

The answer to the question is easy for soil, as long as the grout is harder than the soil you should be able to achieve your ultimate bond stress for soil. But in rock, the grout may never get as hard as the surrounding rock...

 

[hokie66]

Strength testing of grout wouldn't tell you much about the performance of tiebacks. Strength testing is done with unconfined specimens, while the actual grout in place is confined.

 

[PSlem]

I had an old article that said minimum grout strength was related to the relative modulus of the two materials.

 

[PEinc]

Low grout strength could be the cause of failure for a tieback or a micropile but it is hard to prove. Also, if a contractor is sloppy with the grout mix, he is probably sloppy with other aspects of the installation, such as cleaning the hole, tremie grouting, and use of centralizers and/or spacers - any of which can cause or contribute to failure.

http://www.PeirceEngineering.com

 

[BigH]

For all the above - we use are using 20MPa at 3-d and 40MPa at 28-d - in peridotite.

 

[dirtydude]

In either case, micropiles or tieback anchors, there should be a testing program that verifies the grout strength. Micropiles usually have one or two verification tests initially on a project. That in turn will not only yield results for the grout to rock bond stress, but that the grout strength is inherently adequate.