Design method for rock bolts in shear 2

[mte12]

For a structure supported on rock, which experiences shear load only, is there a design method available for design of rock bolts.
The steel structure would be supported on either pedestals or a slab on top of the rock.

All of the literature that I've come across so far only deals with tension in bolts.

One of the mechanisms I came across (I think) was for preload in rock anchor that compressed the rock at surface and this may have been the mechanism for resisting shear. But I think preload was no different to snug or TB specification for a column base plate or structural steel.

I know that rock bolts are used on underground rock faces, to prevent a shear failure in the rock. But the shear load in this instance seems to me, to be well inside the rock face.

Are there any tips to start with.

 

[azcats]

I've not worked much with rock anchors - are they ever battered?

 

[human909]

How far are you embedding your bolts? Do you have a typical strength value of the rock? If it is strong rock then your bolt is going to shear.

 

[mte12]

Thanks for responses.

azcats, I believe bolt orientation is typically normal to face.

human909, the embedment length is of the order of 1m to 2m, Geotechnical Strength Index is 1, Young's Modulus 15GPa.

 

[human909]

I've never heard about a Geotechnical Strength Index or Geological Strength index but I brief google suggests 1 is very low strength. The 1-2m embedment would also indicate very low strength unless your bolts are massive.

Shear strength would likely be very unpredictable as surface crushing would have a good chance of occuring putting your bolt into bending.

You really should be chatting to your geotechnical engineer on this one.

 

[JLNJ]

I've used rock anchors just a few times, so take the following with a grain of salt.

To get good shear values, you need a defined shear plane. Rock bolts are often used on rough/variable/unpredictable surfaces, so applying shear right at the rock face is a dicey assumption.

 

[mte12]

Thanks for the additional comments.

It is difficult to come up to speed on this as there is very little prescriptive information out there.