Rock Anchors

[spaseur]

I am designing a mat foundation using SAFE. We have significant overturning issues and are considering using rock anchors into the bedrock that our foundations are bearing on. We are wanting to model the rock anchors as springs. We are looking at #10 bar. How would you determine the spring constant for these rock anchors? Ks=AE/L but what L do you use?

 

[dik]

Best you talk to a local geotekkie... they are likely familiar with the rock mechanics of the local bedrock...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bones206]

Agree with Dik that this needs to be confirmed by a geotech, but a good starting point to estimate the active length of the anchor is:
[active length] = [unbonded length] + 1/2 [mobilized bonded length].

 

[dik]

Thanks... I've just used the unbonded length.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[S020]

By only assuming the unbonded length for your AE/L calc you could be cranking too much stiffness by way of too short a length. Suggest you use a sensitivity of L.unbonded to L.total. Using L.total may seem excessive but rock is not a homogeneous material and filled with potential imperfections that require movement to completely mobilise the anchor tensile capacity. I guess your L.bonded stiffness will keep you conservative until you receive advice from your geoteqq.

 

[dik]

concur... Thanks... go by the installed tension load... I guess based on a longer installed length.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[spaseur]

Just to be clear. When you say unbonded length, you are referring to the length of the rock anchor not bonded to the competent rock. Correct?

 

[dik]

That's correct with an additional length equal to half the bonded length... that accounts for a little extra 'stretch'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Agent666]

Don't forget as well that if you have preloaded anchors (in most situations you will), that you need to consider this in the stiffness. Until you exceed the preload force there is no movement. Unsure if safe does nonlinear springs.

If not it would be an iterative approach, find reactions, alter to secant stiffness, rerun, iterate until convergence.

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