soil nail bond strength

[Soupbone9437]

I am constructing a soil nail wall. A few of the grout results have come in below 3000 psi. Does anyone know of a resource to help determine the grout to soil bond strength based on these weaker grout results?

 

[PEinc]

Grout for tieback anchors and soil nails is not usually tested. As long as you mix the grout to about a 0.45 to 0.5 W/C ration and do not let it sit too long, the grout should be OK and should not affect the soil to grout bond.

I assume that you will be testing at least some of the nails. If the test nails hold the test loads, the grout is obviously OK also. Grout strength is relatively unimportant for tiebacks or soil nails. The proof is in the pudding. Test the anchors and nails, not the grout. What does it mean if the grout test fails but the nail or anchor test passes??????

 

[born2drill]

PEinc:
Grout for tieback anchors and soil nails is usually not tested - unless you're working for the DOT...

Soupbone9437:
If a verification nail or two was successfully tested, then the assumed bond stresses are good. Those values don't change if you mix lousy grout.

If you really need some reassurance on paper, why don't you do a calc to "back into" the minimum grout strength you need for the maximum load the nail will see. We don't have any of the particulars for your job, but for your soil nail loads I'd bet it's quite a bit less than 3000 psi.

 

[PEinc]

born2drill is right about DOT's wanting to test grout. It's hard to break their old habits. But the bottom line still is whether or not the anchors test successfully. If the anchors fail, the grout tests could help determine the reason but the contractor still has to install supplementary anchors whether it was the grout's fault or not. Grout strength is one of the less important factors in an anchor's performance.

 

[born2drill]

PEinc:
Oh I agree 100%, and especially for tiebacks where you're proving each and every one by test. The anchor is either good for the load or it isn't. It's not about the sum of the parts. But your anchors can be fine and some inspector looks at a couple of grout breaks that don't meet spec and starts causing trouble. Unfortunately common sense arguments don't always take you as far as they ought to.

Soupbone9437:
In short - if you haven't been doing any testing already, run a jack out there and get on a couple of nails so you have some information. If those are 28-day breaks, you ought to have a lot of wall already built, and you really should have *some* performance data.
Less importantly - you said: "a few of the grout results have come in below 3000 psi." Does that mean that the majority have been "acceptable" (per spec)? Are you taking your own cubes? Were the low results all from one batch? Do you know that the cubes are being made, stored, & handled properly?

 

[Soupbone9437]

We have one week of production with the weak grout results. We have tested 5% of the nails and they passed the proof tests. The grout strengths have since come up to acceptable levels. Right now, the designer is holding firm on the 3000psi requirement. I am trying to figure out a way look at it from an engineering POV to determine how the lower strength affects the long term performance of the wall. It is my understanding that the grout to soil/rock bond strengths are taken from a design table that assumes 3000psi grout. Is there another way to calculate the bond strength?

 

[PEinc]

I have designed many soil nail walls - both temporary and permanent, private jobs and DOT jobs. Never has the grout strength entered into the design. A few times cubes have been made by the owner's inspectors but they were never used for anything. Grout cube strength also, I believe, can vary significantly depending on the size and shape of the sample and on the type of mold. Perhaps someone else can comment on this.

FHWA calls for 1500 psi (10.5 MPa) at 3 days and 3000 psi (21 MPa) at 28 days. PTI calls for 3000 psi grout strength at the time of anchor stressing. However, PTI also says, "Grout cube testing is not normally required, but may be utilized to evaluate the time of anchor stressing and the quality of the grout mix. Insufficient cube strength shall not be a cause for rejecting a successfully tested anchor."

What is the designer looking for? A credit for lower strength grout? Ask him how he used grout strength in his design. The grout strength is just not a required input value for the design of a soil nail wall or a tiedback wall. Even weak grout is much stronger than the dirt it bonds to; and the bond between the grout and bar is very much stronger than the bond between the grout and the dirt. If an anchor fails, you will not know if it was caused by weak grout unless, maybe, the tendon pulls completely out of the grout. Even then, the cause could be a dirty tendon rather than weak grout. If your anchor tests are good, the grout is good enough.